After a short hiatus while I sorted out some tech issues with automation that helps me create these posts, we’re back! The Speaker Spotter is a recurring feature curating interesting speakers for sale on Craigslist from around the Pacific Northwest. Links to Craigslist postings might go down at any time if they’re sold or pulled from sale.
The L65 Jubal (1975–79) is the three-way in JBL’s mid-1970s lineup that sits between the L36 and L100 in size but above both in price, with a 12-inch woofer, the LE5 midrange and the 066 dome tweeter under a smoked-glass top — one of the more coveted seventies JBLs. The seller reports the woofers were recently professionally refoamed, all drivers are original, and the veneer is mostly very good apart from some storage wear.
Near-perfect drivers, updated binding posts and original grille frames with the buyer’s choice of Quadrex foam or fabric inserts; the seller candidly notes the customary planter stain and cigarette burn. The L100 Century is the consumer version of JBL’s 4310/4311 studio monitor and arguably the best-selling speaker of the 1970s, its punchy 12-inch three-way sound and orange Quadrex grilles now firmly iconic.
Seller describes a near-mint pair that looks freshly unboxed. The Reference Standard 4b is an early-1980s Infinity tower built around the company’s EMIT planar-magnetic tweeter and polypropylene-cone woofers, and that EMIT top end is what keeps the RS series on collectors’ lists. This seller is taking only phone calls to keep the noise down to seriously interested parties only.
One-owner pair in great shape, lightly used over the last 13 years and now stored in a shop. The Chorus II (1990–96) is the biggest of Klipsch’s non-Heritage horn designs — 15-inch woofer, horn-loaded midrange and tweeter, and a 15-inch passive radiator on the back — delivering La Scala-style dynamics in a cabinet that fits a normal living room. It’s a real shame Klipsch isn’t making anything like this anymore.
Very good, fully functional condition per the seller. The CDM 7NT (circa 2000) sits just below B&W’s Nautilus 800 series and borrows its metal-dome tweeter, Kevlar midrange and Flowport, which makes it one of the more affordable ways into Nautilus-era B&W sound.
Restored circa-1974 pair the seller has owned for eight years and calls stunning; the price was recently reduced. The L36 was the three-way of JBL’s Decade series, pairing a 10-inch woofer with the LE5 midrange and a dome tweeter in a compact walnut cabinet — essentially a scaled-down L100.
Described as excellent condition. The CS-99A was Pioneer’s early-1970s flagship, a six-driver system with a 15-inch woofer, dual midranges, horn tweeters and a super-tweeter behind the elaborate lattice grilles that make it one of the most visually distinctive Japanese speakers of the era. A little kabuki, but you’ll get decent sound out of it and it’d look good in an MCM living room.
Vintage pair in great shape with only minor scratches, closet-stored for a decade, and including the essential active equalizer. Amar Bose’s 901 (1968) is the original Direct/Reflecting design, firing eight of its nine 4½-inch full-range drivers at the wall behind it — polarizing then and now, but a landmark of audio history. I have a detailed write-up about these speakers here on Retrovoltage:
Good working condition with the SDA interconnect cable and stands included; the seller notes a big, spatial sound. The CRS (Compact Reference System) is the smallest of Polk’s Stereo Dimensional Array speakers, which feed a second driver an inverted crossfed signal to cancel interaural crosstalk — one of the more unusual and effective stereo-widening designs of the 1980s. I haven’t seen these before, would be curious to hear how they sound.
Good cosmetic shape, functioning flawlessly, and about 28 by 16 by 11.5 inches. The Monitor 10 was Polk’s flagship through the late 1970s and much of the 1980s — dual 6½-inch drivers, a 10-inch passive radiator and a silk dome tweeter — and remains a favorite budget vintage tower.
Seller says this military-exchange variant of the D-1 is considerably rarer, with beefier components and a slightly larger cabinet; 8-inch woofer, 1-inch horn tweeter, 92 dB sensitivity, roughly 20 by 11 by 10 inches. The D-series was Cerwin-Vega’s 1980s bread and butter, and the small D-1 is the entry point to that red-surround, high-efficiency house sound.
Still in the original Sears boxes with the Styrofoam, plastic and manuals — sold untested since the seller hasn’t unpacked the 53-pound cabinets. LXI was Sears’ house hi-fi brand, and a 15-inch three-way with dual 5-inch midranges is classic late-1970s catalog big-box fare; sealed-in-box examples of anything from this era are rare.
I went to a local electronics flea market recently, and came home with a couple pieces of gear that will be good additions to my test bench:
First up is the HP 53181A Frequency Counter, 0-225 MHz, 2 channels, and some basic math functions built in along with an external reference input. Seen here after powering on and warming up for a few minutes, measuring my GPSDO clock reference – spot on! Not bad for $80.
Then there’s this Tektronix 434 Storage Oscilloscope. This one was free: another patron at the flea market purchased it while mounted to a Tek scope cart, but only wanted the cart. It’s dirty, both outside and inside judging from the noisy and sticky controls, but it powers on and displays a trace. It’s a “storage oscilloscope” where there’s an extra set of grids that absorb a charge from the electron beam and reflect it onto the phosphor, giving a primitive view of where the beam has been – and accounting for the splotchy appearance on the screen. I think something is stuck with that circuitry, because it showed a clean trace the very first time it was powered on, but ever since it’s behaving like this.
The Tek 434 was a portable scope produced from 1971-1984, and it used an early switching power supply that’s basically impossible to service if it goes bad due to using a bunch of custom transformers and a custom controller chip. I’ll probably clean and recap this one eventually so it doesn’t die. That’s a low priority project, though: this is a pretty low-bandwidth (24 MHz) scope, and I already have a Tek 465 analog scope, plus an older 2-channel and a newer 4-channel Rigol digital scope.
I just need to find a decent spectrum analyzer, and that’ll be the last piece of gear I really need for the bench to do just about anything.
