Commodore C64 Modifications
Noise Reduction
By: AlphA

Reducing Noise Interference
before and after noise reduction mod

Noise interference from nearby circuits has plagued the C64 since its release. Many believe that the interference occurs at the SID output. I have discovered that the noise originates from the SIDís audio input. Signals at the audio input have two possible routs to take. They are either sent to the SIDís analog filter, or they bypasses the filter and go directly to the audio mixer. There is no option to disable incoming signals. As a result, the SID mixer spends much of its time amplifying residual noise. This noise can be eliminated completely and easily by simply grounding the audio input line.

MP3 sample of noise with very high gain, before and after noise reduction mod.

The most flexible way to accomplish this is to install 1/4" jacks with normally closed (NC) switch contacts. Switch contacts give you options for multimode use.

If you plan to use the audio input and the feedback loop, you can do what I did and rout the incoming feedback to the switching lug. The tip and ground lugs are wired up in the usual fashion. With this configuration, SID audio output goes through the pot (not shown in schematic) and loops back to SID audio in via the switching and tip lugs until a plug is inserted. An ordinary plug breaks the feedback loop and replaces it with the audio signal at the tip of the plug for normal audio input. Finally, inserting a shorted (tip and shell soldered together) dummy plug breaks the feedback loop and replaces it with ground. Now I have "feedback mode", "audio in mode" and "regular noise free mode".

Just for kicks, I made a stereo sample of both SIDs with very high gain while I plugged in the dummy plugs.
NOTE:You should NEVER connect or disconnect anything while the C64 power is on. I did so in the name of science (sounds like a good excuse).

Secondly, if you donít plan to add the feedback mod, you can simply solder the switching lug to the shell (ground) lug. Normally you will have regular noise free mode. Plug something in, you have audio in mode.
you'll hate yourself in the morning

If youíre lazy and donít want feedback or audio in (shame on you!), you can solder the audio-in directly to ground. Wire the audio-in solder point to any of the ground points. Or, scratch up the ground area near the audio-in point and join them with a big blob of hot solder, you lazy bum. :)

If you find that this mod does not significantly reduce noise, check to make sure that EXT/MIX IN is disabled.

Double any of the above procedures, as I have, for dual SID use.

If you have two SID chips installed and routed their output to a stereo jack, plug in a pair of headphones to really hear a difference.

So, say good-by to that noise which allegedly gives the C64 itís "character"
(now you can stop pretending to like it).





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