Drive a VU meter negative from "0dB" to show compression with VCA compressor

Question

I have a simple VCA compressor based around the THAT 2180 VCA. http://www.thatcorp.com/datashts/dn115.pdf

I would like to use a standard "VU meter" to indicate compression, but work in "reverse." Meaning, the VU meter rests at "0" then moves to the left/negative to show dB of compression. (If you are familiar, this is how compressors like the 1176 indicate compression.)

Somehow bias the meter to "0dB" Then subtract the Sidechain signal from the Meter bias?

The answers I have gotten provide good info. Any circuit ideas or schematics?

Answer 1

These THAT chip's have a log detector, from which is derived a DC voltage which controls the VCA. (Everything is matched to about 7mV/dB from memory.) That voltage is then run through some circuitry to set threshold and ratio, you end up with a control voltage which is applied to the VCA to give gain reduction. Looking at your circuit this looks like the voltage on R16.

If you want to indicate gain reduction on the meter, you need to drive the VU meter such that the meter is at 0 (which needs a standing current through the meter) when this voltage is zero, and then REDUCE that current as gain reduction is applied to the VCA. This is basically an exercise in applying scale, DC offset, and possibly inversion to get the results you want. It can all be done with standard inverting and non-inverting opamp circuits, perhaps with some preset adjustments.

The easiest approach would be to build this circuit and then measure the control voltage with various settings to determine the polarity and mV/dB of control voltage, and then figure out how to scale that for you chosen meter. A bit of fiddling with a calculator and maybe a little trial and error should get you there.

(NB you DON'T want to use the actual control voltage of the VCA, because that ALSO contains a component of "makeup gain" (from VR8) which is NOT part of "gain reduction" as metered on a classical compressor. You only want the "gain reduction signal, after applying threshold/ratio, which is on the common contact of SW3A.)

Answer 2

It depends on what your 'standard VU meter' is. If it actually is something pre-calibrated (with decay curves and stuff) Tony Stewart answer is very interesting. A barebone 'needle' VU meter is actually a microamp meter with a different scale painted on so you just need to cook an opamp circuit giving the required deviation.

Be careful that the timing of an 'active signal meter' are not the same as those of a compressor (in fact usually the timings of a compressor are user selectable) so you may want to look for a way to defeat these.

Answer 3

The VCA has a dynamic range of >115 dB with intercepts of ;
0 dB gain @ 0V
-80 dB gain @ 500 mV
Thus the gain is 6.25 mV/dB and offset = 0 plus the calibration setting for max amplitude and resulting gain must be use to null the meter to 0dB compression.

For an VU rated for X volts full scale= 0 dB and a minimum range on the scale of Y dB so it requires an Op Amp to convert the Vc control signal to the meter.

It is unlikely to use all the 115 dB of compression which is about equal to the human experience from silence to pain threshold. Yet some portion of this range will be used with the maximum input signal sustained .

• The minimum gain Vc(min) with a calibrated output level need Vout = X volts for 0dB.

• the max gain Vc(max) is used for the compression chosen is amplified with a negative gain and an offset of X volts.

• however the meter attack and decay response may be slower than the compander. This requires some specifications such as attack and decay on holding the meter for visual recognition of compression.

But the transfer function depends on the -dB range of compression to amplify the gain by Y

Vout = X- 6.25 mV/dB * Y/X * Vc

You might only want to display say 20dB of compression on a 60 dB VU display so the scale might have to be changed as per practical specs.