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Old 20th September 2007, 12:22 AM   #1
gni is offline gni  United States
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Default Negative Feedback Gain Control

From discussions in other threads:

Why not use the Negative Feedback as a gain control?

Normal Feedback value would be full gain. (into the LTP)

0 ohm Feedback value would be 1:1 gain. (someone told me)
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Old 20th September 2007, 12:40 AM   #2
KSTR is offline KSTR  Germany
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It hard to optimize a design for variable gain down to 1 by changing NFB. In non-inverting config you don't get less than unity gain, so you'd need some attenuation elsewhere. Inverting would get you close to zero but this has the drawback of lower input impedance and gain variation with source impedance.

- Klaus
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Old 20th September 2007, 12:55 AM   #3
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That is going in the right direction. Very helpful. . .
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Old 20th September 2007, 02:27 AM   #4
Leolabs is offline Leolabs  Malaysia
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Actually there is a combination of NFB+attunator configuration in non-inverting set-up.
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File Type: pdf schematic1 _ page1.pdf (13.0 KB, 191 views)
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Old 20th September 2007, 04:12 AM   #5
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That seems like a viable option. . . .but will it work beyond the opamp?

I think it would be a great option to put on a LTP. The values would
be the key.
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Old 20th September 2007, 04:46 AM   #6
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Odd this topic should come up. I've been experimenting with something simular. I am trying varying feedback based on speaker return current. It's a way to compensate for say, a slightly mismatched passive crossover network or some compensation for a speakers resonance. This isnt the quite the same as regular current feedback. It also has another feedback loop which the voltage at the resistor in series with speaker ground adds or reduces feedback. It use the singleton input (single input trans. like the JLH style) and simply connect the negative end of the dc blocking capacitor to the speaker current sensing resistor. Attached is my expermental circuit I've been testing the past few days. This means real circuit built not just simulated. No comments on the amplifier circuit itself please, it will look quite odd and could be rambled about for some time. I am just interested in the feedback concept. Attached in this post is the circuit under test. I will show simulation examples of its behavior in following posts.
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File Type: gif accompinsationamp.gif (42.0 KB, 655 views)
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Old 20th September 2007, 05:08 AM   #7
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This is the feedback voltage across the speaker current sensing resistor. I used a slightly mismatched passive crossover setup for this example. The basic concept is that this voltage is in phase with the voltage at the VAS collector which is the main feedback point. So the voltage increases the appearant feedback across R9 (the main feedback loop). So with an increase in current through the speaker results in more feedback and the gain reduced. And of course the opposite happens when you get an increase in impendance do to a resonance and the current is reduced. It will remove feedback and try to increase gain.
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File Type: gif speakercurrent.gif (80.0 KB, 579 views)
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Old 20th September 2007, 05:11 AM   #8
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And this is the amplifier trying to compensate for the loss of feedback in that 1khz area.
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File Type: gif accompensationout.gif (98.1 KB, 568 views)
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Old 20th September 2007, 12:43 PM   #9
gni is offline gni  United States
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Beyond me. I hope there are some better engineers out there. . . I was just planning to make a 'simple' gain reduction circuit. jerluwoo
has put some serious thought into this. . . looks like a circuit you
would find in a amplifier/speaker combo box. . .
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Old 20th September 2007, 07:02 PM   #10
AndrewT is online now AndrewT  Scotland
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Hi,
I think it was Cambridge audio that did this on one of their amps.

Watch for the pot wiper going open circuit.
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