A recurring feature curating interesting speakers for sale on Craigslist from around the Pacific Northwest. Links to Craigslist postings might go down at any time if they’re sold or pulled from sale. Some really interesting, highly unusual things in today’s round up.
Danish maker Börresen builds its X-series around an iron-free, ultra-low-hysteresis motor and a planar ribbon tweeter — exotic engineering that normally lives well above this price. This pair is 1.5 years old in as-new white finish and comes with the matched stands, sold to fund the owner’s return to DIY speaker building.
The IRS Delta sits in Infinity’s legendary Infinity Reference Standard line featuring EMIT ribbon tweeters and EMIM planar midranges paired with a powered woofer section, an approach few manufacturers ever attempted at scale. The seller describes them as pristine and long out of circulation. I’m going to be keeping an eye on these myself and if they don’t sell in the next few weeks, might bring them home with me following an already planned trip to the Portland area….
Arguably the most iconic American bookshelf speaker ever made, the L100 was the consumer version of JBL’s 4310 studio monitor, and its orange Quadrex foam grille is a design landmark. (An ugly one, in my opinion!) This pair was bought new in 1973, comes with the original paperwork, and has been recently restored, with one driver reconed roughly 15 years ago.
The Forte is Klipsch Heritage’s rear-passive-radiator design using a 12-inch woofer plus horn-loaded mid and tweeter in a compact-for-Klipsch cabinet, famously easy to drive with low-power tube amps. Oiled oak in near-flawless condition is the finish collectors chase, and the seller reports they work and sound exactly as they should.
The 4312B is JBL’s long-running 3-way studio monitor — a direct descendant of the L100 lineage, with front-panel level controls and a 12-inch white-cone woofer that defined broadcast control rooms for decades. These are still in original packaging and quite possibly never used, with one shipping scuff on a single enclosure.
These are JBL 4671 cinema/studio monitors — industrial horn-loaded systems built for theater-scale output, using the well-regarded 2425J compression driver that audiophiles prize for smooth, non-fatiguing treble. At over 100 pounds each, these are a project for someone with the room and the amplification to match.
A late 1960s KLH Model 20 compact stereo system. KLH under Henry Kloss built acoustic-suspension speakers and integrated systems that punched far above their size and remain a mid-century design favorite. Sold as-is: the FM tuner works but the record player needs attention, and the pedestal speakers are included. These aren’t awful to work on, but for whatever reason full service manuals are tough to find (although there’s a schematic concealed under the chassis glued to the interior of the cabinet) and there’s a mix of difficult to replace germanium transistors along with the easier silicon. Parts units are plentiful on eBay at least.
The Studio 590 was designed under longtime JBL chief engineer Greg Timbers and brings cinema-derived technology home — dual 8-inch woofers and a Teonex compression driver on a High Definition Imaging horn. An imposing, highly dynamic tower that rewards being played loud.
a/d/s (Analog & Digital Systems) built the L810 in Massachusetts using Braun-derived German drivers, earning a reputation as one of the great neutral, imaging-focused monitors of the late ’70s and ’80s. This pair is in excellent working order with a few cosmetic digs, and the seller notes comparable sets fetch considerably more at auction.
Phase Technology traces back to United Speaker Systems, the company that patented the soft-dome tweeter, and the V-10 is a substantial three-way tower from its higher-end Velocity line. Described as pristine from a non-smoking home, complete with the plastic feet and two sets of metal spikes.
Boston Acoustics’ A-series was the New England answer to the AR/Advent school — real wood enclosures and unfussy two-way voicing that still sounds right today. The seller has already refoamed both woofers and reports the domes are dent-free, which removes the usual restoration hurdle on a speaker this age.
Cerwin Vega built its name on high-efficiency, high-output designs, and the AT series carries that party-capable voicing in a vintage cabinet. This set has fresh woofer surrounds and new grille cloth, so the two things that usually need doing are already done.
The Bose 901 Direct/Reflecting Speaker System is as polarizing as it is interesting, a unique system from the golden age of hi-fi experimentation. It’s less about perfect sonic accuracy and more about the experience of listening to your music. And that’s one of the things I love about it!
While cleaning up some of my archives to save cloud space, I found that I’ve serviced nearly 500 Active Equalizers which are the important heart of these speaker systems. I’ve distilled some of the knowledge from all of that experience here, to help anyone else who wants to keep these systems running, and get good info from someone who actually has any clue what’s going on.
There is a truly staggering amount of AI slop out on the Internet about these speakers, ranging from slightly wrong to utter nonsense. In contrast, this article was written by a human who has owned and listened to all of these systems, and serviced and studied hundreds of them over thousands of hours
This article covers the Bose 901 Series IV Speaker System and the Active Equalizer. Keep on reading for all of the details, including schematics, repair tips, original documents, circuit variations, and direct links to buy all the parts you need for the 901 Series IV repair on Mouser.com.
This article is part of a series. Scroll all the way to the bottom for the others.
Bose 901 Series IV Speaker System Background
The Bose 901 Series IV speaker system was introduced in 1978, and is the successor to the Series III in the long series of 901 Direct/Reflecting Speaker System designs that spans more than 60 years from the earliest introduction in 1968.
As with the others in this family, they’re based on research suggesting that most of the sound you hear in a live performance arrives indirectly through reflections. Aiming to give a listening experience like a concert hall which adapts to nearly any room, Dr. Bose designed the speakers with nine small, identical full-range drivers instead of a traditional woofer or tweeter. Eight on the back, one on the front, with no crossover components inside and all frequency shaping done on the external Active Equalizer box.
The Series IV is substantially similar to the Series III, using 4″ drivers. The cabinet continues the same ported configuration as was introduced in the Series III, using helical voice coil drivers with foam surrounds. The speakers themselves were only a small evolution over the Series III, with the real enhancements being in the Active Equalizer box.
