Transformer coupled electrostatic amps

[cetoole]

OK, absolutely positively i should never rely on anyone who thinks they know what they are talking about.

The bias on the koss headphones is absolutely close to 600 volts.

Here is the proof

http://gilmore.chem.northwestern.edu/kesp1.jpg

So the tan resistor has absolutely no markings.

The 10 meg resistor is in series with the +630 volt power supply (plus or minus a bit, on batteries, about +600)

The unknown resistor is to ground in parallel with the cap.

You just can't measure that resistor with it in circuit, which is the failings of everyone else.

http://gilmore.chem.northwestern.edu/kesp2.jpg

and this meter measures to 200 meg.

http://gilmore.chem.northwestern.edu/kesp3.jpg

Now usually when i see resistors like this, they are 1 gigohm.

But even if that resistor is as little as 250 meg ohms, the bias is

still (referenced to 600v) a minimum of 575 volts.

I could bring the thing to work where i have meters that measure

into the gigohms, but there is no reason.

THE KOSS ESP950's ARE HIGH BIAS HEADPHONES!

Your E/90 is that way, mine (now Frank's) wasn't. I don't know if this is a result of Koss adjusting the bias to compensate for squealing, or poor QC, or what, but it is definitely different.

[kevin gilmore]

I think you actually need to measure that resistor. I know what it looks like, but i've been told

that it supposed to be brown black violet which would be 100 meg, resulting in a lowering of the bias by about 9%.

There is no way you can safely do 300 volt bias when you have the amplifier doing +/- 600 volts.

That or someone did the modification wrong. With koss, you can never be sure.

Clearly koss has some manufacturing issues.

[cetoole]

Both resistors there have clearly the same marking though, so unless they are sorting them by hand, they should be the same (minus tolerance). I didn't measure the resistance though. I really have no idea about Koss sometimes, they do some things great and some things, like this bias issue, are just weird.

[spritzer]

Speaking of Koss, look up "Factory Made" S02E21 to see one of the Koss models being made...

[FrankCooter]

Got my ESP950's back from Koss yesterday. They replaced the drivers and a number of other electrical and cosmetic components. They work flawlessly and look brand new. Total cost was $6.00 shipping! I bought these used, I have no idea how old they are, but I do know that they have had the drivers replaced under warranty at least once before. I told them the truth that they were deliberately abused and being used with a custom amp. No problems and no costs! I am very grateful and very impressed. Stax and most other companies could learn a thing or two about customer service from Koss.

[spritzer]

Drivers can be repaired but if Stax would just sell us the parts we need then I'd be happy...

[FrankCooter]

[ATTACH=CONFIG]3391[/ATTACH]

Spent the last week or so listening to my newly revived ESP950's and the transformer amp. Don't have any reference points except an EXSTATA and an SRD-7, but I like what I'm hearing and feel there's more to be had. Time to get some DHT's in the mix. I've got a full on 3 stage 845 in the works, but I was thinking about something simpler to start with. Here's a quick sketch of a modest 2 stage design I may try. The 826 has a mu of 31, very unusual for a DHT. Not rare or expensive, but not used a lot because it needs a minimum of about 600V. The 2E22 is actually a directly heated pentode, that aside from the filament and socket, is electrically equivalent to a KT-88.

[n_maher]

I love how you throw iron around, Frank. That design is simple enough (baring whatever PS is required) that a simpleton like me thinks, "Yeah, I should be able to do that."

[spritzer]

The only "real" hurdle with any of these amps is getting output transformers good enough. With that sorted you can build almost anything you'd like.

[FrankCooter]

I love how you throw iron around, Frank. That design is simple enough (baring whatever PS is required) that a simpleton like me thinks, "Yeah, I should be able to do that."

You're not the only simpleton Nate. A big part of the reason I use a lot of iron is that it greatly simplifies the design process, and enables someone with basically technician level electronic ability (me) to play at a much higher level than would otherwise be possible. I'm not the only one though, the Japanese DIY community has been big into iron from the beginning of the DHT revival. Most of the tubes I like best were originally intended to be used with iron. Iron in the audio path was abandoned in the late '30's more for reasons of cost than performance. Even today, while it may not produce the best test measurements, there are some applications (such as driving a big DHT like an 845) that are better served by transformers than any modern alternative.

