|   Feedback in circuit design | eliot
Aug 28, 2002 11:58 AM | | What is feedback in circuit design? I own a Fourier OTL (Output Transformer- Less) amp, which I love; but a professional preamp/amp designer told me that he didn't like OTL designs because of the large amount of feedback that was required to keep output impedances at reasonable levels. This same designer is also encouraging me to have him eliminate feedback circuitry from my PV 12 Conrad Johnson preamp. His point ,I think, is that feedback causes some degree of distortion. |
| re: Feedback in circuit design | A
Aug 29, 2002 10:50 AM | | You state:
] His point ,I think, is that feedback causes some degree of distortion.
The real question is not whether it causes any distortion, but whether it causes any <i>audible</i> distortion. Anything that is inaudible does not matter, so spending any money at all for inaudible improvements is money that is wasted. I would want to compare the specifications of your equipment with what he has designed. And before you have him tamper with your equipment, you might want to audition something that he has designed. Compare it "blind" with your equipment, and find out if it sounds any different at all. If it does sound different (which it may or may not), then decide which is better while listening "blind". If I owned an expensive preamp, I would be very hesitant to have anyone mess with it.
You might also want to read this:
http://www.dself.demon.co.uk/subjectv.htm
Keep reading; you'll get to some relevant comments about feedback in amplifiers. |
| re: Feedback in circuit design | PJ
Aug 30, 2002 2:00 AM | | Feedback lowers distortion, thats its purpose. It takes the output and feeds it back into the input stage and the amp self-corrects its errors.
At least thats the idea. In practice engineers can relatively easily produce vanishingly low levels of THD using this method...like 0.00001%
However, subjectively, many people prefer the sound of no global negative feedback designs, even though they will have higher THD measurements.
The human ear hears distorion in a different way to your distorion instruments... |
| What is feedback in circuit design | skeptic
Aug 30, 2002 4:29 AM | | What is negative feedback? For those who don't understand it, this mysterious buzzword has become some sort of bugaboo because is is a little hard to understand and has become the issue du jour for certain amplifier marketers since Matt Otala of Harman Kardon raised the subject for audiophiles in the mid 1970s.
Witout negative feedback, an audio amplifier at any stage from the first phonopreamp stage right through to the power output stage, is a series of steps where the output voltaqe of each stage is some direct multiple of the input. This multiple is called "gain." What happens at the output has no effect on what's happening at the input of that particular stage. The stages are cascaded so that the output of one stage is the input of the next. In this way a very small voltage at the input is multiplied thousands of times at the output. The power output stage actually controls the flow of current from a large power supply to your speakers so the voltage gain of that stage may be low but the current gain is enormous.
But each stage also introduces some distortion because the process isn't perfect. One of the most ingenious inventions in electronic history was the invention of "negative feedback." In this type of amplifier circuit, a small part of the output of a stage is combined "out of phase" with the input signal to that stage. What that means is that it is subtracted. The amplifier stage is actually driven by the difference between the shape of the input waveform and the output waveform so that it actually corrects its (nonlinearity) error automatically. The price for this is reduced gain so you need more stages. The advantages are lower distortion, wider flatter frequency response, and lower output impedence, all desirable.
However, there are pitfalls. To reap the benefits of negative feedback, the designer must understand and apply the complex mathematics that describes how it works. Failing to do this skillfully can result in disaster. This is probably why so many amplifiers employing negative feedback have not lived up to their potential. Negative feedback is used in many ways to stabalize electronic circuit operations as well. This is why some amplifiers can be turned on and operated perfectly immediately while others require a long warm up period or worse yet always seem to drift all over the place never really becoming stable.
I personally would not let someone redesign an amplifier especially reducing or eliminating negative feedback unless I was certain in advance that he knew exactly what he was doing. A botched project by a wanabee electronics engineer or technician could destroy your amplifier. You are much better off buying one you like in the first place and sticking with the manufacturer's original design. |
| What is feedback in circuit design | A
Aug 30, 2002 6:40 PM | | You seem to be taking the novel approach of actually answering his question rather than my creative approach to a response. What a strange fellow you are! |
| What is feedback in circuit design | skeptic
Aug 31, 2002 3:52 AM | | That's how we engineers are trained. Anything less is unacceptable. I guess that makes me strange. |
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