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Electronics 101: Use of Negative Feedback; a Q

The topic of negative feedback (NF) has been beaten to death in other posts. Nevertheless, I have a question that touches on another aspect of NF that I do not recall having been explained. The Q starts in the middle of this OP.

By way of summary, many of our technical members explained why NF is used in amplifier circuits, e.g., extends bandwidth, helps to prevent oscillation, reduces distortion, reduces output impedance and therefore increases DF, and so forth. However, there is a cost.

Many of our technical members also explained that NF creates TIM (temporal intermodular distortion) because of the nano-second of time it takes for the NF circuit to tell the input circuit to make adjustments. TIM distortion results in distortion in odd-ordered harmonics (particularly the 5th, 7th and 9th) which our brains interpret as loudness, and in turn acoustic harshness.

Hopefully, I got the basics down. I'm sure my summary will be "clarified" by our techies.

But here's the Q (or 2). I read that NF also enables the amplifier (SS or tube) to regulate its output power to match a speaker's changing impedance stats as frequency changes. Could the tech members please clarify my understanding about this point.

It's kind of important because even though the subject of "tube friendly" and "tube unfriendly" speakers has also been killed, it would appear that electrical matching concerns between an amp and speakers having fluctuating impedance stats as a function of frequency may be mitigated in whole or part by using NF. Of course, I not suggesting that NF will compensate for a short or an open circuit, but somewhere in between there will be harmony. ;>')

Perhaps the technical members can put some "flesh on the bones" with respect to this Q.

I suppose the Q raises a related Q about amps (SS or tube) that do not use NF because of concerns pertaining to TIM distortion as mentioned. Great . . . the TIM distortion problem is solved if no NF is used. But how does the amp regulate its output power to compensate for fluctuating impedance curves?

Thanks.
bifwynne

Showing 1 response by kijanki

kijanki's avatar
kijanki
4,448 posts
 
Marakanetz, I think that negative feedback can make amplifier unstable since feedback can easily become positive at higher frequencies, while "no feedback" should be always stable.

I agree with Kirkus that there is always some form of feedback. It should be used in moderation. By definition it improves all specifications making it tempting to use as a fix for design shortcomings. For instance, TIM can be completely eliminated by limiting amplifier's bandwidth at the input to one that amp had before feedback was applied, but it requires decent bandwidth to start with and that calls for good design and components.
Even with this bandwidth limitation negative feedback lowers distortions - both IMD and THD while also reducing output impedance. Again, I wouldn't go crazy with amount of feedback since distortions, especially THD, might not be that audible while output impedance is not that important. We should take into consideration that inductor in series with the woofer is most likely >0.1 ohm (limiting DF to 80 no matter what) not to mention that speaker/transducer impedance itself is mostly resistive. Some speakers work well with higher output impedance amps while others (like B&W 802) expect amplifier to have low output impedance - to be voltage source doubling current to half impedance. Driving complex load is very complex (as name suggest)and I would follow Kirkus advice to get speaker, amp and try them together. Judging by spects or descriptions can yield poor results IMHO.
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