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 Forum index » DIY Hardware and Software » YuSynth
Some questions on the Minimoog VCF
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Omega



Joined: Oct 11, 2010
Posts: 16
Location: Böblingen

PostPosted: Thu Oct 31, 2019 2:57 am    Post subject: Some questions on the Minimoog VCF
Subject description: Problems with the adjustment procedure
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Hi everyone, I have some problems with the adjustments of the minimoog filter according to Yves instructions.

First of all, I have populated the PCB and made some first checks. So the Cutoff Frequency is working fine, I can mute any signal down to 30 Hertz and bring it back to life. Also the emphasis potentiometer is working properly I guess. Self oscillation is also o.k.

But I have troubles with the V/OCT adjustments:

Apply 0.000V to the V/Oct input
Turn the frequency knob fully counter-clockwise in order to measure 0mV at the base of Q1 (node between R9 to R11)


This part is working.

Apply 1.000V to the V/Oct input
Adjust T1 in order to measure 18.2mV at the base of Q1


This is also o.k. To apply the voltages I am using a Midi/CV converter and a 4 octave Keyboard

Apply 5.000V to the V/Oct input
Check that you have 91.0mVat the base of Q1, if not adjust T1


Since the Keyboard only have 4 octaves I apply only 4V to the V/Oct Input. When I have a look at the values, it seems for me that the 91mV are exact 5 times the 18,2mV from the last step. So for my 4 octaves this whould mean 18,2mV * 4 = 72,8mV. But I do get 58,4mV. So I have to adjust T1. But if I do so, I will change also the value of the 1V input. So I will change both values. And I noticed, that the delta bewtween the measured values and the expected values ´will increase, if I increase the 18,2mV at 1V Input. On the other hand, I can only lower the 18,2mV to about 17,2mV. Then I am at the end of T1. So, this will not work. I measured also the output of the filter with an frequencycounter and I am far away from a good tracking

Do I make any principle mistakes and/or can anyone have suggestions for me?


And one last question of understanding. I noticed, that the frequency of the sinuswave of the self oscillation is changing when I turn the Frequency potentiometer (without any audio input signal). I guess this is o.k. since all 3 CVs (and the 1V/OCT input is one of the CVs) and the Frequency Pot having the same node, the base of Q1. Please can someone confirm this?

Thanks a lot in advance

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Cheers

Bernhard
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