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.


 Schematic diagram.

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.