[sdiy] protection circuit at audio output

[Antti PitkXmXki]

>From: "HL-SDK Synths" <syntroniks at gmail.com>

>I read on the MIDIbox SID forum that some guy heard you could fry your sid
>by grounding it's audio input pins. That, I have tested with the 6581, and 
>it does no damage. I >don't know how turning a mixer off would affect it's 
>output.

According to this page

http://bigmech.com/misc/c64mods/index.html

grounding the input significantly decreases the amount of background noise 
the SID produces (in a C64). Another interesting mod on that page is the 
feedback mod for increasing the maximum resonance amount - after that, even 
filter self-oscillation is possible (in BPF-mode)! Plus this mod can also be 
used for overdrive. The SID's possibilities that are still being discovered 
never cease to amaze me :)

>The only thing I can think of (it's not a bad idea IMHO!) is to put a op 
>amp voltage follower/buffer >and a cap after that... Good luck, and my sid 
>has gone through a lot :-p

Schematics? I'm just a beginner :P BTW, 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. I wonder if it's 
for "current amplification" or for protection etc. 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!

About the MIDIbox SID: Yes, I've seen this and it looks really cool. Too bad 
I've got too many projects already... Also I think the C64 can possibly be 
used as a synth in a decent way with the commercial Prophet64 software and 
with the upcoming MIDI interface for that program.

Since I've got some spare 6581R4AR's, I might buy the 4-SID HardsidQuattro 
PC-soundcard in the future - I heard some sound examples and it's great!

Thanks to all of those who replied to my question. It's great that there are 
other "SID-people" here too ;)

Antti

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