Help me troubleshoot DJ Thomas White Lopass Gate

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Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by Synthiq » Sun Mar 22, 2020 9:47 pm

These signals still looks reasonable so I don't think there is a problem with the 4.7nF capacitor so I'm not really sure what the problem is. Did you go back to VCF mode and listened to the signal after lowering the amplitude? You may have to vary the offset pot as well to find the best frequency for the resonance. If you still don't hear any difference when you vary the resonance pot, there are two more tests you can do.

The first is to set the mode to VCA+VCF (center position) and apply a +/-2V square wave to the audio input and set the offset to max to maximize the filter bandwidth. Then take pictures of the signal at pin 14 of the TL084, first with resonance set to minimum and then at maximum. In the first case you should see a square wave with the same +/-2V amplitude as the input signal and in the second case +/-4V amplitude since the gain should now be 2 in that amplifier.

The second test I can think of is to set the mode to VCF and apply the same +/-2V square wave to the audio input and connect the scope to the out jack. Adjust the offset pot until the rise and fall times of the signal is roughly 25% of the half period of the signal when the resonance pot is set to minimum. There may be some overshoot even at the lowest setting but when increasing the resonance pot you should start to see increased ringing after each transition of the square wave if the circuit works correctly.

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Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by wackelpeter » Mon Mar 23, 2020 8:26 am

Also perhaps check the resistor from the reso pot to gnd (51K) for the correct value... As far as i remember the smaller the value the quicker the resonance gets in and the bigger the more tamed the resonance is above a certain value mostly no resonance.

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