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spritzer

Transformer coupled electrostatic amps

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This concept has interested me for a while now given just how simple the amp could be made. Just two tubes per channel and then a phase splitting output transformer makes for a stark contrast to something like the T2. Now whether this simplicity is better or worse isn't the issue for me, I just want something different. :) I figured it was time to get the concept of these amps outside of the random thread on HF and PM's.

I've been talking to Frank Cooter about this and given how happy he is with his D3a/KT88 amp I'm going to try something similar. I'm always a fan of using what I have (hence my Exstata is a bit different from all the others) so soemthing like the 3d21A or 7403 in place of the KT88 could work well. The 7403 does interest me since it can handle so much voltage. Any suggestions here would be welcome, the more obscure the better. :)

As for transformers, they are a large part of the this project and no doubt, a large part of the over all cost. That said, people are willing to pay a small fortune for the updated output capacitors of something like the Woo WES for which you could get amouphous core Lundahls made. Since the transformers provide so little (if any) stepup they should be fairly linear. Given the currents and voltages involved, we could even get away with using stock interstage tranformers. Frank used the Electra-Print 3436 in his amp and at roughly 100$ each it's less then a V-cap. Now the 35mA might be a bit small but the 3035 model can handle 70mA and isn't that much more expensive. :cool:

Now the true beauty of these amps, the ability to use DHT's without needing 12 of them. The same amp could also be used for dynamic headphones or even speakers with the right output transformer.

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Do I even want to know how much voltage those things can handle? :-[ Something like this would make a 'stat amp for speakers "easy" to build., makes a lot more sense then to use a regular tube amp on an ESL.

Count me down as interested.

Would a mercury vapor rectifier PS be feasible?

The sky is the limit here but mercury is a nasty stuff to deal with. I would probably go for a tube rectified PSU with no electrolytic caps and plenty of chokes. Might even regulate it with tubes too. :D

Edited by spritzer

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Right now the D3a/ KT88 amp is my "daily driver"electrostatic amp. I recently completed an exSTata, which sounds pretty good in it's own right. Over the next week or so I'll be doing extensive comparisons.

My first attempt at upgrading the D3a/KT88 design was the 813 based amp I brought to LA CanJam.This amp was not what I'd hoped it would be. I made a number of mistakes in the output transformer specs, and my SRPP driver stage was not up to the job.

Since there are a few others interested in transformer coupled electrostatic amps, I think it's time to take a second shot. This time the approach will be a little more practical. The output transformers will be speced properly, and instead of an 813, I'll try an 801. The 801, while still a DHT, is much more "real world" than the 813.

Whether this whole approach is a viable alternative to existing designs I don't know. Just have to build a few and see. I'm hoping to make this thread a sort of "miniblog" of the journey.

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Welcome to the other side Frank. :)

Transformers certainly have enough merit for me to make a genuine attempt at the high end. I've been logging a lot of hours on my two transformer boxes lately (Lundahl LL1630 and vintage Koss iron) and even from a lowly Gainclone they are well worth the time and expense. All my time until CJ is booked with finishing current projects but after that I'll order up some D3a's and some custom iron. :)

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I'm thinking that just 2 more people need to appear here, then we will have the top DIYers

of all time all in one place... :D

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Right now the D3a/ KT88 amp is my "daily driver"electrostatic amp. I recently completed an exSTata, which sounds pretty good in it's own right. Over the next week or so I'll be doing extensive comparisons.

My first attempt at upgrading the D3a/KT88 design was the 813 based amp I brought to LA CanJam.This amp was not what I'd hoped it would be. I made a number of mistakes in the output transformer specs, and my SRPP driver stage was not up to the job.

Since there are a few others interested in transformer coupled electrostatic amps, I think it's time to take a second shot. This time the approach will be a little more practical. The output transformers will be speced properly, and instead of an 813, I'll try an 801. The 801, while still a DHT, is much more "real world" than the 813.

Whether this whole approach is a viable alternative to existing designs I don't know. Just have to build a few and see. I'm hoping to make this thread a sort of "miniblog" of the journey.

Hi Frank,

Welcome to HC. Let me know if you'd like a second opinion. :D

I bought Nate's Menace TCLite and love the sound of transformer-coupled headamps.

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Here's a picture of my electrostatic amp under construction. It's my first attempt to post a picture on a forum. Hope it works. I'll get better. Any advice would be appreciated. Right now I'm in the process of rebuilding it in a more professional form. More info. later.

post-2408-12951157447305_thumb.jpg

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Looks beautiful, Frank, but 2 posts in a month does not a "miniblog" make! Let's hear some more about this thing! Do you think it will be ready for prime time if we have another Mayberry get together in February?

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Here's a picture of my electrostatic amp under construction. It's my first attempt to post a picture on a forum. Hope it works. I'll get better. Any advice would be appreciated. Right now I'm in the process of rebuilding it in a more professional form. More info. later.

Hey Frank, it looks great. I advise you to invite me over so I can listen to it too. :D

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Looks beautiful, Frank, but 2 posts in a month does not a "miniblog" make! Let's hear some more about this thing! Do you think it will be ready for prime time if we have another Mayberry get together in February?

Totally agree with Al. We need to hear more from you, Frank. I like how your "under construction" looks like a totally gorgeous finished build. And Al, I like how you're thinking with February rather than October.

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As usual Frank your first attempt is more impressive than most of us could ever hope for. As for feedback, I got nothing.

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Are those 7403's? :) Other then that, no comments from me besides basic questions about the circuitry.

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Wow! My image post worked!

Thanks for all the kind words. I've had a few distractions in the past months, but now I'm back on track and should complete this amp in a few weeks. A "Mayberry" public debut would be great, but first listen will go to my fellow "Santa Barbarian" Ken, after a barbecue and a few beers.

I need to put a kudo in here for Jack Elliano of Electra-Print. For 15 years he's built transformers for me. Almost everything I've asked for has been a non-standard, custom design. He's never let me down no matter how crazy my requests, and his prices are dirt cheap considering the custom engineering involved.

Originally, I used an old recycled pair of 35ma 1:1-1 high level phase splitting interstage transformers for the output of the amp. Although they subjectively worked well, I knew that running 600vdc with a 1kv output swing was way beyond their design parameters. He doesn't normally make this transformer anymore and now reccomends an autoformer for phase splitting duties. I called Jack and asked for a pair of purpose built transformers that could do 600vdc, have enough primary inductance to support a wide variety of tubes, generate a 1:2 step-up,and have a 20hz-30khz bandwidth. He laughed. This is an extrodinary challenging set of specs for a transformer. At first he wasn't sure it was possible, but after a couple of attempts we have purpose-built electrostatic amplifier output transformers. If this amp has any value at all it will be because of his work. Thanks Jack!

On Spritzer's suggestion, I substituted a 3d21/7403 for the kt88 as output tube. In a no feedback, single ended amp, the quality of the tubes makes a big difference in the sonics. I'm fortunate enough to own a pair of original British Genelex kt88. They sound great, but the suppli8es are gone, and they now cost a fortune when and if you can find them. The reissue "Gold Lions", the best of the current production, while very nice, are a step down. Standard Chinese Shuguangs sucked. I've got a pair of 7403 on tap, but I've been running 1970 British STC 3d21a. You can buy them NOS all day on Ebay for $20.00 a pop. In this application, I can't tell them apart from the Genelax kt88.

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