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DC amp for VU meters

Discussion in 'Misc Electronics' started by Bipolar Boogieman, Jul 9, 2004.

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  1. Hey all... I have a small home studio setup for doing voiceovers and
    sound work, mainly for video/PC games. Nothing especially demanding of
    "audiophile" quality but at least decent. For a long time, I used an
    ancient mono Gates Studioette 80 console, but I finally needed to upgrade
    to something stereo with little to no money to work with, thus a new
    console was simply out of the question. So, before you flame me for the
    following hack job, this was the best I could do with what I had at the

    I really needed to have both AUD & PGM output busses in stereo, so I
    gutted the Gates completely and mounted two identical Radio Shaft stereo
    mixers inside the Studioette chassis. Sorry, but I'm "old Skool" and find
    the rotary pots much easier to work with, blame that on my radio
    experience back in the 70's...

    Considering the hack job that it is, it does the job quite well, except
    for ONE little bug...

    The RS mixers originally had a pair of those typical "cassette deck"
    style VU meter movements in it that I'd guess were about 50uA FSD. I
    found a pair of Modutek 4" meters, complete with mounting bezels, so I
    used them to replace the large Weston movement in the Gates. However, it
    seems to take a *lot* more to drive these to full-scale, and they're
    reading at least 10dB below "actual" levels.

    Anyways, I've maxed out the meter trimmers on the mixer and it's still
    pinning the VU meters on the compressor following it when the Modutek's
    are indicating only +1dB, and there is great distortion anywhere beyond
    that. The metering circuit inside the RS mixer rectifies the signal to DC
    via a half-wave bridge. The Modutek meters are also DC movements (nio
    internal rectifiers) and I have even tried disconnecting one side of the
    internal 600 ohm shunt, with no effect.

    The console does has an unused 1-amp variable PS as well as an extra
    stereo headphone amp in it, with volume and tone controls.

    -IDEA # 1... run the existing metering signal into the headphone amp and
    using the output to run to the meters, but I'm not sure if it would work
    very well with the pulsating DC as an input.

    -IDEA # 2... I remove the meter lines from the mixer circuitry
    altogether, run them to the actual audio outputs coupled through a series
    150uF electolytic. I can run that into the headphone amp, it's output
    feeding to a full-wave bridge across the meters. Calibrate with
    volume/tone controls accordingly to match the input level shown on the
    compressor's VU meters.

    I'm toying with idea #2... sounds easiest to me and probably more
    accurate in the long run. I don't really need it to be 100% accurate, but
    I do want it to be at least within a dB or so of "actual" rather than the
    convoluted mess which I have now.

    Ideas? Comments? Free stereo console you're sick of tripping over?? :)

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