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uneksija
Registered: Jan 2002
Posts: 44 |
connecting SID to mixer etc.
Hi,
I've been using emulators on PC to compose SID music because my old TV that I use with my C64 does not have a headphone output, so I can only hear the mighty sound of the SID through the little TV's stinky little speaker... So what I should do is to connect the audio OUT of my C64's SID to my mixer and for this I need a proper cable. However, I've heard that I might also need a protection circuit for the SID's output so that possible voltage spikes from my mixer (for example, when I turn it off and forget to unplug the C64's audio OUT before that) don't blow up my C64's SID. What do you guys and gals think, is this true? Do I need a protection circuit between the audio OUT of my C64 and the input of my mixer? Or does the C64 already have some protection on the SID's output? Does anybody have schematics for the circuit? I think some protection circuits like this have diodes. I also wonder if the C64's audio output is not AC-coupled and so would also need a capacitor at the output...
...so, how do YOU connect your C64's audio OUT to your mixer or stereo system? Please help! :) Can't wait to hear my C64(s) through my headphones and also through my stereo system ;) |
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... 6 posts hidden. Click here to view all posts....
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Frantic
Registered: Mar 2003
Posts: 1627 |
Actually, someone with the proper skills really should build and mass produce this kind of protection device. I hate killing SIDs this way. It has happened 3-4 times at least. It is such a waste of überfresh hardware. |
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Raf
Registered: Nov 2003
Posts: 343 |
hmm.. mass production of c64 hardware nowadays means ... 100 pieces? ;-)
www.vulture.c64.org |
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ready.
Registered: Feb 2003
Posts: 441 |
it would be interesting to know what kind of device was connected to these blown up SIDs. If plugging the SID output into a TV is fine, why sholdn't it be fine with a good amplifier? Good amps always have built in protection for their inputs. I plug my SID into LINE IN input, which I belive to be protected. Check your amp specification datasheet.
Ready. |
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uneksija
Registered: Jan 2002
Posts: 44 |
Quote: I fried SID by connecting high power amplifier to c64's audio output. both were powered on. surely SID went down , because inserting another one brang sound again.
looks like you have to turn off at least one of devices to have SID safe when connecting the stuff (I usually turn off the amplifier(s) - and this way I haven't broken anything so far) , but preferably both.
usually I also power down C64 first and then amplifier - it's a kind of habit actually.
so - these are not rumours , those are just facts. that circuity is actually protecting but it seems it doesn't protect in all cases ;-) . that broken SID could only make clicks coming from volume changes onwards (yes , it can still play digis).
www.vulture.c64.org
Hmm, I thought the danger was in turning off the mixer/amp when the cable is connected, but hmm, I understand that it possibly might be "dangerous" to plug/unplug cables while the machines are turned on...
The three diodes idea is interesting. According to the Commodore 64 Programmer's Reference Guide, the SID's maximum output is 2 volts peak-to-peak, so I guess two diodes in series from signal to ground and two diodes in series from ground to signal would conduct at +/-1.4 volts (2.8 V peak-to-peak), and possibly protect the SID.
Another idea I've heard would be using "an
opamp as unity-gain follower"... However, I think the transistor at the output is already doing something similar, but if it's not enough...
I've also heard an "insulation transformer" being mentioned, I gotta check that one too.
Thanks for the information! |
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ready.
Registered: Feb 2003
Posts: 441 |
I haven't tested it with multimeter (I'll do it tonight), but reading from various docs (C=hacking20 and SID homepage), the "zero" level of the volume output is around 6V, so the actual swing 6+/-2V= 4V - 8V. In these conditions we should apply 11 diodes in series (11*0.7=7.7 V) for protecting our SID from over-voltages from the + side of the amplifier.
So first I'd add the circuit shown in the Programmer's reference guide to the C64 audio output, which cuts the DC voltage level (1kOhm signal-->gnd and 10uF in serie after resistor to the + of the amplifier). After that the SID output should really be from -2V to +2V. After that I stop here. It's usless to go on for me without testing on the C64.
One question: how do I insert images in this forum? I'd like to add a sketch of a sample circuit.
BR,
Ready. |
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uneksija
Registered: Jan 2002
Posts: 44 |
If I remember correctly, the SID's output goes trough a DC-blocking capacitor in the C64, resulting in that there is no DC in the output. Check the C64 schematics (not the programmer's reference guide). |
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ready.
Registered: Feb 2003
Posts: 441 |
Yes, you are right, I checked the schematics and the SID output is connected to the base of a transitor, then the actual audio output comes from the emitter of the transistor. It is coupled thourgh a 10uF capacitor to the output and 1kOhm resistor to gnd. I also measured the output with multimeter and I saw that at power up the capacitor discarges: after a while you get 0V. I also checked the audio output while playing music, but just with a simple multimeter, I don't have an oscilloscope. The SID datasheet says that the SID output has a 2V p-p output, but the actual audio output might be different. So even the first circuit shown at Prophet64 forum with 2 diodes might work in case the audio output swing is from -0.7V to +0.7V. I have to buy an oscilloscope some day.
Still I would like to know this audio output swing before using such circuit, since if the output gets higher than +0.7V (or lower than -0.7) it is like shorting it to ground, look at the schematics: if the signal to gnd diode conducts, there's no resitance to block the current!!!
Ready. |
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Mason
Registered: Dec 2001
Posts: 459 |
I dont have any drawings but when we used the SIDs for the radio program in 1985/1986 then we had a simple cable with a din stick to put into the c64 and then we have a big jack stick in the other end.
Worked fine and didnt cause any problems for the c64 or the mixer.
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ready.
Registered: Feb 2003
Posts: 441 |
Yes, I think that a general rule is just not to plug/unplug stuff when power is on. This goes for serial bus connection, cartdrige,....
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uneksija
Registered: Jan 2002
Posts: 44 |
Quote: Yes, I think that a general rule is just not to plug/unplug stuff when power is on. This goes for serial bus connection, cartdrige,....
OK - It's just that the Prophet64 manual says not to turn off mixer when the C64 is connected... But yes, what you said does make sence.
Yesterday there was a new post on the Prophet64 forum of yet another SID being blown: "i blew another sid yesterday when i knocked the cord from my sid2 audio out jack"
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