VIC-20 audio modification

From: Richard Atkinson (
Date: 2008-08-06 14:03:08


I was listening to some VIC-20 demos and it occurred to me that the VIC 
sound output always sounds rather muffled, as if a treble tone control had 
been turned down. So I had a look at the schematics of the new and old VICs, 
and found that both have a 1.6kHz low pass filter (single stage RC network, 
6dB/octave) in the audio output stage. On the old VIC (324001) it is R16 
(1K) and C20 (100nF) and on the new VIC (251027) it is R8 (also 1K) and C10 
(also 100nF).

So I tried replacing C10 on a new VIC with a 10nF capacitor, to bring the 
cutoff frequency of the filter up to 16kHz, and the result was substantially 
brighter sound. The effect is most noticeable on demos such as pwp's Robotic 
Liberation, where the new waveforms (viznut waveforms) have a lot of high 
frequency content.

What's also interesting as an aside is that on the new VIC (the cost reduced 
model) there is an external audio input from pin 2 of the cartridge port, 
connected via another 1K resistor R40, which is not present on the old VIC.


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