Membrane Pot Mega Problem

Discussion in 'The Projects Forum' started by trad3mark, Aug 19, 2008.

  1. trad3mark

    Thread Starter Member

    May 11, 2008
    17
    0
    Hey all,
    Ok, so i'm planing a new guitar build at the moment. But in it, i'm looking into replacing the standard pots with membrane ones. That's being massively optimistic, i know, and it's asking for problems right from the start. I'm a bit of a beginner when it comes to circutry, like i know the basics, but thats about it. So here's where i stand.

    Basically, i need two pots. A 250k ohm one, and a 500k ohm one. I can have membrane versions of these made, but at $2000 a pop, thats out. HOWEVER, i can get two 10k ohm ones for about $20 each. Now, obviously 10k is a long way off 250k. So my new problem is, is there a way i can use a 10k ohm, in place of the 500k or 250k. Like can i put extra resistors in or something?? or does anyone have any other suggestion as to how i could have a touch controled variable resistance of 250k/500k.

    thanks all.
    tm
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You can get free samples at Spectra:
    http://www.spectrasymbol.com/typo3/site/en/quotes-samples.html

    Sparkfun Electronics has membrane pots for $15/ea when you buy singles:
    http://www.sparkfun.com/commerce/product_info.php?products_id=8607

    Pots are usually used to select a voltage or signal between two levels. I have no idea why you need a resistance that high. Is it to match the input impedance of an amplifier?

    Will there be power available inside the guitar? (I really don't know; I've never had an electric guitar myself.)
     
  3. trad3mark

    Thread Starter Member

    May 11, 2008
    17
    0
    the pots are like mediators to control the Volume and Tone between the pickups and the amplifier. They have no power source. The guitar basically goes pickups > pots > amplifier. Basically, my problem really is, can i convert a 10k ohm pot into a 250k ohm pot?
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Wow, I had to do some Googling to find out about these things. It's a bit ironic to see that they are supposed to be more economical than standard pots.

    I see that there has to be a mechanical pressure applied to make the contact for the wiper. For any accuracy, the membrane pot will have to have some length to it. That might get in the way of playing more than a knob.

    For the money, fitting a motor to a standard pot might be cheaper. It's hard to beat the price of a guitar pot.

    You can't swap in a 10K pot in place of a larger value without modifying the circuit to accommodate the change. You can get 100K sliders, but they won't go in much easier.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Like Beenthere said, you would either need to add a powered impedance matching box between the amp/preamp and the guitar, or mod the amp/preamp. Apparently, the input impedance of a guitar amplifier is pretty high. Adding an impedance matching box implies adding noise to the signal. If your client were a heavy metal rocker, they might not care. But just about anyone else probably would - including yourself.

    I've seen some impressive reliability figures on those pots; 1,000,000 to 10,000,000 cycles expected lifetime. But that could go right out the window were it dropped one time just the wrong way, or something fell on the pad.

    I'd give it a year or so. The prices and selection will likely improve enormously.
     
  6. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    It´s because the output impedance of the pickups is something like 1meg, and inductance of cca 1H, so you need high value pots so you don´t loose the high frequencies or amplitude.
     
  7. trad3mark

    Thread Starter Member

    May 11, 2008
    17
    0
    thats a pity. it would have made one hell of a guitar...
    thanks anyway.
     
Loading...