[sdiy] SVF High Pass Mode not Cutting Off

[Stewart Pye]

Hi Rob,

Usually the LP output (which is already out of phase with the input 
signal) is fed back to the input summer. In this case the LP output is 
in phase with the input signal so is fed to the + input of the input 
diff amp, while the input is fed to the - input. It's a bit hard to read 
the values of R14 and R15 but I suspect these are the ones you have to 
adjust.

Regards,
Stewart.


rob at emulatorarchive.com wrote:
> 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. 
>
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