Feedback is when a part of the output signal of an = amplifier is=20 fed back to the input. NEGATIVE FEEDBACK is when some of the output = signal=20 is fed back to the input in such a way as to PARTIALLY cancel the = input=20 signal. It can't completely cancel the input signal because if it = did there=20 wouldn't be any output to feed back to cancel the input and then = there would=20 be output again but then it would cancel it ..... you get the idea.=20Canceling part of the input lowers the gain and that may seem = like an=20 undesirable effect. The good things feedback does far outweigh the = bad. =20 Feedback lowers the distortion, widens the frequency response, = lowers the=20 output impedance and is neutral when it comes to signal to noise = ratio. =20 With all that going for it you would think the more the better. = Well,=20 maybe. =20
This is a=20 modest amount of feedback. But suppose the design engineer had done = a=20 better job. He could have used local feedback (around only one = stage,=20 likely requiring more stages) and let's say he got the distortion = without=20 overall feedback down to 1.0 %. Then when he applied 20 dB of = overall=20 feedback the distortion would have dropped to a very good (for = tubes) 0.1 %.=20What are the numbers?
You may be wondering exactly how = much=20 negative feedback changes all of these things. Let's let B equal = the=20 fraction of output signal that is fed back to the input. Let A = equal the=20 gain of the amplifier before the feedback is connected. To find = what the=20 gain will be after the feedback is connected divide the gain = by
(1 +=20 AB).
Let's say the gain is 280 and the fraction of output = signal fed=20 back is 0.03. The new gain is
280 / (1 + 280 x 0.03) =3D = 29.8.
If=20 the same amplifier has a distortion of 5 % the distortion after = feedback=20 is connected will be
5 % / (1 + 280 x 0.03) =3D 0.53 %. =
Now if the=20 frequency response was 100 to 10,000 cycles, the new frequency = response=20 will be
100 / (1 + 280 x 0.03) =3D 10.6 cycles and
10,000 = x (1 + 280=20 x 0.03) =3D 94,000 cycles.
The new frequency response is 10 = to 94,000=20 cycles. If the output impedance is 8 ohms the new output = impedance will=20 be
8 / (1 + 280 x 0.03) =3D 0.85 ohms. That gives a damping = factor of 8=20 / 0.85 =3D 9.4.
The damping factor =3D (1 + AB) =3D (1 + 280 = x 0.03).
The noise from tube hiss and hum will be reduced by the same = amount as the=20 gain so the signal to noise ratio remains the same. Good thing! = In all=20 of these examples the amount of feedback is very close to 20 dB.=20What is 20 dB of feedback?
Audio people talk in terms of = dB of=20 feedback. Decibels are a way of expressing a ratio of two = voltages or two=20 powers. The formula is
dB =3D 20 Log (V1 / V2). As it = applies to=20 feedback
dB =3D 20 Log (1 + AB). If you don't happen to have = a=20 scientific calculator Here is a quick way to evaluate dB.
Voltage Ratio =3D dB
0.00316 =3D -50
0.01 =3D -40
0.0316 =3D -30
0.1 =3D = -20
0.316=3D=20 -10
1.00 =3D 0
3.16 =3D 10
10.0 =3D 20
31.6 =3D = 30
100 =3D 40
316=20 =3D 50
That should be enough to establish the trend. In the case of = the=20 examples above the value of (1 + AB) was 9.4 which is very close = to 10. =20 So that amplifier had about 20 dB of feedback. =
The Right Way and the Wrong Way.
Back in the tube days there = was a=20 saying among audio engineers "There are two ways to design an = amplifier, the=20 right way and the wrong way". The right way goes like this. Select = the=20 best output transformer you can find for the desired power level or = even=20 have one made especially for your company. (If you are going to = order=20 several thousand units you can do that). Don't just rely on tube = manual=20 data, set up the output tubes and adjust plate, screen and bias = voltages for=20 the lowest possible distortion. Design everything carefully for = lowest=20 distortion. Add a lot of local feedback to get the distortion even = lower. =20 After you have an amplifier which is very clean sounding even = without=20 overall feedback add a modest amount, say 20 or 30 dB, to make it = even=20 better.=20The wrong way goes like this. Get the least expensive output = transformer=20 you can find, use what ever tubes you can get, slap a phase splitter = together and then clean it up with as much feedback as you can cram = on it. =20
What's Wrong with Transistor Amplifiers?
