SID2SID & stereo SID (dual SID) Mod
I’ve had a thing for the Commodore 64 SID chip ever since i’ve known about it and despite only briefly knowing the C64 breadbin/breadbox in childhood from the kid down the street. I’ve wanted to get that particular one in recent times and I do have a preference to the original 6581 SID chip.
Last year I got a C64 breadbin, it wasn’t a great experience as I had hoped since it was goofed up when the seller said it was fully working. This resulted in somebody kindly donating a C64c to me since he had a spare and didn’t have the room for it. From this I got a taste of the 8580 SID.
I was happy to have a C64 but did miss the 6581’s beefy sound, though I also got a taste of the 8580 ‘s sound which had a bit more funkiness to it, which can be apparent in some SID tunes.
Not long after I heard about the SID2SID mod which goes with the mssiah cartridge. It is a kit you can buy that allows you to use two SIDs. Initially I thought i’d be able to use both the 6581 and 8580 together and hear in stereo, one being on the left stereo channel and the other being on the right. As well as being able to use them in Mssiah mode which offers 6 channels. (At this point I was still learning about the SID since being fascinated with it).
I soon discovered that the SID2SID mod does not allow stereo SID, in fact it doesnt even allow you to use 2 different SID chips because each SID works at a different voltage, different enough for them to not be interchangeable. So you could only use two 6581s which operate at 12v or two 8580s which operate at 9v but not mix them. I had already ordered the kit and mssiah cartridge just before realizing all this, because the mssiah software itself was enticing me, I want to have a go at creating SID music despite being so used to creating tracker mods on the Amiga.
Circuit Diagram for the Breadbin/Breadbox C64:
i have just been informed: Whether you’re doing this or just the Mssiah SID2SID.
if you install the 8580 on SID 2 of the SID2SID board, you must NOT install resistor R28.
This is not in the SID2SID documentation.
Circuit Diagram for the C64C
I had an idea even though I was too scared to go with it in fear of damaging a SID chip (these things are really not cheap anymore) The power issue would be no problem since all I need to do is get a 7809 voltage regulator, cut the track to pin28 (DC power in) on SID2 of the SID2SID board and step down the breadbin C64’s 12v to 9v and connect the 9v out from the 7809 regulator to the SID2 (8580) pin 28. Thats the power sorted and quite simple.
Now on the standard SID2SID, the Chip Select on SID2 (pin 8) connects to pin 7 of the cartridge port, this is what assigns the second 3 channels in order to get mssiah mode to work, so it would be like having a single 6 channel SID chip. I had a thought to just connect the SID2 pin 8 chip select onto SID 1 pin 8 chip select so that they would be identical and parallel. However I didn’t act upon this.
I did more researching and came across GadgetUK164’s video about saving the data bus, if I understand correctly, I was right in my thinking of paralleling them with the chip select and it would work, but I was missing out something vital. GadgetUK explains that both the SIDs would be trying to write to the data bus at the same time therefore shortening their life or even damaging them. So he kindly put the circuit diagram into his video which I then drew out into my own.
Everything more or less went smoothly with the mod except for a few serious goof ups which I had done when getting confused with the pin layout, working on the underside of the board from the circuit diagram, along with sleep deprivation not helping, I kept forgetting that the underside of the board is mirror imaged and I cut tracks thinking they shouldnt be connected. Really not my proudest moments and was irritated with myself at the time. But I repaired the tracks using thin mod wire. Luckily I realised this before even testing it for the first time or I think I would have done some serious damage.
I tapped the Audio output of SID 1 from the video output DIN from pin 3 (audio out) onto RCA socket for right stereo and the left stereo RCA socket connected to the output of the SID2SID board which outputs only SID 2’s audio.
I would also urge to test continuity between positive and negative of the SID board before testing. Mine had a short between positive and negative which I didn’t know about until testing it. I had blown the fuse inside the C64 and scared the freak out of myself when realizing my c64 isn’t working anymore. Luckily I had a fuse spare from the first C64 and decided to test my modded SID2SID board discovering this short from tiny blob of stray solder. After testing everything worked fine except SID 2, the 8580, wasn’t working at all but SID 1, the 6581, was. I went back to my desk and checked the amp circuit with the transistor, the base wasn’t making contact with what it should due to a damaged pad. I repaired that, took it all back, tested, still nothing on the 8580. Getting tense and slightly upset. I just went to the darn bench and looked at it, re-checked every thing, removed the 8580 SID2 chip and reinserted it.
I tiredly went back to my retro corner to test corner after all this 2 hours of troubleshooting along with, powered everything up, connected everything, loaded up a test music and cried, teared up and got all emotional after hearing my C64 Breadbin with Stereo SID, 6581 beefing out of my hifi’s right speaker and 8580 funking out of it’s left speaker as i’ve been wanting for the past year.
Watch the two part videos here: