I’m mostly interested in audio equipment and related repairs as a hobby, but there’s plenty of crossover skills to other things that need to be fixed. I’ve fixed a electronics module from an old Mazda, the control board from my dryer, and resoldered more than a few broken charging ports on Bluetooth speakers. Most recently, though, it was time to work on my old Crock-PotSlow Cooker which had become unreliable and would power itself off randomly. That’s not great when you’re counting on it to cook something up to temp, safely, and keep running for a long time.
Time to take it to the shop and see what’s going on. These are certainly not built with repair in mind, and I’ve had this one for at least 12 years, probably longer, so if it’s not repairable I wasn’t going to be too sad about it. I figured it was worth an hour or two to investigate over the weekend regardless.
There’s not a ton going on inside. After removing a pair of rivets going through the bottom, the silver heating liner lifted right up. There’s a long wirewound resistor wrapping around most of the base; one side is connected directly to the AC line and the other to the controller. A heat insulating pad protects the back of the control unit and the wiring from the heat of the pot.
A thermistor in a diode package provides temp sensing to the control module. Inside, four screws on the back of the controls separate the plastic control clamshell and let you pull the board.
Removing the shield, which is also the heat sink for the triac, shows the front of the board. Kayue Electric is a Hong King-based company that OEMs kitchen appliances and similar. There’s a small microcontroller on the back which handles the timing and temperature feedback, running a small algorithm (maybe a PID) and triggering the gate on the triac to control the heating element. I didn’t trace it out, but it looks like the AC line comes in and gets dropped by the pair of resistors, rectified through a diode, and then a single electrolytic capacitor provides power and filtering for the digital side.
It’s a little hard to tell in the photos, but it was easy enough to spot (with a trained eye) in person: the 470 uF, 10V, 105C capacitor had a slightly bulging top and a tiny spot of leaked electrolyte. Sounds about right. If it’s anything more than this, it’s going to be destined for the trash, but this is a likely place to start.
Confirming what I already knew, this one is definitely bad.
Compare the readings on the bad vs. good capacitor: 35% vs. 1.1% V_loss; 3.6 Ohm vs. 0.14 Ohm ESR, and the capacitance on the original had drifted down by about 25%. The replacement is a Nichicon 470 uF, 16V, 105C rated capacitor. It’s very slightly larger than the old one, but still fits.
Unfortunately, I just moved quickly through the reassembly and didn’t get a photo of the new component installed. I also cleaned and reapplied thermal paste where the triac sinks heat to the metal shield. (It’s a very small amount of heat.) Then, time to button everything back up.
I’ll need to make a run to home depot and find some flat head machine screws and use those in place of the rivets I removed, so right now the (hot, energized) liner and heating element are just sitting in place held by gravity. But functionally, it seems to be holding up to a test run on high at 6 hours. It’s fun to be able to do this sort of thing around the house, and save a kitchen appliance from heading to the landfill.
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 bit light on interesting stuff this week, but that’s how it is sometimes.
Klipsch’s roadie-grade La Scala Industrial in the rarer split configuration, with the horn top-hat separated from the folded-horn bass bin and mounted on wheels. Asking $2,000 OBO in the Eugene area; a touring-spec cousin of a classic Paul W. Klipsch horn design. Well worth it if you’re willing to do some cosmetic cleanup. (Also, look at this seller’s other stuff!)
A Decade L36 from the original owner, re-foamed in 2020 and otherwise original, with the 10-inch woofer, 5-inch midrange and 1.5-inch tweeter in oak veneer. These are the classic 1970s West Coast JBL sound at a fair $750 on the Oregon coast.
A Belle in oiled oak with consecutive serial numbers, which the seller describes as exceptionally clean. The Belle is the corner-horn-flavored sibling of the Klipsch Heritage line and rarely turns up in this shape, which explains the $3,600 ask.
Second-owner Forte IIs in oak, 99 dB efficient and a famously good match for low-powered tube amps. At $1,500 in NE Portland, they are a fair entry into the Klipsch Heritage line. I’d love a pair of these myself, but I think that price is a bit high.
A KEF B139 woofer loaded into a transmission-line cabinet, which the seller is pitching as a very low-reaching man-cave subwoofer. Only $100 in Spokane, and a fun project for anyone who likes unusual bass loading. Maybe you alraedy have a KEF system and want some more punch on the low end?
Mint, second-owner JBL S38s with 8-inch woofers, which the seller notes can sell for $250-$400. A well-liked Made-in-USA JBL design, listed at $250 in Boise. Getting into the more “modern” era but still a good quality product.
Late-1960s Radio Shack Realistic Solo 3 speakers with an 8-inch woofer and 3-inch cone tweeter, with wood cabinets. A cheap $40 slice of Tandy-era hi-fi history in Bellingham. These seem to be re-listed every week or two, and definitely need some cosmetic TLC, but it’s not a horrible deal.
The D-1E is a rarer export version of the Cerwin-Vega D-1, sold mainly through overseas military exchanges, with a slightly bigger cabinet and tougher components. With an 8-inch woofer and 1-inch horn tweeter at 92 dB, this pair is $165 in Bellingham. Better as “nice garage speakers” than for a hi-fi setup, though.
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.
