OTL Headphone amplifier

[Thom]

Hi all !

I'm going to build a power amplifier for my Stax SR-404 electrostatic headphones. I've been searching the internet for information and inspiration for quite some time now, this is what I've come up with. Please look at it and give me your opinions about it.

If you recoqnize the first stage...yes it's taken from Lynn Olsons Amity, I should not go wrong with that I suppose.
The same way as in Stax commersial amps, the power stage plates are directly connected to the headphone stators (these drive the ESL membrane in a push-pull fashion, membrane located right between the stators). They should be near 0 V. 580V headphone bias is fed to a separate pin on the headphone connector.

I was thinking about using the Lundahl LL1667 anode choke,
2x135H/15mA. Or do I compromise HF response with such high inductance?, will it be better to use the LL1668 2x50H/25mA?
(both are the same price).

If my math is right I will have a Zout of about 12k with the chokes.
The phones have Zin of 70k@20kHz, 140k@10 kHz (they are like 110pF caps) Do you think a Zout of 12k is sufficiently low?
If I use load resistors (say 10-15k) instead of chokes, then I will lower Zout to about half that, Am I correct?

I've not made up my mind about the powersupply yet..I will post it when I'm done. Suggestions about it are very welcome.

I hope I'm not pestering you with too many questions in my first post, I'm grateful for any input.

[fdegrove]

Hi,

The phones have Zin of 70k@20kHz, 140k@10 kHz (they are like 110pF caps) Do you think a Zout of 12k is sufficiently low?

Should be OK.

If I use load resistors (say 10-15k) instead of chokes, then I will lower Zout to about half that, Am I correct?

Yes,but than you will have to change the 15V PSU as well and I feel the chokes are a much better solution.

Here is a link you may want to read:

STAX SRX

Cheers,

[Brett]

I'll ditto Frank, and add that you can buy 540H/5mA LL1667 from www.diyparadiso.com in Belgium. I haven't asked Benny, but I think they're a custom made for him, as the local distributor here didn't have them on his list.

I also have two pairs of STAX Lambda Pro's and want to do a driver for them, and your is similar to what I had in mind, though I was planning on EL84 / 7189As for the output tubes, basically because I have a lot of them. My guess is it will work well and sound great.

Cheers

[Thom]

Thanks for the input, much appreciated.

fdegrove: fact is, I was almost determined to build the SRX amplifier but changed my mind. I did some computer simulations of the SRX, without feedback the frequency response was very bad, beginning to roll off at 10kHz or so.

Brett: I didn't see the 540H on the Lundahl site either..yes probably custom, or a maybe a new model?
The STAX lambda Pros are said to be near identical to the 404's.
I haven't had the opportunity to hear any STAX, not even my own because nobody import the headphones or amplifiers in Sweden.
High expectations..

[fdegrove]

Hi,

I pointed to it since it had some interesting comment by Mr.Nardelli.

Did you simulate his modifications as well?

I haven't had the opportunity to hear any STAX, not even my own because nobody import the headphones or amplifiers in Sweden.

Just like Brett I used to have the Lambda Pro.
You'll find the Stax quite addictive and a very analytical listening tool.

Enjoy,

[Petter]

These sound great, don't worry. You would be hard pushed to get high end sound at a lower price. I would go so far as to say that i have yet heard speakers that compare

Segway down to www.headwize.com and check out Kevin's projects as well while you are at it.

If you want full drive, you need to be able to supply about 15+mA which is quite a lot given these voltages.

Petter

[Brett]

Originally posted by Thom
Thanks fdegrove: fact is, I was almost determined to build the SRX amplifier but changed my mind. I did some computer simulations of the SRX, without feedback the frequency response was very bad, beginning to roll off at 10kHz or so.

I saw it a couple of years back, and didn't like the look of it. A dealer I know (who knows me and the SRX) told me not to bother, and design and build one for myself. The expression on his face said it wasn't good, but he knew where there was a second hand one if I <i>really</i> wanted it.

Brett: I didn't see the 540H on the Lundahl site either..yes probably custom, or a maybe a new model?

I get the impression the Benny has them made to spec. My local distributor also told me there were many other types available, not commonly listed on distributor or dealer sites.

The STAX lambda Pros are said to be near identical to the 404's.
I haven't had the opportunity to hear any STAX, not even my own because nobody import the headphones or amplifiers in Sweden.
High expectations..

I've had my Lambda Pros since the mid 80's, and used them daily for many years. Recently I was offered a newer pair very cheap, by a guy who wanted to upgrade to 404's. When he got them, he was underwhelmed by the difference. By that time I'd paid him for the old ones.
Recently (1 year ago) I went around all the dealers and listened to every pair of headphones I could find. None were even close to the STAX LP's (my old pair) in sound quality, and few were as comfortable. I know from experience I can wear the LP's for many hours at a time.
When you first hear them, they may sound a little bass light, but what you're actually missong is all the distortion.

I hope you enjpy them, and keep us informed of your progress with this amp. I will slowly collect the chokes etc for mine, and probably not build for a year yet.

Cheers

[Brett]

Forgot to metion in my first post, that the ECC99's really love being run at about 20mA Ia. Reduce the negative rail if you need to get the dissipation down, 'cos with the chokes you'll swing plenty at the top, and 350Vp-p (max for the diaphrams iirc) is INCREDIBLY loud with these earspeakers.

Cheers

[Thom]

Originally posted by fdegrove
Hi,

I pointed to it since it had some interesting comment by Mr.Nardelli.

Did you simulate his modifications as well?

Enjoy,

Hi,

Yep, interesting comments I agree.

Yes I simulated both balanced and unbalanced drive and 6SN7GTs. The balanced seemed to work best. In unbalanced mode, I never got the amplitudes in the last stage equal. The first stage trimmer only seemed to affect the relative position of the two signals and not their amplitude.. Maybe it has to do with the tube models I used?

[Thom]

Originally posted by Brett
Forgot to metion in my first post, that the ECC99's really love being run at about 20mA Ia. Reduce the negative rail if you need to get the dissipation down, 'cos with the chokes you'll swing plenty at the top, and 350Vp-p (max for the diaphrams iirc) is INCREDIBLY loud with these earspeakers.
Cheers

Thanks for the advise. You're right that I probably don't need all that 350V Va. My 404 datasheet say that 100VRMS=100dB.