DIY Stax amp?

[Sjoenne]

Hi everybody!

Here goes - My first post on head-fi.org:

I've just bought myself an old pair of STAX Gamma Pro's with a SRD-7 SB MKII driver adapter thingy.

I would like to put together another BIAS supply, and possibly a whole different way of powering the STAX.

The BIAS supply shouldn't be a problem, for the Pro's its 580V right?

But the power-section is another deal:

Is it possible to tweak an existing preamp or build a small tube stage to power the stax directly? Without the transformers? - if I use the SRD box + a small tubepoweramp I would end up with to stages of transformers - output from tubeamp plus stepup in the STAX driver...

Hope to get a lot of suggestions!

 

[grawk]

If you decide to sell the gamma pros, let me know

There are a number of DIY stax amplifier circuits out there. Here's a thread: http://www.head-fi.org/forums/f6/diy...statics-44199/

 

[Sjoenne]

Quote:

Originally Posted by grawk /img/forum/go_quote.gif If you decide to sell the gamma pros, let me know There are a number of DIY stax amplifier circuits out there. Here's a thread: http://www.head-fi.org/forums/f6/diy...statics-44199/

Thanks for your fast reply - but the thread you are linking to doesn't contain much info.. just a pair of links to irrelevant threads... grado... earbuds...

They mention the Blue Hawaii amp... is it publicly available as a kit og just schematics?

My goal is to find a simple DIY amp, or mod an existing preamp or poweramp.

 

[grawk]

There's no simple way to make an amp for stax, the voltages are way too high. There is a "poor man's" stat amp project that's in the works, but I'd guess it's 6 months or more til they're ready to release it. The blue hawaii and the kgss are the well known gilmore stat amps that are buildable, plus the circuit used in the rudistor egmont is pretty well known. I think tho that the people building them are reluctant to give a lot of details because no one wants to be responsible for an inexperienced builder electrocuting himself.

 

[Sjoenne]

hmm ok - maybe I'll just build a simple poweramp and then mod the SRD-7 driver with better stepups and a seperate BIAS.

 

[spritzer]

For a simple amp you can't really beat the Egmont since it is simple enough for a child to build and cost next to nothing. It needs to be optimized though (different plate resistors and probably some feedback wouldn't hurt) and the tubes have to be replaced with something that can give you some power. The 6SN7 is a horrible choice to say the least.

Then there is the Stax SRX circuit which Single Power used in the ES-1 and ES-2 amps. It too is cheap to build and fairly pain free. Again the tubes should be substituted for something better. I'm building a version of this amp which uses ECC83's in the first stage, then ECC99's driving 6S4A's. That amp should be up and running in a month or so, if nothing explodes.

There are also the amps from highamp.de which should be easy enough to build and the PoorMan project should be ready by the end of the year, at the latest. Now if you have a push-pull tube amp that you can modify a bit then that would be the most cost effective way of driving the Gamma Pro's. You can take the signal off the output transformer primary and feed it though some caps, directly into the headphones.

 

[Sjoenne]

Thank you very much Spritzer! - just what I needed to hear!

I'll take a look at the amps... the SRX seems to be very plain and simple.

If I get to lazy, I might use your tip on using a PP amp with a bypasscap.

 

[Sjoenne]

Could this EL84 PP amp be a candidate? Without the output-transformers? or does it need a constant load?

Would it be possible to use the B+(with a 2M series resistor for safety) for the STAX bias? or is it to low?

 

[spritzer]

Quote:

Originally Posted by Sjoenne /img/forum/go_quote.gif Could this EL84 PP amp be a candidate? Without the output-transformers? or does it need a constant load? Would it be possible to use the B+(with a 2M series resistor for safety) for the STAX bias? or is it to low?

I need a link to see it...

There has to be a load on the output tube plates so you either need an OPT or a push-pull choke.

All Stax headphones use a 5M ballast resistor so I would stick with that. Check out the electrostatic amp designs on Headwize and look at the bias supply designed by Dr. Gilmore. It is easy enough to retro fit it to just about any design since it only needs some high voltage AC and some B+ to operate.

 

[Sjoenne]

It must be sunday......

Here's the link : Simple EL84 poweramp/integrated

 

[spritzer]

That amp could work but the need for iron will inevitably drive up the price quite a bit. The PSU transformer could also be a bit costly (400mA isn't usually cheap) while I built the Egmont with a 25$ R-core I got off ebay (plus a second filament transformer due to the hungry tubes).

 

[Sjoenne]

Can I leave out the outputtransformers? replace them with another load maybe?

 

[spritzer]

You probably could do that (with plate resistors) but I wonder how stable the circuit would be. I'm also not sure that this amp would give enough gain and/or voltage swing. I'm certainly having gain issues with ECC99's as drivers in my Egmont.

 

[Sjoenne]

Hmm... so the Egmont would be problematic gainwise? And a pushpull EL84 possibly can't also put out the required voltageswing ? I'm confused.

Maybe your suggestion with the STAX SRX amp is the best.

 

[Sjoenne]

What is the impedance of the stax? Is that not a significant variabel, when designing an amp?