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: 3100$ plus shipping
Here we have the super rare Stax SRM-Monitor, intended for studio use back in the 80’s. This might be the rarest Stax amp from this era, even fewer units than the SRA-14S or even the SRM-T2. This one has the black front panel which is rarer still as most were silver. What’s inside are a fully fledged SRM-1 Mk2 and an ED-1 equalizer in the same box. It’s a lovely setup with a SR-Lambda or SR-Lambda Pro and the amp also has an output for the EQ on the back to pass the signal onto another amp.
I’ve fully refurbished this unit, replace all the electrolytic capacitors, adjust it and cleaned all the contacts. It’s in lovely condition and those marks on the front panel are just because of the flash photography as the light here is so bad. I can set it to any voltage before shipping it out.
Price: 2400$ plus shipping
Here is another Stax SRM-007tA that I’ve fully refurbished and modified. I’ve replaced the capacitors, change the tube type to 6S4A, added a CCS to the tubes and done my usual mods as well. This turns it into a mini KGST and an amp that matches really well with the 009, 009S and X9000’s. Plenty of power to drive them too.
The camera didn’t want to play ball on the back panel but it is like new like most of the amp. The only cosmetic blemish is a bit of the paint has rubbed off around the tube covers. I can set the amp to any voltage before shipping it out.
Price: 2000$ 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 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
Here is one of the rarest Stax amps ever made, the Airbow SRM-222 with the matching power supply. This started life as the SRM-212 before being overhauled with better parts and some slight improvements too. It’s has also been “Cryo” treatment but I put very little stock in that, it is certainly better than the stock unit though. Not many of these made and it might actually be nr.1, based on the serial number. No way to confirm that but I’ve never seen a lower serial number.
The power supply can be run at either 100V or 117V and features a medical grade switch mode power supply. Much higher grade than the stock units and my upgraded ones. It has a captive US plug on it too which is perfect as it can’t run on 230V anyway.
Price: 700$ 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 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
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
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 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
A late model Stax SRM-1 Mk2 in nice condition that I’ve fully refurbished with new capacitors and updated to the current standard. Very nice starter amp and works very well with the new SR-X1. The amp can be set to any voltage before shipping it out.
Price: 375$ 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
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
This is a an early model of the Stax SRM-313, the amp that started all of this electrostatic fascination for me so I’m quite fond of them. They can even drive more difficult sets like the 007’s to a reasonable degree so a great starter amp to have. This one I’ve fully refurbished and replaced all of the electrolytic capacitors so it is as good as new. It’s in good condition too with just some small marks in the paint here and there.
As with all of the other Stax amps, it can be configured to any voltage before shipping it out.
Price: 470$ 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: 325$ 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
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.
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
Here is a fully original Stax SRM-006tS that I’ve performed my mods and added a CCS to. This is almost the same amp design as the current 500t and 700t so with the CCS and mods it is greatly elevated in performance over them. The condition is good overall but not quite perfect, some small marks here and there. The amp can be set to any voltage before shipping it out.
Price: 1150$ 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 fully refurbished and modified SRM-T1S, fitted with the CCS and all capacitors have been replaced. I’ve also fitted all of the modern safety items, updated the bias supplies and just the usual mods I do to these amps. It is in very nice condition with barely a mark on it and it can be set to any voltage before shipping it out.
Price: 840$ plus shipping
A SRM-T1 that I’ve fully refurbished and done all of my updates too plus fitted a CCS. It’s on good condition but the back cover paint has oxidized or something so it has this white sheen to it. Nothing major though and no other large blemishes. The amp can be set to any voltage before shipping it out.
Price: 750$ 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: 500$ 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: 200$ 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 19.05.2025