VCF Four Pole 24dB/Oct with CV, Calibration Problem

Stavros · Joined: Oct 30, 2007 Posts: 36 Location: Athens-Greece

I have just finished building 3 modules of the Ray Wilsons VCF 4 pole 24dB/Oct design.
I have build on my PCBs all the suggested improvements for the :Range of Cut Off Frequency" control and the Improved Tracking.
Following the Instruction published for the calibration of the Calibration ..

I have:
attached a 0.000V keyboard voltage to one of the CVs,
turned the Cut off Frequency knob to 8 o’clock,
turned the resonance control up until it sustains oscillation over the full range of the cut off frequency, and then…the oscillation does not go lower than approx. 2240 Hz.

It should be possible to go down to 100 but it does not!

Furthermore if I increase the CV Voltage to1, 2 or 3 Volts it makes no difference.

The exactly same problem appears in all 3 Modules.

I will very much appreciate any help / suggestion / comment on what I might have done wrong.

toms809 · Joined: May 15, 2016 Posts: 2 Location: the Netherlands

Does all resistors and caps have the right value? Please check the capacitors and resistors from cutoff (R3) to Q1 and Q2.

Stavros · Joined: Oct 30, 2007 Posts: 36 Location: Athens-Greece

Hi, I very much appreciate your responce, many thanks.
Yes I have double checked resistors and the single cap,
R1= wire bridge,
R6=75K,
R12= 1k58 (combined value of new parallel 7k5 and old 2K res.),
R5=100Ω multi trim.,
R8=475Ω,
R2=27k8 (combined value of new parallel 68k and old 47k),
R9=10k,
R13=1M,
C1=100pF (measured with my DGM 160pF).

I should note that I have not proprerly matched the 2n3904 and the 2n3906 transistors.

I have also used extra 100K linear pots as attenuators for CV2 and CV3, but I do not think this causes a problem.

Best Regards

Stavros

toms809 · Joined: May 15, 2016 Posts: 2 Location: the Netherlands

Hello Stavros,

How is the behaviour from 2240Hz to 10KHz?
Have you tried 1 filter without the suggested improvements?
Is it possible to measure the voltage with the cutoff knob fully closed and the resonance fully open at pin 1,2,6,7 of U1.

kind regards,

Marc

Stavros · Joined: Oct 30, 2007 Posts: 36 Location: Athens-Greece

Hello Marc,

1. Turning the cutoff freq. knob clock wise the frequency increases smoothly.
2. No, I have not tried the filter without the improvments.
3. Measurements with the cutoff knob fully closed and the resonance full open

U1 Pin 1 = 0.18V, Pin 2 = 0V, Pin 6 = 0V, Pin 7 = -0.71V

Many thanks for your support and Best Regards

Stavros

Stavros · Joined: Oct 30, 2007 Posts: 36 Location: Athens-Greece

Hello Marc,

I think I have identified my mistake and I will very much appreciate your confirmation of that.

In order to clarify that, I should note that I am not using the MFOS PCB.

Based on the MFOS - Ray Wilson's schematic, I have designed and developed my own PCB to match the standards that I use in my modular.

In doing so I have used the Pin layout of standard NPN and PNP transistors for the 2N3904 and 2N3906.

Looking again and again for the possible "standard" mistake I should have done, and after eliminating all others, I focused on the Transistors.

So I went through the MFOS PCB silkscreen comparing the Transistor positioning and subsequently the datasheets of the 2N3904 and 2N3906 and identified that the collector emmiter positions are reversed.

Am I correct?

Then I should just take out the allready soldered ones and replace them with new ones (which I will match before hand), positioned reversed.

Many thanks and regards

Stavros