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 we have a modified and refurbished Stax SRM-007t with the desirable external voltage selector.  I’ve taken the unit and replaced all the capacitors, cleaner all pots and switches, then the unit was heavily modified to change the tube type to 6S4A’s and fitted with a CCS to drive those tubes.  This turns it into a mini KGST amplifier with most of the same performance at a lower cost.  

The unit is in very good condition and looks almost good as new.  With the external voltage selector it is easy to configure this to any voltage and I will do so before shipping it out. 

Price: 2300$ plus shipping

Here is a Stax SRM-727 that has received the full set of mods I developed for it.  This is a major upgrade from the old feedback mod I came up with back in the day and it essentially turns the 727 into a KGSSHV.  More power on tap, a much cleaner sound signature and more neutral.  Now it lacks some of the finesse of the KGSSHV as there is only so much I can do in that chassis but this makes for a very large upgrade over the stock unit.  It can be set to any voltage before it ships out. 

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: 2200$ plus shipping

Here is another rare one, the SRM-T1W.  This rare beast uses the SRM-T1S main circuit board but adds more inputs, a passive preamp output and adjustable bias for the left most output socket.  I doubt more than 500 of these were ever made compared to the T1 and T1S which were made in the thousands. 

With this one I’ve done all of my usual T1/T1S/006t mods to the it, added a CCS to the tubes section, replaced all capacitors and upgraded the bias supplies to modern standards.  I’ve also treated all the moving parts with deoxit. 

The amp is in pretty good condition, some patina on the front panel and marks on the back but nothing too bad.  Operationally it is excellent and it can be set to any voltage before shipping it out. 

Price: 1900$ plus shipping

Here is a pretty recent SRM-007tA that I’ve modified to use 6S4A tubes and they are driven by a CCS.  This is a huge step up in performance over the stock unit.  This makes it a mini KGST, a lot of power in such a relatively small chassis. 

This unit is pretty much flawless in appearance and feels very close to new.  It can be set to any voltage before shipping out. 

Price: 2300$ plus shipping

Here is a late production SRM-3 that I’ve completely refurbished (replaced all the capacitors, cleaned the volume control, updated the bias supply to the current spec and improved the thermals of the output stage) and it is in rather nice condition.  The SRM-3 was the first major step up from the venerable SRM-1 Mk2 and the blueprint for the 313 and 323 amps.  There are some small marks on the back cover but nothing huge and I can set this unit to 117V or 230V before shipping it out. 

Price: 500$ plus shipping

Here is a second SRM-3 I have for sale.  This is an older unit but it has been fully refurbished as I usually do with these units.  It has some marks on the back of the unit and I’d rate it as fair condition.  I can set this unit to 117V or 230V before shipping it out.  

Price:  460$ 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: 670$ plus shipping

Here is a Sennheiser HEV70 amp.  It works perfectly and has no leaky capacitors inside so I left it as is.  The unit comes with a Euro power adapter as well as an adapter so it can be used with Pro bias Stax headphones. 

Price:  400$ plus shipping

Here is a Stax SRM-252A which is all but identical to the current day 252S unit.  It is in like new condition and comes with my replacement power supply in either 117V or 230V versions. 

Price: 300$ 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

Here is a stock SRM-252S I got recently with some headphones and have no use for.  Looks pretty much new and comes with the original box.  It will also come with my replacement power supply so it can be easily used anywhere in the world. 

Price:  350$ plus shipping

This is an odd one, at its heart is was the Soltanus Electrostatic transformer box.  I got it with a set of their headphones last year and yeah… this was very bad and utterly unsafe.  The bias supply was run straight off the mains and the 600V bias sits a mere 1mm away from the chassis wall.  The unit is also not really earthed as the back panel is, but none of the others actually connect to that electrically. The transformers were good though so I decided to strip out all the bad stuff and fit the guts of a SRD-7 in the chassis, reusing the transformers. 

That’s what we have here, new mains transformer fitted and main PCB plus three teflon output sockets as the stock ones were all trash (and two XLR’s which are wholly unsuitable for this role).  The unit has a built in voltage selector so it can be set to either 115V or 230V before shipping it out. 

Price: 875$ 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.

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: 1300$ 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:  1200$ 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:  1250$ 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: 1300$ 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:  300$ plus shipping

Here is a another small Chinese amp but this time, for the Shure KSE1500.  When I got it  I had already sold my KSE’s so I never spent any time with it.  The circuit is an old Stax circuit so it should be an upgrade over the rather mediocre Shure design.  Comes with a built in battery, charges through a micro USB and normal 3.5mm input. 

Price: 300$ 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:  180$ 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:  600$ 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: 800$ plus shipping

Here is a DIY amp I got from Japan as a combo with some headphones I wanted.  The plan was to do something with it but I just don’t have the time so I’m offering it up for sale.  This is the classic Stax SRX amplifier and is very well made.  It comes with all of the tubes and a very nice Tango transformer.  Therein lies a problem though as the transformer is 100V only and no way to adjust that.  There are replacement transformers available from ISO/Tango and other Japanese firms but they are rather pricey. 

Still it would be a fun project for somebody as it can easily be swapped to CCS load for the output tubes and other mods. 

Price:  300$ plus shipping

Last updated 15.05.22