As with all 901 speaker system generations, the equalizer really is necessary to get the best sound out of the system – and sadly, it gets lost or damaged a lot of the time. Without it, the midsized drivers don’t get the drive power they need on both the low and high ends of the frequency range to perform properly and you end up with the derrogatory description, “No Highs – No Lows”.
If you’re absolutely in a pinch, you can crank the lowest bass and highest treble adjustments on your amplifier up as high as they’ll go, but unlike the first generation of 901 equalizers, the Series III, IV, and above use a summed complex curve that’s very difficult to replicate. You’ll need a multi-band EQ at a minimum. Find yourself a real one, or a mini-DSP replacement, for best results. Take a look at the Owner’s Manual for all the details on how these are connected to your system, and the principles of operation.
Like the other series, these were sold worldwide, and had a pretty solid following in Japan both in their domestic market as well as with overseas US military servicemembers.
Bose 901 Series IV Speaker Drivers
Starting with the Series III and continuing through the end of the series, Bose switched to using a more pliable and faster-responding foam surround on the speakers. This helped with the efficiency, but unfortunately foam does crack with time and will very likely need all 18 drivers to be reconed. Kits are readily available on eBay and from other vendors, and while it’s not a fun job, it’s more tedious than difficult. You can see some of the cracks in the drivers here:
Drivers are connected with wire wrapping, so buy a wire wrap tool or solder them when replacing.
When refoamed, they’ll last another few decades.
Don’t run the speakers other than just for basic testing with dead foam, or you’ll burn out the voice coils which are otherwise highly reliable.
Bose 901 Series IV Active Equalizer
The Bose 901 Series IV Active Equalizer is doing the heavy lifting to shape the signal and provide the depth of bass, clarity of treble, and wide soundstage the 901 speakers are known for. It’s an interchangeable frequency curve with the Series III Active Equalizer, and the Series V and later are very close so you’d be unlikely to notice a difference if you had to mix-and-match for some reason (although I always encourage matching sets whenever possible for aesthetics.)
The Series IV equalizer keeps the same familiar sliders for low and high frequency adjustments and the wide, squat form factor in a wallnut case. There were 5 production variations of this equalizer, but the repair process is going to be substantially similar for all of them, and the most common variant is shown here. (The next section describes the differences.)
The biggest change is the switch from using discrete transistors to adopting a more modern configuration and using op-amp chips to do the work shaping the signal. This brings the Active Equalizer into the modern age of electronics design as of 1978.
With the advantage of using modern construction techniques and components that withstand the test of time, there’s relatively little to replace in these units. This variant, shown above, is the most common mass-production version. Starting with the Series IV, different regional units used an appropriate power transformer for their region (220V) which could be converted back to 120V easily (see schematic), but the 120V are not readily convertible just by changing a resistor or moving a wire. A huge number of these systems were sold on overseas military bases and eventually made their way back to the US, so this was a great bit of convenience for anyone who brought them home from overseas.
Above, the schematic for the Series IV. This shows the right channel; the left is identical. These are extremely easy to service, you can remove the whole board from the case without much difficulty.
In general, you’re going to want to replace the electrolytic capacitors on these boards as parts that are more than 50 years old fail, leak, and cause the equalizers not to work anymore. Like the earlier series, the capacitor leads are crimped on the bottom from the assembly process and need to be clipped to remove. These use 2 x 470 uF capacitors in the power supply section, and 8 x 10 uF capacitors through the signal chain.
You may also need to replace the chips:
On January 3rd, 1983, Bose issued a Product Service Bulletin covering the 802 and 901 IV equalizers concerning the equalizer IC chips:
Failure tracking has shown that a higher than average incidence of failures in equalizers has been due to a parts problem. Early production runs of the 802 and 901 Series IV equalizers used National LF 353 op-amps. It has been shown that some of these components have lead contamination problems and are prone to failure. Typically, they exhibit high noise, although many will fail completely.
National has since modified their processes so that this failure mode is no longer prevalent. Additionally, we have changed vendors to Motorola TLO 72 or Raytheon 4558 and 4559.
It is suggested that for all equalizer failures that are using the LF 353 IC, that all four IC’s be changed for the Motorola or Raytheon IC’s mentioned above. This should reduce the likelyhood of additional feature failures in the customer’s unit.
Questionable use of apostrophes orignial, and they meant “TL 072” for the Motorola chip. If you come in with a unit with LF 353 op-amp chips, absolutely replace them. I have run into units where original TL072 or RC4558/4559 have gone bad for one reason or another, but this is fairly uncommon.
If you’re lucky, you have a board with factory socketed ICs: shown above, (bad) LF 353 op-amps in factory sockets. If you’re less lucky, you’ll have to de-solder them from the bottom and replace them manually. I usually add DIP-8 sockets at this point, which ensures that you don’t damage the ICs themselves from excess heat while soldering. Earlier models seemed more likely than later models to have socketed ICs.
One important note: the Series IV RCA jacks for input, output, and tape loop are soldered directly to the boards; over the years of connecting and removing cables, the jacks are prone to cracking. I strongly recommend re-soldering all of the connections on the bottom for the center signal pin and grounds. You can clearly see in this photo how the joints crack and lose their connections.
Finally, I recommend using control cleaner on the switches and sliders and working them through their travel several times, then apply fader lube to the sliders to keep them in good condition. This is the final equalizer curve from a properly working Active Equalizer:
Circuit Variants
There are at least 5 circuit variations of the Series IV Active Equalizer which exist. Two of them seem to be just a minor change in the power supply physical layout. Three others have significant variations, although all use op-amp chips, and are not well documented.
Mass Production Version
Most commonly, you’re going to find this version. There are two tiny variants, with no schematic differences that I was able to note. The earlier variant has staggered 470 uF filter capacitors; the later puts them in a straight line.
Early Production Prototype Version
I did come across one model which was a real head scratcher when I saw it. I believe this to be right from the start of production: the circuit board was the same PCB material used in the Series III, and some of the finishing details on the case strongly reminded me of the previous generation.
