Sifam VU Meters in a rack

[wackowill]

Hi all - not sure if this is the right place to ask this question.

I'm currently racking up some gear and want to include a tasty VU meter to finish the job off, switchable betweeen the two channels and out with an on-off-on switch.

My first question is regarding Sifam VU meters -

Farnells seem to stock two ranges of Presenter VU meters. One is called "Standard" and cost around £30 while the other "High Performance" is £70. What's the difference? Can I use the cheaper ones with good results?

Secondly, will my switching mechanism actually work? I'll be using a dual pole switch and tying the meter via the switch directly to the output of the modules (Siemens V276).

Thirdly, should I use some kind of buffer amplifier or will the modules drive the meter easily enough?

Well, that's all (!). Thanks in advance.

[Tim Farrant]

Certainly the best place to ask!

The cheaper meters will not conform 100% to the VU standard, namely this is the time it takes for the needle to rise to 0VU and fall back. The AL29 or AL39 meters are pretty close to the standard and are commonly found on a lot of gear and consoles.

You can drive the meter directly, if you use a 5k ohm trimpot between the output and the meter (instead of the 3.6k resistor they recommend), you can calibrate it for 0VU = +4dBu. The switch arrangement sounds fine. If the output is balanced, hook the HOT and COLD to the meter, with the 5k pot in one of the legs.

The rectifier diodes inside the meter generate distortion, and this may appear on the module output. In some cases I have measured 0.1% extra distortion added to the line with a VU connected directly. Using a buffer amp of some sort will eliminate this problem.

Hope this helps
Tim.

[wackowill]

Thanks Tim. How very helpful people here are!

Funnily enough, putting this question on the forum spurred me on to actually get some answers myself, so I trotted off to the Sifam Website where I discovered the two ranges (sold at RS): the cheaper "Presenter" range which include the AL29 and AL39 don't conform to the VU standard while only the R32AF in the "Clarity Focus" range, priced around £60 upwards conforms to the VU standard.

Apparently, there's a book called "Handbook for Sound Engineers - The New Audio Cyclopaedia" which includes various circuits for VU meter buffer amplifiers and the such. They have it at my local library so I'm going to borrow it for a few weeks.

But Tim, here's another question. Can you explain to me exactly what the buffer amplifier does?

Thanks, Will

[brianroth]

I'm not Tim, but... <g>

Depending upon the circuit that drives the VU meter, the rectifier inside the meter can cause odd loading of the audio circuit and introduce distortion. Hence the need for a buffer amp to eliminate that chance.

Here's a circuit I culled from previous projects that will nicely iso the audio path from the meter.

http://www.brianroth.com/library/vu-buffer.pdf

It "looks like" a balanced center-tapped 20K line input transformer to the driving audio circuit, and uses cheap parts.

As for the meter "ballistics", there's an old ANSI spec that describes how much time the meter can rise and fall to a test-burst signal, how much needle overshoot is allowed, etc.

I think Jay McKnight has the test procedures at the MRL website. If not, I have a copy of Jay's test methods here somewhere.

Bri

[wackowill]

Thanks for the diagram Bri - it's great! I'll get some stripboard and try it out.

Will :O)

[sonichermit]

I have been searching for the correct type of bulb as replacement for a Componex AL29 VU meter as used on my master meters for trident series 65. Anyone here know what sort of bulb is required. I know it is a festoon , 6V 1 watt. Or if I can use leds that would be fine too. Thanks

[Songcatcher]

here's a kit for some more thoughts

VU Buffer kit page

D.