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Any electronics wizards out there? Preamp passive bypass Question


Pete1967
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Hey All,

I'm planning to install a Seymour Duncan STC3P which has prewired pots, and I'd like to include a passive bypass switch for those 'oh sh*t my battery's dying' moments. The pickups are passive DiMarzios. I have the mod diagram from SD and it recommends that 100k - 250k pots should be used. The only two pots implicated in the mod are the blend and Vol pots - passive tone not required because it's only a disaster recovery design! The blend is already 250k, but the vol, because it's designed to work with the output from the pre, is 10k.

So my question is - what's the best compromise here? Do I replace the vol pot with a 250k which would presumably attenuate the pre, or stick with the original 10k which would presumably influence the passive performance, or do I try and find a 100k pot and split the difference??

Diagram below if it helps.

[attachment=190048:Drawing1.jpg]

TIA for any help.

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Hey Luke - thanks so much for a superquick reply and from a 'local' too I see.

I'm just not getting it despite your uber helpful reply - I wish I was quicker with this stuff. As I read the diagram with the switch thrown to passive (top terminals closed in the diag) the signal is passed straight from the blend across the vol and to the jack, but in 'active' (bottom terminals closed) the signal passes from the blend through the pre so it's the preamp output that is passed across the vol. Wouldn't a 250k pot kill the preamp output in this scenario?

Sorry for being dim...

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Hey Pete,

A 250k wouldn't 'kill' it, but it may become a very sharp rolloff. Fully up the vol control is the same whatever the value, the resistance comes into play when you turn it down.

I think what LukeFRC means is that if you put the vol. control before the pre-amp (after the blend control) then the passive circuit will behave as it should, the signal is then fed into the pre-amp after the vol. control.

So... Pickups > Blend > Volume > [Bypass Switch]
(ON) > Preamp > output jack
(OFF) > output jack

...if that makes sense.

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LukeFRC and Brensabre79... thanks again for your help guys. I really appreciate it. I've redrawn the cct based on your insights. Please could you do me a favour and check it for me....

[attachment=190107:Drawing2.jpg]

Edited by Pete1967
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[quote name='brensabre79' timestamp='1429547817' post='2752487']

Fully up the vol control is the same whatever the value, the resistance comes into play when you turn it down.

[/quote]

That's not really correct. With a 10K Vol Pot the pickup is loaded by 10K (ignoring the tone contriol) at full volume and with a 250K pot it's loaded by 250K.
This is a big difference and the conventionally correct choice of the two with a standard type of Hi-Z pickup feeding the volume pot would be 250K (cue lots of handwaving about loading, current draw and inductive/reactive impedance).
A 250K pot on the ouput of a preamp will do no real harm. It's not necessary to have such a high impedance and there will likely be a little more noise than with a lower value pot but it's minimal compared to the effect of a 10K pot on a standard passive pickup.
Essentially a higher value makes less demand on the pickup and the resulting signal will have more sustain and a better frequency response.

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