SID external input questions

[Roter_Flieger]

Hi there,

I am on to start a MBSID for my own. I already finished soldering the core module and now I want to proceed to the SID module. What I came across is the SID input...

I heard from a number of people who fried their SID chip when fiddling around with the audio input of the C64. As far as I was able to find out using Google, the SID is very fragile at this pin and will die immediately if it is connected to ground.

For the optimized PSU, it is recommended to ground the input lines in order to reduce unwanted noise. I will not doubt that a number of people are using happily working MBSIDs with grounded input lines, but how does this comply to the stories of C64 users frying their SIDs? MBSIDs input wiring does not seem to differ a lot from the original wiring of the C64.

I am just concerned about the SID chips. I got two 8580 (which had three perfectly working voices last time I tried them) and for me those are precious ICs which I do not want to expose to any kind of risk.

Is there anything else that needs to be paid attention to when it comes to the external audio input section of the MBSID?

Regards.

[Wilba]

You might want to read the thread about Feedback Loop on SID ... the link presented: Commodore C64 Modifications :: Adding A Feedback Loop also discusses the grounding of the input pin for low noise, on the next page.

Also, the SID module has a capacitor between the SID input pin and port J4 (the audio in port) and you "ground" the audio in at J4, not at the SID input pin. This is supposed to protect the input pin. Since this is an input pin, I can't see how that pin even if it was tied directly to ground could cause any problems... there would be no current going into the SID, and no current going out. I don't know the specifics of how other people have fried their SIDs... where are these tales of tragedy? Post some links please!

I have plugged and unplugged a lot of 8580s and 6582s into my PCBs which have the audio in pins bridged (i.e. always grounded) and have not fried any SIDs because of this. I've even listened to the noise go up and down while grounding/ungrounding the audio input, and still not fried a SID this way.

So stop worrying  ;D

[Roter_Flieger]

Also, the SID module has a capacitor between the SID input pin and port J4 (the audio in port) and you "ground" the audio in at J4, not at the SID input pin.

I know, but that was also the case with the original circuit in the C64. In the SID specs I read something like: "Any signal applied directly to the pin should ride at a DC level of 6 volts and should not exceed 3 volts p-p."

So I guess an open SID input line rides at 6 volts. Connecting an external input without a coupling capacitor would be very bad in this case.

I don't know the specifics of how other people have fried their SIDs... where are these tales of tragedy? Post some links please!

I heard those stories directly from two people I know and whom I told about the capabilities of the MBSID. They both (independently) tried to connect a guitar amp to the audio input of the C64 and sent their SIDs to death. So no links for them. :)

However, Google Groups found this for example:

http://groups.google.de/group/comp.sys.cbm/msg/4d0f7ad691379dc6?hl=de&

He states that the coupling capacitor will discharge into the SID if it is grounded with the C64 turned on and that a solution would be an additional resistor that connects the input line to ground in order to let the capacitor charge and discharge in a defined manner. Actually, later C64 boards  had exactly this resistor he is talking about while early versions were lacking it.

Well I do not know how the capacitor can discharge into the SID. What I could imagine is, that if the SID's input line is really at 6 volts and the capacitor is grounded directly, that it momentarily will drain a high current from the SID's input pin which may destroy it.

I have plugged and unplugged a lot of 8580s and 6582s into my PCBs which have the audio in pins bridged (i.e. always grounded) and have not fried any SIDs because of this. I've even listened to the noise go up and down while grounding/ungrounding the audio input, and still not fried a SID this way.

So stop worrying  ;D

Agreed! ;D

[Sebo]

Hi:

I have my SID input conected to a switched jack, that grounds the

input when nothing is connected and when I plug something let's

the signal get in. The SID works great this way, as Wilba said, when

I plug something I can hear the noise going up.

I connected the out of my sound card and processed some drums

with the SID and works OK.

But connecting a guitar amp could be more dangerous, may be they

connected the speaker out (that could blow almost anything), or

may be the amp was bad grounded.

Sebo

[Wilba]

Thanks for that info, Roter!

I'm still not clear about how the SID could get fried.

If the audio in is open, the capacitor doesn't have a ground, so how can it charge up?

I still believe though that if the audio in was permanently tied to ground, before inserting the SID and turning on, then you would never have a problem. In fact, if you always have the audio in tied to ground, it makes that extra 100K resistor redundant.

But if the later C64 schematics have that 100K resistor tying the audio in to ground, then it can't hurt to add it in.

[c0nsumer]

I have my SID input conected to a switched jack, that grounds the

input when nothing is connected and when I plug something let's

the signal get in. The SID works great this way, as Wilba said, when

I plug something I can hear the noise going up.

I'm going to echo this sentiment. My MIDIbox SID-NUXX mainboard design does exactly this, and it helps cut down on noise by a great deal. There are five of these in existence and none have had the problem you describe above.

-Steve