Stereo to C64 with DualSID

I finally assembled one of my DualSID kits. It’s an add-on board for C64 that makes it possible to install an additional SID chip with several different configurations. Detailed description can be found at the DualSID home page or in my earlier post.

I made this one as the 8580 version. Here’s the board – without and with SID chips.

For the convinience of installing it from one C64 to another if needed, I made all external connections and jumper settings using pin headers. When the board is like this, the second SID address selection is done by setting the jumper block in the 2×4 pin header to correct position. Instead of manually changing the jumper block’s position, I’m able to replace the block with 2×4 female pin header that is wired to e.g. a rotary swithch. That way the changing of second SID address can be done externally.

I also did the priority hack (described in DualSID FAQ, A13). I made it in a little different way than is described in the FAQ. See the two small jumper blocks next to the SID #2 socket in the above pictures? That’s the jumper grid of the priority hack, only arranged as a row. A view from underside:

When the jumpers are in place the address lines are not swapped. I am not able to swap the address lines using the jumper blocks in my mod. Instead, I can again insert a header that is wired to a double pole double throw switch. If I am going to swap the priority of the installed chips, I want to do it with a switch anyway. Done this way the hack doesn’t take practically any extra space.

The address lines of second SID

To be able to select different addresses for the second SID chip four wires needs to be connected to the motherboard. The instructions tell you to take two of the addresses from KERNAL chip and two from expansion port. I wanted something cleaner so I started looking for alternate spots from the motherboard where these address lines could be found. Luckily all four lines could be found close to each other next to the RF modulator not very far away from the SID socket.

Note that this C64 is the newer C64 C model with the ‘short’ motherboard. To be precise it’s ASSY 250469 rev.3. In other models or revisions these locations may not be the same. If you are planning to take the address lines like this, make sure that they can be found here using a continuity meter. Better safe than sorry.

All done, everything ok. Plugged in:

The stereo sound

I have to say – it sounds good! The quality of stereo SID compositions I have listened so far varies a lot, but the better ones do sound very nice. I especially like the ones that have been made to stereo as opposed to six channel mono SID tune. Both approaches exists.

And if someone wonders why I have two 8580 chips configured as dual mono in the pictures, let me tell you – it ain’t so stupid than it might seem.

I listened to some non-stereo SID tunes with this setup using headphones. What came out of it was a surprising effect.

As you might know, SID chips are not identical, not even the newer 8580 chips. The SID chips are semi analog, the filters behave differently, the external filter capacitors affect the filter behaviour. In dual SID configuration both chips have their own output circuitry which are not completely identical (I mean, on the component level. The circuit design of DualSID is identical to C64’s. The capacitors and other components are not likely to be identical). All these make slight differences to the output even from two similar chips. These differences would normally go unnoticed, but when you hear them as left and right channel, those small variations bring character to the sound.

The sound becomes more lively, it gets a feel of space. Sometimes even effects that sound like stereo reverb may appear! I recommend anyone with the possibility to try out the dual mono with similar SID models and headphones.

Dual SID modification for a C64 is a nice hack thanks to the stereo or six channel possibility as it is. More than that, it also brings new life to the traditional mono sound! This is what makes this DualSID design so great – the mode of operation can be set with jumpers or switches as desired. Highly recommended!


9 Responses to Stereo to C64 with DualSID

  1. c64berrydejagercom says:

    Nice of you to share your implemention of dual SID into your c64.

    • ilesj says:

      I’m glad to share my experiences also in hopes of other hobbyists catching some ideas. A nice little hardware projects like this (the DualSID) deserve the attention!

  2. archangelmichael2 says:

    Dad wants to know if there is a way to get Creative Microdesign sid stereo cartridge? You can plug it in the back of a C64 which is what we did for a long time until it stopped working.

    Thanks for any input. *Turns on the Commodore 64 theme*

    • ilesj says:

      Hi there! I know the cartridge by name, but I’m not familiar on how common or rare they are nowadays. I’m sure those pop up on eBay every now and then. If you still have the old cartridge, it could be that an electrician with experience with Commodore hardware would be able to fix it. And I’m sure your dad remembers that there is a battery inside the cartridge that powers the extra SID chip ;)

      However, there are alternatives to the original CMD SID Symphony Stereo Cartridge. For example this DualSID board discussed here should be fully compatible with CMD’s SID Symphony once configured to reside in the same address space. Also other similar stereo SID boards should be compatible as long as they are configurable. Of course they are much more complex to install.

      Probably the most convenient alternative to CMD’s SID Symphony would be Digital Audio Concept’s SID Symphony II cartridge. It’s pretty much an enhanced redesign of the original and is operated the same way:

  3. Nelson Addison says:

    Could I use this for 6 SID Channels instead of the MSSIAH SID2SID board? I’m not necessarily as interested in stereo sound as I am extending my C64’s sound to 6 channels for use with MSSIAH. I don’t know how to solder yet (I started working with computer hardware a few years, yet it’s only been for repairs, and never my own creations), so I can’t use SID2SID, and I am not sure whether or not this will work with MSSIAH though.
    Thank you!

    • ilesj says:

      For starters, the DualSID can surely be used with MSSIAH, as it can be configured in exact same way as SID2SID. It’s just the matter of jumper settings on the board.

      The difference between six SID channels or stereo SID setup is if the sound from both SID chips are mixed together or not. In either case we are talking about six SID channels. Here DualSID does not differ from SID2SID.

      At the time I ordered mine, the DualSIDs could be ordered as kits or preassembled. Better ask the designer/seller if you can still have them assembled and configured for you.

      As a kit, the DualSID probably needs more soldering experience than SID2SID, as there are more components, more configuration options, and the DualSID pcb is smaller in size.

  4. Rob Lo says:


    Is there a way to create a single output for dualSID config? Like route the output of SID1 to the audio out for the SID2 on dualSID board?

    • ilesj says:

      You mean like mono out with sound from two SID chips? That would require mixing the line level outputs from both SIDs’ output stages into one signal. Simply connecting the outputs together is not a good idea (like a Y-splitter, but inverted), but a simple mixing circuit can be made using resistors.

      Some good reading on the topic:

      Or do you mean something else by single output?

    • ilesj says:

      …like something like having sound output from both SID chips on one stereo plug?

      With this DualSID, there is the audio out for SID2 on the PCB, but SID1 audio comes from C64 A/V port – so yes, they are like two different audio outputs.

      Some options could be to:
      -Install a stereo jack or two RCA plugs for two channel sound to the C64 case, and wire the sound out from C64 motherboard and DualSID board to these as L and R channels.
      -Modify the A/V port with a second audio out for stereo sound, and use a modified AV cable (no case modifications needed). See my article on C64 video cables.

      Routing audio from one SID to the other is a no-no!

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