[sdiy] protection circuit at audio output

[Michael Buchstaller]

>.., I checked the C64 schematics and 
>noticed that the audio output isn't taken directly from the audio out pin, 
>but before that the signal goes through a small circuit that has some caps 
>etc. and a transistor in common-collector configuration.

This is the one i spoke about.

>I wonder if it's 
>for "current amplification" or for protection etc.

both. As the SID output needs a load to ground for working (commonly used
1 K), it would have difficulties in driving low input impedances. But the main
reason is to protect the output - shorting it to ground will definitively zap the
output driver in the SID. (tried this several times during development of the
Monovoice)

>I also wonder if it's 
>that transistor that people have fried instead of the SID. Of course fried 
>transistor also means no sound, but replacing the transistor must be a lot 
>cheaper than replacing the SID!

Usually, the transistor is not so easy to kill.

-Michael