Designing and Building
Audio Amplifiers.

A Low Distortion Gain Block.

A gain block is a circuit consisting of one or two triode
amplifiers that can be replicated as many times as needed in an integrated
amplifier or preamplifier to makeup for losses in tone, volume, and balance,
controls. These gain blocks are most often found outside of a global
feedback loop so they need to have inherently low distortion. A single
stage with an unbypassed cathode resistor is usually not good enough unless
it operates at a very low level. Gain blocks usually consist of two triodes
with feedback from output plate to input triode cathode to diminish
distortion to a very low level and tightly control the value of the gain.
Here is a circuit I developed that will give a fairly wide range of gain at
very low levels of distortion.

Here are a couple of solid
state versions of the same circuit.

For a verbal description click
here.

This circuit has a lot of open
loop gain which is traded off for low distortion. Somewhere else on this
site I know I said that this is often done because gain is cheap and easy to
come by. The gain may be changed by changing the cathode resistor, Rk, in
the first triode. Changing this resistor has a very small effect on the
operating point of either triode. The numbers are given in the table below.

Cathode Resistor, Rk.

Gain.

Distortion.

Output Voltage.

36 k ohms.

6.67

0.035%.

10 v.

27 k ohms.

8.47

0.04%.

10 v.

22 k ohms.

10.1

0.045%.

10 v.

9.1 k ohms.

21.7

0.045%.

10 v.

5.1 k ohms.

33.6

0.060%.

10 v.

A very good approximation for the gain is,

Av = 1 + Rf / (Rk + 1100)

Where Rk and Rf are as labeled in the schematic and Av is
the gain.

You see that the feedback is DC coupled, not through a capacitor, from
the plate of the second triode to the cathode of the first. An earlier
design used AC coupled feedback. This version was unstable at low
frequencies. Bode analysis showed that a 3 microfarad coupling capacitor
would have been needed to make it stable. That circuit was posted here in
the early days of March 2005. If you saw it and decided to use it please
update your schematics.

The 220 k ohm resistor makes up most of the load for the second triode
which is where most of the distortion occurs. I don't recommend changing
the value of Rf because it effects the operating point of the second triode.
If this resistor is changed the cathode resistor in the second triode will
have to be readjusted for minimum distortion.

Someone emailed me with the information that when this circuit was run on
a spice simulator there was a 1 dB peak in the frequency response below 10
cycles. This really isn't very important but it still might bother some
people. It was caused by the fact that the feedback which goes to the
cathode of the first triode and is negative also is fed in a small amount to
the grid which is positive feedback. I downloaded a spice simulator and
checked it out and confirmed the peak. The trick is to adjust the relative
sizes of the inter-stage coupling capacitor and the input coupling capacitor
so the positive part of the feedback is rolled off before it can effect the
gain. I also tested it with the volume control set at half resistance which
is the maximum impedance condition. The roll-off is now smooth at both
ends. I trust everyone, perfectionists included, can now sleep. Am I a
perfectionists? Well, grudgingly, yes.

You may be wondering why I have gone for such low distortion while on
other pages of this section I didn't indicate it was that important. This
amplifier is NOT meant to be inside the global feedback loop. As such it
needs to have an inherently low distortion level because nothing is going to
improve it later. This circuit may be used where ever a stage with low
distortion, relatively low output impedance, and closely controlled gain is
required.

Happy building and good luck. Keep watching, there will be more in the
future for the builder or the curious experimenter.

Next;
Distortion in RC Amplifiers.

Previous;
How Much Power is Enough?

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This site begun March 14, 2001

This page last updated June 8, 2005.