I suspect this is inconclusive/some circuit tweaks you might consider.

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Posted by Ralph (M) on January 10, 2024 at 11:19:10

In Reply to: RE: When negative feedback is not beneficial posted by cellailuca on January 10, 2024 at 01:36:40:

I've been building voltage amplifier circuits like this for nearly 40 years.

This circuit is sorely in need of a CCS in the cathode circuit of the bottom tube.

You can't apply feedback properly to the cathode circuit of a differential amplifier. So IMO I see this circuit as drawn as also being compromised.

The differential cascode circuit seen here also employs feedback of its own, which is R13 and R12. That is part of why an input coupling capacitor is used. However to be effective, there should be a series resistor with the input circuit similar to how feedback is mixed with incoming audio in an opamp circuit. The input stopping resistor might be handy for this; my surmise is the divider networks will need to be adjusted if this tack is taken.

We need to also consider that a voltage amplifier that has a feedback input to it will not have a signal at its output that is like the input. It will have a corrected signal such that when its applied to the output, the correction does what that output section needs.

IOW if you are to break the feedback loop you need to look at the effect on the signal at the loudspeaker output.

Further, when the feedback is applied to the grid or cathode of the tube, the linearity (or lack of it) will distort the feedback signal, preventing it from doing exactly what its supposed to do.

If this were my amplifier I'd have a B- supply at the very least for the voltage amplifier; it might also double for the bias supply of the output section. I would place a CCS between the B- and the cathodes of the bottom of the differential cascode.

I would then omit the feedback resistor R12 and 13 and have a direct-coupled input so the input grids were at ground. But I would employ the stopping resistors, along with dual feedback loops from the output. This can be done if the output transformer has 4,8 and 16 Ohm taps by grounding the 4 Ohm taps instead of the common. By mixing the incoming signal with the feedback in this manner, we avoid the distortion of the tube from distorting the feedback signal. Its an easy technique that better allows feedback to do its job.

I would also consider a CCS for the output section, also tied to B- as this would improve linearity. IMO the designer hobbled this circuit by electing to only have a B+ supply.

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Follow Ups Full Thread

Topic - When negative feedback is not beneficial - cellailuca 01:31:15 01/10/24 (13)

RE: When negative feedback is not beneficial - Tre' 18:54:19 02/12/24 (1)

Well said. Tre' _nT - Cleantimestream 14:05:43 02/13/24 (0)

Which negative feedback is not beneficial ? - Ian L 14:49:58 01/22/24 (0)

RE: When negative feedback is not beneficial - rage 11:30:27 01/19/24 (1)

RE: When negative feedback is not beneficial - Best audiotransformers 02:14:28 01/20/24 (0)

RE: When negative feedback is not beneficial - walge 07:57:18 01/17/24 (1)

larger image - Ian L 14:46:42 01/22/24 (0)

The output stage needs it - Triode_Kingdom 05:33:25 01/10/24 (0)

RE: When negative feedback is not beneficial - cellailuca 01:36:40 01/10/24 (4)

I suspect this is inconclusive/some circuit tweaks you might consider. - Ralph 11:19:10 01/10/24 (0)

RE: When negative feedback is not beneficial - cellailuca 01:41:23 01/10/24 (2)

RE: When negative feedback is not beneficial - cellailuca 01:47:06 01/10/24 (1)

RE: When negative feedback is not beneficial - cellailuca 01:54:15 01/10/24 (0)

Follow Ups

I suspect this is inconclusive/some circuit tweaks you might consider. - Ralph 11:19:10 01/10/24 (0)

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