VU meters & buffer amps?

[hollywood_steve]

Can anyone recommend a good resource on the need to provide buffer amps (or "make-up gain") when connecting a VU meter somewhere along an audio signal path? To the ignorant among us (me), it all seems so arbitrary; in some cases a VU meter appears to be just hung off existing circuitry. In other cases, a single VU meter is fed by a huge "buffer amp module" that serves no other purpose. 'splain, please.

steve
sjp@pacbell.net

 

[DanKennedy]

Steve,

A VU meter is a fairly nasty non-linear load
on a line. The diodes and capacitor in the meter
don't conduct for the entire portion of the audio
signal, generating a discontinuity at the transistion point, much like a bad class B output
stage.

The circuit that matters here is the device driving the line, a good low impedance source will
not be affected much, but a lot of gear isn't built with high horsepower outputs and will suffer
some audible distortion with a meter hung across it.

That's what the buffer is for, it provides (or should) a differential (balanced), bridging (high
impedance) load to the line, then a calibrated
gain stage that provides the power to drive the
meter and the distortion causing parts, completely
isolating the line.

I always use buffers when driving meters.

 

[hollywood_steve]

Thanks Dan. Your reply confirms what I was able to discern from reading a few semi-related articles. But it does leave me with one obvious question (guess I need to do more reading...) Now that the meter is actually reading the output of the buffer amp, not the original signal, how do I insure that the buffer amp output reflects the original signal level? Any books that cover this subject?

Thanks again.

Steve
sjp@pacbell.net

 

[DanKennedy]

The most universal buffer is the instrumentation
amplifier. This is easily made with a dual opamp.

You pretty much build the circuit up with good components and calibrate against a reference signal and then trust it. Same as the meters in your tape machines. Some 1% resistors, a TL-072, doesn't really take much. Especially if you can tap +/- supply voltages out of a console. A power supply would be the most complicated part of the thing.

I've built meters for folks that were wired to the patch bay as a set of mult jacks, that way they could hang it across any portion of the signal chain.

 

[matthewbarnhart]

Please forgive me if this is stupid question, but I have a late-80's Neotek Elan with LED meters. Obviously, I'd have to take a look at the schematic, but does anyone have any idea just how difficult it might be to replace them with VU meters, or if it would make any sense to do so? Can anyone reccommend a good meter in general?

Thanks for any and all help.

mb