VCF CV IN MOOG OPUS III

The Moog Forum Moog Music General Topics Forum
1

hi again guys, i want to add a VCF In, (not an audio in to the filter, i want CV in, to the filter) of my Opus III to control the filter with an VCO in audio range or other LFO in other wave, the Opus III only has triangular wave, the idea is use a switch jack to use when you want the on board lfo of the opus III or use an external signal to modulate the filter. The filter of the Opus is very similar to the minimoog filter and in the minimoog filter you can control the filter via cv in.

Other question is, can i control de filter with keytracking?, the resonance always is in the same frequency.

any ideas??

Thanks in advance.

Fernando.

:slight_smile: :slight_smile: :slight_smile:

2

Shouldn’t be hard. You just need a tech to wire it for you. Schematics are easily found online.

Other question is, can i control de filter with keytracking?, the resonance always is in the same frequency.

Not likely. The Opus III is a top octave system in which pitches for each and every key is generated simultaneously. It’s basically a fancy organ. When you press a key, you are routing the pitch for that key and producing a trigger for the EGs but it does not generate any CVs. The only way to add filter keyboard tracking is to add another set of contacts and buss to the keyboard assembly, those parts are simply not available and even it they were it would be more expensive than the instrument is worth.

3

heyy, thanks for the answer, ok I discard the filter keytracking, thanks for the information, but i want the cv in to the filter, i want use an external source or other waves, for this machine i think goes to another dimension.

I have schematics, like you know is very easy to get it on internet.

I was looking the modulation section and I made a mod to get a very fast lfo, adding a capacitor of less value together to C10 in parallel, you only have to put together negative legs of capacitors soldered, the positive leg of each capacitor is redirected to a SPDT, and the common, the center pin is wired to the boards, where was the positive leg of C10, and when you want you have the original lfo or the ultrafast lfo. You can get a more extreme speeds or very slow rates, increasing or decreasing the value of capacitor. It’s very easy to do!!!

When i compared the filter section of the minimoog to opus 3, I saw that mini filter works with 10v and opus 3 works with 15v, the value resistors and capacitor resistors are different, and ladder transistor is made with diferent transistors, but in essence is very similar.

I don’t know if I am doing well. Ok i try to cutting the pad that goes from switch S2 that selects lfo to VCF or FM, this pad goes to the filter and connect in R177 and R178. I insert a SPDT and solder a wire to R177 and R178 side to common (center of the three connectors of the switch), the other connector of the switch to S2 VCF and the third connector of the switch to a female jack and ground of the jack to ground of the three jacks of the back synth.

When i choose the normal mode on the switch the moog is in normal mode, but when I connect the jack to the MS50 lfo saw out or other, the lfo not makes nothing, all is working but there is not external modulation.
I think i am not doing well.
could be a reason that the opus 3 needs 6.8 V in a waveform to get modulation?, in schematics you can read that in U7 pin 6 you get a triangular wave to 6.8V.

Any ideas before i will die electroshocked?

Thanks in advance.

Fernando

4

i was reading the minimoog service manual filter section, where say

The cutoff frequency control current originates
in an linear voltage to exponential current converter
consisting of a matched NPN-PNP pair Q26 and Q28.
Control voltages from the external input, the keyboard
switches, modulation, and the contour generator
are summed by their individual control resistors
and fed to the exponential convertor. Resistor R49
sets the keyboard scale at one volt per octave and
R39 calibrates the front panel filter CUTOFF FREQUENCY
control.

in the opus 3 Q26 and Q28, has the equivalent in Q12 and pins 12,13, 14 of the U23. There is a note in the pin 12 on the opus schematics where say, Converts lineal voltage to exponential current, I assume that this is point to inject the cv in,

what do you think?. any ideas?

Thanks in advance.

Fernando

5

I did the mod about 8 years ago, so I can’t exactly recall what I did. The VCF input was about the same as you suggest. I just found an easy spot to solder on an external wire that went directly to pin 12 on U23. Attached the input resistor on the 1/4" jack. Not sure the value, but I probably started with 47K and used a deca-box (resistor substitution box) to dial in the response I wanted from an external CV.
I have a low res photo of the back of the board with the attached wires. The black was the VCF CV input, green S-trig in and red the audio input.

As I recall the audio input involved a resistor and cap in series. Again I used a deca-box to dial in the audio level I needed.
The audio in mod was great because now you could loop back the left or right output back into the VCF. With front panel mixer and pan controls, it was easy to either route the string section through the filter. Or a positive feedback loop into itself.
The VCF CV input gave to ability to modulate the filter with any CV source from a sample-hold, to sequencer, to whatever.
The S-trigger input required a buffer to make it work with all my Moog gear, but most would not bother and convert that input to a positive gate type input.
I tried to make the mod look as stock as possible.

Great mods though, really improves the versatility of the Opus 3.

6

Cz Rider thanks a lot for your answer, your mods are great!!!, I put my hands on the mods inmediatelly!,

I keep you informed.

fernando.

7

Hi again, VCF in done!!!, and working, i added a 100k pot to change the ammount of modulation, and include this in the front panel in the left next to the volume, thanks to all, and specially to CZ, now i going for the Vcf audio in, the strigger gate is not very clear for me, CZ Rider did you remember where is the point to get this mod?

thanks in advance.

Fernando.