Voltage swing of Heffa?

[StoneT]

Hello eveyone I'm new but I'm sure many of you will be familiar with the amp I am trying to build. Its a Heffa, a derivation of the Williamson using 807's:
Documentation
This is my first amp so pease tell me if I am being stupid on anything I say, but I have a couple of questions:

Voltage swing: The tech documents say this is designed to be fed from CD line level (500mV RMS I am led to believe, so ~ 1.4V P-P) to the first LTP inverter. However, looking at the charts for a 6SN7 with the way it is biased this would produce an anode swing of approx 18V P-P between anodes, which as far as I can see would be way too high to feed to the next 6SN7 LTP and stay in class A1. Am I missing something? The Amp book I have (Mogan Jones) stated that a LTP has the same gain as a standard common cathode stage (but neglects to say which biasing method) but I think this may not be entirely true.

Feedback: Sorry to all whom I have offended by the use of the word but the speakers I will be using (at least at first) will not have a very flat impedance response and so I will need to dampen with NFB somehow. As all the stages are differential the only point to apply feedback without lowering input impedance seems to be the unused grid of the first differential pair which would make it global feedback. I was hoping to only use local feedback on the output stage but can't see how to do it. This may not be too critical since I am hopin to DC couple the first two stages. Also would people reccommend taking feedback from the 807 anodes or the output of the transformers?

Hope these questions are not too basic, I did search, honest Any help is greatly appreciated.

[SY]

I'll answer the question you should have asked- to apply feedback, you'll have to stagger the LF time constants for stability. A 10:1 ratio is probably adequate as long as the shorter time constant corresponds to a frequency at least a decade above that of the LF -3dB point of the output transformer.

I don't see the resemblance to the Williamson at all. This is cascaded diff amps.

BTW, you can do local feedback by putting some small resistors in series with the second diff amp's cathodes, then bringing the feedback to those points.

[StoneT]

Thanks for the reply SY.

I'm assuming your comment-

...corresponds to a frequency at least a decade above that of the LF -3dB point of the output transformer

only applies if the transformer is in the feedback loop?

Also, you say:

...local feedback by putting some small resistors in series with the second diff amp's cathodes, then bringing the feedback to those points.

There are already these resistors on the 807 cathodes, could I not take the feedback to there instead? As I say, I just want to lower the output resistance rather than reduce distortion so the number of stages (and therefore time constants) included can be minimized.
My preferred method is as shown in the attached .gif but I am unsure of the values for the (Rnfb) resistors.
And yes I do realize as shown the speakers would be at ~ 160v above ground...
Any comments would be appreciated on this or the Voltage swing issue.

[SY]

only applies if the transformer is in the feedback loop?

Yes.


Also, you can use the entire transformer secondary to provide local feedback to the 807s. Take a look at any of the classic Audio Research amps to see how that connection is done. Your only difference from their topology, because you're using cathode bias, is to run the bias resistor from the 4 ohm tap to ground instead of grounding the 4 ohm tap directly.