Inside, the physical layout was drastically different from the linear arrangement of chips found throughout most of the series runs, and there were plenty of extra jumper wires. I regret not tracing out the schematics when I had this on my bench at the time. It worked identically after replacing the op-amp chips and electrolytic capacitors.
I encountered exactly 1 of these to be repaired over the years.
Production Evolution Version
A second variant adopts the more linear style, but still has some variations. It used the same components, but one op-amp chip was offset at a 90° angle from the others:
I repaired a total of 2 of this version over the years.
Late Production Version (“Series 4.5”)
Finally, towards the end of the production run, the board style evolved, components became the 1980s variants, the layout was further optimized to remove electrolytic capacitors, and the power supply went from being an unregulated supply to the same regulated supply found on the Series V.
This version uses only 4 x 10 uF capacitors from above, along with the same 2 x 470 uF main power supply capacitors. It also adds 2 x 1 uF 50V capacitors, one used with each power regulator. (I’ve specified an NP but you can use a polarized cap.) I also encountered 2 of this variant over the years. Owing to the reduced use of capacitors, this one almost certainly has some significant differences in the signal path, but they’re highly reliable and by this point you most likely only need to replace the capacitors to restore full operation.
Conclusion & Postscript
The Series IV was one of the most popular models of speakers, and was variant I worked on most second only to the Series I/II, with a total of 51 units repaired over the years. I’d highly recommend this series if you are looking for a 901 system of your own.
I am not accepting repairs currently, but I hope this will help you with yours.
Mouser parts were correct and current as of publishing (August 2026). They may change over time. And probably will: when I started with these equalizers in 2012, I was using Xicon 1% metal film resistors, and Nichicon axial electrolytic and film capacitors. By 2021 when I stopped doing this as a business, Nichicon axial electrolytic caps were unavailable. Now, their film caps are out as well. I’ve selected good quality replacement parts which I’d be happy to receive if I paid for the repair in another shop, and which I’d confidently use if I were doing commercial work at the moment.
A lot of modern amps don’t have a tape loop anymore, and there can be some issues with sensitivity mismatch on modern gear if you use the Bose EQ in the pre/main loop. That’s the only real downside. These speakers and their companion amp are best used as part of a vintage system. If you’re planning to use them with a modern system I recommend you first try using a modern EQ or the EQ functions built into your gear and crank the bass/treble; if not, a MiniDSP-based solution would be an OK bet but can require a little bit of know-how to program.
Leave a comment if you have a question, and I’ll try to answer as best as I can.
Other Resources
You can find my other articles about different 901 systems here:
A recurring feature curating interesting speakers for sale on Craigslist from around the Pacific Northwest. Links to Craigslist postings might go down at any time if they’re sold or pulled from sale. Slightly fewer in today’s roundup than usual, but a few interesting options out there!
Really uncommon omnidirectional speakers, sometimes considered the holy grail of that genre. The Ohm F uses Lincoln Walsh’s inverted-cone driver to radiate the full frequency range in every direction from a single transducer, no crossover required. Genuine Fs turn up rarely and the seller describes very little use with an excellent exterior, making $1,100 a serious bargain for a speaker that still sounds unlike anything else made.
The speaker that put a Quadraflex foam grille in every 1970s living room, and arguably the most iconic American monitor ever built with a 12-inch white-cone woofer, midrange and tweeter and front-panel level controls. The seller reports flawless cabinets and grilles with zero nicks, scratches or water stains, which is uncommon for the now fifty-year-old L100.
Klipsch’s KG4 was the value entry to the family in the mid-1980s, pairing a 10-inch woofer with a passive radiator on the rear for surprising bass extension out of a compact tower. High sensitivity makes them a natural match for low-power tube amps, and the seller admits the only reason they’re going is an overstuffed listening room. I used to own a set of these, and used them as my test bench speakers: they were efficient enough to work with literally anything, and had a wide enough frequency range to be useful for testing all kinds of music. Even driven by a 1/2W per channel SET tube amp they sounded fantastic. I do think the price is a bit high for these, but the exterior looks very nice.
A mid-1980s British floorstander from B&W’s DM series, using a 2.5-way arrangement with dual 200mm drivers and a 26mm tweeter for 91dB sensitivity and a notably open soundstage. At $285 this is one of the cheaper routes into vintage Bowers & Wilkins house sound, and the seller offers a demo. Worth a look.
Wharfedale’s W70 dates to the era when Gilbert Briggs’ company was building substantial sand-filled and heavily braced cabinets in Yorkshire, and the mid-century styling has made them as much a furniture find as an audio one. Tested and working, though the seller is upfront about an attempted side repair and water rings on the top.
Sansui’s SP-200 is Japanese golden-age engineering at its most unapologetic, and predates the quality decline that’s associated with Sansui speakers as they moved through the 70s and into the 80s. These are 42 pounds each, with a 12-inch woofer, two 5-inch midranges, twin horn tweeters and a rear brightness control. These all used rubberized surrounds, so nothing to repair on the drivers, either. Again, I think these are priced just a bit high, but not crazy.
Soliloquy was a short-lived North Carolina boutique that built solid-hardwood cabinets and voiced its speakers specifically for low-power tube amplification, earning a devoted following before the company folded in the mid-2000s. The 5.3 retailed near $1,900; at $600 with only moving scuffs and flippable spike feet, it’s a lot of speaker for the money.
MCS was JCPenney’s house audio brand, and the better models like this 683-8320 were built by Technics — the “Linear Phase” badge refers to Technics’ driver-alignment work aimed at getting all drivers arriving time-coherent at the listener. Department-store branding keeps prices low on genuinely well-engineered Japanese speakers.These were high-end enough that they’re worth collecting, but pededstrian enough that the prices aren’t bid up.
The RP-600M II is the current-generation Reference Premiere bookshelf, and the mark-two revision brought a redesigned Tractrix horn, a new woofer and a rear port that made it far more forgiving of placement than its predecessor. Listed at $475 against a $699 retail, described as like new.
