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.
A worn but working pair of KLH Model Twenty-Four bookshelf speakers for $60 OBO in Redmond; the cabinets are scuffed and the two woofers don’t match, so this is a project pair. KLH was a pioneer of acoustic-suspension design in Cambridge, MA, and these make a cheap way into the brand’s vintage sound, if you’re willing to work a bit for it. It’d be a good base to learn cabinet refinishing and woofer repair.
A clean-sounding pair of floor-standing 3-way Technics SB-2715 speakers with both grilles, $100 in Redmond, with cosmetic wear on the woodgrain cabinets. A solid 1970s-80s Japanese hi-fi piece that’s easy to refoam and recap. I briefly had a pair of these myself – they’re actually not bad, even though they don’t have a great reputation. Would make a good part of a basement/party/early 1980s period set, or a project to learn to refoam drivers without a lot of risk if it goes badly.
All-original ADS L630s from 1979 at $850 in Portland, part of Analog & Digital Systems’ professional-leaning Lxx30 line with claimed bass down to 33 Hz. ADS was known for tight, detailed, low-distortion sound and compact cabinets, and the L-series is still well regarded among collectors. I think this price is a little high, but they do look really clean.
A DIY-built 1970s four-way KEF transmission-line tower, about 46 inches tall, for $400 in Spokane. It uses KEF B139 ‘Racetrack’ woofers, a B110 midrange and a T27 tweeter, plus a Celestion HF1300 tweeter, a classic KEF driver complement. The B139 is among the most sought-after bass drivers of its era. Not entirely sure what to make of this one, though, I almost wonder if these weren’t used as PA speakers.
A 15-inch-woofer Jensen 3-way floor-standing pair at $99 firm in Spokane, described as very loud and in excellent working shape. Big-woofer Jensens like this are a fun, affordable vintage option for tube-amp or garage-system builds with their efficiency and frequency response, if not strict sonic accuracy.
A compact KEF Q10 bookshelf pair for $120 in Oregon City, in good working condition. KEF’s Q series brought Uni-Q coaxial drivers to a budget price, and these small monitors are well suited to nearfield listening.
Boston Acoustics CR7 two-way bookshelf speakers (6.5-inch woofer, 0.75-inch dome tweeter, 88 dB sensitivity, magnetically shielded) at $120 in Oregon City. Boston Acoustics speakers have a following for punchy, easy-to-drive sound in small cabinets.
A KEF Q300 pair for $100 in Bellingham; one speaker has broken bi-wire tweeter terminals, which only matters if you bi-wire and could be fixed by replacing the plate. A lot of Uni-Q speaker for the money. I’d strongly recommend this if you’re nearby, I own a set of these myself which I’ve used inside of a couple antique radio enclosures.
A late-1960s Realistic Solo 3 pair for $40 in Bellingham, with an 8-inch woofer and 3-inch cone tweeter in cabinets described as excellent. Realistic was Radio Shack’s house brand, and these early two-way speakers are a charming, inexpensive vintage curiosity. The price looks right, and with some refinishing and probably a fresh set of foam, you’d have a cute period system.
A complete system of a Creek 4330 integrated amplifier and Monitor Audio Silver RS6 floor-standers in rosenut veneer, $800 in NE Portland. The Creek has a British reputation for clean, warm sound and the RS6 uses Monitor Audio’s metal-dome drivers, a good turnkey hi-fi start.
A pair of JBL PRX835W powered 3-way PA speakers with 1,500W of amplification each, $1,800 in Boise, with scuffs from gigging. A rugged pro-audio setup that’s more mobile-DJ than audiophile, but a notable one for JBL fans.
A pair of JBL LX22 bookshelf speakers with 6.5-inch polymer woofers and 1-inch titanium dome tweeters (90 dB sensitivity, 14 lbs each) for $75 in Albany. Heavy and efficient for the size, and a bargain starter pair from JBL’s Northridge line.
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 V 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 901 Series V repair on Mouser.com.
This article is part of a series. Scroll all the way to the bottom for the others.
