Don't Want Potentiometer in Circuit

Discussion in 'The Projects Forum' started by bubbagump0, Jul 21, 2007.

  1. bubbagump0

    Thread Starter Member

    Jul 10, 2007
    26
    0
    I ordered a small 1W amp kit and it came with a 10K potentiometer which is too big and bulky, and I just want to connect the holes in the PCB for that potentiometer so that it is at full volume always (I want to adjust the volume on the mp3). By looking at the schematic, can anyone tell me the correct way to wire it so that it is at full volume always? I have found one way for full volume always, but when i turn off the mp3 and the amp is still on, it makes a lot of static noise. Thanks.
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    It should be as simple as connecting pin-7 and pin-4 of channel 1 and channel 2 respectively to the corresponding capacitor by putting a jumper between the wiper and the high end of the volume control. You will then need to add a 10K resistor from the wiper to ground so that the input sees the 10K that it is used to seeing.

    hgmjr
     
  3. bubbagump0

    Thread Starter Member

    Jul 10, 2007
    26
    0
    Thanks for the help... but I don't know what a wiper is or whatever. I am fairly new at this. Any other suggestions?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The "wiper" is the part of the potentiometer that moves. On the schematic, the wiper is represented by the arrow.
     
  5. JohnnyD

    Well-Known Member

    Aug 29, 2006
    79
    0
    Do you mean pin 7 and pin 6? cos pin 4 is ground.

    Anyway, with the volume control pot removed you should have 6 holes. 2 of them will be connected together and also connected to pin 4 of the chip. This is Ground. You need to get 2 10k resistors and 2 wire links and solder them like in this awesome quality drawing I did lol.
    [​IMG]

    Whichever holes are connected directly together and to ground, you need to solder the resistors between them and the middle 2 holes.

    Hope this helps.
     
  6. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    If you don't want to use the pot simply connect each input directly to where the potentiometer wiper should be. Your potentiometer has six legs (three for each channel). Also, you may notice that each channel is grounded (each potentiometer dish has a leg connected to ground). So, if we number the potentiometer pins this way:
    1 4
    2 5
    3 6
    and if pins 3 and 6 are grounded, you should connect directly the hole corresponding to pin 1 to the hole of pin 2 and the hole of pin 4 to the hole of pin 5. If pins 1 and 4 are grounded, you connect 2 directly to 3 and 5 directly to 6 instead. You can identify which pin holes are grounded since those are the only ones that share the same printed circuit path. All others are separated.
    You won't need resistors, just jumper wires.
     
  7. JohnnyD

    Well-Known Member

    Aug 29, 2006
    79
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    Wouldn't leaving out the resistors cause a few problems? like the input impedence would be changed and also the capacitors would have nowhere to discharge, except through the IC which might cause a nasty popping noise.
     
  8. JohnnyD

    Well-Known Member

    Aug 29, 2006
    79
    0
    Also, aren't C2 and C3 connected the wrong way round? Surely the input should connect to the + side of the cap?
     
  9. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    No. In this case the input impedance is not critical. Since there is no voltage dividers (the double gang potentiometer), you can leave the resistors out with no problem.

    That depends on the amplifier. In the case of the LM386 you use the caps connected with the input at the + side. In the TDA2003, for example, you use them the other way around. It preety much depends on the internal circuitry of the amplifier itself (namely it the transistor of the input stage are PNP or NPN).
     
  10. hgmjr

    Moderator

    Jan 28, 2005
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    214
    You are right. I meant to refer to pin-6 instead of pin-4.

    Thanks JohnnyD for catching that typo.

    I would still include the 10K resistor in the design since the original design would have seen a 10K resistance to ground with the potentiometer in the full volume setting.

    hgmjr

    PS. Nice sketch JohnnyD....
     
  11. bubbagump0

    Thread Starter Member

    Jul 10, 2007
    26
    0
    Well at first I had the correct holes jumpered, and it was making a popping noise when it had no signal running through it. But now it stopped making the popping noise, (only a very quiet static is left) so I guess i will leave the resistors out unless the popping noise comes back. Thanks so much.
     
  12. hgmjr

    Moderator

    Jan 28, 2005
    9,030
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    Omit the 10K resistor if you must but be aware that the manufacturer's datasheet clearly recommends the use of a 10K to ground on both inputs.

    hgmjr
     
  13. JohnnyD

    Well-Known Member

    Aug 29, 2006
    79
    0
    thanks! :D

    I would keep the resistors personally even if it was just for the sole reason that it's how the original circuit would be with the volume control at full volume.
     
  14. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    I believe that without the resistors, the inputs will not necessarily return to ground like they should during periods of no signal on the input.

    hgmjr
     
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