The Bose 901 Direct/Reflecting Speaker System is as polarizing as it is interesting, a unique system from the golden age of hi-fi experimentation. It’s less about perfect sonic accuracy and more about the experience of listening to your music. And that’s one of the things I love about it!
While cleaning up some of my archives to save cloud space, I found that I’ve serviced nearly 500 Active Equalizers which are the important heart of these speaker systems. I’ve distilled some of the knowledge from all of that experience here, to help anyone else who wants to keep these systems running, and get good info from someone who actually has any clue what’s going on.
There is a truly staggering amount of AI slop out on the Internet about these speakers, ranging from slightly wrong to utter nonsense. In contrast, this article was written by a human who has owned and listened to all of these systems, and serviced and studied hundreds of them over thousands of hours
This article covers the Bose 901 Series III Speaker System and the Active Equalizer. Keep on reading for all of the details, including schematics, repair tips, original documents, and direct links to buy all the parts you need for the 901s on Mouser.com.
This article is part of a series, which is being expanded. Scroll all the way to the bottom for the others.
The Bose 901 Series III speaker system was introduced in 1975, based on the same principles as the original: with research suggesting that most of the sound you hear in a live performance arrives indirectly through reflections, Dr. Bose designed the speakers with nine small, identical full-range drivers instead of a traditional woofer or tweeter. Eight on the back, one on the front, with no crossover components inside and all frequency shaping done on the external Active Equalizer box.
The Series III used similar 4″ drivers as the Series I/II, but the design took into account another decade’s worth of speaker design research, and moved to a tuned ported cabinet configuration which offered much greater efficiency than the originals meaning that these could be powered more effectively with a small amplifier. The drivers themselves were also update with a “helical voice coil” delivering more efficient performance.The Series III used similar 4″ drivers as the Series I/II, but the design took into account another decade’s worth of speaker design research, and moved to a tuned ported cabinet configuration which offered much greater efficiency than the originals meaning that these could be powered more effectively with a small amplifier. The drivers themselves were also update with a “helical voice coil” delivering more efficient performance.
Those ports, responsible for a lot of the power and efficiency on the low end, really push some air. If you crank it up on a bass-heavy song, you’ll be able to feel notable air movement. Some listeners report it being able to extinguish a candle at 3 feet.
Take a look through the owner’s manual for details about the hookups:
As with the Series I/II, and all other 901 speaker system generations, the equalizer really is necessary to get the best sound out of the system – and sadly, it gets lost or damaged a lot of the time. Without it, the midsized drivers don’t get the drive power they need on both the low and high ends of the frequency range to perform properly and you end up with the derrogatory description, “No Highs – No Lows”.
If you’re absolutely in a pinch, you can crank the lowest bass and highest treble adjustments on your amplifier up as high as they’ll go, but unlike the first generation of 901 equalizers, the Series III and above use a summed complex curve that’s very difficult to replicate. You’ll need a multi-band EQ at a minimum. Find yourself a real one, or a mini-DSP replacement, for best results.
The Series III speakers have an odd wiring design, and includes a speaker terminal marked * in addition to the standard +/- terminals. This is a center-tap, and was used with some specialized Bose amplifiers to provide additional spatial imaging.
It does not appear to have been actually used with any Series III generation hardware, and I’m a bit unclear what a compatible amplifier is actually doing when connected there. It does get some optional use with the Bose Spatial Control Receiver, which was contemporary to the Series IV and is backwards compatible here. Supposedly, the spatial controls allow a suitably shaped signal to introduce variable “leftness” and “rightness” and control the width of the stereo image to a greater degree, but it doesn’t appear to have made much of a market receiption and quietly faded.
While the Series III and later speakers are much more efficient than their predecessors, the generation change does come with one major downside: the speaker driver surrounds are now a more typical foam, rather than rubberized cloth. These do crack with time and will very likely need all 18 drivers to be reconed. Kits are readily available on eBay and from other vendors, and while it’s not a fun job, it’s more tedious than difficult. You can see some of the cracks in the drivers here:
Drivers are connected with wire wrapping, so buy a wire wrap tool or solder them when replacing.
When refoamed, they’ll last another few decades.
Don’t run the speakers other than just for basic testing with dead foam, or you’ll burn out the voice coils which are otherwis highly reliable.
Bose 901 Series III Active Equalizer
The Bose 901 Series III Active Equalizer is doing the heavy lifting to shape the signal and provide the depth of bass, clarity of treble, and wide soundstage the 901 speakers are known for. It’s an interchangeable frequency curve with the Series IV Active Equalizer, and the Series V and later are very close so you’d be unlikely to notice a difference if you had to mix-and-match for some reason (although I always encourage matching sets whenever possible for aesthetics.)
As with the major changes to the speakers themselves, the equalizer also got a major overhaul, becoming a wider and flatter box and returning to a solid wood case as was used on the earliest Series I equalizers.
There’s also a black case variant floating around:
The Series III was sold around the world and came in both 120V and 220V versions. If yours was originally a 220V model, there’s a large wirewound resistor installed near the power input. There are three terminals: common, the 220V terminal (power delivered to the transformer through the resistor), and a 120V terminal (no resistor.) If you’re converting from 220V to 120V, you can simply move the power input to the correct terminal and you’re done. Should you need to convert to the other direction, however, you’d need to populate that resistor.
The overall design of the Series III is a bit more complex and chaotic, but it’s still using transistors; in this case, 12 NPN transistors (BC239C, I usually replaced those with 2N5088s) and 2 PNP transistors (2N3906).
The equalizer is doing quite a bit more work, it’s a true curve, not just boost centered around two peaks:
Servicing this model is generally fairly easy, and you can remove the board from the case without disconnecting anything.
The terminals of the passive components are crimped on the bottom, so after de-soldering, you’ll still need to cut the bottoms off to remove the old parts.
