All About Audio Amplifiers

Overall Feedback, Pros and Cons.

Feedback is when a part of the output signal of an amplifier is
fed back to the input. NEGATIVE FEEDBACK is when some of the output signal
is fed back to the input in such a way as to PARTIALLY cancel the input
signal. It can't completely cancel the input signal because if it did there
wouldn't be any output to feed back to cancel the input and then there would
be output again but then it would cancel it ..... you get the idea.

Canceling part of the input lowers the gain and that may seem like an
undesirable effect. The good things feedback does far outweigh the bad.
Feedback lowers the distortion, widens the frequency response, lowers the
output impedance and is neutral when it comes to signal to noise ratio.
With all that going for it you would think the more the better. Well,
maybe.

What are the numbers?
You may be wondering exactly how much
negative feedback changes all of these things. Let's let B equal the
fraction of output signal that is fed back to the input. Let A equal the
gain of the amplifier before the feedback is connected. To find what the
gain will be after the feedback is connected divide the gain by
(1 +
AB).
Let's say the gain is 280 and the fraction of output signal fed
back is 0.03. The new gain is
280 / (1 + 280 x 0.03) = 29.8.
If
the same amplifier has a distortion of 5 % the distortion after feedback
is connected will be
5 % / (1 + 280 x 0.03) = 0.53 %.
Now if the
frequency response was 100 to 10,000 cycles, the new frequency response
will be
100 / (1 + 280 x 0.03) = 10.6 cycles and
10,000 x (1 + 280
x 0.03) = 94,000 cycles.
The new frequency response is 10 to 94,000
cycles. If the output impedance is 8 ohms the new output impedance will
be
8 / (1 + 280 x 0.03) = 0.85 ohms. That gives a damping factor of 8
/ 0.85 = 9.4.
The damping factor = (1 + AB) = (1 + 280 x 0.03).

The noise from tube hiss and hum will be reduced by the same amount as the
gain so the signal to noise ratio remains the same. Good thing! In all
of these examples the amount of feedback is very close to 20 dB.

What is 20 dB of feedback?
Audio people talk in terms of dB of
feedback. Decibels are a way of expressing a ratio of two voltages or two
powers. The formula is
dB = 20 Log (V1 / V2). As it applies to
feedback
dB = 20 Log (1 + AB). If you don't happen to have a
scientific calculator Here is a quick way to evaluate dB.

Voltage Ratio = dB

0.00316 = -50
0.01 = -40
0.0316 = -30
0.1 = -20
0.316=
-10
1.00 = 0
3.16 = 10
10.0 = 20
31.6 = 30
100 = 40
316
= 50

That should be enough to establish the trend. In the case of the
examples above the value of (1 + AB) was 9.4 which is very close to 10.
So that amplifier had about 20 dB of feedback.

This is a
modest amount of feedback. But suppose the design engineer had done a
better job. He could have used local feedback (around only one stage,
likely requiring more stages) and let's say he got the distortion without
overall feedback down to 1.0 %. Then when he applied 20 dB of overall
feedback the distortion would have dropped to a very good (for tubes) 0.1 %.

The Right Way and the Wrong Way.
Back in the tube days there was a
saying among audio engineers "There are two ways to design an amplifier, the
right way and the wrong way". The right way goes like this. Select the
best output transformer you can find for the desired power level or even
have one made especially for your company. (If you are going to order
several thousand units you can do that). Don't just rely on tube manual
data, set up the output tubes and adjust plate, screen and bias voltages for
the lowest possible distortion. Design everything carefully for lowest
distortion. Add a lot of local feedback to get the distortion even lower.
After you have an amplifier which is very clean sounding even without
overall feedback add a modest amount, say 20 or 30 dB, to make it even
better.

The wrong way goes like this. Get the least expensive output transformer
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.

What's Wrong with Transistor Amplifiers?
Answer: In my opinion, they
were designed the wrong way. In the 1960s there was somewhat of a
disconnect (generation gap) between the older tube engineers and the young
transistor guys. (I came along just at the right time with just the right
attitude and took in the knowledge from both groups). The standard
transistor power amplifier circuit produces tremendous amounts of distortion
10 or 15 % with the feedback disconnected. The transistor engineers solved
the problem, they thought, by cramming on the feedback. Add another stage
to get more gain and pour on even more feedback. In Tube amplifiers the
phase shift in the output transformer places a practical limit on how much
feedback you can put on. There is no such problem with transistor amps. If
you are familiar with transistor circuits you will remember that little
inductor in the output lead. That's to do a phase correction caused by the
transistors at ultrasonic frequencies so they can cram, jam, pack and load
on the feedback, as much as 80 dB in some amplifiers I have seen.

