Used and special one-off amplifiers

Time to split up the special’s page into two so one for headphones and another one for electronics.  Here I’ll list what my more unusual amplifier projects, used amplifiers that I’ve refurbished/rebuilt plus any other piece I’m selling.  Feel free to contact me at if you are interested in any item here.  I will reply to every mail so if you haven’t received a reply within 24 hours then feel free to send me another email.  Email gets lost all the time or is flagged as spam.  All prices in US dollars (USD).

Here we have a Stax SRM-717 which I’ve fully refurbished and applied my full mods to it.  I replaced all the capacitors, cleaned all contacts, turned the circuit into the KGSS derivative it should have been in the first place.  The end results makes this, the best amp Stax ever made, even better.  It can drive anything you throw at it as my mods increase the power by 20% which makes for a welcome upgrade.

It is in very nice condition with just a few marks on the painted back panel.  The amp can be internally switched to any voltage so I’d do that before shipping it out and it even comes with the original box. 

Price:  2400$ plus shipping

Now here is something very rare, the Airbow SRM-222 with the matching SRM-222P power supply.  This was a custom modified SRM-212 where they replaced a bunch of part with higher end units such as Black Gates and silver mica caps.  These are very expensive when they pop up so I just had to take a look inside.  🙂  I left it all stock but full of nice parts and the PSU is a TKD switching supply with Sanyo OS-CON caps on the outputs.

In terms of sound, this is an upgrade over the stock 212 but not by much.  It’s more of a novelty though. 

No original boxes with these and the SRM-222P will only work at 100V or 120V so US or Japan use only.  The amp can be used with my own custom plug packs if you want to use it on 230V. 

Price:  650$ plus shipping

Now here is something fun, I’ve had a few of these original SRM-1 amps collect here over the years but nothing much I could do with them.  The transformers were all 100V only, the amplifier circuit was not that great and it was filled with obsolete parts which have no substitutes.  I’ve been working on the next batch of the Octave so why not marry the two, use the SRM-1 chassis but replace all of the internals.  So this is the new Octave V4 but it uses the SRM-1 chassis, the original Stax sockets and the power switch, everything else is new.  The Octave has always been my answer to the Stax SRM-353X and the new 400S, similar power envelope but more neutral sounding and has more power to the headphones.  Same R-core transformer as the Octave V2/V3 but the circuit has numerous changes. 

The amp has an internal voltage selector switch so it can be set to either 115V or 230V.  I have a few of these and normally they are setup with two Pro bias outputs but I could change one of those to normal bias. 

Price:  930$ plus shipping

Now here is a rare one, the Sound Tech MC7 Pro bias amplifier.  I got this one with the headphones (which is what I wanted) but these sold almost exclusively in China a few years back.  The circuit is a variation of am old Stax design but updated so this makes for a good, budget tube amp.  RCA and XLR inputs (with a proper input selector switch) and the classic ECC83/6SN7 tube layout.  This is no monster in terms of power output but it should work well with all of the Stax L series, Lambdas, SR-009 and X9000. 

This one comes with all of the original tubes, the original bamboo box and it is sadly 220-240V only.  There is no way to change the voltage internally. 

Price:  1200$ plus shipping

This SRM-1 Mk2 started out life as the normal bias only version but since they use the same PCB as the Pro bias units, I just populated all the missing parts on the PCB so this one now has one of the outputs converted to Pro bias.  The amp has been fully refurbished with new capacitors, updated the bias supplies and cleaned the volume control so it works perfectly. 

This unit has the voltage selection hard wired so I can set it to any voltage before shipping it out.  The amp is in over all nice condition, some marks on the painted back panels but they all suffer from that.  Over all though, it is a nice little amp which will drive most electrostatics perfectly well. 

Price:  395$ plus shipping

Here is a very early Stax SRM-313 with the external voltage selector.  The SRM-313 was my first ever Stax amp and sounded good enough to hook me for life when paired with a SR-007Mk1.  🙂  This unit is in nice condition for its age, some marks on the chassis and the top of the front panel but nothing to loose any sleep over. I’ve fully gone of the amp and replaced all of the electrolytic capacitors and cleaned the volume control so it is ready for the next 20 years of service.  With the external voltage selector it is easy to set it to any input voltage. 

