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previous: David Manson Anyone wanna take a crack a explain... -- 11/6/1999 8:58 PM

Re: Advanced feedback loop questions..

11/8/1999 2:16 PM
Stephen Conner	Re: Advanced feedback loop questions.. Hi David, In amps like the Valvestate, Rocktron Velocity, and Peavey Trans Tube, the current feedback is used to increase the output impedance of the amplifier, which makes it sound more valve-like. Normally, solid-state amps have a low output impedance (under 0.1 ohm) which damps the mechanical resonances of the speaker. Valve guitar amps have a higher output impedance (my project amp has 5 ohms) which lets the cone flub around more, giving you a bass and midrange boost. The exact effect depends on the speaker. The current feedback is usually done with a low-value resistor which senses the speaker current. A feedback signal is taken from the voltage across this resistor, and mixed with the normal voltage feedback. The more current feedback you use, the higher the output impedance will be, but you can't use current feedback alone, because the amplifier would become unstable. It has to be used along with normal voltage feedback. There are also current-feedback hi-fi amps, which were popular a while ago. The current feedback is used differently, and they still have low output impedance. Confusing eh? If you're interested in this stuff I can heartily recommend 'The Art of Electronics' by Horowitz and Hall. Steve.

11/8/1999 2:16 PM

Stephen Conner

Re: Advanced feedback loop questions..

Hi David,

In amps like the Valvestate, Rocktron Velocity, and Peavey Trans Tube, the current feedback is used to increase the output impedance of the amplifier, which makes it sound more valve-like.

Normally, solid-state amps have a low output impedance (under 0.1 ohm) which damps the mechanical resonances of the speaker. Valve guitar amps have a higher output impedance (my project amp has 5 ohms) which lets the cone flub around more, giving you a bass and midrange boost. The exact effect depends on the speaker.

The current feedback is usually done with a low-value resistor which senses the speaker current. A feedback signal is taken from the voltage across this resistor, and mixed with the normal voltage feedback.

The more current feedback you use, the higher the output impedance will be, but you can't use current feedback alone, because the amplifier would become unstable. It has to be used along with normal voltage feedback.

There are also current-feedback hi-fi amps, which were popular a while ago. The current feedback is used differently, and they still have low output impedance. Confusing eh?

If you're interested in this stuff I can heartily recommend 'The Art of Electronics' by Horowitz and Hall.

Steve.

Replies:

anonymous ok.. so negative current feedback i... -- 11/8/1999 4:43 PM