The Bose 901 Series V speaker system was introduced in 1983 and ran up until 1987. It’s the successor to the Series IV in the long series of 901 Direct/Reflecting Speaker System designs that spans more than 60 years from the earliest introduction in 1968. This photo from a German advertisement shows their evolution leading up to the Series V:
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. 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 V keeps the ported cabinet and 4″ helical voice coil drivers from the Series III and IV, with a handful of changes to the cabinet: it’s now an aluminum shell with plastic sides, the speakers feature a round cup for the speaker terminals on the bottoms, and now there’s poly fill behind each driver for some better damping.
Bose also finally dropped the extra * terminal that showed up on the Series III and IV which was designed for exclusive use with the Bose Spatial Control Receiver, and made the speakers ambidextrous rather than having a defined Left/Right that were found on some of the earlier speakers.
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 derogatory 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 like the Series III and IV, the Series V uses 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 V is the most efficient 901 family speaker yet: the owner’s manual calls for a miniscule 10 watts per channel. For optimal listening, Bose recommends that the point of the “V” on the back grille be 8-16 inches away from the rear wall, with a minimum of 18 inches and 2-4 feet recommended from the sides, floor, and ceiling. They specifically caution against putting the speakers exactly halfway between the floor and ceiling as this could cause some unpleasant multipath distortion. Take a look at the Owner’s Manual for all the details:
Starting with the Series III and continuing through the end of the series, Bose used a more pliable and faster-responding foam surround on the drivers. This helped with the efficiency, but unfortunately foam does crack with time. As of September 2026 when this article was published, reports are mixed – some examples have good foam surrounds, others are cracking. You’ll probably need all 18 drivers to be reconed after some listening, if not immediately. Kits are readily available on eBay and from other vendors, and while it’s not a fun job, it’s more tedious than difficult.
Those are photos from a Series III speaker, but it’s substantially similar.
Partway through the Series V run, Bose reportedly switched the surround material from polyester foam to polyether foam to cut down on rot, starting around 1986. These hold up better, but at this point they’re still 40 years old, so your mileage may vary. If the foam is bad, don’t run the speakers other than just for basic testing or you’ll burn out the voice coils which are otherwise highly reliable.
Bose 901 Series V Active Equalizer
The Bose 901 Series V 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.
The speakers look pretty similar, but the extra efficiency and revised porting design mean that it’s a substantially different curve from the Series IV and earlier – the maximum amplification provided at bass and treble frequencies is only about +10 dB, compared with as much as +20 dB on some earlier models. The overall shape is somewhat similar, so in a pinch I’d be willing to substitute a Series IV or III with the bass and treble turned down, but that wouldn’t be highly recommended. The curve is identical on the Series VI, so that’s a good drop-in substitute as well, and may be more readily available due to its younger age.
The less intense curve is also good for electrical reasons – +20 dB on a 1V signal would be getting up to about 10V RMS, which is close to clipping an op-amp on its supply rails, so there wouldn’t be much headroom. This series was released around the same time as CD players started to enter popularity, with their higher standard signal level, and so everything is designed to play nicely together.
In addition to some electrical changes, starting with this series the equalizers have adopted the more modern look of a metal-cased device, and are basically fully modern internally. The case is now a wide aluminum chassis with a black control area, two pushbuttons, the Mid-Bass and Mid-Treble sliders, and an LED power indicator in place of the old neon bulb. The Bass switch has two positions, and position 2 gives an alternate low-end curve with reduced bass response which could be handy if you’re in a boomy room with a lot of echo.
We’re using a fully modern, op-amp-based design in the Series V and later along with a regulated power supply for stability. The biggest change is the power supply: the Series V moved from the unregulated supply on the previous generations to a regulated supply for both the positive and negative rails. (If you’ve read the Series IV article, this is the same power supply that showed up on the late production “Series IV B” boards.)
It’s quite easy to remove the board from the case to access the underside for soldering, just remove six screws and it pops right out with nothing further needed.
In general, you’re going to want to replace the electrolytic capacitors on these boards as parts that are around 40 years old fail, leak, and cause the equalizers not to work anymore. Each successive model uses fewer and fewer capacitors, so there are only 12 total to replace in this unit: 2 main filter capacitors, 4 power regulator bypass capacitors, and 6 in the signal path.
The Series V does also have one “gotcha”: the larger, LS404CB quad op-amps are highly prone to failure. Every single one of the 16 units I’ve serviced suffered from a high noise floor and popping/static in the outputs. For good measure, you should replace both quad op-amps and both dual op-amps while you’re in there. Use the TL 074 and TL 072 respectively.
