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 the very rare Stax SRM-T1W which I’ve fully refurbished and modified. This comes from the same family as the SRM-T1S and uses the same amplifier pcb but with some extras, a passive preamp capability and adjustable bias on one of the outputs. This was the flagship amp of the reborn Stax from 1997 and until the 717/007t were introduced.
With this unit, I’ve performed my usual T1/T1S/006t mods and upgrades so this has been fitted with a CCS for the tubes. On top of that I’ve fully refurbished the unit, replaced all the capacitors and gone over all of it to make sure it is is 100% functional.
This unit is in very good condition overall with a few marks on the chassis. The amp has an external voltage selector so it is easy to set it to any voltage before shipping it out. Sadly no original box for this one.
Price: 1650$ plus shipping
The Stax SRM-717, arguably the best amp they ever made, that I’ve fully modified and refurbished. I’ve cleaned up the output stage, increased the output power and removed the protection stage as it just makes things worse. The end result is a far superior amp to the stock unit with more output power on top. I’ve also replaced all of the electrolytic capacitors so the amp is good for another 20 years of service.
There are some marks on the chassis but overall it is in lovely condition, close to mint. The amp can be set to any voltage before shipping it out.
Price: 2100$ plus shipping – reserved for a customer
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
Another Stax SRM-007t, here the later version with three Pro bias outputs. This unit has received all of my usual mods, tubes changed to 6S4A’s, fully CCS for them and mods to the rest of the amp. Then all of the electrolytic capacitors have been changed and beefed up too as the amp is now more powerful than the stock unit. In essence this is a mini KGST and close to it in performance. Given the abundance of Pro bias outputs, I replaced one of them with a 6 pin socket and added a normal bias supply to it. These make for great amps for the normal bias sets and have enough power to even wake up the SR-Sigma.
The amps has the usual tiny specs missing in the paint but overall condition is very good. There is a tiny spec of paint somebody has put to indicate the position of the volume knob so I just left that in place, as it can be hard to see from any distance. The amp can also be set for any voltage before shipping it out.
Price: 1750$ plus shipping
During the development of the Octave V4 I made a small number of test mules in Stax SRM-1 cases I had sitting around to verify the design and the PCB layout while I worked on the chassis design. This is the last of these test mules and inside is a full Octave V4 but spread out a bit on the larger chassis. It also has a RCA only input as that part of the Octave V4 had not been set in stone. I fully refurbished the unit and the chassis is in okey condition, marks here and there and I repainted the back cover as a part of another experiment. The amp has the same voltage selector as the stock Octave V4 but is internal on this one.
Price: 930$ plus shipping
Here is very early model Stax SRM-007t, fitted with the external voltage selector. This is the same core amp as the 007tA with just a different chassis finish and some minor changes inside. It’s a great amp when modified and with the normal bias, makes all the older Stax sets really sing.
This has been treated to my usual workup, all electrolytic capacitors have been replaced, the tube type changed to the much better 6S4A, added a CCS for the tubes, added all of my usual mods and cleaned all of the contacts. It’s in almost perfect condition though, just two small marks on the back of the top cover and on top of the tube covers. The amp can be set to any voltage before shipping it out.
Price: 1850$ plus shipping – reserved for a customer
Here we have a Stax SRM-323S, what I’d say is arguably the lest of the good small Stax amps before they went off the deep end with the 353X and 400S. This is rather powerful and neutral little amp, capable of driving rather power hungry headphones in a pinch. It is in lovely condition with just a few small marks on it. It unit comes with its original box and it can be set to any voltage before shipping it out.
Price: 625$ plus shipping – Reserved for a customer
Stax SRD-7Mk2 in rather nice condition that I’ve gone over and cleaned up. This is the last version of the SRD line and allows Stax headphones to be used with power amps to make for a nice and easy setup. These have both Pro bias and normal bias outputs and can be used on any voltage. They have a US style plug but I can supply a Euro style adapter for these.
Price: 375$ 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: 1100$ 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 something rare for me, a non Japanese version of a Stax amp. This is a SRM-006tII from Europe with the excellent black front panel. This unit looks nearly new and has received all of my usual mods, had a CCS fitted and I’ve replaced all of the capacitors with new units. This unit was set to 230V but can be set to any voltage before shipping it out.
Price: 1100$ plus shipping
This one is 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. It can be set to any voltage before shipping it out.
Price: 950$ plus shipping
This may look like a very early SRM-T1 but the PCB inside is actually a T1S. I combined two amps as I had a T1S that was damaged in shipping and a rather noisy sounding early T1. Since this was an early T1, the preheat function worked differently from the later units, let alone the T1S. The button is simply bypassed here and the led’s simply flash in tandem until they both light up as the tubes are warmed up.
Now aside from the chassis and input connectors, this is a normal T1S and has received all of my usual mods, CCS for the tubes and all new capacitors. These have an internal voltage selector under the bottom panel so they are easy to set to any voltage before shipping out.
Price: 875$ 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
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
This SRM-T1 had 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
This is a Stax SRA-10S which is the Japanese only version of the SRA-12S preamplifier. This dates from the late 1970’s and was the top of the line Stax amplifier until the SRA-14S was introduced in 1985. These are great little amps and a really good match to the more bright units Stax are making now. The output stage is essentially single ended class A so it runs hot and is warm and inviting.
I had a couple of these in storage and since they all shipped with 100V only transformers, I started out with new custom made torodial transformers. Then the amps were fully rebuilt with new electrolytic capacitors, I added a Pro bias output (as these were normal bias only), updated the normal bias to the current spec and finally added an IEC connector on the back instead of the captive lead. All controls were cleaned and treated and the back panel polished to make it look nicer. This is the nicer of the two amps no major blemishes and it can be internally switched between 115V and 230V as I added on of my voltage selectors to it.
Price: 450$ plus shipping
The legendary Stax SRM-1 Mk2 amplifier and this one fully refurbished with new electrolytic capacitors, the volume control cleaned and the bias supplies updated to the current standard. It is in lovely condition, barely a mark on it and will continue to run for the next 40 years just fine. This unit has the voltage selector on the back so no problem switching it to any voltage before shipping it out.
Price: 480$ plus shipping
This is a stock Stax SRD-7Mk2 in pretty good condition. This is the last 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 Koss E/90 amplifier which I’ve modified for Stax use. Whenever I get one of these here I harvest the Koss socket for use in adapter cables and fit my own Stax sockets and a new front panel to the amp. This makes for a cheap little amp which can easily drive the less demanding Stax headphones. It comes with a US spec 117V power adapter.
Price: 160$ plus shipping
Last updated 25.04.2026