Cool, you’ll find the scope a big help. Thanks for posting the schematics, I’ll take a look when I get some time. Regarding the bus out and accompaniment out and mods… Once I’ve looked over the schematic I’ll be able to give you some more help. If I understand correctly you’d like to have separate outs for bass and acc and you’re happy to keep the drums together on their own bus and output?
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Almost! We are actually aiming for individual outs of the Drum parts in the first place for further processing. I regarded to the mix-out as answer to this:
You could add a mixer stage and have normalled jacks in the input path so you could send an individual instrument out and still create a mix output with individual level control.
Whereas a mix out might come in handy in some situations I thought of therefore just using the one that already is in place by default, turn down the two other sections in the Mix of the Main-Out to have a quick drum-bus without having to add anything at all.
Indiv outs for the bas and acc would be ace, but considering we would likely need to rely on the pre-existing rhythms in there it is not necessairly something we aim for. If however it would turn out to be doable to have those parts to be triggered as well as pitch-controlled accordingly indiv.-outs would be a really cool addition as the bass in there actually sounds quite nice 
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Cool I get what you mean. In that case if you use normalled jacks for the individual drum voice outputs the socket will break the connection to the bus when you insert a jack, just what you want. If you were to just tap off the existing circuit then that drum voice would still be on your bus output…, no way to turn its level down. You could break the connection to the bus, wire in a pot and buffer then return it’s output back to the bus but how often would you want a direct out as well as that same voice on a bus, especially if you’re using external processing anyway… the convention in drum machines is that individual outs break the connection to the mix bus.
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If you’re not familiar with switched jacks here’s a pic.
If you notice the three ‘fingers’ that run across the top of the jack? They have pins or solder tags on their ends (hidden behind the body in the pic) the other ends of the fingers connect to the pins we can see and form a switch. When a plug is inserted the fingers move up and disconnect from the pins we can see. Now the hidden pins are connected to the plug but the pins we can see don’t connect to anything. So if you connect your drum output to the hidden pin and then connect the other pin to the bus with no plug inserted the signal will go via the finger to the bus. Plug in a jack and the signal is isolated from the bus. This called a normalled connection because with no plug inserted the signal takes its normal path.
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I’d still feed the drum signals through buffers and then to the jack sockets for the reasons I mentioned earlier of isolating the outside world from the Elka and avoiding any loading/impedance issues.
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We eventually got hold of a digital oscilloscope from a friend and were able to track the internal triggers of the rhythms, of the so far installed trig connections. Interestingly, and as @Bunker already suggesthat that this could be the case, two of the trig singals were detected in a positive Voltage range, whereas the other two were detected in negative voltages. All with a Peak to Peak of approx. 6V
BD Trig
CL Trig
Bongo 1 Trig
Bongo 2 Trig
What would these measures mean for our further proceeding in regard to the trigs? Is it reasonable to assume that we could work with the +/- 5v we could get from the DFAM or should we aim for a different approach for now in the building phase as well as in general
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Aaaand it is time to revive this thread! Summer has been long and bautiful but it is time to head back to this unfinished project ^^
Given that I am a newbie to Buffer and Mixing stages, I’ll most likely decide to stick to the following video: Designing a 3-channel mixer with diode distortion from scratch - YouTube
in order to get a grasp of buffer stages for all indiv. outs as well as a mixing stage for the sounds that require a mixing of noise and drum-sound.
I’ll probably start to built a first prototype on a breadboard as seen in the video and then try to reimplement it on stripboards. Does this sound right, is the posted video missing anything crucial?
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I’d prob forget the diode clipping in the first instance. Concentrate on getting your mixer stage right with the correct input resistors. If you’re using variable resistors that’s fine but the mixer input resistors will still govern levels and you might want to experiment with values so when your pots are ‘all the way up’ you’ve still got the balance of levels you’re looking for. For example your noise source maybe a lot hotter than your drum sound and using identical input resistors would mean that with both pots all the way up you’d have much more noise to drum sound.
Using a dual or quad op amp for the buffer stages is super easy but again, take one thing at a time and breadboard them first as you’re planning for the mixer.
If you want to add some distortion treat that as a separate sub circuit and prototype that after you’ve got the mixer and buffer stages working as you want them.
A Virtual earth summing stage is pretty much the de facto standard way of implementing a mixer so there’s tons of info out there.
A quick word about supply rails too. Are you using a bipolar supply or unipolar? If it’s unipolar you’ll need to implement a rail splitter of some kind to feed into the non inverting input. The vid shows it tied to gnd which is correct for bipolar rails. A quick and easy method is two resistors of equal value (100k for example) connected in series between +ve and ground. The centre point will be half the supply and that would connect to the non inverting input of the summing op amp (you’d have to do the same for any other op amp stages too). A better way is to use an op amp as a rail splitter as you’d then have a low impedance reference voltage that can feed multiple op amps.