VU-meter multiplier

[gurkan75]

I have a pair of Daven TA1000-1 vu-meter multipliers I'd like to use. They consist of resistor networks B and C in the picture below, A is a meter zero calibration rheostat. I hooked each one up like this:

Ignoring the 600hm line conditions (hooking the meter + attenuator up across hot and cold of converter output, tone at -20dBFs, converters set to -20=0dBu), the scaling of the attenuator is not good. There is also some loss introduced meaning I can no longer calibrate the 0.775V output of my DA-converter to a 0VU reading. I am assuming this has to do with the mismatch of power vs voltage measurement standards, dBm vs dBu.

If I created an isolated 600ohm line, would that work? I'd like to try. This is what I came up with:



I am hoping this would put the meter + attenuator between a 600ohm source and load, with a voltage equal to the input voltage. Considering unbalancing the input with a transformer.

The value of the feedback resistors is just a guess, apart from being equal. Maybe they should be higher to not disturb the voltage across 600+600ohms?

Any comments appreciated!

Edit: came up with this to see if 600ohm line helps:


Connect to a dedicated output calibrated to produce 0.775V across the 600ohm. Would still prefer something closer to the first suggestion for the meters to be a bit more universal in the analog world, but this should at least tell me if the multipliers work as expected.

 

[RuudNL]

What is the point amplifying the signal 6 dB and then attenuate it by 6 dB?
I would use a unity gain buffer and connect the meter through a 600 ohm resistor to the output of the OpAmp. (1/2 5532 schematic.)

 

[gurkan75]

Thanks for your suggestion. Thats what I ended up with if the meter should bridge a ”line” with 600 ohm load and source. The 600 ohm shunting r would be what the 1mW would develop across, the series r setting source impedance. I might be overthinking?

 

[Brian Roth]

Is that a "real" "true" such as a Simpson, Weston, Dixon, Modutec, etc? Or merely a chinese meter with a VU scale, that wiggles in response to audio? A true VU meter requires a 3600 (or 3900) Ohm series resistor for proper ballistics.

Bri

 

[gurkan75]

Hi, yes they are westons. The attenuator/multiplier has a 3900ohm constant output impedance. (7100 + 400ohms input impedance)

 

[RuudNL]

Thanks for your suggestion. Thats what I ended up with if the meter should bridge a ”line” with 600 ohm load and source. The 600 ohm shunting r would be what the 1mW would develop across, the series r setting source impedance. I might be overthinking?

In the circuit diagram you draw there is no terminating resistor (at the input) at all.
The question is: how important the 600 ohm termination is. 600 ohm is IMHO something of the past.
A lot of current equipment can't even drive a 600 ohm load...

 

[gurkan75]

In the circuit diagram you draw there is no terminating resistor (at the input) at all.
The question is: how important the 600 ohm termination is. 600 ohm is IMHO something of the past.
A lot of current equipment can't even drive a 600

Yes the whole thing is a thing of the past. I’ll try all three possible scenarios and see if it starts lining up.

 

[abbey road d enfer]

Ignoring the 600hm line conditions (hooking the meter + attenuator up across hot and cold of converter output, tone at -20dBFs, converters set to -20=0dBu), the scaling of the attenuator is not good.

VU-meters have the right scale only when fed with +4dBu via a 3600 ohms resistor. Below that, the rectifiers non-linearities and thresholdmake the lower part of the scale useless.

 

[gurkan75]

VU-meters have the right scale only when fed with +4dBu via a 3600 ohms resistor. Below that, the rectifiers non-linearities and thresholdmake the lower part of the scale useless.

Ok thank you! I’ll try to calibrate for +4db se if i get closer just doing so. There is a 1mW +4, +6, , +8 and so on until ”off” (steps on the switch) and I started at the top, trying to calibrate the 1mW position. +4 is nearly 1dB off then it gets worse as it is right now. To me it is 0dBu then maybe another 12dB that is of interest so +4 could be a starting point.