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Old 16th July 2013, 09:45 AM   #1
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Default Variable feedback vs. input attenuation

I'm looking for the best way to vary the input sensitivity of my tube power amp. As far as I know, there are at least two ways of doing this: variable global feedback or an attenuation pot at the input. Both have their pros and cons, so I was wondering what you guys would prefer and why?
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Old 16th July 2013, 09:56 AM   #2
DF96 is offline DF96  England
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Variable feedback changes not just gain but also frequency response, output impedance, distortion, noise. It may lead to instability, or at least peaking, at frequency extremes. Attenuation has much smaller side effects. However, the real answer is that an amp should be designed to do what the user needs it to do.
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Old 16th July 2013, 10:11 AM   #3
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I forgot about the output impedance! I believe I'd be able to guaranty stability within the range of the variable feedback, but that's a long list of variables with just one control...

The purpose is to set the input sensitivity, so I'm able to 'chose' the volume/gain range of the preamp to get the poweramp to full output.
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Old 16th July 2013, 10:45 AM   #4
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Use attenuator type controls, perhaps you've notice that every single commercial amplifier does it that way - there are good solid reasons for it.

With valve amps it's even more important, as the whole point of using a valve amp is for the lower quality and distortion, altering the feedback is going to alter how it distorts the signal. Many guitar amps don't use any overall feedback at all.
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Old 16th July 2013, 10:59 AM   #5
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Quote:
Originally Posted by funk1980 View Post
I forgot about the output impedance! I believe I'd be able to guaranty stability within the range of the variable feedback, but that's a long list of variables with just one control...

The purpose is to set the input sensitivity, so I'm able to 'chose' the volume/gain range of the preamp to get the poweramp to full output.
In that case you can get by with one or two switched sensitivities, using an attenuator at the amp input.
Like 'normal', '-10dB', and '-20dB'.
A single SPDT switch with mid position 'off' and a few resistors.

jan
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