Hey everyone, a friend pointed me to this link: Commodore C64 Modifications :: Adding A Feedback Loop
Thought you all might find it interesting… There’s some good samples in there, too.
-Steve
Great link, I especially like the examples about the self oscillation 
Best Regards, Thorsten.
What a cool site!
It was only the other day that I had a “what if” thought about routing SID output to input… and then someone’s done it (and proved it’s worth doing!).
The mod he did to the SID2SID board to use the audio in of the second SID uses a 100k resistor to ground, which I assume is just copying the C64 schematic? TK’s SID module uses a 1nF cap to ground. I’m wondering about the difference and whether the 100k resistor would reduce noise even more… he seems to get some really low noise!
Nice site indeed. What a geek
Hey TK, is there room in a SID patch to store the feedback parameter if it’s done with a digipot?
That reminds me quite a bit of some of the most fun I’ve had musically. That is, taking some analog filters, feeding them back on themselves, throwing in some additional effects, and making simply interesting thumpy bouncy dancy sounds. Which were completely impossible to reproduce nor sync with anything. 
I need to get / build more analog stuff soon…
completely impossible to reproduce nor sync with anything.
It’s possible, grab your calculator 
It’s possible, grab your calculator
Well, if I hung a bandpass filter and a freq counter off of it…
Actually, they are Electrix boxes, so they sent their position via MIDI, but I don’t think it was accurate enough to reproduce this. The devices are weird and you could tap the knob to change the sound, even though they didn’t move enough to send a new position out via MIDI.
I can confirm that a pot in between a SID module’s input and output does not blow up the SID ;D and does make some funky sounds. I’ve also observed that if you lower the SID’s resonance then you can tweak this feedback knob to get better (and more!) resonance, to the point of funky self-oscillation squeal.
What I plan to do now is hack a pot apart and break the resistor strip near the anticlockwise end… so that end is ground, the wiper goes to input, and the clockwise end goes to output. This will make a sort of switch, so when the knob is fully anticlockwise, the input is tied to ground (less noise), otherwise it will range from 1M to 0 ohms as you turn clockwise.
don’t they do pots that have a ‘click-off’ setting? like with radios, you turn it fully anticlockwise until it clicks to turn off the radio, but if it’s on, the pot just does volume.
Why not replacing the pot with a VCA with ratio 0 to 1 driven by an AOUT?
(just an idea - don’t tell me how to do 8) )
That’s another way to implement midi control but the big question is whether the new MBSID has space in a patch to save the feedback levels… If not, then the level couldn’t be saved in a patch…However, a DCA or VCA could be set to 0 feedback and switched to ground when a patch is loaded, and set by a CC with just a little extra code, and you could send the CC at the beginning of your song to set the feedback as required… This is starting to sound like a daughterboard hey?
don’t they do pots that have a ‘click-off’ setting? like with radios, you turn it fully anticlockwise until it clicks to turn off the radio, but if it’s on, the pot just does volume.
Yeah, but I haven’t found one with a good range… it needs to be at least 500k, and dual gang…
Also note that any kind of feedback, even feedback going in through 1 Mohm, introduces noise… there’s a distinctly audible difference in noise between a grounded input and input connected to output through 1 Mohm.
From my initial playing with this new knob, I don’t anticipate the need to automate its setting, I doubt I’ll be tweaking it during live playing or recording… but I’ve noticed controlling the SID filter resonance can tweak how much the feedback has an effect - it’s like they get multiplied together or something, I’m no filter expert… but maybe tweaking the resonance parameter is enough for most people.
I fully recommend spending a couple bucks on a 500k or 1M pot and giving it a go…
What if you decoupled the feedback and kept the ground on the input? Would that perhaps lower noise?
I doubt I’ll be tweaking it during live playing or recording…
Ahh but what about loading patches? That’s the real reason to want to save it or control it by midi…
it’s like they get multiplied together
That’ll be the log curve on the outputs, the resonance is a peak and gets amped more… Maybe… I think
@wilba, you can get pots with a push pull switch too. they’re used in electric guitars.
yeah, probably not dual ganged for stereo though… I know what the ideal solution is, it’s just a matter of finding these perfect pots… my DIY solution is cheap and involves no hunting around for ages, which frankly I am sick of doing, I still can’t find 22nF polystyrene caps, or <2% tolerance silver mica caps, or ultra bright diffused 3mm green LEDs, or knobs like TK’s MB-SID…
LMFAO ;D
I take it you’re joking about the 22nF caps and diffuse LED’s! They really are ten-a-penny around here. You can get them both from Maplin if you’re in the UK.
I can get your standard stuff, but not but not polystyrene caps or ultra bright diffused LEDs. Maplin does not have what I want.
stock number : 466-3481 for the LEDs. (10,000mcd enough?) £0.80 each(!)
they also do ±1% mica caps. they are expensive though.
3039640 - 22nF 63V Polystyrene cap.
9520430 - 22nF 63V Polystyrene cap with RoHS compliance. both £1.12
hope this helps.