General testing for newly completed projects

skidmorebay · Mar 4, 2017

Hello,
When I started building DIY audio projects, if the unit didn't smoke and seemed to do what it was supposed to I was happy.
More recently, I've started running sweeps through the eq's I've built with FuzzMeasure to look at the resulting curves. Of course, this has often showed up problems that need fixing.
I've also dealt with ground loop noise by looking at the plot on a spectrum analyer plugin in my DAW with no signal through the unit.

I've got a signal generator, oscilloscope, and DMM. And a computer, of course. My question is, when I have finished building a new project what are the standard things I should measure, and how should I do it?

I can test frequency response with FuzzMeasure.
What about noise level? What values should I expect for an IC-based processor? Or a tube-based processor?
What about testing harmonic distortion?
And, how should I check the time constants of a compressor?
Anything else I should check before assuming that everything is working as it should?
thanks,
JS

alexc · Mar 4, 2017

3 words ... Room EQ Wizard !

Its got the all important real time THD+N readouts for characterising the basic distortion performance.

As well 'The' standout real time spectral analyser.

Sweep measurements are all there, including super low distortion function generator.
Magnitude, Phase, IM, THD vs freq plots and all that is there in spades.

May be the best real time/historical charting I've seen .. including the ability to 'layer' historical traces for display.

....

There's also a bunch of other stuff, done detailed! that more advanced types can use

And its donation based ! With a decent audio interface, it's as good as audio measurement *for the poorman* gets.

alexc · Mar 4, 2017

If one gets really involved, one can diy a 'signal probe' for ... 'checking as you go' for noise, THD and dynamic range.

The signal probe is basically a passive affair with some capacitor blocking of any dc voltages that may be present at a circuit's test point.

For tube stuff, that's important - the dc voltages may be high enough to overwhelm your audio interface's 'input curcuit'
protections.

Once you have the hang of it it's a great tool especially in diy builds where one integrates a bunch of modules.

The other indispensible is an 'attenuator' box - it 'scales' down the voltage signal taken from the probe, so that it can be displayed in REW with the best resolution.

ie. testing power amps : attenuator box has big power resistors to soak up the power and scale down the test signal

You can then really develop your sense of 'what's what' in terms of 'gain', 'thd' and dynamic range etc.

skidmorebay · Mar 6, 2017

REQW - I hadn't thought of using that. I've got a copy already. I'll definitely play with it.

A recent project seems to have 60 cycle hum, and associated harmonics above that, but it's hovering around -80dbFS to -90dbFS on an analyzer plug in. Would that be acceptable in a commercial product? (My converter is set for -16dbFS = 1.0dbu)

alexc · Mar 6, 2017

You need to know the amount of gain you have applied - or how much gain you will apply in service.

So for a 'unity gain' device, where you aren't going to typically apply gain, then your -90dBFS (add 17 for -73dBu) may well be fine.

But if you then add 15dB of gain, you are starting to crap out ... noise will be worse than -60dBu .. which isn't appealing at all.

SO - what's your expected gain range to use this thing? Set it and measure the noise floor and max desired signal output.
The difference will tell you what's what.

I shoot for -95dBu hum floor and a signal-hum margin (dynamic range more or less) of up to +110 dB.

That's what you would find in a basic mixer channel with good components.

You can take a look at some of my snapshots of this stuff to give you an idea of what REW tests are about ..

https://groupdiy.com/index.php?topic=64897.0

I annotate them so they are pretty 'self explainatory' with the info necessary to quickly get the picture .. The ones towards the ends is where I have 'standardised' the snapshots with the main info.

Cheers

ruffrecords · Mar 6, 2017

alexc said:
3 words ... Room EQ Wizard !

Its got the all important real time THD+N readouts for characterising the basic distortion performance.

As well 'The' standout real time spectral analyser.

Sweep measurements are all there, including super low distortion function generator.
Magnitude, Phase, IM, THD vs freq plots and all that is there in spades.

May be the best real time/historical charting I've seen .. including the ability to 'layer' historical traces for display.

....

There's also a bunch of other stuff, done detailed! that more advanced types can use

And its donation based ! With a decent audio interface, it's as good as audio measurement *for the poorman* gets.

I second that recommendation. I would only add that it will provide better results with a decent external sound device. I use a Focusrite Scarlett which is USB powered. I also have built my own passive interface box with a 600:600 output transformer to provide a fully floating output (essential for testing mic inputs that may have phantom power switched on). an output attenuator to produce mic pre signal levels, switchable termination for measuring noise and on the return side a switchable 600 ohm load and 20dB attenuator for measuring distortion at high output levels.

Cheers

Ian