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.

Update in April 2018: The DualSID home page is gone, so I add the latest(?) versions of the assembly instructions here, plus link to Lemon64 forums.

• dualsid_assembling_instructions_6581_v3
• dualsid_assembling_instructions_8580_v3
• Discussion, mods and fixes on Lemon64 thread

Note: the transistor orientation is incorrect in the pcb silk screen in pcbs prior to pcb v3. However, letters E, B, C are printed correctly on the board, so check your transistor pinout. Pictures seen on this post have the transistor installed wrong way around. Transistor’s flat edge should face towards edge of the board. I’ve fixed mine since. </update>

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, link to Internet Archive). 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!