For the Series III, you’re usually looking at a fairly simple repair job most of the time:
Electrolytic capacitors are universally going to be bad, but the film capacitors are typically stable and high quality. Slightly annoying, there are two axial capacitors but the rest are all radial. Replace the 500 uF capacitors with 470 uF capacitors which is the modern replacement size. I’ve gone up a voltage level in the parts list below to offer extra longevity.
You can replace the 1 uF axial capacitors with 1 uF film capacitors if you’d like, which may be easier to fit into the footprint.
Resistors are typically coated carbon and are stable; only a handful of resistors are carbon composition resistors subject to extended drift, and those few are in non-critical positions and can safely be ignored unless you’re going for 100% perfection.
Transistors are seldom bad, and although matching can be spotty from the factory, the differences are usually only to a level you can see with an analyzer like my Audio Precision and are swamped by individual differences in room placement and/or your own ears.
Faders should be cleaned and fader lube applied, as these often get scratchy. The Bass and Tape switches also can get grubby, and so should be cleaned.
The indicator lamp on these is also an NE-2A neon bulb with integral dropping resistor, connected directly across the AC line. You can replace it with an integrated assembly, or a 20V LED powered from the first filter capacitor. The lamp “not powering up” does not actually indicate anything about the equalizer’s functional operation. (Not included on the parts list.)
There are some gotchas, which I’ll cover further down the page.
Series III Active Equalizer Schematic
Click to zoom in on the schematic. The bass frequencies have a small amount of summing going on, with wide spatial separate on the mids and highs.
Series III Active Equalizer Parts List
Here’s what you’ll need for most overhauls. You’ll probably only want the electrolytic capacitors, but I’ve also included the transistors.
When you’re done, you’ll only have a pretty small pile of parts:
Series III Gotchas
The biggest gotcha with the Series III equalizer itself is the rear connections: these are crimped onto a phenolic board, which becomes brittle with age.
The phenolic can easily crack, and the entire set needs to be replaced at that point.
I typically recommend using new panel-mount RCA jacks. Shown here, the owner requested I replace only the main loop and chose to leave the original Tape Loop section as it was found.
Conclusion & Postscript
The Series III seems to have been much less popular and widely distributed than the earlier, and later, series and was only in production for a couple of years before the design evolved. In about a decade of repairing Bose equalizers for clients, I only encountered a total of 28 Series III equalizers through the shop out of about 500 total repairs.
I am not accepting repairs currently, but I hope this will help you with yours.
Mouser parts were correct and current as of publishing (June 2026). They may change over time. And probably will: when I started with these equalizers in 2012, I was using Xicon 1% metal film resistors, and Nichicon axial electrolytic and film capacitors. By 2021 when I stopped doing this as a business, Nichicon axial electrolytic caps were unavailable. Now, their film caps are out as well. I’ve selected good quality replacement parts which I’d be happy to receive if I paid for the repair in another shop, and which I’d confidently use if I were doing commercial work at the moment.
A lot of modern amps don’t have a tape loop anymore, and there can be some issues with sensitivity mismatch on modern gear if you use the Bose EQ in the pre/main loop. That’s the only real downside. These speakers and their companion amp are best used as part of a vintage system. If you’re planning to use them with a modern system I recommend you first try using a modern EQ or the EQ functions built into your gear and crank the bass/treble; if not, a MiniDSP-based solution would be an OK bet but can require a little bit of know-how to program.
Leave a comment if you have a question, and I’ll try to answer as best as I can.
Other Resources
You can find my other articles about different 901 systems here:
A recurring feature curating interesting speakers for sale on Craigslist from around the Pacific Northwest. Links to Craigslist postings might go down at any time if they’re sold or pulled from sale.
This $2,300 pair in Eugene is the rare ‘Split’ configuration of Klipsch’s legendary La Scala. This is a horn-loaded, high-efficiency Heritage design dating back to the 1960s that was originally built for portable PA use by traveling speakers, with the top-hat mid/tweeter horn separating from the folded bass bin for easy transport on the built-in wheels. Really cool, and customizable.
At $7,900, this is a pristine pair of Dutch & Dutch 8c active monitors. They’re a modern DSP-corrected, cardioid-radiating design out of the Netherlands that’s widely regarded as one of the best-measuring loudspeakers made today, prized by mastering engineers and audiophiles alike for room-independent bass and pinpoint imaging.
This $1,000 mid-80s pair from the Portland area is the ‘professional’ mirrored-pair sibling of JBL’s iconic L100 and is known for punchy, musical studio monitoring in one of the most enduringly collectible JBL cabinets.
Priced at $275 in Portland, the DQ-1W is a companion bass module built to extend the low end of Dahlquist’s famed DQ-10 phased-array speakers from the 1970s – basically a subwoofer. This is a rare piece of a system that pioneered time-aligned, deliberately imperfect driver placement to improve imaging.
A working $75 pair from Port Angeles, these oak-cabinet JBL 2800s are a solid entry point into vintage JBL home audio, with the large-driver, warm-voiced sound and midcentury wood veneer that made JBL’s consumer line a fixture of 1970s–80s living rooms. Not as iconic or collectible as some of the earlier models, but solid enough for what they are.
This $80 pair from Coeur d’Alene pairs dual 4-inch Kevlar woofers with a 1.25-inch planar ribbon tweeter. SLS Loudspeakers built its reputation on exactly this kind of exotic driver combination, delivering the fast transients ribbon fans chase at a fraction of typical ribbon-speaker prices.
This $75 Klamath Falls listing pairs two BSR DR-1550R five-way acoustic-suspension speakers with a JBL SC-305 cabinet. It’s an unusual mixed-brand system, since BSR is far better known for turntables, built on the sealed-box acoustic suspension principle pioneered by Acoustic Research in the 1950s for tight, extended bass from a small enclosure.
