How to convert VU-meter to VU + PPM-meters ?

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soundcave

Member
Joined
Sep 13, 2005
Messages
22
Location
Hamburg/Germany
After my first DIY steps with building a gyraf gssl (Thanks to Gyraf!!!), I recapped a AMEK M2000 console and changed all pots.
All works great. :grin:

But now I looked for something to convert the VU-meters of the console in ppm-meters. Because VU-meters are not really good for digital recording.
I searched this forum and found nothing. In the net I found the RPM-1 KIT from benchmark media.

Here the manual:

http://www.benchmarkmedia.com/manuals/rpm1manual.pdf

Has someone schematics for such a module?

Or somthing simular?

This Mudule from Benchmark costs 180,-$ and that will be over my budget cause I need 24.

(Sorry for my bad english!)

Dirk
 
> such a module?

Rod Elliot #55

> Sorry for my bad english!

What bad english? You write the language better than the boys in Florida, California, and New Jersey.
 
I have trouble thinking about PPM on a VU scale. Most PPMs also include a log-amp to make all "dB" the same size, instead of the linear scale used on VU where the top dB are much bigger than the bottom dB.

However, if you use it mainly to indicate overload, not to indicate program level (loudness), I suppose the top dB are the most important.

Rod's circuit is classic, but maybe too simple. The decay time is set by transistor effects, and you will probably notice a difference among 24 of them.

Here is a similar idea but decay time set only by passive parts. Also simpler to switch from Peak mode to buffered VU mode.

PPM-VU-PRR.gif


V1 and U4 are a simulated audio source feeding the "In" terminal at R1.

Opamp U3 should be FET-input, and it is convenient to make all three opamps TL072 or TL074 type. U1 is a buffer, U2 is an inverter.

D1 D2 catch the positive peaks and dump them on C1. Germanium diodes will be a little more accurate at low levels, but that does not matter much for an overload indicator, so use 1N4148, 1N914, 1N1004, or whatever diode is in your pile. >30mA, >30V.

R5 limits the rise time so that super-short peaks (which won't cause audible clipping) are ignored. R6 bleeds C1 slowly, so you have time to notice the peak but the meter does not stay at the peak level forever. (If you want "infinite hold", try disconnecting R6, but the stray leakage will not let C1 "hold for infinite time".)

U3 buffers the small charge on C1 into the heavy load of the mechanical meter and its calibration resistors. Feed a steady tone so that your recorder just blinks OVERLOAD, then adjust pot R8 for the desired needle indication (slightly in the red). Do this at 1KHz; the TL072 chips may not handle 11V peaks at 20KHz, but no listenable music has full power at 20KHz without also high level at middle frequencies.

Opamps must be fed at least +/-15VDC. +/-18VDC is the limit for most chips.

I show the attack and decay curves. Attack is within 1dB at about 3 milliseconds, similar to many PPM standards but a little fast since this is for overload. The decay curve shows a signal that stops at 100mS, and is down 6dB at 800mS later, so the "-20dB" point (often cited in PPM standards) is about 2.5 seconds. This is longer than many PPM standards, but we want to see overload, not integrate program level. Anyway without a log-amp, the meter will drop faster than in a customary PPM, the longer time constant will look more like what PPM users expect.

My schematic shows a second resistor/pot network and meter working as Buffered VU. Obviously you can use the same physical meter by switching one wire on the meter (but my circuit program does not have an easy SPDT switch symbol). You do need two resistor/pot networks because the calibration for VU won't be the same as the calibration for Peak Overload. Set up the VU mode in the usual way: feed 1.23VAC at about 1KHz and adjust pot R12 for zero VU on the meter.

Switching 24 meters, together, from Peak to VU is trickier. This and other multi-gang features are a part of why the commercial part is $180. You could switch the meter with a CMOS switch like CD4016/CD4066 powered with +15VDC and ground. Use two of the four switches in a 4016 for Peak or VU, use a third switch with a resistor as a low-performance logic inverter to get the Peak-OR-VU action, then feed a +15V/0V signal to all 24 units to switch them together.
 

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