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 spritzer@mjolnir-audio.com 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 is something special, the one off amp I made for Tyll Hertsens when he started Innerfidelty so he could test any electrostatic headphones that he came across. There was a bit of discussion about how under served electrostatics would so after a brief “mafia” meeting it was decided to build Tyll something and naturally… it fell to me to do so. Now we had some basic design ideas, we wanted it to be a solid baseline for measurements but also not a fire breathing monster out of reach of anybody. At that time, the Headamp KGSS was the most common “non-Stax” amp out there so it made sense to base the amp on that… with some twists naturally. At its core is the Headamp KGSS pcb but with upgraded parts, larger heatsinks and some part substitutions so it could push out more power. It runs on +/-400V rails, fed from a custom version of the KGSSHV power supply designed to fit into this case. It also has one other PSU that I’ll get to later.
Now the 4 output sockets were setup to make sure Tyll to could test pretty much anything out there. From the right there is a regular Pro bias socket, a normal bias socket (230V bias), a Sennheiser HE90 socket (500V bias and the correct ballast resistor) and finally, an extra 5 pin socket. This has a special PSU on the side of the amp which feeds a 0-1000V DC/DC converter. Simply plug probes into those sockets on the side and adjust the trim resistor above to set the bias. This means the amp can run Jecklin Floats and other oddball designs plus there was talk of higher bias voltages back then so it was a way to future proof the amp.
Now when Tyll retired the amp was sent to Kevin Gilmore where it sat for a couple of years. On my latest trip to visit him I decided to thin out my hoard of amps so this one came back home with me. I then went through it completely, replaced the volume control, new feet fitted and put in a new custom made transformer (as it had three originally). It can be set to either 115V or 230V via the external voltage selector on the back panel which also houses the power switch. To make the front panel look clean, there was no power switch or LED fitted to the there so instead the switch is on the back and the LED’s are on the bottom of the amp, firing down.
Price: 3400$ plus shipping
A Stax SRM-007t amplifier that I’ve fully recommissioned and then modified/upgraded. These amps are now of the age where electrolytic capacitors can become an issue so I first replace all of them, clean the volume control and then modify the amp to use 6S4A tubes. I then add a CCS for the tubes and perform some further mods to the rest of the amp. This completely transforms the sound of the amp, making it much more neutral and far more powerful.
This amp is in nice condition, some small marks on the painted cover. It can be set to any voltage before shipping it out and with that normal bias output, it is one of the most powerful amps out there for the older Stax sets.
Price: 1850$ plus shipping
Here is one of the limited edition Stax SRM-600’s that were released to celebrate the Lambdas 40’th anniversary back in 2009. I bought one back then and quickly sold it as it was just a fancy 006tA and I never used it. I’ve always wanted another one though and now I really wanted to apply my mods and fit a CCS to one maximize its performance. I bought the nicest one I could find and it is a lovely piece of kit. There is a well known issue with these where the plate resistors fail and this amp was no different. No matter though as they are removed for the CCS modification anyway. So this amp is very close in terms of circuitry to the current day 500t but uses the ECC99 tubes instead of the 6CG7. That allows for a bit more power and more voltage swing.
The amp can be set to any voltage before shipping it out.
Price: 1700$ plus shipping
This is a very nice SRM-007tA amplifier that I’ve performed my full suite of mods on, added a CCS to it and changed the tubes to 6S4A’s. This totally transforms the amp and makes it into a mini KGST with performance far outstripping the stock units.
This amp is nice condition and can be set to any voltage before shipping it out.
Price: 2150$ plus shipping
Here is a Stax SRM-727 that I’ve fully modified and updated. The initial mod I designed for these, “the Spritzer feedback mod”, was a good first step but this one goes much further and turns it into a mini KGSSHV. More power, much more stable output stage and a far cleaner sound signature. This makes the amp ideal for use with SR-007, 009 or X9000 on a budget.
This unit is in overall good condition but there are some marks on the back panel and a faint scratch on top of the front panel. It can be set to any voltage before shipping it out.