Price: 600$ plus shipping

Here is a small amp project which dates back to 2019 but never really went anywhere.  The plan was to look into cheap (sub 1k$) amps and a simple way of making them quickly.  I had known about the Little Dot units for years so I bought a couple of them, stripped them down and replaced pretty much everything to turn them into an electrostatic amp.  The main chassis, transformer cover and the back panel are all Little Dot Mk2 but the holes for the tubes were enlarged, a new transformer fitted inside the cover and a new PCB designed for the chassis. 

The circuit is something I’ve played with before, ECC83’s feeding 6CG7’s but in a very compact build.  The power supply is quite simple but the bias is fully regulated just make something new.  The plan was to make an aluminum face plates but it never went past the matte black PCB’s fitted to the units.  It terms of sound the circuit has just one cap in the signal path so while being extremely simple, it sounds really good if you aren’t trying to drive SR-007’s with it.  Throw a SR-407 or L700 on there and it sounds really good for such a small unit. 

Now what happened with this project… well it just kinda died when Covid hit.  The two amps were finished and going through trouble shooting and tinkering but yeah… other projects took over so these have sat for 3 years gathering dust.  One of the issues I never fully fixed was a bit of background hum due to the proximity to the transformer right behind the output tubes.  It is drowned out as soon as the music plays but still, it bothered me.  🙂  I will likely never attempt an amp in this price range again so now that the new Octave V4 is getting close to being ready, the number of prototypes is growing and time to move these on. 

The units can be wired for either 115V or 230V, I have one unit here in Iceland while the other is with Kevin Gilmore so it would ship from there. 

Price: 850$ plus shipping

Here is a very early SRM-717 amplifier (nr. 43) with the rare external voltage selector.  This was always the best amplifier Stax ever made but I’ve made it better with my extensive mods which gives about 20% more power over the stock unit.  The unit has also been fully refurbished with new capacitors and fully adjusted as well. 

It’s in very nice condition, with only small marks on the back panel.  It can be set to any voltage before shipping it out via the selector on the back panel. 

Price:  2300$ plus shipping

Here is something very rare, a Stax SRA-14S.  This was the successor to the SRA-10/12S from the 70’s and is a large upgrade in every way.  The preamp is pretty great and the earphone section is an upgraded version of the SRM-1 Mk2. 

This unit I found in Japan without the external power supply and I just had to have it.  This may be the most difficult product Stax have ever made in terms of servicing but I just like it so much.  Now the external power supply isn’t one really, it’s just a transformer and some rather crude standby circuit as all the actual power supplies are inside the main chassis.  So I did the most logical thing, I sacrificed the add in bay on the back and put a nice custom made R-core transformer there.  The umbilical spot was fitted with an IEC socket and it even has a built in voltage selector now. 

All of the wear items have been swapped out and all moving contacts (of which are many) have been treated with Deoxit. It’s condition is fair, the front panel looks nice but there are some marks on the back cover.  The replacement transformer can be switched to either 115V or 230V using the internal voltage selector. 

In terms of sound, this was the best amp Stax had at the time and it holds up well today.  They are very rare as I believe only about 300-400 were ever made as it was very expensive, around 200000Yen back in the 80s’. 

Price: 1540$ plus shipping

As a contrast to the one above, here is a very nice SRM-1 Mk2 amplifier, close to being in mint condition.  It has gone through my full refurbishment, all capacitors replaced, volume control cleaned and the amplifier fully adjusted.  Very nice sounding starter amp either for Pro bias on normal bias models.  The amp can be set to any voltage and I will do so before shipping it out. 

Price:  510$ plus shipping

Here is a SRM-727 I have for sale.  It has all the same mods as I apply to these amps which turns it into a mini KGSSHV so to speak.  Major updates to the output stage which increases the output power by 20% or so over the stock unit, plus the updated feedback mod is applied. 

It is is good condition over all but small pin prick marks on the back of the chassis.  The amp can be configured for any voltage before shipping it out. 