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.
With everything replaced, here’s what it may look like:
This is the final equalizer curve from a properly working Active Equalizer:
The service manual is available on HifiEngine, but I’ve reproduced it here as they’re often down or have registration blocked for new users:
The schematic is spread across a couple of pages, though, so I’ve edited it together into a single sheet which is easier while tracing signals. Click to expand.
Other than the LS404 op-amps needing to be replaced, there wasn’t really much to note for this series:
RCA jacks: Generally stable, but I did run into one set where the solder connections to the board had cracked. Inspect and re-solder as necessary.
By this point, these units used separate 120V/240V transformers and can’t readily be converted. (If you’re handy with Ohm’s Law, you could figure it out, but your best bet at that point would be a small step up/down transformer.)
The Series V is by this point expecting levels closer to the modern standard 1V line vs. the prior standard 150 mV line, so you’re less likely to run into sensitivity mismatch issues like clipping or excessive volume here, but there was a lot of variability in the stereo world at the time so it could crop up.
Other than that, the Series V is one of the simplest and easiest repair jobs you could run into – just a few caps, and the ICs, and you’ll probably be back in business. I never saw any with a more complex problem.
Conclusion & Postscript
Many Bose fans would argue that the Series V was the beginning of their corporate decline, as happened to many companies in the late 1980s. I only serviced 16 of these models over the years, compared with many more from before and after, so it’s not likely they sold super well. Bose was pivoting towards car audio and more inexpensive pieces by this time, and I had trouble finding much in the way of original advertising for this series.
I am not accepting repairs currently, but I hope this will help you with yours.
Mouser parts were correct and current as of publishing (October 2026). They may change over time. 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:
Do you need a Bose Active Equalizer to go with your speakers? They turn up on eBay regularly. If you search and buy with our referral link, we’ll get a small commission on the sale to help keep the site running. Click Here to see what’s available right now.
I’ve come across an original copy of rare document, the KLH Research and Development Corporation SOLID STATE COMPONENTS CROSS REFERENCE GUIDE (6 Februrary 1970).
This shows recommended substitute transistors for many of the components used in KLH equipment in the late 1960s/early 1970s, like the Model 20, Model 24/25, Model 21, and. You can use this information to substitute a different transistor on most models if needed, including many transistors which were identified only with house part numbers.
I’ve transcribed the guide here in full because of the quality of the original, this will be more readable, and it will also be more easily searchable this way for anyone looking for specific substitutes with a search engine, LLM, or AI agent search.
KLH Research and Development Corporation
SOLID STATE COMOPNENTS CROSS REFERENCE GUIDE
(6 February 1970)
Code used with some transistor types:
+ Different Base Configuration * Poor Reliability History
This is the full transcribed text of the original document, hopefully it helps you with replacing transistors in a KLH device from the late ’60s like a Model 20 or Model 24/25.
One good source for replacement parts for KLH equipment is a parts unit, which are readily available on eBay. This is especially true if you need a mechanical component like a lever or switch. If you shop eBay with Retrovoltage, we’ll get a commission if you buy something, which helps keep the site running. Click here to see what KLH items are available today on eBay.
I’m working on a few projects right now, and hope to have several of them fixed up and ready to post before the end of the year. Likely a few more ad-hoc, too.
KLH Model 20 set with speakers and tulip stands, this one had seen some modifications and wasn’t playing at all.
KLH Model 24 in an integrated provincial-style cabinet, this one was playing but very poorly, with the balance cranked almost all the way to one side.
KLH Model 20 desktop model, the latest produtcion version with silicon output transistors and a rocker switches instead of slide switches.
1965 Silvertone 5066 console stereo. Someone’s been in here before – several wires on the harness are cut and/or loosely spliced and the turntable isn’t original (and isn’t hooked up). Not to mention the entire dial string and dial pointer are missing. But it otherwise looks to be very clean and in good shape.
1960 Grundig Majestic SO191U console stereo. The absolute highest end unit sold by Grundig at that time, this one was purchased on base in Germany by a former U.S. service member, but ultimately spent the last 20 or so years inside an abandoned home in Eastern Washington. Fitted with the original record player, reel-to-reel tape deck, and comes with the original microphone too. Except for a few small nicks on the edges, it’s in nearly perfect condition.
