Puzzled by VU meter

[rankot]

I have build Gyraf G9 mic preamp, and wanted to add some more bells and whistles, so adding VU meters was a logical step. Bought two round meters and a driver board. Now what puzzles me is how to make sure they show correct value? Tried to connect them like this, with 10k resistors in series, to + wires on XLR outputs. Is it OK?

 

[C.B. - Boudio]

As Ian (Ruffrecords) would say, a VU meter is a passive mA-meter with certain ballistics, connected to a 600 ohm source via a load resistor and a full-bridge selenium rectifier ;-) (now I hope I didn't get anything wrong here!)

But in your case, I'd compare it to a known meter, and adjust from there. How much db of amplification do you get in the highes gain setting? Then use a signal generator putting out a, say, 1k tone of - x db, and adjust the VU to zero.
Atleast that's how I would do it, in my rather 'punk' way.

 

[ccaudle]

Tried to connect them like this, with 10k resistors in series, to + wires on XLR outputs

If the signal is symmetric and active outputs connecting that way will show you -6dB relative to the actual signal. If the signal is transformer coupled you have not connected one side of the transformer so you would see no movement on the meter.
Balanced connections are measured between pins 2 and 3, that is the only defined voltage to give an accurate measurement of the signal level.

Do you have a link to the driver board schematic? If the driver board is active you may be required to connect in a specific way for proper operation.

how to make sure they show correct value?

Generate a sine wave signal using whatever means you have available (signal generator, sound card, in a pinch a synthesizer might do) and adjust the level until the voltmeter reads 0.775V between pins 2 and 3. That is 0 dBu. Keep in mind your voltmeter may have limited frequency range over which it is accurate, so use primarily low frequency signals (any meter should be accurate with 50Hz or 60Hz since that is a primary application for volt meters). If you have a higher quality meter then it should have a user guide indicating accuracy at other frequencies.

 

[rankot]

If the signal is symmetric and active outputs connecting that way will show you -6dB relative to the actual signal. If the signal is transformer coupled you have not connected one side of the transformer so you would see no movement on the meter.
Balanced connections are measured between pins 2 and 3, that is the only defined voltage to give an accurate measurement of the signal level.

Do you have a link to the driver board schematic? If the driver board is active you may be required to connect in a specific way for proper operation.



Generate a sine wave signal using whatever means you have available (signal generator, sound card, in a pinch a synthesizer might do) and adjust the level until the voltmeter reads 0.775V between pins 2 and 3. That is 0 dBu. Keep in mind your voltmeter may have limited frequency range over which it is accurate, so use primarily low frequency signals (any meter should be accurate with 50Hz or 60Hz since that is a primary application for volt meters). If you have a higher quality meter then it should have a user guide indicating accuracy at other frequencies.

Thanks a lot! I don't have a driver board schematic, but it is based on TA7318P. It seems to be designed for power amp output metering, not for this purpose. But I will try to adjust it.

Like this one: https://www.amazon.com.be/-/en/TA7318P-Power-Amplifier-Driver-Lndicator/dp/B08576JF4N

 

[ccaudle]

I don't have a driver board schematic, but it is based on TA7318P.

That is a single ended input driver circuit of some kind. The datasheet I found says it is for the "1/4 root compression circuit," but I don't know how to make sense of that description.
Where are you trying to connect this? To the output connectors of the mic preamp?
The Gyraf G9 DIY page says it is transformer balanced output, so the connection you drew will definitely not work.

My instinct would be to use an instrumentation amp to connect to the output and derive the single ended drive signal, so you don't disturb the output signal by connecting this meter driver.
Then you have separated the connection issue from the calibration issue, and can easily switch out to a different meter circuit entirely if you want.

 

[rankot]

Yes, to output of mic preamp. It works, but I suppose measurements are not correct.

 

[ccaudle]

It works, but I suppose measurements are not correct.

I'm curious how it works at all if you only connected one leg of the output transformer to the input of the circuit.

 

[rankot]

AFAIK, transformer is referenced to ground anyway, so it doesn't matter if only one leg is connected. Another leg is just the opposite phase.

 

[ruffrecords]

That is a single ended input driver circuit of some kind. The datasheet I found says it is for the "1/4 root compression circuit," but I don't know how to make sense of that description.

I looked at the datasheet too. It is a strange chip designed to drive a 1mA dc meter movement. Rectification, ballistics and this strange quarter root thing are all internal to the chip

Where are you trying to connect this? To the output connectors of the mic preamp?

I would have thought the same - probably regular line level so +4VU would be about 1.228V rms or 1.735V full scale. The datasheet says the input resistor should be such that the input current is 1mA at full scale so its value should therefore be 1.735K or threreabouts.

The Gyraf G9 DIY page says it is transformer balanced output, so the connection you drew will definitely not work.

I think it should work. The transformer signal exists between pins 2 and 3. He has pin 3 grounded and pin 2 connected to the meter input

My instinct would be to use an instrumentation amp to connect to the output and derive the single ended drive signal, so you don't disturb the output signal by connecting this meter driver.
Then you have separated the connection issue from the calibration issue, and can easily switch out to a different meter circuit entirely if you want.

My feeling is this is the wrong board for the job and unfortunately connecting pin 3 to ground unbalances the output. JLM makes an excellent VU meter buffer board with balanced inputs:

http://www.jlmaudio.com/forum/viewtopic.php?f=2&t=239

Cheers

Ian

 

[ccaudle]

He has pin 3 grounded and pin 2 connected to the meter input

To me, the picture looked like pin 3 was not connected to anything on the metering circuit.

AFAIK, transformer is referenced to ground anyway

Can you point out in the G9 schematic where that is the case? In the schematic I found here:
https://www.gyraf.dk/gy_pd/g9/g9_sch.gif
the secondary connects only to pins 2 and 3 (as it should).

 

[ruffrecords]

To me, the picture looked like pin 3 was not connected to anything on the metering circuit.

View attachment 142963



Can you point out in the G9 schematic where that is the case? In the schematic I found here:
https://www.gyraf.dk/gy_pd/g9/g9_sch.gif
the secondary connects only to pins 2 and 3 (as it should).

You are absolutely right. I completely misread his schematic. Doh! Old age? poor eyesight? Onset of dementia??? Take your pick

Cheers

ian

 

[ccaudle]

Take your pick

It's only been a week since New Year, takes longer to recover as you get older.

 

[ruffrecords]

It's only been a week since New Year, takes longer to recover as you get older.

Not to mention the stinking cold I had before Christmas, the subsequent chest infection and the swollen foot I have had for the last four months (not bragging you understand)

Cheers

ian