Electrostatic Tube amp?

i keep wanting to build that.

but then im put off by other projects.

i think with good tubes it should sound very sweeeet.

I built this amp a couple of months ago. It was my first attempt at a diy tube based stax amp. I have only 303s, so whatevever I hear is not going to be the last word in stax performance. Initial results were a little dissapointing. Everything worked, but the sound was thin, and the base was pretty weak. Not enough gain or voltage swing. Never the less, a stax amp with only two tubes and a transformer is a very attractive concept. I reworked the design so that the output tube is now a triode connected kt88 running at 580v on the plate ("pro" bias). The 6sl7 was replaced by a high gm 12gn7a (thanks, Pete Millett!) and the coupling is via interstage transformer. Huge improvement! Base, depth, and some real balls! Still brutally simple. I'm curious to see how it would compare to a "real" Stax amp. Definately worth a try if into diy Stax!

snicker, snicker

Looks like your redesign would be pretty fun to listen to though.

To some this is actually a "real" Stax amp as it is the circuit used in the Single Power ES-1 and ES-2. It is a design by Stax released in the 1968 SR-3 instructions manual in Japan for owners to DIY. Amps have come a long way since then and so have have the headphones. I've been slowly gathering parts to build one like this, though with a quad of 7n7's and EL34's for output duty, but it keeps getting knocked back in the pecking order, most recently by a new Blue Hawaii. I'll include the original schematic here below which includes the PSU.



The only thing of note in the PSU schematic is the heater arrangement which floats the output tubes as the tubes pass the heater/cathode limit. I'm also going to have a separate HV PSU for the output stage but that should be overkill. Your take does sound interesting though, what is the ratio of interstage transformer?

Frank,

I'd love to see a schematic of that if you have one. And of course any pictures of your work are always appreciated.

Actually it was the Cuifoli Stax amp not the Bonavola amp that I built. Sorry, got my Italians confused. This amp is just a simple two stage single ended design with a 1:1 phase splitting transformer on the output. Still, beefed up, it seems to work pretty well. Besides the use of the 12gn7a/kt88 combination, the main difference from the Bonavolta design is the power supply. I'm using an off-board 580vdc tube rectified LCLCLC filtered supply for the kt88s then regulating it down to 325vdc for the 12gn7a drivers. As stated earlier, the12gn7a drives the kt88 via a 1:1 interstage transformer. The "output" transformer is a 30ma 1:1 phase splitting transformer. At 580vdc, this transformer is considerably out of its design specs. No problems yet, but I like this design enough so that I had Jack Elliano of Electra-Print build me a couple of 60ma phase splitters capable of handling 1kv. These will go in the finished version of the amp I am currently constructing. There's lots of room to play here. ultimately I'll try an 845 or a gm70 for a "poor mans'" dht Stax amp. Sorry no pics or schematic (no digital camera). If someone is really interested pm me and I'll surface mail a schematic.

One further note. I replaced the interstage transformer of the 12gn7a with a 5k:32 ohm output transformer. I now have an amplifier that drives both dynamic and electrostatic headphones! In Stax mode the primary of the 5k headphone transformer becomes a plate choke, coupled to the grid of the kt88 with a pio .33mmf cap. It's fun being able to hear both types of headphones from the same amp at the same time.

Very interesting indeed and it's this basic design?

The DHT's are definitely the way to go for voltage swing and there was some talk about electrostatic amps using them a few moths back. Gilmore has even designed a fully dual differential monster featuring 3 stages, all DHT's, but refuses to share the schematic to anybody but those that are willing to drop the considerable sum to build one. The Emission Labs tubes alone are a few grand...

Spritzer, the amp you linked to is indeed the amp I built.

Like you, I am intrigued by the possibility of an all dht Stax amp. My Stax experience is very limited, but I've been building big dht amps for years. My main speaker amp is a three stage all dht (46/211/gm70). I built the gm70 k-1000 amp at the "Mayberry on Acid" meet. I think there are enough sonic similarities between the k-1000 and the Stax to try a similar amp for the Stax. This is my ultimate goal here. I've never heard one of Dr. Gilmores' amps but I'm familiar with the designs on "Headwize". My design philosophy is more primitive but I hope equally valid. Especially with dhts, I like to keep everything single ended as long as possible and do the phase splitting in a transformer at the end of the signal chain. This substantially reduces the parts count of the amp and provides an inherently balanced signal. If I build such an amp I'd like a 1kv at the output (I know you like lots of headroom too!). To get 60db of gain in a three stage dht amp is tough but not impossible. Those EML tubes are nice but there are readily available and much cheaper alternatives. The Eimac 35t or 3c24 come to mind. Use a 211 as the second stage and a gm70 as the final. An amp like this would probably run about $4k in parts. A major investment but not an impossibility. Potentially, this could take Stax to a new level. Right now I'm learning to crawl before I try to run. The kt88s' sound surprisingly good in this application (I have an elderly pair of original Genelex). Bet a quad would sound nice in your "Blue Hawaii"!

A SET tied to an OPT is certainly a way to make things easier and opens up a world of options. I do feel that the headphones need a lot of power behind them so a lot of voltage swing with plenty of current reserve is a must. The amp that I'm working towards will have four 450v PSU's yielding a +/-900v output which is all the CCS parts parts can handle. The SR-007 soaks up current as well as voltage so it will interesting to hear the result when there is plenty of both.

There certainly are cheaper alternatives to the EML's but they are releasing some interesting new twists to the old designs and the build quality is astounding. There could even be some obscure tubes out there perfect for use with electrostatic headphones such as the 6S4A has proved to be in the smaller amps.

Four 450v power supplies? +/- 900v ces? Your amp is nothing, and I do mean NOTHING compared to my new "Brown New Jersey"! Dual stacked 833 transmitting triodes in mu follower configuration, outputting directly into veterinary grade cardiac defibrilators! Bias is supplied directly to the listeners brain stem by a well worn, but still functional Alabama Dept. of Corrections surplus electric chair skull cap! In a stunning technological breakthrough, all 17 filament power supplies have been reduced to well under 400lbs. apiece! For those on the go, a portable version will be offered that easily fits into a U.S. Army M1A1 tank transporter! All cabling is triple cryoed Monster Cable that has been treated with hormones from extinct Abyssinian hummingbirds in a unique proprietary process! For a slight additional charge, a "green" version will be available that uses a wind driven phased array of tesla coils as a power source! One man can't do everything, so I'm thinking of approaching a certain well known maker of high end amps to handle the disribuion and warranty work.

F-ing brillinat!!!! I really like the option of green wires for those select few that do appreciate the sound of properly made corrosion... with cryo.

Speaking of DHT's, you might like to look at this Morgan Jones 845 amp that is featured in one of his books. The schematic is huge so some detail is lost in the printing due to the small size of the page:






Interesting use of a push-pull choke to mimic the absent OPT.

Double post due to HD800 madness...

This amplifier is not a "Bonavolta design". It is a copy and paste without permission from my article "Un amplificateur pour casque électrostatique" in the french magazine L'Audiophile" (May 1990). In my article I clearly indicated this circuit is based on the "Stax Circuit B". This amplifier works very well with some modifications : use 180K and 270K feedback resistors and + 245 -370 volts power supply voltages.

 

P. H.