Why does this LM13700 VCF design have a volume drop?

frequencycentral · Joined: May 25, 2008 Posts: 185 Location: UK

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?

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JingleJoe · Posted: Thu Mar 15, 2012 3:41 pm Post subject:

Add an amplifier to the end, most filters have a drop in signal amplitude, one I made recently requires a very low input signal or it doesn't work very well at all.

frequencycentral · Joined: May 25, 2008 Posts: 185 Location: UK

Yeah I did, made the unity gain buffers into amplifiers (330k feedback resistor, 10k from -ve input to ground)..........with the added benefit that feeding the bandpass output back into the input now causes it to self oscillate nicely.