Class A hybrid tube-SS openloop power amplifier

[PMA]

Class A hybrid tube-SS openloop power amplifier

How about to try a “less serious” design 🙂. It would have some excellent parameters, like bandwidth, dynamic range, low noise, free of crossover distortion, and some less good parameters, like non-linear distortion, which would however remain low-order only.

The concept is well known, a tube gain stage followed by a class A solid state output stage, with gain close to +1, no global feedback from output to input, and very low distortion (speaking about output stage). No race for the best possible parameters, however avoiding known issues like crossover distortion, noisy circuits etc.

I think I have all the building blocks prepared. Is it worth trying, what do you think?

I am not sure if this belongs to Solid State or Tube forum, so if mods would decide to move it, please feel free to do so.

 

[Bigun]

Pick whichever forum you like, there are dozens of tube + SS hybrids all over place, some are in the chip amp forum too although not Class A, and there’s a couple in Pass forum. You could call it an integrated amp, not much different than a tube pre + SS power amp/follower.

 

[jfetter]

I would build them as separetes.

Start with prepamp and test on whatever then do power amp.
I did exactly that and learned a lot along the way.

The Sziklai type schematic here is what ended up sounding the best. You can use 6N1P and simply adjust the heater circuit.


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[PMA]

This is how it looks on a test bench. On the top there is a tube gain stage, below is a class A SS buffer, parameters as this one:

PM-A4 power amplifier

 

[Nrik]

It would have some excellent parameters, like bandwidth, dynamic range, low noise, free of crossover distortion, and some less good parameters, like non-linear distortion, which would however remain low-order only.
....
Is it worth trying, what do you think?

Of course it is worth trying.
But to get low non-linear distortion and low output impedance (to have great dynamic range) it would help with some feedback, global or local.

 

[PMA]

There is a local feedback in the output stage, that helps it to keep distortion in 0.0001%, when the output is in class A. Tube gain stage has higher distortion, though low order.

Schematics is basically as attached. It is a SRPP gain stage + slightly modified ExtremA output stage.

 

[PMA]

I have created a thread with a listening test, where the original file (rip) is compared with a file recorded through this amplifier that operates to real loudspeakers.

The thread is at

Can you tell original file from tube amp record? - test

So feel free to give it a try 🙂.