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Howdy Folks,
The original VU & PPM Project 55 from Rod Elliott uses a single IC to make up the rectifier driver for a 50uA meter. And here's the schematic.


Dennis

Now, I've laid out a circuit using a quad LF147/347 (had them in the bit box) to drive two meters as a power amp needs.
Prepping to make up this board this evening to make sure it works OK.
Any interested gurus inclined to take a look see if all is well.
Sorry, didn't bother with a new schematic.

Dennis

The diodes, 1N34 and 1N4148 on the right side of the board are backwards.
Otherwise everything works, no smoke. Now need only to calibrate the meters on an amp.

Dennis

Seems there are not many true VU meter scale devices to be had. Followed the build recommendations and bought a pair of 50uA meters from Jameco. And have made a VU scale to replace the original scale.
The circuit seems to work just fine to drive the meters. However the meters jump up from zero on the scale as soon as power is applied and cannot be zeroed out with the meter mechanical adjustment. And I'm still not quite clear on how to calibrate it. Got a nominal 250 watt amp to test this on. Maybe set up a sound generator or pink noise and measure some known output voltage that corresponds to 0 VU?

These statements from the project notes should make sense to someone, but not to me.

"The speaker input has a maximum sensitivity of 5V, and can be adjusted to allow for any higher voltage to suit nearly all power amps."
"The output voltage from the rectifier for full scale is designed to be about 5V, and the 4k7 and 220 ohm resistors around the meter provide the attenuation needed and give excellent electrical damping."
"you may need to adjust the 4.7k and 220 Ohm resistors so that full scale is achieved in VU mode with an input of 5V into the speaker input. The calibration control must be at maximum resistance for this test."
So now, is full scale 0 VU at 5 volts ac or +3 VU at 5 volts ac?
And is it 5 V from the speaker input or the rectifier output?
Does this calibration set up for the PPM mode also?

Sorry to be so dense. Anyone understand how to calibrate this?

Regards,
Dennis

Have you looked at the Sifam meters.

Back when I worked in broadcasting I used a lot of thier AL-XX products.

They are a bit expesive.

Very pretty meters, company located in the UK, no price listing on the page, no US seller. Kinda difficult to purchase, eh?
But many thanks for the lovely pictures. Any chance you know how to calibrate one to a power amp?

Prosit,
Dennis

OK, after makeing an international call to the UK, look here.

http://www.meterdistributor.com/SIFAM/S ... ntents.htm

INSTRUMENT METER SPECIALTIES
Ike Moore
1756 Victory Blvd.Glendale, CA. 91201
(818) 243-0416 (800) 926-3837 FAX (818) 243-3736
The only US distributor of Sifam meters.

Has the "old" VINTAGE VU METERS
Types 20A SQ & 25A SQ round or square face meters.
$69.63 each.

See also,
http://www.sifam.com/meters_home.lasso

Still hopeing someone knows how to calibrate Project 55 for a power amp.

Prosit,
Dennis

The Volume Unit is an old telephone company invention. Its origin relates to the need to standardize the proper level for good modulation across long lines and into amplifiers without overdriving them.

There is an old definition of what constitutes 0 Vu. It?s a short paragraph that talks about so many volts being dissipated across a 600 ohm load something, something?
http://en.wikipedia.org/wiki/VU_Meter
(true RMS reading a sign wave)

If any one really needs this language I can look it up in the Audio Cyclopedia that is don?t currently have with me.

I got this idea somewhere on the ESP website. It seems reasonable. Using some kind of signal generator, find the peak output (before significant distortion) of your amplifier. Adjust the pot on the ESP ckt so that this causes the meter to defect to the +3dB position on the meter display.

When input signal is backed down to read 0 Vu, this should tell us that the amplifier is now operating at ? its useful output.

When a modulated signal is introduced to the amplifier, the meter should give you a pleasant display when the amplifier is being operated near full power. As the needle moves to 3 dB and above, you have a nice visual indication that the amplifier is likely being driving into distortion.

Howdy,
Given "that full scale is achieved in VU mode with an input of 5V into the speaker input", is there any particular reason one might not use a variac to produce 5V that is fed into the speaker input? Then the particular amplifier power is a moot point.
The round face Sifam meters are on the way to me now.

Prosit,
Dennis

yah,

5 volt across an 8 ohm load is only gonna yield a little more than 3 watts.

I expect you have a bigger amp than that.

The variac would work for the meters, but I think its gonna look like a big old inductive load across the amplifier. Not too ideal for sound quality.

VR-1 in the ckt should allow you to adjust the deflection for higher output amplifiers.

Likewise you could skip the speaker level input and use the line input. That would give you a different set of calibration issues.

OK, suppose I use the clipping indicator from Prof. Leach and drive the amp just into clipping and adjust VR-1 for full scale 0 VU?

Prosit,
Dennis

I built the leach clipping indicator and hooked it up.
This amp has +/- 80 dc V rails with 20,000uF caps on each rail driven by a transformer of 15 A. As a load I connected a big ole 250 W 10 Ohm resistor to the speaker binding posts. This was fed by a pink noise signal from a spectrum analyzer and cranked up until the resistor got hot and began to "sing". No led indication of clipping. The rail voltage drops about 6 volts.
So another clipping indicator was chosen and got the same result. Kinda hard to believe this amp does not go into clipping.
Is a pink noise signal not a good testing source?
Dennis