[sdiy] SVF High Pass Mode not Cutting Off

[rob at emulatorarchive.com]

Hi
David is right, although the solution appears more dramatic than 
matching to 0.1%. The circuit I am attempting is a Roland 703 SVF. By 
varying the value of the resistor feedbacking signals from BP and LP 
into the first Op Amp (100k down to 22k) I can get the filter cutting 
off in HP mode,  but loose LP volume. The schematic is slighly odd in 
that the LP and BP signals are not feedback independently, so I will try 
changing this - maybe a mistake in the original schematic. However I am 
on the right track thanks to David and the list. I have the same issue 
with an ARP2500 UAF, which I will try and fix next weekend, this could 
well be a matching issue on the first Op Amp.

Best Regards - Rob


On 30/10/2011 04:17, David G Dixon wrote:
> I don't think so, at least not to an annoying extent. As I said 
> before, I'm pretty sure that the problem here is imperfect 
> cancellation of the LP and input signals at high cutoff. The cure is 
> definitely to tighten up the resistors on the input summer, and even 
> perhaps to make one a tad smaller than the other and add a trimmer in 
> series which can adjusted to perfect cancellation. The problem is that 
> even a few mV of bleedthrough will be quite audible. I had this 
> problem with the Korgo, and we fixed it (mostly) with 0.1% resistors.