Price:  2200$ plus shipping

Here is a Chinese Stax amp which is for instance sold on Aliexpress and on the local Chinese market.  I’ve already taken apart some previous amps by this builder (botched KGSS and KGSSHV amps)  so when one of these was offered to me, I had to check it out.  Now it didn’t disappoint as I had to do extensive mods to it before I could even turn it on.  This is essentially a Stax SRM-T1/006t/500t/700t amp (it uses the same circuit) with a cut down CCS on the output.  The CCS transistors were stuck to the side of the chassis with double sided tape… and only one was actually stuck down when I got the unit.  I came up with a quick fix to that so now they are secure and actually conduct heat into the chassis. 

In terms of sound, it is what I’d expect from the Stax amp it is based on but a small upgrade from that.  It’s not as large an upgrade from stock design as with my CCS and mods but not bad at all.  The only quirk with this are the dual volume controls and they they operate backwards.  This builder has form with having no idea how to wire up volume controls so yeah… was expecting that.  Other then them being backwards and split up, they do work just fine. 

This unit is 230V only and can’t be changed to 117V.  It is in pretty good condition bar the scratches on top of the front panel. 

Price: 495$ plus shipping

This is a late production Stax SRM-Xh that I’ve completely refurbished with new capacitors.  This the later bipolar version which is very similar to the later SRM-212 and 252.  It will come with my aftermarket PSU with either a US or Schuko plug. 

Price:  230$ plus shipping

A Stax SRM-252S which is pretty rough shape cosmetically.  Small marks all over, as can be seen on the pics, but it works perfectly.  These are still the best way to get good sound on a budget and it walks all over the Topping EHA-5 in terms of performance.  Like all of the small 12VDC Stax amps, this one will come with one of my custom power supplies with either a US or Euro plug on it. 

Price:  280$ plus shipping

A Stax SRM-212 amplifier in pretty good condition.  There are some marks on the top cover but nothing major and these are excellent desktop amps.  The 212 was the upgraded version of the SRM-Xh, latter version, and a precursor to the 252A.  Lovely little amp which drives something like the Stax Lambda and L series well plus the Koss ESP-95X and the Nectar Hive.  As is usual with these small Stax amps, they will come with my own upgraded power supply with either a US or European plug. 

Price:  260$ plus shipping

Here is something fun, the small portable amp which was included with the Monolith electrostatic headphones but converted to the Stax standard.  This small battery powered unit is based on the Stax SRM-001/002 design and sounds good for something so small, only being let down by the DIN plug it came with as stock.  For this mod I replaced that with a Stax plug and fixed the bias supply to make it Stax compatible.  Normal 3.5mm input on the back of this and charged via micro-USB but it lacks a volume control so the source has to take care of that. 

Price:  170$ plus shipping

Here is one of the newest Stax compatible amps to come out of China, the Holo Acoustics EHA-1.  This was just released in China at the end of the last year and it manages to cram a version of the Stax SRM-001 amp design with a transformer into a tiny desktop amp.  There is also a DAC in there but I’ve never tried it, I always prefer to use my own.

This is a small amp, not all that powerful, but will drive the Stax L series, Lambda and Koss ESP95X etc. perfectly well. 

The amp is 230V only as there is no way to reconfigure the internal transformer.  It uses the C13 plug which is a bit of a pain but easily found on most laptop power supplies. 

Price 255$ plus shipping

Ahh the SRM-300, the weirdest Stax amp ever made.  This one was in a sorry state when it arrived, most the paint had flaked off the cover and it was bent out of shape due to shipping damage.  The amp it self was just fine, measured great so I adjusted it and bent the covers back into shape and painted them.  I had to sand them completely down so I tried a new way to a textured paint finish (as it was stock), it is a coarser than it should be but I think it looks nice. 

I can set the amp to any voltage before shipping it out. 

Price:  350$ plus shipping

The venerable SRM-1 Mk2 PP which was a special edition of the SRM-1 Mk2, dual pro bias outputs, upgraded wiring and improved parts.  I’ve gone through it and replaced all of the capacitors, upgraded the bias supply and rebiased it.  The finish on these doesn’t hold up well so it has some marks on the edge of the front panel and the painted back cover but nothing major.  It can be set to any voltage before shipping it out.  

