VU Meter ADD distortion!

rlaury · Oct 11, 2004

I just bought a pair of vintage style Triplett
VU meters for a project. When I connect them
across the balanced output, the distortion rises from .013
to .4!
I've always heard the rectifier type VU meters caused distortion
but I never dreamed it would be this bad. Any ideas?
Or is it buffer time.

RonL

gyraf · Oct 11, 2004

That depends entirely on the output impedance of the source. If source impedance is low enough, the VU-meter circuit will do little or no difference. But if output impedance is high, you may be in trouble..

Jakob E.

clintrubber · Oct 11, 2004

If I remember it corrcetly (don't have a schematic here), the Trident comp/lim had an arrangement that the output was sort of clamped by the diodes+meter arrangement.
There is a series resistor in the metering-circuit of that unit, but when at minimum or low values of this trim-pot the clamping can become noticable.

So even with an opamp capable of the usual/enough mA's for driving the output, clipping can result.

Or is it buffer time.

Probably. Would sure solve it.

NewYorkDave · Oct 11, 2004

Ron, did you add the necessary 3.6K series resistors to the VU meters?

rlaury · Oct 11, 2004

I DO have the 3.6K resistor in series with the meter.
I also have a 500 ohm balanced T attenuator in the output.
The meter is connected after this attenuator.
This actually raises the output impedence.
This may be the problem. I have to monitor the actual
output level so it looks like I'll be building a buffer amp.

:-(

RonL

NewYorkDave · Oct 11, 2004

Ah, that probably is your problem. Check out this excerpt from the VU meter spec:

The harmonic distortion introduced in a 600-ohm circuit, caused by bridging the volume indicator across it, is less than 0.3 percent under the worst possible condition (no loss in the variable attenuator).

"Variable attenuator" in the above statement refers to a switchable VU meter range extender, if used.

Since the output impedance of your circuit has been built-out by your T-attenuator to a value close to 600 ohms, I guess you could say your meter is meeting the spec :wink: 0.3% of added distortion was considered pretty minimal back in 1939, when the VU standard was adopted. Of course, the distortion added by the meter would be less if the source Z were less than 600 ohms; but in your case, if that added 0.3% is not acceptable, I guess you're looking at an active buffer.

If you really want to avoid the active buffer, there are two other solutions you might find workable:
1. Switch the meter out-of-circuit after lining up your operating levels.
2. Add a range-extending attenuator to the meter, or simply increase the 3600-ohm series resistor to a higher value. 0VU will no longer equal +4dBM, but this will buffer the line from the meter somewhat and reduce distortion. You'll just have to remember that your "zero" level falls at a particular point below zero on the meter. You could make this range extender switchable so that you could have it switched out while setting your levels, then switch it in before you begin recording.

A T- or bridged-T attenuator designed for 3900 ohms is a suitable range extender for a VU meter, when used in addition to the 3600-ohm series resistor.