I also have a copy of the KLH Model 20 service manual, and a KLH Transistor Substitution Service Manual. They’re not in great quality, so rather than scanning them, I’m planning to transcribe and post both here soon. The Model 20 service manual is also only partially useful – some of the illustrations reflect earlier versions of boards which are close but not quite on the later models – but the bias, symmetrical clipping, and radio alignment procedures are handy.
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.
Someone’s dad’s 1977 pair, reportedly acquired from a musician and possibly once used in a church, with working drivers, birch cabinets and removable cloth grills, set up for demo in the garage. The La Scala is Paul W. Klipsch’s horn-loaded, wall-friendly Heritage classic and one of the most sought-after Klipsch models.
Original-owner pair in oiled oak, bought new in 1989 and recently put into storage ahead of a remodel; the seller calls them superb. It’s a bit like a scaled down La Scala, but I’m admittedly not sure of the differences. These seem a bit more rare compared with the others.
A rare, massive audiophile speaker from Merlin Excalibur’s boutique line, described as very flat with tight bass; the seller is upfront that the driver photo is from the web, not their pair. Priced far below what boutique Merlin designs usually command.
Magnepan’s 1.7i: a 3-way quasi-ribbon planar panel known for transparency and a big, open soundstage, in black fabric. At this price it’s a way into the planar-magnetic sound without the cost of newer Magnepans.
The Advent/3, a compact bookshelf from the Advent Corporation, the company founded by Henry Kloss. Seller says they look good and function flawlessly; at $180 they’re a handy way into a classic 70s design.
Spendor A4 floorstanders in oak, in excellent condition per the seller. British BBC-lineage voicing, a famously natural midrange and a classic thin-wall cabinet make these a perennial audiophile favorite. They don’t look like much but have a good reputation.
Klipsch Quartet in oiled walnut, claimed to be in original pristine condition, and the seller is open to offers. Lots of heritage/semi-heritage Klipsch from this era with some small variations; all are generally worthwhile.
Early-1970s Sansui SP-3500 four-way speakers, in good working order with age-appropriate blemishes; the seller has no source to demo them but invites buyers to bring a receiver. A decent project or vintage Japanese-hifi pickup at $150. Lattice grilles look to have some expected minor damage but that would clean right up with a finish pen. The crossover capacitors on this model will probably need replaced, too, but it’s a super easy job.
A pair of Sansui SP-X9700 five-way, seven-driver speakers in good shape per the seller. An over-the-top 1980s Japanese multi-driver design that’s more curiosity than reference, and $245 is cheap for the spectacle. This was past the point where Sansui was “good” to the extent it ever was – even in their prime, they had a reuptation for aesthetics over sound – but it’d be a great bar/basement/party speaker.
Harman Kardon HK-30 speakers from the 1970s in solid wood cabinets; the listing is sparse, but at $40 and in Kalama, WA these are a cheap vintage-cabinet gamble. I don’t think they’re known for being especially interesting, but if nothing else the cabinets might be a good starter for a DIY project.
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, so you can mix-and-match within those two series. (The Series V and later use a similar shape, but has about 6 dB less boost at the low and high end, so you could use it but might find that the speakers have less punchiness than you’d expect.)
Physically, 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 differences are all described later in this article.)
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. (Click to enlarge.) 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 have a unit with LF 353 op-amp chips, absolutely replace them. I have also 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 IV B
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 encountered 2 of this variant over the years. They didn’t use any bad op-amp chips, but it’s not a terrible idea to replace them with fresh ones if you’re in there anyway. (Use 4 x TL 072 from the parts list above.)
This equalizer produces the identical curve as the earlier production Series III and IV models, but it’s from the very end of the production run, and so the details for this model are actually found in the Series V Service Manual.
It’s on multiple pages in the service manual, however, so I’ve stitched it into a single sheet here. Click to zoom in:
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. 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:
Do you need a Bose Active Equalizer to go with your speakers? They turn up on eBay regularly. If you search and buy with our referral link, we’ll get a small commission on the sale to help keep the site running. Click Here to see what’s available right now.
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.
