Tube OTL and Zotl amps

[Mr. Moscode]

A long winded talk with Berning:
Some screen grabs:

 

[Mr. Moscode]

I have not come across this, outside the the work of JC Morrison. His Silbatone Amp's using this, last time I heard them on antique WE speakers at Munich in 2019BC (before COVID) sounded fine.

I would be interested in other examples on existence.

Thor

Existence of tube/classD amps? I haven't been to a hifi show in years. Here's what under "Class D amplifier with tubes" google search:
https://hifiheaven.net/shop/Rogue-Audio-Hydra-Tube-Class-D-Hybrid-Amplifier-Blackhttps://audioxpress.com/news/new-hy...ds-tube-tone-with-class-d-power-and-bluetooth
It's really not much different than using a tube preamp with a D amp.
I once considered it. Just put a tube cathode follower or amp in front of the D amp and put a cathode follower in NFB loop. But that was 14 years ago. How fast are class D amps now?

 

[thor.zmt]

Existence of tube/classD amps?

No. Tube OTL with solid state current booster. Like my comment here.

For "OTL Tube Amp" I would probably just do a Futterman style Amp using a sensible output tube that can drive 600 Ohm (maybe a few 6N30 in Parallel) and add P-Channel Mosfets in a compound feedback pair to drive low impedance loads.

Yes, it's not strictly "OTL Tube" BUT is will give really high power and sound very much like a tube OTL Amp.

That is what I referred to

It's really not much different than using a tube preamp with a D amp.

Hmmm, that depends. A lot.

I once considered it. Just put a tube cathode follower or amp in front of the D amp and put a cathode follower in NFB loop. But that was 14 years ago.

No, no followers. Not if you want something that sounds like a real tube amp

How fast are class D amps now?

Depends on who designs them. Last one I did, the one with a tube frontend, it ran at 1.411MHz or 1.536MHz.

I'm look at multiphase designs (quadrature and beyond) at the moment that should give the equivalent of > 2.9MHz and 40V RMS into 4 Ohm.

Thor

 

[Mr. Moscode]

Hi,

For "OTL Tube Amp" I would probably just do a Futterman style Amp using a sensible output tube that can drive 600 Ohm (maybe a few 6N30 in Parallel) and add P-Channel Mosfets in a compound feedback pair to drive low impedance loads.

Yes, it's not strictly "OTL Tube" BUT is will give really high power and sound very much like a tube OTL Amp.

ZOTL? Why not just make a Class D Amp with a Tube in the first stage?

View attachment 107244
Thor

I found a medium power chip that runs at runs at 2.1 mhz. 45 W
https://www.ti.com/lit/ds/symlink/t...tps%3A%2F%2Fwww.ti.com%2Fproduct%2FTPA6304-Q1

 

[Mr. Moscode]

No. Tube OTL with solid state current booster. Like my comment here.



That is what I referred to



No, no followers. Not if you want something that sounds like a real tube amp

Thor

That's the Moscode, my amp, 2 gain stages, bootstrapped cathode follower, complementary lateral mosfets, like the Hafler amps of that era. There are a few thousand in the world.

 

[Mr. Moscode]

www.moscode.com

 

[thor.zmt]

That's the Moscode, my amp, 2 gain stages, bootstrapped cathode follower, complementary lateral mosfets, like the Hafler amps of that era. There are a few thousand in the world.

Harvey "Gizmo" was a friend, I know his amplifiers.

Thor

 

[Lampie519]

There are audiophile record labels that do care but it isn't a big market

Check these ones out :

https://trptk.com/studio/
https://funkstunde.com/?cat=16
https://www.tacet.de/main/seite1.php?language=de&filename=welcome.php
https://www.northstarconsult.nl/
https://challengerecords.com/catalogue/1216200682/Turtle Records

 

[Mr. Moscode]

Harvey "Gizmo" was a friend, I know his amplifiers.

Thor

I don't doubt it. I was the chief engineer at NYAL. You may not know the whole story.
A few pix - HR's Balls and him and his violin playing girlfriend at my wedding.

 

[Mr. Moscode]

Check these ones out :

https://trptk.com/studio/
https://funkstunde.com/?cat=16
https://www.tacet.de/main/seite1.php?language=de&filename=welcome.php
https://www.northstarconsult.nl/
https://challengerecords.com/catalogue/1216200682/Turtle Records

Add to that:
https://chesky.com/www.HDtracks.com
Telarc

 

[thor.zmt]

Add to that:
https://chesky.com/www.HDtracks.com
Telarc

And:

Reference Recordings | Since 1976, The Best Seat in the House

A lot of Music on Decca and sublabels is rather good, Decca/l'Oiseau-Lyre is excellent.

