Balance Pedal

Discussion in 'The Projects Forum' started by TrevorP, Dec 21, 2008.

  1. TrevorP

    Thread Starter Active Member

    Dec 8, 2006
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    0
    Hey,

    I'm just poking around an idea for a Balance pedal for a guitar and I have the basic idea down of what I would like.

    Basically the input AC signal (assumed to be at most 1.2V RMS) is first biased using a voltage divider and a 9V source. Biasing to be determined by the saturation voltage of the balancing opamps/transistors.

    Next the 9V source (Not divided) is divided but now using a variable resistor. This will be considered the "direction" control. So I've specified it that when the direction control is 9V then the LEFT channel output is equal to the input signal. 0V = RIGHT output = Input signal.

    So my problem lies in figuring out how to do this balancing. I was thinking of using two opamps. One that takes in the direction control and the biased signal to produce a buffered LEFT output. The other one would also take both in but produce the RIGHT output that is inverted compared to the LEFT output. EG: Signal = 1V RMS LEFT out = 0.4V RMS, then RIGHT = 0.6V RMS.

    However, the 741 requires 10V minimum. So should I be using opamps or go to transistors?

    That "direction" control is also used to drive 9 leds using an LM3914 Driver. (Where 1V lights up the first led, 2V lights up the second...and so on.) This part I'll have to figure out later and isn't really the important part.

    I'm in second year Electrical Engineering...we've done opamps but not transistors yet though. So if you do recommend something could you also explain why I might want to do that.

    Thanks for any suggestions and whatnot,

    Trevor P
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    So you want to control the audio level to a left and/or right output jack? How is the control voltage to be applied? Can you post up a schematic?

    What is the 10 volt minimum that a 741 op amp requires? Are you open to op amps other than 741's? Those dinosaurs are a 40 year old design, and not really "it" anymore.
     
  3. TrevorP

    Thread Starter Active Member

    Dec 8, 2006
    55
    0
    Ok yeah I've attached what I've got right now.

    I'm basically having a few problems right now:

    -Figuring out how to calculate the resistor values for the referencing in the LM3914. I'm thinking that I'm not going to be able to have it so that the input on pin 5 is at most 9V. Because the Power supply to the LM3914 is also 9V. I may have to put a voltage divider after that to make it like max 5V or something. Also am I alright using 9V for the LED supply? And am I to assume that Vref is always 1.25 (Can be seen in the LM3914 Datasheet on page 8) or is that supposed to be equal to my highest possible voltage?

    -Figuring out how to have the control line (coming out of the channel select and adjust section) control the output of the respective left and right outputs. I figure I could think up a way using an opamp. But it could definitely be done with transistors (although I don't know how to go about figuring out what to use for that).

    As for the 10V on the 741 I was referring to the minimum supply voltage. (Usually takes 15). And no I would not be opposed to using lower power opamps. They need to not affect the sound very much though.
     
    Last edited: Dec 21, 2008
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    You must use an opamp to provide a very high impedance load to the guitar and a low output impedance to drive the balance circuit.
    Since you are using an LM3914 voltmeter then you need two voltage-controlled attenuators for the audio which will be a nightmare to design and build.

    Why can't you use a dual pot? One part for audio balance and the other part to send a variable DC voltage to the LM3914.
     
  5. TrevorP

    Thread Starter Active Member

    Dec 8, 2006
    55
    0
    Hmmm ok then.

    But I was thinking could I not make that with two transistors? One being an NPN and the other being a PNP. That way they would act in reverse of each other. Then the control signal would control how much of the signal was getting through. The only problem I see with this I guess is that I'm going to have a pretty large voltage into the gate or collector of the transistor. (I really don't understand transistors enough to do that I'm afraid.) And then I have to worry about impedance as well I guess.

    Ok well I guess I'll try the dual pot configuration then. But now how do I got about doing the math for this LM3914. My big question now is if I'm supposed to specify what the internal Reference Voltage is or just assume it is always ~1.25V. It seems that it might be the second option. But I'm wondering why on page two of the datasheet (the 0-5V example) they use 3.83k for R2. I see why they choose the 1.21k for R1 (wanting ~10mA for LED current). But when I calculate the Ref Out V I get 5.2V. Why wouldn't they use a different value that would give them 5V?
     
  6. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    It takes a lot of transistors to make a voltage controlled audio attenuator.

    I don't know why National Semi uses resistors that give a full scale of 5.2V instead of 5.0V. It is only 4% high so maybe they don't care.
     
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