Engineer's Thumb question...

[digi2t]

Hey folks! Building an ET (my first compressor, yay!), and I don't have a 10M resistor for the input to ground. Highest I do have is 6.8M. Is that enough, or should I chain a 6.8M and 4.7M together?

Would a 3.2M difference really be critical here?

[jimilee]

Throw it in and see what happens, it's what diy is all about.


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[digi2t]

Throw it in and see what happens, it's what diy is all about.

Already done. Just need someone to mentally prepare me to yank it out, if need be.

[jimilee]

It shouldn't really be a problem. I have found that I get pops more on tube amps than solid state. I really don't see it being an issue.


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[samhay]

My take on this is that Merlin's an EE, so he designs in orders of magnitude.
Further, if you look at his circuits, you will see the all tend to use the same/similar values on the input side. Why did he choose them?

The designed input impedance is 1M, which is the same as/similar to a valve amp. With a non-inverting op-amp, we can set this with a 1M biasing resistor from op-amp (+) input to Vref. If a pull down/anti-pop resistor is added to prevent switch pop (this is optional/not needed in buffered bypass designs) then if we make this 10 x the bias resistor (i.e. 10M), it essentially has no effect on the input impedance - it lowers it by 9% (1 || 10 ~ 0.91 M), which is within the tolerance of some resistors

Short answer - a 6M8 resistor will be fine there. You have lowered the input impedance to 870k cf. 910k, which is almost certainly not something you will be able to hear.

[digi2t]

Thanks for the insight Sam! Greatly appreciated.