Answer: In my = opinion, they=20 were designed the wrong way. In the 1960s there was somewhat of a=20 disconnect (generation gap) between the older tube engineers and the = young=20 transistor guys. (I came along just at the right time with just the = right=20 attitude and took in the knowledge from both groups). The standard=20 transistor power amplifier circuit produces tremendous amounts of = distortion=20 10 or 15 % with the feedback disconnected. The transistor engineers = solved=20 the problem, they thought, by cramming on the feedback. Add another = stage=20 to get more gain and pour on even more feedback. In Tube amplifiers = the=20 phase shift in the output transformer places a practical limit on = how much=20 feedback you can put on. There is no such problem with transistor = amps. If=20 you are familiar with transistor circuits you will remember that = little=20 inductor in the output lead. That's to do a phase correction caused = by the=20 transistors at ultrasonic frequencies so they can cram, jam, pack = and load=20 on the feedback, as much as 80 dB in some amplifiers I have seen. =20What's Wrong with That?
Well, it's a matter of delay. To = get all=20 that gain they used quite a few transistors. Each transistor has a = little=20 time delay. (Tubes have it too). Say a quick transient comes along = in the=20 music such as a snare drum or a woodblock. The burst of signal goes = through=20 the amplifier and then is returned to the input by the feedback = loop. By=20 the time it gets back there part of the transient has already gone. = The=20 transient comes out distorted. The trouble with this type of = distortion is=20 there is no way to measure it in the laboratory. Distortion tests = are made=20 with steady state sine wave tones. Those modern transistor amps = come out=20 with distortion figures of 0.0007 % but to the ear they have a harsh = sound.=20Another part of the problem is that the basic circuit of a = transistor=20 power amplifier has not changed since the 1960s. I have a GE = transistor=20 manual on my book shelf which has the copyright date 1964. It has = circuit=20 diagrams of amplifiers ranging in power from 2.5 watts to 15 watts. = The=20 transistors have been improved since then, allowing higher power, = but the=20 circuit is essentially unchanged. The most prominent features are = that the=20 speaker is driven alternately from the emitter of the top transistor = and the=20 collector of the bottom one. That's got to cause a lot of = distortion. The=20 pre driver has a bootstrapped resistor in its collector. This is = actually=20 positive feedback and increases distortion.
By no stretch of the = imagination can=20 you say that transistor technology was mature in 1964. Better = amplifier=20 circuits could have been designed if any competent engineer had put = his mind=20 to it. I designed and built a 200 watt per channel amplifier in the = first=20 half of the 90s. I did it the right way using lots of local = feedback. The=20 distortion with no overall feedback (none around the complementary = outputs)=20 was 0.25 %. After adding 27 dB of feedback I got 0.015. It has the = classic=20 tube sound. Sue says I should call it the tubeless tube amplifier. = If you=20 are curious about the circuit, click=20 here. But I digress.
Hold on a second, doesn't any = amount of=20 feedback have the delay problem?
The obvious answer is "yes" but it's a matter of degree. When = feedback is=20 used in moderation, the delay effect is not large enough to be = audible. The=20 enormous amounts of feedback used in transistor amps brings the = problem up=20 to the level where it is not only audible but objectionable.=20Some updated information
Maybe my knowledge is a little out = of date.=20 That can happen with a retired guy like me. I received an email = from a=20 gentleman in Norway who provides this information.=20I read the article about "What's Wrong with Transistor=20 Amplifiers" and there was mentioned a 10 to 15 percent open loop=20 distortion in transistor amplifiers. There a lot of good = amplifiers with=20 no feedback (Counterpoint got less then 0.04% THD). The main = problem with=20 adding feedback with transistors has been the performance of the = power=20 transistors. The bandwidth of the power transistors in the = seventies was=20 so bad and the result was all the problems you mentioned. By = combining=20 new quality transistors (Sanken) with moderate local and global = feedback=20 loops, new transistor amplifier design has got an excellent = performance=20 (Krell, Mark Levinson). The Ft of power transistors has improved = from a=20 few hundred khz to 50 - 80 Mhz and that revives the old negative = feedback=20 strategies.=20Thanks for an interesting web site
Feedback In Tube Amplifiers.
As I have already mentioned the = phase=20 shifts in the output transformer place an upper limit on the amount = of=20 feedback that can be put on a tube amplifier. That was, and still = is, what=20 saved it from the same problems that plague transistor amps. Here = is the=20 circuit of a very simple amplifier to show how negative feedback is = applied.=20 =20