Voltage levels and VU meters...

[Haible Juergen]

	>I'm designing an external input pre-amp cum signal level meter
panel for my
	>synth. I use Disgisound specs so it's 10V P-P audio and +/-15V
supply. The
	>pre-amps will all be un-balanced single-sided things based on 1/4
of a TL074
	>probably, unless someone is willing to make a convincing case for
using
	>something better/more expensive....

Build a little compressor. 10Vpp output means that everything above +6dB
will
lead to nasty clipping (the kind of opamp outputs getting stuck on the rails
...),
so you better take precautions. I recommend a compressor, but a fast clip
LED
is a must, IMO.

I have a very simple circuit for that on my homepage (built around the
SSM2017
and a vactrol), and it does a good job at the input of my HiFli clone.
You even get a balanced input for free.

	>If it needs DC then what do I do, if I rectify it with signal
diodes, how do
	>I go about smoothing it when the signal might be anywhere between
20Hz and
	>20kHz?...

Use an opamp in inverting amplifier configuartion, and use a bridge
rectifier
instead of the feedback resistor. The meter is connected across the bridge
then.
A capacitor across the meter will do the smoothing. Provide a small resistor
at the opamp output (before it goes into the diode) to keep the opamp
stable.

JH.