VU meter driver assembly

Hey everyone... a friend suggested I pop over here and inquire, i'm getting into the DIY scene and need some resources.

I'd like to build a pair of VU meters into a rack panel as an extension of my passive monitor... most likely Sifam analog VUs, not sure of the model yet. I've been told I need an opamp buffer/driver in order to feed the meters the proper impedance of 3600 ohms and also so i can split off my monitor path without affecting it in a negative way. Are there any good schematics out there for this? I found the Elliot Sound VU meter schematic, but am not sure of it's reliablility. No need for ppm, VU/rms will be adequate for my needs.

Thanks in advance everyone!

You can put a 3600 ohm resistor in series with the meter, hang it right across the buss, and it will work just fine (0VU = +4dBM). That's how they did it in the old days. But the nonlinearity of the rectifying diodes in the meter will introduce a little distortion into your signal. Nowadays, this is generally considered unacceptable in professional applications, so we buffer the meter with an amplifier having a high input impedance, flat frequency response and precise gain. Any amplifier circuit meeting those criteria will make a fine VU buffer. There are many "nice to haves" with regard to this application, but that's the short story :wink:

C'mon guys,
Someone had a meter driver circuit on their site. Keef, Pete ???

I saw this over at Gear-S-l-aaarts but just could find an appropriate link. :sad:

Why bother to dig around? My point was this: if we're talking about a real VU meter, any good buffer amplifier will do.

Hey, JP! Welcome to my main hangout on the web. :thumb:

If these folks can't help you, nobody can. :guinness:

[quote author="NewYorkDave"]Why bother to dig around? [/quote]

only because I think there is a PCB or a veroboard layout to go with it.
.... so now new work required and he can be up and running fast.

but , yes any good buffer amplifier will do.

you can use a tl072 wired as a buffer amp. It has a very high impedance input and won´t load your source.

Yes, that'll do just fine: TL072 as a unity gain buffer, then a 3600 ohm resistor, then the VU meter. You can get fancier if you like, with balanced input, variable gain or attenuation for ranges outside of 0VU=+4dBM and so on...

If I'm not too busy with my homemade knishes later this evening, I'll draw up a circuit.

Hey Consul! Nice to see a familiar, um, face!

I have a feeling i'll be here a lot... after completing my passive monitor, i'm now in the process of building some Neve 183-based line amp stages to add some iron to my digs, and will also be diving into the Gyraf Pultec very soon. Still plan to do Brad's passive stereo EQ as well, and some Seventh Circle A12s... hoping to improve my studio and ultimately the quality of my work by the end of the year entirely with DIY gear!

[quote author="NewYorkDave"]Yes, that'll do just fine: TL072 as a unity gain buffer, then a 3600 ohm resistor, then the VU meter. You can get fancier if you like, with balanced input, variable gain or attenuation for ranges outside of 0VU=+4dBM and so on...

If I'm not too busy with my homemade knishes later this evening, I'll draw up a circuit.[/quote]

my monitor is unbalanced, so no need for anything fancy in that area. This is primarily for checking L/R balances, and of course eye candy for clients. I'm more worried about imposing a load on my signal. It's a 6-way Grayhill rotary feeding a 10k TKD stereo pot. Various stereo input sources such as Masterlink, CD player, console outputs etc. I'll be multing the signal post-selector switch, pre-volume pot.

Thanks guys

I made 6 sets of quad buffer cards for a 24-channel meter bridge. They had balanced to unbalanced buffers, then 0VU trims on the secondary stages. The range of the trim was determined by the choice of trimpot and range padding resistor values.

Simple enough... but it would take time to find the layouts...

Keith

This might be a dumb question, but is there any way to use one opamp buffer for both left and right signals?

i thought i had a copy of the opamp cookbook around here somewhere, still digging though...

>but is there any way to use one opamp buffer for both left and right signals?

You can use a dual-opamp IC chip... and wire each opamp for each channel.

> This might be a dumb question, but is there any way to use one opamp buffer for both left and right signals?

Better to ask the so-called "dumb" question than to sit in ignorance.

The answer is "no". Oh, in special cases there are ways to put two signals through one amplifier. But a single op-amp costs 19 cents, and a dual op-amp costs 19 cents, while any kind of multiplexing costs dollars. Worrying about minimizing op-amps went out of style in 1970. Your real cost is power supply (at least a $2 9V battery; wall-wart is more convenient in studio) and box and knob and connectors and tail-fins and etc.

[quote author="PRR"]> This might be a dumb question, but is there any way to use one opamp buffer for both left and right signals?

Better to ask the so-called "dumb" question than to sit in ignorance.

The answer is "no". Oh, in special cases there are ways to put two signals through one amplifier. But a single op-amp costs 19 cents, and a dual op-amp costs 19 cents, while any kind of multiplexing costs dollars. Worrying about minimizing op-amps went out of style in 1970. Your real cost is power supply (at least a $2 9V battery; wall-wart is more convenient in studio) and box and knob and connectors and tail-fins and etc.[/quote]

Gotcha... i'm more concerned with the circuit, since that's the part I know next to nothing about. Mounting the meters on a rack panel and attaching a power supply and connectors is cake.

isn't a TL072 a dual chip?

This is what I made to nail a meter on an unbalanced signal....anit worked a treat.

peter

Thanks peter!

just to clarify, VR4 is the trimpot, and U1 could be half of a TL072?

The only problem with that circuit is that the gain will always be more than 1, and that's too much gain if you're monitoring a +4dBM nominal signal. However, the fix is easy: make R3 variable, and R4 fixed. Then you can vary the gain from 1 up to 1+(R3/R4).

If you need loss, you'll either need to add attenuation at the input, or use an inverting stage instead.

Dave,

Is it the fact that there's a resistor in the feedback loop that tells you the circuit will have a gain of more than 1? To configure the op amp for unity gain there needs to be a feedback loop but with no added resistance?

Thanks.

http://pg.photos.yahoo.com/ph/takei...1&.dnm=6a1d.jpg&.src=ph&.tok=phpF9tBBxa6y_IUN

here are some trident meter amps I just got.
Haven't had a chance to draw up the circuit yet.
Not sure if I can get to it for a couple of days though.

curious what kind of power supply this needs
as well as whether it runs balanced or unbalanced ( I think unbalanced but I'll have to check again with the friend who gave them to me.
there are
2-LM741 op amps
2-mc1741s op amps
2 RCA 40361 transistors (metal cans)
4 bc413 trans.
caps are
10uf 25v tantalum (red gumdrop thing)
22uf 25v tantalum (green gumdrop thing)
and a 470uf 25v electrolytic

sleeper