Negative feedback, voltage and output impedance


Hi,

Can someone please explain the correlation between the above in a tube amp?

Since voltage output and output impedance are not commonly listed specs, how does one determine whether one amp or another is better in these areas?

TIA.

Mike
1musiclover

Showing 4 responses by sean

As a general rule, higher amounts of negative feedback will lower the output impedance. If you want a technical explanation of this, your best bet would probably be to head over to the Tube Asylum and ask there. Sean
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Wasn't there a relatively recent review of a tube amp in Stereophile that had user adjustable negative feedback? If i can remember correctly, this not only affected the output impedance but also the bandwidth / linearity of the entire design. Was this the ASL amp??? Sean
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Negative feedback aka "error correction circuitry" is kind of like trying to bail water out of a boat that has a major leak. Plug the hole i.e. build a linear circuit first, then worry about corrective measures later ( if needed at all ). After all, you can't respond to an error until it's already happened, making error correction introduce it's own errors due to the lag time involved. As such, designing the circuit for maximum speed and linearity negates much of the need for such "band-aids" as feedback. This is because the circuit is both fast and stable enough to keep up with any of the demands placed upon it without introducing its' own non-linearities into the equation.

As a side note, high levels of negative feedback is what makes a large percentage of SS designs sound hard and sterile. Most of the SS amps that offer excellent high frequency "air" while retaining midrange "liquidity" are of a low or no feedback design.

For those of you that have never heard a low / no feedback SS design that is fast with wide bandwidth, i would suggest checking it out. For those that have never heard an Atmasphere OTL amp with suitable speakers, i also suggest checking that out. Both of these types of products are what "accurate musicality" sounds like. Sean
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Greg: I should have clarified that my "NO" feedback was in regards to "global" feedback. The use of small amounts of local feedback is not a big deal and results in less time lag. Sorry for my lack of clarification.

Tom: That was a good find. I'm sure that Theta won't mind the re-post here either : )

Marakanetz: I agree with your basic statements. See my response above to Greg.

El: My analogy applies more to "global feedback" than to "local feedback". Distortion and the correction of that distortion is like gossip. It can be quelled much faster and in a more effective manner "locally" than it can when trying to fix the problem "globally".

Think of "local feedback" as having a dedicated supervisor for each stage of amplification. If everything is good in section A, B & C but section D is a mess, only section D's supervisor has to get involved.

On the other hand, global feedback is kind of like having one supervisor by himself trying to keep track of what is going on in the entire "plant". The potential for more problems to slip through the cracks un-noticed is far greater. Once those problems are noticed, it is possible that the amount and type of correction applied may not be directly to the source of the problem, resulting in other problems. This is why global feedback is less desirable. Sean
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