Do BH 2-channel amps use local negative feedback?

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  • Guest
on: July 19, 2020, 01:16:09 PM
I ask b/c I've read that the amps have "zero global negative feedback", so no feeding of (inverted) signal output of the amp to the signal input. But that leaves open the possibility of local negative feedback. But I see only one possible candidate: tying the signal return from the output tube to the cathode (after dropping across parafeed cap and primary impedance of the OPT).  But even here I think any feedback would be tiny. Assuming a stock Kaiju loop of kthode -> anode -> 10uF parafeed cap -> 3K primary -> 100uF cathode resistor bypass cap forms a 3-part voltage divider, the reactance of the bypass cap is dwarfed by the combined impedance of the the two preceding devices in the loop (comprised almost entirely of the 3k primary impedance).  But some tiny amount of signal voltage must drop across the bypass cap, and so appear at cathode. But is it enough to reduce harmonic distortion a tiny bit?  The 300B only has a gain of approx. max 4X.  So this seems doubtful.  But I'm usually wrong about these things, so I thought I'd ask.

cheers and thanks,  Derek

Offline Paul Joppa

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Reply #1 on: July 19, 2020, 02:25:17 PM
No, we don't. Well, Crack has a cathode follower, that's kind of a local feedback. The Seductor did have two local feedback loops, but it's long out of production.

I suppose you could argue that the regulators and current sources use local feedback, but they are generally outside of the signal current loop.

The parafeed output has two loops:

1) tube-PF cap-OPT-tube

2) tube-plate choke-PSU cap-bypassed bias resistor-tube. This is only active at infrasonic frequencies, where the plate choke reactance is low and the bypass impedance is high - but the power supply cap is also high impedance at those frequencies, and the choke's DC resistance is still there.

Paul Joppa


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Reply #2 on: July 19, 2020, 02:46:16 PM
Many thanks PJ.