Op-Amp Output to High-Impedance Load?

Discussion in 'The Projects Forum' started by swipesy, Apr 29, 2014.

  1. swipesy

    Thread Starter New Member

    Oct 1, 2010
    Hi everyone,

    I'm working on an on-board guitar preamp for the magnetic pickups to compliment a separate on-board acoustic preamp, namely, the Graph Tech Acousti-Phonic preamp. I've been in contact with their tech people and they tell me the minimum pot value for the Acousti-Phonic is 250K.

    Now, in my design I want to have a master volume (magnetic and acoustic) plus a blend between the magnetic and acoustic. Since I can't get either a dual-value dual-gang pot for volume or a dual-value blend pot, and taking into consideration the Acousti-Phonic requirement of a minimum 250K pot value, that means I have to use two 500K pots for the volume and blend in parallel (250K) for the Acousti-Phonic preamp (acoustic-side), but it also means I have to use those same values for my magnetic preamp as well. Unless I'm mistaken, that is!

    Attached is the diagram for the Graph Tech preamp. To have both a master volume and blend I thought to wire both pots piggy-back, with the wiper of one feeding the hot of another, at least for the that preamp's "acoustic volume". I've done that before with low-impedance circuits and it's worked well, at least for what I needed.

    In my previous on-board circuits I've always used lower-value pots, basically nothing larger than 50K, and they've always worked fine. However, since I have to use 500K pots for this (as I understand it, anyway), which I've never done, are there any special things I have to do except use a 470K input resistor and a 500K post on the feedback loop of the op-amp? Also, can an op-amp output handle feeding a 500K blend pot as shown in the diagram without affecting tone? The magnetic circuit would also include balanced pickup pres, tone and pan, but that's all before the volume and blend and connecting to the Acousti-Phonic.

    I hope I've explained this well enough. I've searched for something similar to what I'm trying to do - here and elsewhere - but came up empty. Can someone here, obviously with a heck of a lot more knowledge that I have, help with this by chance?

    Thanks in advance!
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    Last edited: Apr 29, 2014
  2. DickCappels


    Aug 21, 2008
    After that long explanation of what you are doing, what kind of help are you looking for?
    Last edited: May 3, 2014