What's Wrong with That?
Well, it's a matter of delay. To get all
that gain they used quite a few transistors. Each transistor has a little
time delay. (Tubes have it too). Say a quick transient comes along in the
music such as a snare drum or a woodblock. The burst of signal goes through
the amplifier and then is returned to the input by the feedback loop. By
the time it gets back there part of the transient has already gone. The
transient comes out distorted. The trouble with this type of distortion is
there is no way to measure it in the laboratory. Distortion tests are made
with steady state sine wave tones. Those modern transistor amps come out
with distortion figures of 0.0007 % but to the ear they have a harsh sound.

Another part of the problem is that the basic circuit of a transistor
power amplifier has not changed since the 1960s. I have a GE transistor
manual on my book shelf which has the copyright date 1964. It has circuit
diagrams of amplifiers ranging in power from 2.5 watts to 15 watts. The
transistors have been improved since then, allowing higher power, but the
circuit is essentially unchanged. The most prominent features are that the
speaker is driven alternately from the emitter of the top transistor and the
collector of the bottom one. That's got to cause a lot of distortion. The
pre driver has a bootstrapped resistor in its collector. This is actually
positive feedback and increases distortion.

By no stretch of the imagination can
you say that transistor technology was mature in 1964. Better amplifier
circuits could have been designed if any competent engineer had put his mind
to it. I designed and built a 200 watt per channel amplifier in the first
half of the 90s. I did it the right way using lots of local feedback. The
distortion with no overall feedback (none around the complementary outputs)
was 0.25 %. After adding 27 dB of feedback I got 0.015. It has the classic
tube sound. Sue says I should call it the tubeless tube amplifier. If you
are curious about the circuit, click
here. But I digress.

Hold on a second, doesn't any amount of
feedback have the delay problem?

The obvious answer is "yes" but it's a matter of degree. When feedback is
used in moderation, the delay effect is not large enough to be audible. The
enormous amounts of feedback used in transistor amps brings the problem up
to the level where it is not only audible but objectionable.

Some updated information
Maybe my knowledge is a little out of date.
That can happen with a retired guy like me. I received an email from a
gentleman in Norway who provides this information.

I read the article about "What's Wrong with Transistor
Amplifiers" and there was mentioned a 10 to 15 percent open loop
distortion in transistor amplifiers. There a lot of good amplifiers with
no feedback (Counterpoint got less then 0.04% THD). The main problem with
adding feedback with transistors has been the performance of the power
transistors. The bandwidth of the power transistors in the seventies was
so bad and the result was all the problems you mentioned. By combining
new quality transistors (Sanken) with moderate local and global feedback
loops, new transistor amplifier design has got an excellent performance
(Krell, Mark Levinson). The Ft of power transistors has improved from a
few hundred khz to 50 - 80 Mhz and that revives the old negative feedback
strategies.

Thanks for an interesting web site

Feedback In Tube Amplifiers.
As I have already mentioned the phase
shifts in the output transformer place an upper limit on the amount of
feedback that can be put on a tube amplifier. That was, and still is, what
saved it from the same problems that plague transistor amps. Here is the
circuit of a very simple amplifier to show how negative feedback is applied.

For a verbal description click
here.

This is that first High Fi amplifier I
ever owned. It was designed the wrong way. Its distortion figure is in
excess of 1 % at all power levels. The amount of feedback is really rather
small being about 10 dB. (The measured open loop gain is 83 giving 10.7 dB
of feedback.) This was an economy amplifier and the output transformer has
a lot of phase shift at the high end. That limits the amount of feedback
that can be applied before oscillation begins.

If you want to build a 6V6 amplifier use a better quality output
transformer and bring up the feedback to at least 20 dB, more if you can get
it. To get enough sensitivity you would need to add another stage outside
the global feedback loop but after the volume control. Even with its high
measured distortion this was not a bad sounding amplifier. I listened to it
and to two copies of it, in stereo, for many years.

This amplifier is shown because it's the one I started this series with.
Note how the power supply is connected to the various stages of the
amplifier. The negative feedback goes from the 16 ohm tap on the output
transformer back to the cathode of the tube right after the volume and tone
controls. This is a mono amp so there is no balance control. The feedback
always goes to the first tube after the last user control. Placing any
control other than balance trimmers or bias adjustments inside the feedback
loop is about the worst idea I can think of. The feedback would try to
regulate out their effect. One interesting note, the Amplifier is a Model
D-10 made by the Newcomb Audio Products Company, Hollywood Calf. There also
is a notation of patents used which belong to AT&T and Western Electric.
I guess negative feedback was still covered when this amp was made.

The phono preamp shown is not the one in the original amplifier. That is
so bad I wouldn't wish it on anybody. The truth is that I used to have a
Sam's photofact on this amp but I lost it somewhere along the line. It has
a switch to select 6 different equalization settings. I'm just too lazy to
trace out that switch.

What is shown in the schematic is the phono preamp used in a Harman
Kardon model A-300. It's an excellent sounding preamp.

Well, I guess that's about it. If you are left
with questions don't hesitate to email me.

The End.

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This site begun March 14, 2001

This page last updated March 20, 2005.