Listed at $250 in Albany, these vintage Peavey Black Widow 12-inch drivers are a workhorse of the guitar-amp and PA world, built around Peavey’s Super Structure/Focused Field Geometry magnet design that’s kept them prized for decades for their power handling and rugged reliability. It’s not quite hi-fi, but more in the PA speaker genre – but still pretty interesting.
At $4,700, these Boise floor-standers use Peerless XLS subwoofers with passive radiators and a copper-phase-plug midrange in a cherry cabinet. Peerless XLS drivers are a favorite among serious DIY and boutique speaker builders for their low distortion. These look like a DIY project, but a very high-end one from someone who knows what they’re doing.
This $50-and-up Boise blowout is a grab bag of American speaker history: Klipsch KG series, Advent Prodigy/Mini, JBL, Cerwin Vega DX-9, Bose, and Polk SDA all in the mix. A good hunting ground for anyone assembling or restoring a vintage system, but who knows what the actual prices are by item.
Listed like-new at $500 in Bellingham, the JBL Studio 590 towers carry forward JBL’s signature waveguide horn tweeter for controlled, detailed high-frequency dispersion paired with the brand’s classic dynamic low end.
I’m not doing repairs for commercial clients at the moment, but I am working with a local furniture restorer to help her expand her offerings into mid-century (and adjacent) speaker and stereo repair. We just completed our first project together, which was a pretty simple one overall: refurbishing a set of 1961 Audax CA-70 “Sonoteer” speakers from 1961.
(Side note: some of the photos here are in whatever the “HEIC” format is. They embed and load for me, but if you can’t see them in your browser, leave a comment and I’ll re-encode them and replace.)
These are an interesting, and unusual, speaker from what was a fairly boutique manufacturer at the time. Not to be confused with the well-respected French brand Audax, the Audax USA brand was actually produced by New York-based manufacturer Rek-o-Kut, who were primarily a manufacturer of record players and vinyl cutting equipment. I wasn’t able to learn whether this was in fact a joint venture or licensing deal of some sort, or if two companies on opposite sides of the pond happened to come up with similar branding given the marketing conventions in the industry at that time.
The September 1961 issue of Stereo Review has an original advertisement for these — unfortunately revealing that the included optional legs have gone missing at some point in the past. These really were quite a high-end piece in the day, retailing for $79.95 in 1961, or nearly $900 today.
The Audax CA-70 is an open-baffle design, which gives a really wide sweet spot, and offers a lot of mounting flexibility. Each speaker contains five drivers: two 6 woofers, two 4″ midrange drivers, and a tweeter. It’s rated for up to 45W power handling, and 40 Hz – 18 kHz frequency response. (In this time period, “full frequency response” was considered 50 Hz – 15 kHz, so this set of speakers offered exceptional performance indeed.)
These arrived in good original condition. The legs were apparently missing, but that’s not the worst to replace or use an aftermarket floor stand. Other than that, the rattan covering and delicate latticework grilles were in excellent shape. While a bit dirty and faded with age, the wood cases were fully intact, making this an easy project for an expert furniture restorer. The fronts were attached using tiny flush-set finishing nails which were impossible to remove without causing damage, so we went in through the back (which ended up being the better approach given how the drivers were mounted) by carefully cutting around the edges, which were set into the baffle board. This allowed the rear rattan to be carefully removed in one piece with full coverage, so it could be reattached using spray adhesive. This is basically how it was attached originally, so it’s perfect.
The drivers are arranged in series-parallel with the tweeter in parallel with the whole stack, through a simple crossover, for a nominal impedance of 8 Ohms. The simple first-order crossover used a single 4 uF capacitor. This is an oil-paper capacitor block. These are somewhat more reliable than other styles of vintage capacitor, although given the age and manufacturing tolerances, it was definitely due for replacement to ensure it would perform reliably for a long time to come. Each was replaced with a new 4 uF polypropylene capacitor, rated for a much higher voltage, which should offer decades of life compared with more precise transient response.
These originally came with basic screw terminals mounted on phenolic board; that’s not a great pairing for modern equipment and high end speaker cables. The spacing was perfect for a swap over to modern, standard spacing 5-way binding posts for your choice of banana plugs, spades, pins, or bare wire. (Interestingly enough, “5-way” referes to the fact that the assembly is made from 5 components, but there are in fact only 4 different ways to hook up cables to this type of conector.)
While waiting for these to finish their glow up, I hooked them up and started their break-in. As you’d expect, these needed some time to stretch their legs after resting in storage for decades. After about 20 hours of break in time playing a variety of styles of music at moderate volume, they’d had a chance to fully unlimber, and were sounding fantastic and ready to go to a new home. They have a really wide soundstage and fantastic mid-range definition, offering excellent listening from far left to right, unlike some of the excessive directivity you get with some more modern speakers. They sounded best when listening to jazz, vocals, classical music, acoustic performances, and early rock – just about anything that was current in the 50s and 60s. These had a very different character from many other designs of speakers I’ve heard throughout the years.
With a freshly refinished wood case and wood lattice, carefully cleaned rattan covering, and refurbished electrical components these are better than new and will make an absolutely beautiful centerpiece of someone’s vintage audio system, or a wonderful addition to their mid-century modern home decor. And in fact, they did! Within just a few days, these were snapped up, selling for $695 to a local collector. Happy listening!
A recurring feature curating interesting speakers for sale on Craigslist from around the Pacific Northwest. Links to Craigslist postings might go down at any time if they’re sold or pulled from sale. There’s a few interesting things in here today, and we’re a bit heavy on JBL in this issue.
A pair of the legendary 1970s JBL 4311B three-way control monitors — the studio workhorse that defined the near-field sound of that era — offered at $450 in Portland as-is with some woofer/cone damage that the seller notes is repairable with sub-$200 kits. If you click through you’ll clearly see it. Looks like an easy project for someone in the know, and then you’ve got a great pair of speakers for a good price.
This $950 Boise pair is the coveted ALNICO-magnet version of JBL’s 4311 studio control monitor, with original drivers, freshly re-foamed tweeters, and clean cabinets. The ALNICO variant is prized among collectors for its distinctive midrange, and the seller offers free local delivery from Boise to McCall.