Price: 425$ plus shipping

Here is a late production SRM-313 with dual pro outputs.  This being the late production type it has the same internal PCB as the SRM-323A and is one of the best amps Stax have ever made.  Far better than the trash fire which is the SRM-400S.  I’ve replaced the electrolytic capacitors and cleaned the volume control with Deoxit.  The amp can be set to any voltage before shipping it out. 

Price:  650$ plus shipping

Here is a second SRM-Xh, this is a very early serial number but the last version of the internal PCB.  Usual Stax and serial numbers BS.  🙂  This makes it very similar to the SRM-212 and 252.  I’ve replaced all the electrolytic capacitors and cleaned the volume control with Deoxit so it is as good as new. 

The amp has some marks on the back panel and will come with my own PSU, either with US or EU plugs. 

Price: 240$ plus shipping

Here is something simple, a Koss E/90 amp which I’ve modified the bias supply to Stax pro bias and fitted a Stax socket.  It is a 6 pin one but I’ll plug the center pin so only Pro bias sets can be plugged into it.  This makes for a nice starter amp for anybody thinking of trying Stax. 

The included PSU is US only voltage.  It could be substituted with a linear 9VDC/1000mA power brick for other voltages but one of those is not included. 

Price: 140$ plus shipping

Stax SRM-X Pro which is one of those oddities Stax made back in the day.  It is essentially a SRD-5 with a built in amp to drive it, all powered off either batteries or a 12VDC power supply.  This is the later Pro bias version so not quite as weird as the earlier normal bias only version but still, it is neat.  Now these are a pain to work on so it is fully stock but it fully works.  The condition is is not great, the rubberized coating it coming off in spots and the battery door is missing.  It comes with a 12V power supply with either EU or US power connectors.  Neat little starter amp which can even drive some of the more difficult headphones. 

Price:  225$ plus shipping


The following amps are all used Stax units which are based on the same circuit and have received similar modifications.  The main essence is how the tubes are being driven, the stock Stax amps use (to this day) resistors to drive the output tubes but what I do is remove those and fit a solid state constant current source (CCS).  What this does is almost eliminate the loss you get with the resistors so more power and more voltage swing which completely changes the amps performance.  Once modified these are the best bang-per-buck to be had as they can drive even the SR-007’s or SR-009’s. 

Stax SRM-006tS which has received my full mods and CCS upgrade as well as I fixed that mistake Stax did on the PCB with these.  There is a small mark on the top of the front panel and some small marks in the painted finish but otherwise it is in very good condition.  It can be set to any voltage before shipping it out. 

Price:  1300$ plus shipping

This is a rather unique T1 as it is one of the very first, dual transformer units.  Not only is it lacking the automatic preheat function but the circuit is also different to all of the later T1 and 006t units.  This more like a SRA-14S with a tube output and it does sound rather lovely.   I’ve fully refurbished it new capacitors and I’ve fitted a CCS to the tube output.  It is in very good condition as well and can be set to any voltage before shipping it out. 

Price: 1000$ plus shipping

Stax SRM-006tA which has received the full CCS mod.  The amp is so recent that there was no need to update the bias supply to the modern spec so it was left as is.  It is in very good condition aside from those small marks on either side, presumably from gear being pusher right up against it.  It can be set to any input voltage and sounds just lovely after the modification. 

Price: 1200$ plus shipping

This one is the same as above but a bit of a rarity, as it is the 006t but with three Pro bias outputs.  Stax only made these for just over a year as they discontinued support for the Normal bias sets but the new 006tA wasn’t quite ready.  Their solution, just add the third Pro bias socket and the amp is now a bit odd.  🙂  Same performance and with the CCS mod, this amp can actually drive three sets at the same time.

Price: 1100$ plus shipping

Here we have a Stax SRM-T1S in very nice condition which has received my standard mods plus a full refurbishment.  Not really a mark on it and since it comes with the internal voltage selector, it can be set to any voltage before shipping it out. 