[dsavitsk]

I love how you throw iron around, Frank. That design is simple enough (baring whatever PS is required) that a simpleton like me thinks, "Yeah, I should be able to do that."

Simple in terms of parts count, perhaps, but, getting that thing quiet and stable is real work.

The only "real" hurdle with any of these amps is getting output transformers good enough. With that sorted you can build almost anything you'd like.

[spritzer]

...and your point is? Does any iron made today match the quality of for instance the old Peerless iron? I certainly don't think so...

[dsavitsk]

Regardless of the difficulty of sourcing the transformers, building that amp is difficult. There are lots of "real" obstacles to having it in one's living room. There's a reason there is only one Frank around ...

[kevin gilmore]

There is more than one frank...

I have a MIG2 dynamic amp (similar to BH) with 8 x 6c33's

6 filament transformers alone. Consumes about double the power of the T2.

Even though i published schematics, no one but me ever built one.

I have an all DHT direct coupled electrostatic amp. Its so expensive i can't even

afford it. (plus i never finished doing the high frequency isolated filament transformers)

$8k for the tubes alone.

[FrankCooter]

Simple in terms of parts count, perhaps, but, getting that thing quiet and stable is real work.

You have a good point. Only way to find out if it's practical is to build it. Fortunately, it's simple and I have all the parts at hand. No great loss if it doesn't work out. I've built 2 stage all DHT amps for headphones before. Simple design, but a bitch to get quiet enough for headphone use. You need really well filtered power supplies, especially for the filaments ( I had to add additional filtering to the otherwise exellent Tentabs supplies), the gain needs to be as low as possible, the tubes need to be shock mounted, and you need lots of tubes to sort for the quietest and most stable individuals. PIA, never totally without residual problems, but to me at least, totally worth it. Once you hear a 26 or 112A in a 6SN7 circuit, you'll never be satisfied with the 6SN7 again.

In this case, I'm hoping that the low sensitivity of electrostatic headphones will work to the advantage of DHT's. Hopefully, any residual noise or ripple will be too small to make any difference at all. I don't expect any problems with the 2E22 outputs. The 826 first stage is a "pig in a poke" until actually built. Never worked with the 826 before. My guess is, that because of it's high mu (31) and heavy filament requirements (7.5V @ 4A) it won't be suitable. Nothing new for me though, by far the majority of the stuff I build I eventually discard. I have to remind myself now and then of the Thomas Edison story where he was asked by a reporter about how the light bulb was coming. At this point Edison had been working on the light bulb for eight years without success. His reply was something like: "I can't tell you exactly what makes a light bulb, but I can tell you 10,000 things that don't."

The fun for me in all this stuff is the journey. Uncertainty and failure make success, should it come all, that much sweeter. I like to think there's still a place in the modern world for the occasional empiricist.

[n_maher]

the occasional empiricist.

I think you just earned yourself a custom title.

[FrankCooter]

Had this up and running for about a week. It's a choke loaded DHT 826 cap coupled to a transformer loaded directly heated 2E22. It's a bit shy on gain (about 400), and the bass rolls off due to the 100H choke on the 826 (need about 300). Still, a very interesting project. Surprisingly little noise and hum. Enough promise here to warrant pursuing this with appropriate parts and a little more sophisticated design. [ATTACH=CONFIG]3597[/ATTACH]

[padam]

Wow, nice!

[Dusty Chalk]

I love what you did with the copper.

[spritzer]

Truly, stunningly beautiful.

[kevin gilmore]

Outstanding. I would have no idea how to make a chassis like that.

[Voltron]

Fucking gorgeous Frank! Wow.

[Nebby]

At first I thought those copper pipes were heatpipes, so I was trying to figure out where the heatsink was Beautiful work Frank!

[Grahame]

Frank, a work of art!

I expect you get that a lot

[guzziguy]

Wow, nice!

I love what you did with the copper.

Truly, stunningly beautiful.

Outstanding. I would have no idea how to make a chassis like that.

Fucking gorgeous Frank! Wow.

Frank, a work of art!

I expect you get that a lot

x N.

Gorgeous work Frank. Would it be possible for me to see it and listen to it before you tear it down?