BMP experiment revisited: question about (optional) pull-down resistance

[KarenColumbo]

I already built 5 versions of it - never git it right. So I try it yet once again.

This is gonna be a BMP with active tone control (parametric mids) and an LPB as a switchable "boost" at the input.

I plan to switch the boost in and out via a 2PDT (it has an LED as indicator).

Here's a quick and dirty schem without the tone section - I will fill that in later, have to board it first to get the values right.

As you can see I have two inputs ("IN_BOOST" and "IN"). The original BMP (at least those 20+ versions I looked at) doesn't have any pull-down resistors at the input. Somehow I feel they are missing. So I drew them in: "RXX" and "RYY" right below the input pins.

My question(s) is: Do I need them? And should I go the >680k way to avoid loading down the guitar?

[marcelomd]

Hi,
That first 1M or so resistor, in most pedals, is there to prevent switch pop. I'd keep it and I'd also redesign the switching scheme.
The way it is drawn, if you switch the input jack between IN and IN_BOOST, you'll get some interaction with the BOOST pot, even if the boost is not engaged, because IN is connected to all that stuff around the BOOST pot.
If you don't ground IN_BOOST when the boost is off, you'll also get noise from that stage.

[KarenColumbo]

Thx, that's a good point - that's what quick & dirty will do me
So, this is the ticket, I presume? (postimg is down right now so I had to upload it to my server, sry!)

[antonis]

Have mercy on our poor eyes, Andreas.. 

Yes, that should be OK..!!
(for 1M or bigger resistors..)

And make D5 Schottky diode (1N5817 or similar..)

[marcelomd]

Hi,
This should work now.

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