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 my take on the BHSE, single chassis amp measuring just about 33cm deep with the volume knob. There were two of these amps under construction for a while of which this is the latter unit. This a full blown BHSE running at 20mA per tube and with the +/-400V power supply built into the chassis. It’s a tight fit but it all works nicely though the transformer can hum a tiny bit as it is suspended from the top panel. There is no hum present in the headphone output though.

The amp will shipped with a matched quad of JJ EL34’s and all the adjustment for the tubes is made through the top panel of the amp. I don’t endorse tube rolling at all but it’s a good idea with any BHSE amp to monitor how the tubes are doing from time to time. The amp can be set to either 117V or 230V via an internal switch. There is just one of these amps and I won’t build any more for a while at least.

Price: 5100$ plus shipping

Here we have a Stax SRM-006tA which I’ve upgraded to use a constant current source to drive the output tubes.  The main issue with these amps in stock form is how they just run out of steam at even moderate volume levels.  The mod fixes that to a large extent so the amp has much more power and better power delivery into the headphones.  This also makes it in many ways a mini KGST.

Cosmetically it is in fairly good shape, no dings I can spot but some pain on the back cover is missing.  That is quite normal for the Stax amps as it flakes off easily.  The amp was originally 100V only but I’ve converted it so it can work with with either 117V or 230V and I will preset it before shipping it out.

Price:  1350$ plus shipping

Here we have a Stax SRM-T1W which I’ve upgraded to use a constant current source to drive the output tubes, same as the 006tA above.  The main issue with these amps in stock form is how they just run out of steam at even moderate volume levels.  The mod fixes that to a large extent so the amp has much more power and better power delivery into the headphones.  This also makes it in many ways a mini KGST. This amp is also pretty rare and this is the only upgraded one I know of.  On top of the CCS mod, I replaced all the capacitors in the unit and redid the bias supplies so they match the current Stax spec.

Cosmetically it is in fair shape, some dings on the front and the back cover is missing some paint.  That is quite normal for the Stax amps as it flakes off easily.  The anodizing on the front has also seen some damage, clearly from sunlight shining on it for an extended period and something on it which makes the finish not uniform.  Again, it is something common on this vintage of Stax amps.  The amp has the voltage selector on the back so it can be set to any voltage.

Price:  1450$ plus shipping

This is the amp included with the Massdrop ESP-95X and I have no use for it.  I’ve made a Stax adapter cable so this can work with any Stax pro bias model.  It’s not the best amp in the world but it is capable enough.  I have both 117V and 230V power supplies for it.

Price: 120$ plus shipping

This is the last of my Dynalo amps and was my personal unit.  It uses the newest version of the circuit which was supposed to be used in the next run of these but I just don’t have the time to build them.  Too many electrostatic projects to build.

Price:  590$ plus shipping

Ahhh the Jade 2 amp.  Me owning this and exposing how shockingly bad its build quality is has caused Hifiman to loose their cool but now I just need to get rid of it.  The amp is what you would expect from Hifiman but I did some changes to it so it’s at least now safe to use.  I cleaned up the internal wiring, made sure it is properly earthed and replaced all the no-name capacitors with proper units.  I also redid a lot of the soldering as one capacitor just fell off the PCB.

Sound wise this is pretty thick sounding with not a lot of extension.  Uninvolving also springs to mind but it could be messed with and improved greatly.  I just don’t need any more projects so I’m offering it up for a lot less than retail.

Price:  450$ plus shipping

I was asked to a fully encased version of the KGSSHV as I did a few of these a couple of years ago.  The heatsinks can be a bit sharp so with children around, it can be an issue.  I said why not and made a small batch of these, same power supply as the regular KGSSHV but a new amplifier board with on board heatsinks.  Same specs as the regular KGSSHV too and these amps are the first to sport the new custom made Stax sockets I had made.  As with most of my other amps they have an internal switch to select between 117V and 230V but as always. will be configured correctly before shipping it out.

The KGSSHV is my take on the maximum performance for a price.  When the Stax T8000 is just a badly done tube input version of the 727 (with some real degrade in terms of tech) the KGSSHV has more voltage swing, more current, fully regulated power supply and better performance for half the price.

Price: 2200$ plus shipping

Here is one I thought I’d never sell but then again, I never use it. So this is a Stax SRA-7S which as the first popular Stax amplifier and designed for the SR-1. This was back in 1967 so before even the SR-3 or SR-X mk1 was released. It’s an all tube design and quite small for the amount of circuitry that is in it. So I got this amp from Japan and it has been completely rebuilt. The only original parts in this as the chassis, output sockets, tube sockets, amplifier PCB and the original tubes as they all measured good. The rest is all new and improved. There is a thread on head-case.org where I documented the lengthy rebuild process but basically when I got the amp it was 100V only and the power supply was basic to say the least. There was also no place to put the improved power supply so the first thing I did was to scrap the phono input as it took up so much space and was terrible to begin with. The transformer was scrapped and a new unit from Hammond was installed with the +300/+600V power supply from the Octave. The amplifier board was completely rebuilt with far better parts and moved further away from the transformer.

I really went to town here though as everything else was changed and upgraded. The front panel markings were peeling off when I got the amp so I cloned them and had my laser shop etch them onto the same plate once I had cleaned off all the old labels. The back panel RCA sockets had not aged well so I replaced them with new gold plated Neutrik sockets and redid all of the input wiring. There was a new input switch fitted plus a new volume control and balance pot, both Alps RK27. The power and mono/stereo switches were also both replaced with new units from Japan plus one output was converted to Pro bias output to make it more modern.

This amp has one unique feature which Stax stopped adding in the 70’s… it has a tone control. Basically this is a switch which has 4 settings and it increases the amount of series resistance there is inline with the output of the amp. Now the stock configuration was odd as it treated just one half of the push pull output so when I rebuilt it, it’s done right with matched sections for each output phase. Finally the amp was fitted with new feet, a proper power cable with strain relief and the top cover was powder coated. Last but not least…the knobs used on these are still being made in Japan so I custom ordered new ones as the old ones were unsightly.

The prices for these amps are crazy in Japan now even with tatty samples hitting 1000$ or more. I paid way too much for this one and the rebuilt was…insane so that explains the high price. I also kinda don’t want to sell it but that’s how it is… 😉

Price: 1700$ plus shipping

This is a preamp I used for testing and I got off ebay a while back.  The basic circuit was sound and used good transistors but there is always room for upgrade so I replaced all the capacitors with Black Gates, Panasonic FC and Phillips caps, redid the input wiring with pure silver wire and silver plated copper and replaced the volume control with a DACT unit.  The end result was an excellent little preamp but I just upgraded so I don’t need it any more.

The unit is set to 230V now but I should be able to change it to 115V easily.

Price:  250$ plus shipping

Last updated 19.05.19

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