Price:  1050$ plus shipping

This is the third SRM-006t I have in stock but a rather early one with the T1S circuit board.  Lovely little amp and fitted with my mods, CCS and all capacitors replaced as I usually do.  This one has some marks on the back of the top panel but otherwise it is in very good condition and I can set it to any input voltage before shipping it out. 

Price:  1050$ plus shipping

Here is a 006t of the last version with three Pro bias outputs.  It’s received all the same mods as these amps get plus all of the capacitors have been replaced.  As all of these amps, it can be set to any input voltage. 

Price: 1100$ plus shipping

Here is a Stax SRM-006tA with all of the mods, CCS and further upgrades to the circuit.  The unit is in very good condition over all and can be switched to any voltage before shipping it out. 

Price:  1150$ plus shipping

This is a Stax SRM-T1 which has been fitted with the CCS mod and all the capacitors have been replaced.  The front panel is in fair condition but the back has been resprayed rather badly in the past so that is reflected in the price.  Can be set to any voltage before shipping it out

Price:  900$ plus shipping

This is a really nice “gold” SRM-T1S.  I’ve gone through it and replaced all the capacitors, done all of the mods and fitted with CCS.  It has an internal voltage selector so it can be set to any voltage before shipping it out. 

Price: 1050$ plus shipping

This SRM-T1 has been shitting on a shelf for years here as it arrived with a missing volume knob and the volume control was broken.  I was always on the lookout for replacements but nothing has shown up so I just did it my way.  The amp was fitted with a new Alps RK27 volume pot and then I found pretty much the only available volume knob which fit in that cutout.  Due to the length of the volume control shaft and the height of the knob, the control is not as pronounced on the front panel as it was with the Stax units but it works perfectly. 

The amp was then treated to my usual mods, the output protection added for the amp channels, bias supplies updated to the latest spec, all electrolytic capacitors were replaced and the amp fitted with a CCS for the output tubes. Cosmetically the amp is pretty good condition for a 35 year old unit, with not too many marks on the front panel or the back cover.  This one can be set to any voltage before shipping it out. 

Price:  840$ plus shipping

Here is an odd little thing, a portable electrostatic amp from China.  This is sold with a matching set of headphones but I’m keeping the headphones so the amp is up for sale.  It has a built in battery so it charges from a micro USB and the input is standard 3.5mm.  It comes with a propitiatory connector but also an adapter to 5 pin Stax so it can drive any pro bias Stax headphones.  Neat little desktop thing but I have too many units at hand so it is never used. 

Price:  260$ plus shipping

Here is a classic, a Beyer Dynamic N1000 energizer for the ET1000 system.  The set I received had some serious issues with the headphones so I replaced the cable with a Stax normal bias unit so this energizer is surplus to requirements.  It will with two adapters as well, one which allows Stax normal bias headphones to be used with it and adapters from the DIN inputs to banana sockets. 

Price:  160$ plus shipping


Burson Conductor V2 which I was using as a preamp.  Very nice unit, sounds great and is a far better unit than the new Conductor amps.  This one doesn’t have the dac module so it is just a headphone/preamp. Can be set to either 115V or 230V.

Price:  400$ plus shipping

A completely original Stax SRA-8S preamp/amplifier from 1967.  This one is completely untouched and while it came from Japan the transformer does have a provision to run on 117V as well.  Simply move one wire and you are set.  Now this amp needs to be refurbished but I have so many that I just want to sell it.  These are super rare and a great match for anybody with a SR-1 set or even the super rare SR-2.

Price: 700$ plus shipping 

Ahh the Topping EHA-5, people had so much hope for this but it is not a good little amp.  It doesn’t measure or sound well but I’m done with this unit so I might as well list it for sale. 

I bought this when they were first released and it only arrived with the Euro plug on the PSU, even though they can be easily swapped out.  I stripped it down, ran tests on it and then designed a new PCB which adds protection to the output as the stock unit has none.  This amp has been fitted with that fix that did require me slightly moving on of the transformers and drilling an extra hole.  Other than that, the unit is fully stock and even has the plastic cover still on the front panel.  It comes with the original box and manual. 

Price: 330$ plus shipping

Last updated 27.05.2023