Muff's Modules & More

[frequencycentral]

I found this really simple multimode in an old copy of E&MM, part of the 'Synbal' circuit. I did a PCB for just the VCF part, adapted for bipolar power. The filter itself is nice, doesn't self oscillate, the modes are cool. But there's a significant volume drop with respect to the input signal. The filter itself is nice, doesn't self oscillate, but the modes are cool. Maybe an error on my layout, but I'm pretty sure I checked it thouroughly. Why does this VCF design have a volume drop? Is there any glaring reason that I'm missing? Any quick fix suggestions?

[L-1]

If you install 0.1 uF cap with 10K impedance you have -3dB at 160Hz and optimal low freq response at 1600 Hz. IOW this cap cut low freq. 0.1 uF is good for 100K impedance. For 10K 1 or 2 uF is needed.

[frequencycentral]

Ok thanks, I used 0.47 in place of the 0.1. I'll try bigger values. But.....I think there's something else too (?).

[L-1]

Use this calculator http://www.v-cap.com/coupling-capacitor-calculator.php

[falafelbiels]

There's always something else...

[Dr. Sketch-n-Etch]

Did you terminate the 4.7k resistors from the Darlington buffer outputs to -V or to ground? The symbols look a bit like ground symbols, but they are in fact the negative rails. Also, did you take the HP signal from the output of the inverter? The schematic is drawn kind of badly there, but the HP output must definitely be taken from the output of that inverter.

Other than that, there is no reason why this circuit should not give fairly robust outputs. (Incidentally, those three output buffers are not really required, since their input signals all come from buffers anyway, but since you will probably use a quad opamp here, there is no harm in using them, I suppose. In any case, they could simply be replaced with 1k output resistors, after the capacitors (which are definitely required).)

This is actually a pretty terrible circuit as drawn. Here are a few suggestions for improvement:

1) The LM13700 distorts really badly unless the differential inputs are limited to less than about 50 mV p-p. I have confirmed through simulation that this circuit, as drawn, will distort something terrible (and I'm not talking about good distortion -- I'm talking about shitty-looking and shitty-sounding output waveforms). This is VERY easily fixed. Replace the two 22k resistors with 100k, and the four 1k resistors with 220R. You could also replace the 4.7k resistors with 10k, but this is not really necessary.

2) The ac coupling caps are too small, and will filter signals near the bottom of the audio range. Replace with 470nF (at least).

3) Unless your input signal is expected to have a significant dc bias, you don't really need that input coupling cap, and it is going to filter your input signal fairly significantly unless it is much, much larger (like, 10uF nonpolar or two back-to-back 22uF aluminum electrolytics).

Of course, I'd only build an SVF with 2164 now (and my circuit self-oscillates quite nicely and gives very clean quadrature sine waves from all outputs), but this could be a tight little circuit with the correct component values.

Hope this helps.

[ultrashock]

Dr. Sketch-n-Etch
original 2164 are hard to find. do you use them or Cool Audio clones? As I see, lots of Intellijel modules incorporate this chips (Dixie expoconverter, corgasmatron. HEX vca etc.)

[Neutron7]

look at the input of the first LM13700. it has a voltage divider consisting of the 22k and 1k resistors. then look at the output buffers. they are only unity gain buffers.

just change them to an amplifier and it ought to be fine.

there might be other signal losses or gains in the LM13700s and associated circuitry, so its best to measure the outputs before calculating how much gain you need.

[Dr. Sketch-n-Etch]

[Dr. Sketch-n-Etch]

[frequencycentral]

Thanks for the advice Dr. Sketch-n-Etch, I'll try those mods and report back.