Price: 2000$ plus shipping
Here we have a close to brand new Stax SRM-400S. I wanted to get one of these to take apart and tinker with and now I’m done with it. I do prefer the older solid state Stax amps to this one but this is a very nice example and it comes with the original box. Stax may have changed to yet another voltage selection standard but that won’t stop me, this unit can be configured to any voltage before shipping it out.
Price: 800$ plus shipping
Here is something I haven’t worked on in years, the Stax SRA-12S preamp/headphone amp. This was the top of the line amp in the 70’s and served as the inspiration for when I was designing the Carbon back in the day. Nice sounding amp and rather different from the SRM-1 Mk2’s that came later. Thicker, warmer sound which works really well with the more modern Stax sets such as the SR-009 and X9000.
I’ve completely gone through the amp, replaced all the electrolytic capacitors, fully adjusted the amp and added a pro bias supply to one of the sockets. I can easily change it so that both outputs are pro bias too. The amp can be set to any voltage before shipping out but it come with a captive power cord with a US plug. I can fit a Euro type plug on that too.
Price: 650$ plus shipping
This is a lovely SRM-323S that arrived here with a 100V only transformer. I don’t let that stop me so this unit is now fitted with an upgraded transformer which also gives it a bit more power compared to a stock amp. I also fitted the unit with a voltage selector switch so it can be easily switched between 115V and 230V. This is a very nice SS amp to begin with and my mods make it even better.
Price: 650$ plus shipping
Here is one of the last Stax SRA-14S integrated amps ever made, and a matching set at that. I was once told that only 250 of these were ever made and here we have nr. 249. This is a combo unit, a preamp and a Stax amplifier in one unit with an external power supply. This was the most expensive Stax amplifier until the SRM-T2 was released and features a much more advanced version of the SRM-1 Mk2 circuit, plus a preamp and a fully regulated power supply. This unit also has both of the option bays filled, one with the extra RCA input “jumper board” and the second is the MM-1, moving magnet phono stage.
These are a royal pain to work on, due to all of those front switches which then extend to the back of the unit. I’ve fully taken it apart though and replaced every electrolytic capacitor in the both the power supply and the amp unit. All the switches were also cleaned and lubricated for the next 40 years of service.
Over all condition is very nice, some scuffs here and there but close to mint for a 40 year old device. I also included a picture of the unit powered up as not many have seen the light show these have… so 80’s. This unit can be set to any voltage before shipping it out.
Price: 1700$ plus shipping
Stax SRM-313 with the desirable external voltage selector. This makes it one of the earlier units and its been fully refurbished by me with new electrolytic capacitors and I’ve cleaned and lubricated the volume control as well. It is a great starter amplifier and can even drive some of the more demanding headphones in a pinch. This one is in fairly good condition, no major blemishes, just small marks here and there. Due to the external voltage selector, it is easy to set the voltage for any region and I’ll do so before shipping the unit out.
Price: 485$ plus shipping
Here is the last one of the Octave V4 prototype amps that I built into old Stax chassis. This unit ended up with Kevin Gilmore to testing and I brought it back recently when I visited him. This is a full Octave V4 amp, just in a different chassis. I had a few of these chassis in stock so it made sense to use them for prototyping. The main difference with these are the RCA only inputs and they use an Alps RK27 volume pot.
The amp can be set internally to either 115V or 230V like all of my other amps.
Price: 930$ plus shipping
Here is a new Stax SRM-270S amplifier that I bought when they came out and just opened it up to check out the circuitry. It has next to no hours on it and looks like new, even though my camera simply refused to take a good picture of the front panel. 🙂 This is a Japanese unit but will com with my own upgraded power supplies with either a European or US power plug. It has the original box, manual and RCA cable.
Price: 420$ plus shipping
This is a stock Stax SRD-7Mk2 with the original box. This is the last, upgraded,version of the SRD boxes and suitable for both Pro and Normal bias headphones. This is a good way to drive electrostatic headphones off speaker amps as these units provide the bias supply and use transformers to step up the audio signal to the right level. It comes with a US/Japan power plug but is universal voltage so it can be used anywhere in the world.
