[AMRadio] Re: Frequency Response

Patrick Jankowiak recycler at swbell.net
Mon Sep 27 22:03:57 EDT 2004

Sorry for the long winded post, but here it is..

Since the discussion is regarding distortion and frequency 
response, I would like to share some experience I have had with 
improving the fidelity of otherwise mundane audio power 
amplifers. Although some of the technicques presented may require 
some ingenuity to apply to a very large plate modulator, I feel 
that they have worthwhile and audible merit from the standpoint 
of linearity, 'tilt', and transient response. In the second case 
presented below, A/B tests with various CD recorded music 
including violin, Cuban percussion, and industrial styles, the 
improvements were very noticeable. Surely there is then merit for 
the improvement of the fidelity of the voice waveform.

I would also like to suggest that a triangular wave is an 
excellent way to check the linearity of an audio amplifier. The 
proper circuit configuration around the phase inverter can make 
the sides of the wave obey an extremely straight 45 degree angle, 
to the point where the input and output signal shapes are nearly 
indistinguishable. 'Tuning' is done by superimposing the input 
and output signals on a scope and matching them.

(In an amplifier with poor linearity, the sides of the triangle 
wave can 'bellow' out, or 'hourglass' in, just like the trapezoid 
RF waveform in a poorly designed or operated linear amplifier (go 

The first two circuits embody this method in the phase inverter 
circuit. I realize that many people may prefer a balanced circuit 
throughout, but it may not always be practical.

The first two circuits also embody a cathode feedback scheme 
surrounding only the output tubes and the audio output 
transformer. This application will 'straighen up' and otherwise 
poor transformer quite well, although there are requirements for 
plenty of grid drive voltage and ample B+ voltage along with a 
large value filter capacitor on the plate supply. If the 
transformer is too small, the low frequency power output fidelity 
will be less improved since the transformer cannot handle the 
extra current demanded when correction takes place. A solution is 
to demand less power.

This article describes the conversion of a fairly poor performing 
(actually bad sounding) 70W theater amplifier which suffers from 
lack of iron. The result is a very clear 40 watt amplifier. A 
method of in-circuit measurement of tube matching is shown.

This article describes the improvements made to a decent-sounding 
50 watt public address amplifier to the point where it is as 
faithful as a Quad or McIntosh as far as most people's ears are 
concerned. About 50 watts was still obtained (large iron).

This article demonstrates a method of coupling the output signal 
back to the grids around a transformer coupled high power (50 
watt) AB2 audio stage.

For those interested in a QRP experience, this article describes 
an extremely linear series modulator for an 807 RF stage.


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