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eXStata DIY Electrostatic Amp for Intermediate DIYers  

post #1 of 2969
Thread Starter 
Folks, I believe we have arrived.

This thread is a continuation of this thread:

A DIY Electrostatic amp

started by ericj. Ericj was looking for a reasonably priced, easy to build DIY electrostatic amp that is within reach of diyers who have or want to move to electrostatic headphones.

I did a quick and dirty design on that thread, but this design was only marginal. While we were testing this verion out my consulting practice went haywire for a few months. A good thing as it turned out because while I couldn't work on any electronics for a while this project kept percolating in the back of my mind. The prototype team was remarkably patient during this gap. I want to thank them for their patience.

When I was able to get back to the project a whole new design had evolved along with a new PS. Both designs were much simpler and were straightforward enough to ensure that properly built boards would work without difficulties. The whole design philosophy was "simple but good."

Once again the proto team consisting of ericj, pabbi1, sachu, wiatrob, and me started stuffing boards. And now there are five of these amps working and driving headphones.

We've called it the eXStatA and there are two versions: an all SS version and a tube hybrid that uses 6S4s. The tube version is going into prototyping now.

The same PS will supply either board and the sand and hybrid amp boards (both channels on one board) are the same size with the same mounting holes so you can swap one for the other in a working amp.

A few particulars for now.

1. The rail voltage is +/-300. This number was chosen so that you can use easy-to-get, inexpensive transformers. There is a Hammond transformer and a Triad transformer that fits. The Hammond has the heater winding for the tube version.

2. The PS has two bias supplies: an HV supply that is adjustable from about 340V to about 680V and an LV supply that is adjustable from about 170V to 340V.

3. The boards are: 150mm x 95mm (two channel amp) and 100mm x 100mm (PS including bias supplies).

4. The maximum peak to peak voltage is about 700-800V. This is good enough for many, many situations. Remember that this amp is targeted at intermediate builders.

5. The gain is approximately 1000 give or take some.

6. Balanced or SE input (Edit)

7. The amp is fully DC coupled. No capacitor bass rolloff. In fact the amps have no caps at all in them (Edit)

I think we have met or exceeded the original set of design goals.

The prototypers have been chomping at the bit to post impressions and pics. But, we have waited until all five amps and PSs were successfully built and listened to.

We're going to burn these amps for a while longer just to be sure that their are no problems with use. HV is a different animal from the LV amps that most people build.

I don't have pics because my boards are off with wiatrob getting ready for RMAF. But the other guys should be posting soon.

Thanks for your patience during the gap. We're back.
post #2 of 2969
Congrats! I'll be interested to see this... hmm, electrostats? Oh wifey.... heh
post #3 of 2969
And, chomping at the bit to give impressions... but, first, some eye candy:





As almost everyone knows, I am a tube guy - just basic coding in my DNA. But, we had to go SS first to proof the design, and that is where we are.

My first disclaimer is that I do not have any other stat amps (operational) with which to compare. Perhaps soon. All my impressions to date are with the he60, and limited with the SR-40 electrets.

First impressions are simple: Plenty of gain (I can't get past about 1 o'clock) even as insensitive as my ears have become. This is a fast amp, which really opend up not only the upper range, but also the soundstage. There is plenty of air, precise imaging, and, oh yeah, bass.

Bear in mind the team has at most 100 hours on these units, most with less than half that. Even so, the amp is very stable with 1v offsets all around, and stable (adjustable) bias.

Ongoing listening and burnin, but, for $325 ish in parts (NOTHING boutique), this amp is a no brainer. I will have both the SS and tube versions in Houston for a meet on Oct 17th, so I'll get additional comments then.

One thing that is a little different about this amp is that it is, well, FUN - like a brassy blonde, but not obnoxiously so.

This is the DIY stat amp for intermediate builders that won't require a serious investment in parts, and is as simple to case as any of Alex's dynamic amps, which really sets this apart from most stat amps.

Comparative impressions will be fothcoming in a few weeks. For now, just know it exists, it sounds quite nice, and will be available soon.
post #4 of 2969
Quote:
Originally Posted by pabbi1 View Post
And, chomping at the bit to give impressions... but, first, some eye candy:





<<SNIP>>.
Putting it in a SUN drive box "classes" it up a few notches! Nice Job. Would love to see it "race" against the conventional big iron out there.

.
post #5 of 2969


Some quick impressions on the amp...
Definitely better than the SRM1 Mk-2 that I have. There is a lot more air ..wider soundstage. Truly brings to the fore that my ESP950 is not really "dark".
I love the bass it has but i would want a little more with the ESP950, the instruments seem to have air around them making it feel forward compared to the SRM1-MK2.

I am using a PEC carbon pot and vampire like jacks for the inputs .

The case was chosen to be cost efficient and in keeping with the extreme poormans concept. But I will spruce it up with powdercoating, redo the rear panel for the jacks and add a custom top plate once everything is in its place.

That means having the tube boards as well in the same chassis and with 3 outputs (one low bias, one pro at 580 and one pro at 650 volts).

Best compliment I can give it is that since its arrival the EHHA has taken a back seat and hasn't seen use in a week now.



Another pic



Congrats to Alex for answering the call, Ericj for starting this journey out with his challenge. Also kudos to wiatrob for putting together such a fantastic easy to follow BOM.
post #6 of 2969
Quote:
Originally Posted by sachu View Post
You guys are making me rethink some stuff I have sitting in the Garage. Some of it SUN or SUN clones by the way!
post #7 of 2969
Just throwing out some casing ideas... but, here is my tube case version.



Rosewood front panel, limoncillo back and sides, with walnut feet - just scraps I had laying around the garage.
post #8 of 2969
I actually have two amps working on the bench and am feverishly working on casing - they will be at RMAF Saturday and Sunday - I will not be able to attend Friday (stupid day job).

If all goes well - one of those may be the tube version, fingers crossed...
post #9 of 2969
Quote:
Originally Posted by pabbi1 View Post
Just throwing out some casing ideas... but, here is my tube case version.


Rosewood front panel, limoncillo back and sides, with walnut feet - just scraps I had laying around the garage.

You've been holding out! Wonder how you'll get the board to line up under that tube ring
post #10 of 2969
nice scraps Pabbi, and well done all round.

This may be the final push that sends me flailing into the stat world ..dB
post #11 of 2969
I'd post pictures of my amp, but it's a bit naked as i was the last to complete a build and i haven't gotten it into an enclosure yet.

I don't have any other amps to compare it to either - just the SRD-7 transformer box, and i have to say it compares very well.

I agree completely that this is a very lively amp - not shy or reserved at all.
post #12 of 2969
this is very sweet. Any bom lists? or more details L)
post #13 of 2969
Cool! I look forward to trying it out at RMAF this weekend.

If I got one I would want balanced XLR inputs and 6S4 tubes - is that the standard configuration, or is RCA inputs the standard?
post #14 of 2969
Quote:
Originally Posted by wiatrob View Post
I actually have two amps working on the bench and am feverishly working on casing - they will be at RMAF Saturday and Sunday - I will not be able to attend Friday (stupid day job).

If all goes well - one of those may be the tube version, fingers crossed...
I'll be there about 5PM Friday evening through Sunday.
post #15 of 2969
XLR inputs would be ideal i suppose but I don't plan on ever going balanced so thus I have SE inputs..

And yes for now the idea is to stick to 6S4 tubes..though there has been talk of making it possible to swap between EL34 and 6S4 tubes.
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