Price: 475$ plus shipping
Here is a second, very early SRM-313 amp, but in much nicer condition. Same external voltage selector and this unit has been fully refurbished as well with new electrolytic caps. Some light marks on the back of the mousing but this is close to mint condition.
Price: 475$ plus shipping
A Stax SRM-1 Mk2, the ultimate starter amp for any electrostatic setup. This unit has had all of the electrolytic capacitors changed. the volume control was also cleaned and lubricated plus the bias supplies updated to the modern standard. The amp can be set to any voltage before shipping it out. It’s in pretty good condition for it age.
Price: 440$ plus shipping
The SRM-3 was the follow up from the SRM-1 Mk2 and a far more modern design. This is a great starter amp and puts some more expensive amps to shame. This unit has been refurbished with new capacitors, the volume control cleaned and lubricated plus I updated the bias supply to the modern standard. It can be set to any voltage before shipping it out.
Price: 465$ plus shipping
This is a Stax SRM-1 Mk2, the ultimate starter amp for any electrostatic setup. This unit has had all of the electrolytic capacitors changed. the volume control cleaned and lubricated plus the bias supplies updated to the modern standard. It’s in very good condition over all and it can be set to any voltage before shipping it out.
Price: 440$ plus shipping
Stax SRM-212 amplifier with box and instructions manual. It’s in nice condition and similar to the current day SRM-270S in performance so a great little starter amp. It will come with my upgraded power supplies in either US or Euro configuration that enhance the performance a bit.
Price: 285$ plus shipping
Here is a nice Stax SRM-1 Mk2 amp, fully recapped and cleaned so that it will work perfectly for years to come. These are a great beginner amp and pair well with either the vintage Stax sets or the newer L series. There are some scuffs on the back covers but over all in very nice condition. As with all of these, it can be set to any voltage before shipping it out.
Price: 430$ plus shipping
Here is a near mint SRM-1 Mk2 P.P. which was a special edition with improved internal wiring and resistors. These all have dual pro bias outputs with no normal bias. It is a great starter amp and drives any of the Lambda, L series or Koss ESP950 with ease. This unit was fully refurbished with new electrolytic caps, volume control cleaned and lubricated plus the bias supplies brought up to the newest standard. There are a couple of marks on the chassis (as pictured) but over all condition is very good. It can be set to any voltage before shipping it out.
Price: 485$ 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 works 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: 830$ plus shipping
Here is a Stax SRM-3, which was the follow up from the very popular SRM-1 Mk2. This is a great starter amp and puts some more expensive amps to shame plus it runs pretty cool. This unit has been refurbished with new capacitors, the volume control cleaned and lubricated plus I updated the bias supply to the modern standard. It can be set to any voltage before shipping it out.
Price: 470$ plus shipping
A Stax SRM-252S which was just superseded by the SRM-270S. Underneath the skin, these are almost identical amps so the 252S is great value now. It’s a great starter amp for the Stax X1, L series and the older Lambda units. I’ve gone through it and it is all running as it should be.
This unit will come with my own replacement power supplies, either in US spec or with a Euro plug. This an upgrade over the stock Stax unit. No original box with this one.
Price: 340$ 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$) amps and a simple way of making them quickly. I had known about the Little Dot units for years so I bought a couple of them, stripped them down and replaced pretty much everything to turn them into an electrostatic amp. The main chassis, transformer cover and the back panel are all Little Dot Mk2 but the holes for the tubes were enlarged, a new transformer fitted inside the cover and a new PCB designed for the chassis.
The circuit is something I’ve played with before, ECC83’s feeding 6CG7’s but in a very compact build. The power supply is quite simple but the bias is fully regulated just make something new. The plan was to make an aluminum face plates but it never went past the matte black PCB’s fitted to the units. It terms of sound the circuit has just one cap in the signal path so while being extremely simple, it sounds really good if you aren’t trying to drive SR-007’s with it. Throw a SR-407 or L700 on there and it sounds really good for such a small unit.
