[sdiy] Interpreting PEQ schematic help

[Ben Stuyts]

I don’t see any sonic advantages. If the offset voltage on the output of IC1B isn’t exactly 0 (set by R63) you will have audible clicks anyway.

As far as I can see from online images, the switch is not on the pcb. So it saves one wire to solder/assemble. Maybe it was just for economical reasons.

Ben


> On 5 Oct 2020, at 16:25, ColinMuirDorward <colindorward at gmail.com> wrote:
> 
> Thanks Ben!
> What's the advantage of doing this, rather than using a switch connected to the output buffer that toggles between either the processed or the unprocessed signal? 
> Less click?
> Colin
> 
> 
> On Mon, Oct 5, 2020 at 12:43 AM Ben Stuyts <ben at stuyts.nl <mailto:ben at stuyts.nl>> wrote:
>> On 5 Oct 2020, at 05:28, ColinMuirDorward <colindorward at gmail.com <mailto:colindorward at gmail.com>> wrote:
>> 
>> Here <http://www225.pair.com/audio/waltzingbear/Schematics/Urei/545.JPG> is the schematic for the Urei 545 PEQ. I understand that it's a highpass filter followed by four BP SVFs summed into the dry signal, then a lowpass filter.
>> I'm stuck on the in/out switch, though. What's going on here? I know what it's supposed to do, but I don't understand how it works.
> 
> The switch connects the output of IC1b directly to the output stage IC7. The output of IC1b is very low impedance and is connected directly to the + input of IC7. Therefor, the processed signal (coming in via R52) is completely attenuated (short circuited you might say) by IC1b’s output.
> 
> There might be some bleed but it is attenuated by R52 (10k) over the output impedance of IC1b (extremely low).
> 
> Ben
> 
> 
> 
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