Hypothetically, the Audio In function of the MIDIBox SID seems really intriguing, but I can’t seem to find any concrete sound examples of the feature in action. What sorts of implementations have you guys tried/recommend?
Honestly, beyond using as the feedback loop, I’ve never really seen anything cool done with it
If you start experimenting it is important to use a cap to ac-couple the signal to the input. SID input pin is floating at half supply voltage, not ground.
i am designing the rest of my analog signal path right now, so this has to wait a little, but i am interested in trying.
Basically it allows you to process analog signal like drumloops with all the goodness the SID (or external voltage controlled effects) have to offer. So it´s really cool! I already implemented this with my external filters. You can imagine what you can do with a drumloop, tempo synced LFOs and envelopes, using filters, distortion and vcas…
I expect the sound of the sid input to be a bit grainy compared to using an external filter. Maybe it has a cool vibe to it when going beyound the 2vpp signal level pointed as maximum in the datasheet. And the sid filter of course…
I definitly want to check it out, but as i said, for now i am at the other end of the signal chain so to speak.
About feedback: With my 6581 this does very little but adds a bit of grain, distortion and boost you might find pleasant. 8085s will go into oscillation when in BP mode.
I decidet not to implement it as i am using external filters and a lot of much more powerful voltage controlled processing so it had no reall benefit to me.
If i would implement it it could be considered to use a fixed feedback level. At least for me i had the feeling this was more of a turn on turn off type of effect. - In other words, my pot during experimenting was either fully up or down when playing with this.
I’m using the feedback option in almost all bassline demos, e.g. in this one:
http://www.ucapps.de/mp3/midibox_sid/mbsid_v2_tk2_soundbank_demo.mp3
However, the signal/noise ratio is very poor, which means that the option will only work for loud music… 
Best Regards, Thorsten.
the demo makes me think i should give feedback another try… like the scream it gives.