Now what happened with this project… well it just kinda died when Covid hit. The two amps were finished and going through trouble shooting and tinkering but yeah… other projects took over so these have sat for 3 years gathering dust. One of the issues I never fully fixed was a bit of background hum due to the proximity to the transformer right behind the output tubes. It is drowned out as soon as the music plays but still, it bothered me. 🙂 I will likely never attempt an amp in this price range again so now that the new Octave V4 is getting close to being ready, the number of prototypes is growing and time to move these on.
The units can be wired for either 115V or 230V.
Price: 600$ 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: 2100$ 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: 1350$ 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: 420$ plus shipping
This is another stock Stax SRD-7Mk2 in very nice condition. This is the last, upgraded,version of the SRD boxes and suitable for both Pro and Normal bias headphones. This is a good way to drive electrostatic headphones off of speaker amps as these units provide the bias supply and use transformers to step up the audio signal to the right level. It comes with a US/Japan power plug but is universal voltage so it can be used anywhere in the world with adapters.
Price: 350$ plus shipping
A Stax SRM-323A amp which is just about the same amp as the 323S, the last of the great small Stax amps. The major difference between the A and S model is part substitutions as parts became unavailable. It’s a great starter amp and can drive even the 007’s without falling flat on its face. This unit is in excellent shape and comes with the original box. I can set it to any voltage before shipping it out.
Price: 540$ plus shipping
This is an early production Stax SRM-Xh that I’ve completely refurbished with new capacitors. This the very early mosfet version which is same as the SRM-X, same PCB even. It will come with my aftermarket PSU with either a US or Schuko plug.
Price: 200$ 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: 220$ 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 235$ plus shipping
The SRM-300, the weirdest looking amp Stax 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.
The amp is a neat design that acts like a headphone stand and the circuit is really a scaled up version of the portable SRM-001. Very neat design and even features a small R-core transformer. I can set the amp to any voltage before shipping it out.
Price: 335$ plus shipping
This is a SRM-1 Mk2 amp that has been fully refurbished with new capacitors, updated the bias circuit to the new standard and cleaned/lubricated the volume control. The amp can be set to any voltage before shipping it out and it is in over all good condition with some marks on the paintwork.
Price: 395$ 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: 220$ 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.
Please note, due to so many requests, that I do not offer these modifications and refurbishments as a service. I only sell refurbished and modified amps from my own stock.
Here is a SRM-006tS that has received my full mods and a CCS. This unit was fitted with a 100V only transformer when I got it so I replaced it with an upgraded unit that can be configured for either 117V or 230V. The 006tS was the last of the 006t units until Stax released the 500t, which is the same exact unit. It’s in very good condition except for the pictured scratches on the side of the chassis.
Price: 1200$ plus shipping
This is a rather clean example of a T1S. Some small marks on the back cover and a discoloration at the top of the front panel but I’ve certainly seen worse. It’s been fully refurbished, CCS fitted, tubes tested and volume control cleaned so it’s good to go for plenty of years of service. It can can be set to any voltage before shipping it out.
Price: 890$ 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: 950$ plus shipping
Here is a really clean SRM-T1 that I’ve fully refurbished, fitted the CCS, cleaned the volume control and checked the tubes. Nice little amp that can easily drive most Stax headphones perfectly well. It can be set to any voltage before shipping it out.
Price: 850$ 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: 730$ 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: 200$ 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: 360$ 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: 600$ 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: 230$ plus shipping
I can’t remember which Audio-GD dac this is but I’ve had it for years. It uses four PCM1704 chips so fully R2R and sounds just great. I updated the firmware in the USB module at some point to make drivers far less of an issue (as Audio-GD are infamous for that) so now it works flawlessly. When the unit arrived it was set to 120V but I converted it to 230V for use here but I can easily change it back.
Price: 395$ plus shipping
The Massdrop Airist R2R dac which caused quite a stir back in the day. It’s a nice little dac, not up to the level of the Denafrips stuff but beats the crap out of most budget Delta/Sigma units. Comes with the PSU which is universal voltage so it can be used anywhere in the world.
Price: 225$ plus shipping
Musician Pegasus dac which is an excellent R2R design from China. It is multi voltage so can be used anywhere in the world. There are some marks here and there on the silver chassis but it is in overall good condition and works perfectly.
Price: 650$ plus shipping
Last updated 20.12.2024