I have always argued that a great system can even make very poor recordings of great music involving and listenable. I have a test track for this.

The Chicago Transit Authority live playing "25 or 6 to 4". Terrible recording (maybe straight from the line out of the live mixer), but the band is totally on fire. If you can get past the recording quality.

A good system will do that. Get you to the music, past the recording.

Most rooms at hifi shows absolutely refused to play this (when I was still reporting hifi shows) and cut it after a few bars. In most cases I didn't mind either.

Thor

 

[Lampie519]

Please check out the recording i have mentioned here before (on this forum):

http://www.nakk.nl/Nieuw-Amsterdams-Klarinet-Kwartet/nl-NL/winkel.aspx




This one is killing ! Harmonics all over the place !

If there is distortion in the peaks or interference between the instruments then you know there is work to be done

It is the best test CD i have...

 

[thor.zmt]

A long winded talk with Berning:
Some screen grabs:

Here a better representation of the actual circuit:


Now, how does this differ materially from this (conceptually - not showing tube biasing etc., coupling devices not shown, not verified for specific polarity etc.):



And can my second example be made to materially match the transfer function of the first?

And can my example be made to have < -100dB switching frequency (I like to use the term "Carrier") on the output? Presume ~ 1.5MHz switching frequency.

Thor

 

[Lampie519]

The point is that ZOTL uses power tubes to drive your speakers and not a preamp that drives a couple of power mosfets.

Please see attached schematic (page 10 and 11) so you have it all:

https://davidberning.com/images/zh270/ZH270_Manual.pdf

 

[thor.zmt]

The point is that ZOTL uses power tubes to drive your speakers

It does not actually do that. That is my point. The speaker drive current is generated by a set of full bridge DC-DC converters.

David Bernings solution is very clever, but it is in effect a Class D Amplifier with an unusual way of modulating the output current, by modulating the current in a secondary winding (this doesn't have to be done by a tube anode).

In a Class D amplifier the speaker drive current is generated by a half or full bridge DC-DC converter where the output current is modulated by an input current. This input current can be generated by a range of means, including the current modulation of tube's anode.

and not a preamp that drives a couple of power mosfets.

Depending on your objective this "preamp driving a couple of power Mosfet's" may deliver better subjective and objective results.

I have often wondered what a hybrid Amp based on Jim Strickland's "Trans-Nova" lateral FET common source output stage with a tube frontend would perform like.

In terms of transfer curve, this output stage largely matches push-pull triodes with output transformer.

Well, life's too short.

Thor

 

[Lampie519]

Like this ?

 

[Lampie519]

The speaker drive current is generated by a set of full bridge DC-DC converters

Don’t think you know, know you know.

Best regards,
Frank

 

[thor.zmt]

Like this ?

Maybe, probably not. No need to DC couple.

Make a suitable bias network and DC servos, you have a output stage with an input cap.

Make the driving stage a nice tube with choke or CCS load and unbypassed cathode resistor.

The output stage in effect acs as cascode.

Return the feedback loop to the cathode of this tube.

Add another tube in Front, open loop.

Voila.

 

[Lampie519]

No dc coupling removes all the “magic” as they need to be driven hard (as all output devices).

The coupling cap is at the high impedant stage where it creates the least damage. Obviously you have no experience in high end audio.

This can all be done in PA systems or instrument amps ( like in Hammond organs) but does not belong to the top designs of high fidelity audio,

Study the designs of top designers like Jim and you will (maybe) understand.

Best regards,
Frank

 

[thor.zmt]

No dc coupling removes all the “magic” as they need to be driven hard (as all output devices).

Tell that to Mr Moscode...

The coupling cap is at the high impedant stage where it creates the least damage.

That is meaningless.

Where you place a coupling cap has no real impact, as long as you size it correctly.

Lateral FET's are actually very begnign loads, especially in grounded source mode.

You only need +/- 8V drive and the node to be driven is mainly capacitive.

Obviously you have no experience in high end audio.

Obviously not. Clearly Stereophile Class A rated products and EISA awards plus a raft of lesser stuff don't count.

Study the designs of top designers like Jim and you will (maybe) understand.

Yes, of course.

Thor