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Old 19th February 2009, 08:29 PM   #1
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Question Negative Feedback

Hi, I'm designing and building a high gain guitar amp and am unsure how much negative feedback to use if any and where to apply it.

My initial thoughts were to leave it out and only use it if the amp requires it but I'm not sure what advantages it would have and how I would know if I need it. In general experience feedbacks applied to stabalise a system but it seems that in audio there is more to it than that.

In terms of how to apply it, is it simply a matter of taking a line from the output transformer to in front of the phase splitter and adjusting the amount of feedback with a resistor?

Thanks
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Old 19th February 2009, 09:12 PM   #2
Jeb-D. is offline Jeb-D.  United States
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In a guitar amplifier I wouldn't use feedback. If you have too much gain, reduce it by other means (remove a stage, change tube types or leave cathodes un-bypassed).
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Old 19th February 2009, 09:22 PM   #3
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Thanks for the response - I did think that too but I've noticed that nearly all guitar amp makers use feedback and stick a presence control in there too. What would be the advantage of this feedback?
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Old 20th February 2009, 01:15 AM   #4
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Commercial manufacturers/distributors prefer the gain of their amps to be predictable, rather than having to be individually adjusted to bring the gain into line with the specification for the particular model. NFB helps to stabilize the amp's gain, making it dependant more on the ratio of fixed resistors and less on the performance of individual tubes that will vary from one example to another when new and will deteriorate with age.

NFB also improves damping factor, distortion, noise and bandwidth, but I don't know how important those things might be in a guitar amp.
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Old 20th February 2009, 01:22 AM   #5
Trout is offline Trout  United States
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The effects can be fairly dramatic in some guitar amps especially if the builder went cheap on the OT.

What is getting pretty popular is using variable feedback either with a pot, or stepped switch or lift switch to remove it entirely.

On a couple of my guitar amps I put in a pot, a couple others I just swapped the resistor value.

Personally, in some amps like the Fender Champ aa764 circuit, completely removing it can make is rather fizzy when cranking the volume. The normally loved distortion gets very raspy.

I ended up using 68K in mine which is a very small amount of feedback. Stock it was a 22K resistor.

Its one of those adjust to taste things in most cases.
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Old 20th February 2009, 08:44 AM   #6
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I see - I think the best bet in my case will be to have an adjustable pot so I can vary and see the difference. Thanks for the advice!
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Old 20th February 2009, 04:23 PM   #7
Jeb-D. is offline Jeb-D.  United States
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Quote:
Commercial manufacturers/distributors prefer the gain of their amps to be predictable, rather than having to be individually adjusted to bring the gain into line with the specification for the particular model
That is very true for HiFi, but I'm not too sure if that is the case for guitar amps. It's not stereo, so gain matching between amps isn't that important.

As trout mentioned, it's a tone tweak. It will probably have the most effect if the amp is somewhat de-tuned in open loop to start with (skimpy iron, high harmonic distortion).

No harm in trying it. Making it variable using a pot is a good idea. That way you can find what you like.
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Old 20th February 2009, 08:05 PM   #8
45 is offline 45  Italy
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Default Re: Negative Feedback

Quote:
Originally posted by AmpBuilder225
Hi, I'm designing and building a high gain guitar amp and am unsure how much negative feedback to use if any and where to apply it.

My initial thoughts were to leave it out and only use it if the amp requires it but I'm not sure what advantages it would have and how I would know if I need it. In general experience feedbacks applied to stabalise a system but it seems that in audio there is more to it than that.

In terms of how to apply it, is it simply a matter of taking a line from the output transformer to in front of the phase splitter and adjusting the amount of feedback with a resistor?

Thanks
I don't like feedback in my guitar amps, they always sound lifeless to me. As mentioned above just try to reduce the preamp gain by "natural" methods.

There is also another reason: especially if you want to use a 12" cone, the high output impedance of the pentodes without FB is desirable!
Try to measure the frequency response and CSD for such system and you will see...

Cheers,
45
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