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How would you drive a meter?

post #1 of 3
Thread Starter 
I have a pair of Bach-Simpson larger VU meters that I want to use with a stereo mic preamp that I am building. I bought these meters from a guy in Canada who parts out large mixing boards as sells the parts. I was thinking of using the VU And PPM Audio Metering drive that is on Elliott Sound Products web site.
http://sound.westhost.com/project55.htm
I've read other places that meters aren't standardized. Each company has its own requirements. The meters all read the same at the needle but they vary on how they were designed to be driven.
I disassembled a meter and found the + terminal was connected to a Red resistor. (colors: Green, Brown, Brown, Gold) It's end is connected to a corner of a PC board. The - terminal is connected the opposite corner of the PC board. On this little board are 4 diodes soldered in, what looks to me as, a bridge rectifier. Two points of this bridge rectifier are connected to the input terminals, as I just explained. The other two points of the PC board are connected together through another red resistor. (colors: Brown, Green, Orange, Gold). Can you guys tell what I have by my description?
The meter drive I think resembles what Elliott calls a typical internal simple (cheap) circuit. His claim is there is not much control in this drive. What I am asking you guys is what do you think? I want these meter to work right. Should I remove the drive from the meter and build an Elliott drive or can I just add a pair of wires on the outputs of the microphone preamp, run the + wire through some kind of adjustable resistor, and connect them to the terminals of the meters? I'm assuming the meter was good enough for a pro mixer when it was originally built so it still should?
Thank you,
steve
post #2 of 3
You will need to build a circuit to provide the drive for the meters. It would not be a good idea to drive them from the preamp output directly. You can use the ESP circuit or look for other drive circuits that will work. I know nothing about calibrating them, so I can't help you there.
post #3 of 3
Steven,

From your description, it sounds like your meters are proper VU meters, with the appropriate rectification and ballistics already built in. It is true that such meters vary in quality. Bach-Simpson is a good name though, used in some Neve desks.

Such a meter does not need special drive circuitry. Standard practice is simply to connect it to the line with a 3600 ohm resistor in series. A continuous +4dBu (1.228V RMS - standard professional line level) signal will normally register 0VU on the meter.

Signal degradation due to having the meter connected is likely to be minimal, but it is probably a good idea to use a buffer stage to
eliminate the possibility all together. Also, a buffer with variable or non-unity gain will allow you to trim the meter reading or use it on lines that aren't running at the +4dBu level.

The circuit on Rod Elliott's page is designed to use a meter movement _without_ the built in rectification and ballistics, since these are provided by the circuit. Therefore, you will not be able to use your meters with that circuit. It may be possible to remove the components from inside the meters, but I would be reluctant to do that to what are probably a very good pair of VU meters. Use them as they are, or save them for another project.

Rod's circuit does have the advantage of offering PPM (peak) metering, so if you think that would be useful it might be worth building it and saving your VU meters for another time. The failure to register peak levels accurately is a definite shortcoming of VU meters. For example - a VU meter provides a fairly accurate representation of the level of a spoken voice (this is what it was designed for) but tends to under-read signals with a large transient content, e.g. a snare drum. You have to learn to consider the nature of the signal as you make the meter reading. With that in mind, I would argue that it doesn't really matter whether your meters are the simple/cheap type or not, provided that you can learn to interpret what they are telling you.

Of course, PPM meters are good at indicating peak levels, and do a poorer job at average levels. It's handy to have both types about.

Hope this is useful,

Steve.
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