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Old 5th Jan 2021, 6:24 pm   #1
Strange Brew
Tetrode
 
Join Date: Aug 2009
Location: Derbyshire, UK.
Posts: 50
Default Test tone generator for PPM calibration - information needed

Hi.

I need to calibrate one of my PPM units and I need a test tone generator to send out a 1khz tone at 0db -8db and +8db.

Could anyone help me with this? I can't seem to find a unit which allows this.

I need something cheap too.

The boards in the PPM are Alice PPM304.
Link here - Alice PPM information

On another note, please see pics attached of the unit.

I have a bit of confusion. Pins 2 & 3 of the 8 way connector of each driver board are soldered together. This would suggest an unbalanced configuration but the unit is fully balanced.

I'm not sure if this was done on purpose or from new as the manufacturer (Dyer Audio Systems) are no longer around.

Also, the main board where the PSU and the XLR connectors attach have two Sowter transformers in the signal path.

I wrote to Sowter and they came back to me with this:

"The transformers are type 6387 and were designed and built in 1986/87 and information is a little scarce but it would appear it was to replace a Neutrik NTL1 transformer."

I have attached the Sowter spec sheet.

Can anyone shed any light on the soldered connections of the boards and what the transformers do?

Many thanks
Jo
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Last edited by Strange Brew; 5th Jan 2021 at 6:52 pm.
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Old 9th Jan 2021, 12:01 pm   #2
John_BS
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Default Re: Test tone generator for PPM calibration - information needed

Do you have any test equipment to hand? Eg DVM, signal generator, scope?



John
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Old 9th Jan 2021, 12:54 pm   #3
mhennessy
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Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 4,241
Default Re: Test tone generator for PPM calibration - information needed

Firstly, there's no point worrying about the transformers and the configuration of the connections. Someone designed it that way; it obviously works fine.

(though it sounds like they're using the transformers to go from balanced to unbalanced into the PPM cards - nothing wrong with that, it's just a design decision. Obviously, they felt the transformers would do a better job of balanced->unbalanced conversion than the input stage of the card, and while that's no-doubt correct, electronic conversion is usually good enough for most line-level audio applications. As I say, it's not an issue)

Next, there's nothing magic about the signal you need to apply. You know the frequency; you know the amplitudes. And you know the procedure from the manufacturer. Simple. I've done a few of these units myself, and it's very straightforward.

Of course, I assumed that you're able to convert from dBu to RMS voltage. If you can do that, then you just need any old sine wave generator and an AC voltmeter. Because this is a sine wave, you don't even need a true RMS voltmeter. Just make sure your voltmeter is accurate at 1kHz, as some are not.

(I'd check that by simply setting the frequency to 50Hz - where it should be accurate, and then change to 1kHz and see how much the reading changes. This obviously assumes your sine wave generator doesn't change amplitude when you change frequency, but that is usually a reasonable assumption, but one that is easily verified with an oscilloscope)

But in case you're not sure, 0dBu is 0.775V RMS, so +8dBu is 2.5 times that (1.95V RMS) and -8dBu is 0.775/2.5 (0.31V RMS)

Those general-purpose instruments will be part of any electronics workshop. If you want a dedicated audio test set, you're looking at a lot of cash. Worth it if you're an audio specialist, but hard to justify for odd jobs like this. But it's worth saying that some of the more expensive DMMs will have a dBm mode, and those that do will allow you to enter 600 ohms as the 0dBm reference level (which is where dBu comes from). The Fluke 187/189 and 287/289 models do that, but are expensive. The older Fluke 8060A also does that, but s/h examples tend to be quite pricey, and they are at an age where they need a bit of maintenance now. The Fluke 8920A is a nice meter that will also work. These instruments have other uses in the workshop, so might be a better buy than a dedicated audio test set - but again, that depends on what work you do.

Finally, just a "heads up": of the units that I have recalibrated over the years, they've never been all that far off. If you're not confident in your ability to generate the correct signal levels, then there's a risk you might make things worse.

Good luck!

Mark
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