At $2,800 in Spokane, these JBL 4430 Bi-Radial studio monitors are the high-end pick of the week — Greg Timbers’ landmark constant-directivity design built around original 2235H 15″ woofers, factory compression drivers, and 2344 Bi-Radial horns – occasionally called the “butt cheeks” horn. A genuine professional control-room monitor with fresh veneer and intact crossovers. I confess to having inherited a pair of these while I was in college, having absolutely no idea what they were (or caring at all about audio at the time), and because the woofers had trashed surrounds I eventually took them to an e-waste station. It remains, to this day, one of my big regrets in audio.
A nicely priced $100 pair of DCM Time Window-era three-way towers in Parkland, garaged for years by a sentimental owner. DCM’s cult following stems from their imaging-focused designs, making this a low-risk entry point for vintage tower sound.
Early-1990s KLH AV5000 three-way towers in excellent condition for $400 in Olympia — big 42″-tall 8-ohm floorstanders carrying the storied KLH badge founded by Henry Kloss. Firm price, cash only, and a lot of cabinet for the money. These are getting to be a bit after their really good, famous years but they’ll still be pretty solid as the anchor of a 2-channel system or in a home theater setup.
A $290 Snohomish pair of Klipsch KM-4s in excellent shape with the original box — horn-loaded 8″ two-ways with a 90×60 Tractrix horn and a stout 94 dB efficiency that makes them an easy match for low-power tube amps. Classic Klipsch horn dynamics in a compact bass-reflex cabinet.
A bargain $175 pair of Definitive Technology StudioMonitor 350 bookshelf speakers in Marysville. Definitive’s aluminum-dome tweeter and BDSS mid-woofer punch well above their price, and the terse listing (‘these sound absolutely fantastic’) suggests a quick, no-fuss deal.
The original Andrew Jones-designed ELAC Debut B6 (DB61) in near-mint condition for $200 in Port Angeles — one of the most acclaimed budget audiophile bookshelf speakers ever reviewed. The listing throws in a well-worn Swedish leather listening chair, cat scratches and all.
German-made Canton Karak 30 bookshelf speakers at just $50 in Meridian, with great bass presence and 5-way binding posts. Canton is a respected European hi-fi marque, and the only knocks here are cosmetic scratches on the bases.
A recurring feature curating interesting speakers for sale on Craigslist from around the Pacific Northwest. Links to Craigslist postings might go down at any time if they’re sold or pulled from sale. Today’s roundup features several listings from Olson’s Hifi in Lynnwood. I haven’t seen that name before, but it might be worth stopping in if you live nearby, they look to have some interesting items on offer.
Martin Logan’s Ascent i pairs a curved electrostatic panel with a dynamic woofer for the see-through transparency and pinpoint imaging electrostats are prized for; this matched Eagle, ID pair is listed at $500 in excellent condition — a lot of high-end audiophile speaker for the money. A friend of mine had these in her condo a while back, and the soundstage is incredible.
The Klipsch Forte II is a Heritage-line horn-loaded classic, using a rear passive radiator and horn-loaded midrange/tweeter for the effortless, high-efficiency dynamics that make Forte pairs perennial collector favorites; this Cedar Mill, OR pair is $1,200 with a few minor cosmetic scratches noted.
The KLH Model 23 is a vintage acoustic-suspension bookshelf from the legendary Henry Kloss-founded KLH, and the seller notes the original treated-cloth inverted woofer surrounds are in excellent shape — a warm, musical classic in oiled walnut for $250 near Albany, OR. These would go great with a KLH Model 25 desktop stereo system.
Vandersteen’s Model 3A Signature is a time-and-phase-correct floorstander revered for its natural, uncolored midrange and holographic soundstage; this Tacoma pair wears custom mahogany trim, is about ten years old, reportedly like-new, and can be auditioned before purchase at $2,500.
Lots of electrostatic speakers today. The Martin Logan ElectroMotion ESL X is a hybrid electrostatic tower mating a large curvilinear stat panel with a powered bass section for striking clarity and imaging; this Lynnwood dealer listing (Olsons HiFi) is priced at $2,999.
Infinity’s RS3 is a compact Reference Series bookshelf offering balanced, detailed sound and easy placement — an affordable entry into the collectible Infinity lineage at just $180 from Olsons HiFi in Lynnwood. I’m not familiar with that shop, but they seem to have a good amount of interetsing speakers for sale.
The KEF 103/4 is a classic British floorstander built around KEF’s Uni-Q coaxial driver for coherent, pinpoint imaging and deep bass; a detailed, natural-sounding audiophile bargain at $350 from Olsons HiFi in Lynnwood.
The Advent Loudspeaker (the ‘OLA’, or Original Large Advent) is a hi-fi icon — Henry Kloss’s walnut-veneered, 10-inch acoustic-suspension two-way famous for delivering giant-killing warm, full-range sound at a modest price; this pair is $450 from Olsons HiFi in Lynnwood.
The KEF C40 is a compact vintage C-Series bookshelf known for accurate, smooth midrange performance, ideal for smaller rooms or a second system; a tidy budget classic at $150 from Olsons HiFi in Lynnwood.
A pair of large vintage KLH acoustic-suspension bookshelf speakers in Marysville, said to sound fantastic with only minor storage marks on the cabinets — carrying the warm KLH house sound for $250. I think these are from after KLH were really known for their speakers and were converging more towards the mid-90s aesthetics and functionality. (I had a pair of the large tower cousins of these, and they were decent, if not spectacular.)
The Sansui SP-50 is a vintage Japanese two-way from Sansui’s well-regarded 1970s speaker line, offered here in Portland in great condition with their original boxes — an easy-to-drive 25W/8-ohm classic for $120. These are pretty small compared with some of their larger cousins, but would look great paired with a vintage integrated amp.
