SID protection circuits

[uneksija]

Hi!

This is slightly off topic since it's about the SID chip but not about the MIDIboxSID. Maybe I'll build a MIDIboxSID in the future (it's really cool!), but my question is about circuits for protecting the SID. I'm asking from this forum since I'm sure there's a lot of SID expertise here.

So what I'd like to do is to connect my Commodore 64 to a mixer or to an amplifier. However, I've heard that voltage spikes, when connecting cables, or when turning off the mixer while the C64 is connected, can fry the SID. The C64 does have a little transistor circuit for protection etc. at the output, but it seems it's not enough. So... How are your experiences in this matter with the MIDIboxSID? Has anyone fried any SIDs in the way I described? How is the SID protected in MIDIboxSID? I'd love to hear recommandations for protection circuits so that I don't kill my C64's SID.

So far I've gotten the following suggestions:

-using an isolation transformer

-using diodes from signal to ground and from ground to signal so that they'll make possible voltage spikes go to ground instead of the SID

-using an opamp as unity-gain follower (I guess the protection transistor in the C64 is doing something similar)

Please help me if you know something about this :)

Thanks for any help in advance!

[TK.]

Hi uneksija,

the SID module uses the same transistor circuit as the C64. I made a lot of experiments in the past (e.g. connected the SID to various filter circuits, to different mixers, as audio source for small amplifiers, etc) and never fried a chip. So, it seems to be sufficient at least for DIY purposes. However, an additional OP amp doesn't hurt, and this seems to be a good idea to realize a double protection, just for the case... it could amplify the signal as well, as the SID output is not so loud like common synths.

I wouldn't use diodes, since they could affect the sound, and I'm not sure if a transformer really helps, since there is still a direct galvanic connection.

Best Regards, Thorsten.

[SounDuke]

I'd like to rebring this topic, as I'm working on my mbsid. I'll probably amplify the sid signal through an Alesis IO|26 audio interface, which features 8 mic/line preamp with switchable 48V phantom power per pair. My question is: what if I accidentally press the 48V pahntom power switch with the sid output connected? I don't know if the protection on the sidmodule is sufficient, maybe it's better to put a NON-electrolythic capacitor (10mF 63V) in series on the signal line which could work as a switch opening the line and preventing the 48V to reach and harm the sid? What do you think?

[Jaicen]

For that to work, you'd probably need at least 10uF. However there are two reasons I wouldn't do this. Firstly, when the 48v line is turned on, the voltage will spike, sending a voltage pulse down the other end of the cap. Secondly, the SID has a transistor and a cap on its output anyway, so there's no additional gain.