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Wiring analog VU meters to amplifier

Discussion in 'General Electronics Discussion' started by Solidus, Nov 20, 2012.

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  1. Solidus


    Jun 19, 2011
    Hello all,

    You may or may not remember me as he who builds (or attempts to build) tube amplifiers, and I have a schematic I'm working on now for a combination power pre-amp and headphone amplifier.

    Anyway, I would like to wire two analog VU meters onboard the amp for metering and effect. While being new to the audio amplification concepts of electronics, I doubted that it was as simple as wiring the meters to the appropriate channels on the output cabling, and looked into it online.

    Not many of the articles are helpful as they detail how to make 10-LED segments of VU measurement which as far as I'm concerned from looking at them before are more novelty then accurate. What forum posts I found on the subject raised varying points, with one point made on more than one post saying that a buffer amplification section for isolation may be needed otherwise the impedance of the VU coil may adversely affect the circuit being measured.

    So, it's time to go back to the drawing board with questions -

    1) How would a buffer amplifier be constructed that doesn't significantly amplify or alter the signal characteristics of the amp line to be measured? If I were to think on the issue before my gut impression would be to use an LM386 or dual differential op-amp with R2 set to zero, assuming the formula A = 1 + (R2 / R1) is correct - yielding a gain of 1.

    2) Would there need to be a calibration potential to synchronize the main audio signal to the true reading on the meter? That is, would I need to have a bias running to the meters?

    3) Would it be safe to wire the grounds of the metering circuits to the grounds of the amplifier stages or would that adversely affect the performance of the amp?
  2. john monks

    john monks

    Mar 9, 2012
    The best way is to use a buffer amplifier and a rectifier circuit driving the meter. And use capacitive coupling between the amplifier and the rectifier circuit.
    The buffer will prevent rectifier noise from batting back into your audio stages. And the buffer need not be very substantial. A 741 op amp should work fine.
    The calibration can be done with a variable feedback resistor for the op amp.
    With capacitive coupling there should be no need for a bias supply however your buffer should be biased at mid-point.

    You can look at commercial automatic volume control or limiter circuits for other ways to do this.
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