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-   -   A simplistic distortion-meter (http://www.diyaudio.com/forums/equipment-tools/207513-simplistic-distortion-meter.html)

Elvee 25th February 2012 08:48 AM

A simplistic distortion-meter
 
6 Attachment(s)
Here is a very simple and easy THD meter project for beginners.

Of course, it cannot compare with commercial instruments, or even projects like Cordell's, but the investment in time, money and effort is on a completely different scale.

It only requires standard, consumer-grade parts (5% carbon resistors and 10% capacitors, common opamps), needs no component sorting or selection, and is completely adjustment-free.
It works on a single, non-regulated supply, and is easy to use, making it ideal for educational/didactic purposes.

It has three ranges: 100%, 10% and 1%. On the 1% range, the measurement floor and usable resolution is 0.01%.

The indicator is simply a digital multimeter in the 2V range, the full range voltage being 1V.

The circuit is based on a SVF (state variable filter), making it highly tolerant to components inaccuracies: they have no impact on the depth of the null, only on the exact frequencies.
Compared to a double T, it is a huge improvement, and it makes frequency variation easy with any low-cost stereo potentiometer.
With the values shown, the frequency range is 500~5000Hz, this can easily altered or augmented by changing/switching the integrator's capacitors.

The discrete precision rectifier renders operation possible from 30Hz to 30KHz (harmonics up to 150KHz).

The first version has a discrete residue amplifier.
The second schematic is annotated with DC quiescent voltages, and the third shows a fully integrated version.

Some pics of my prototype follow.

pinkmouse 25th February 2012 09:33 AM

Interesting. Add a couple of switchable attenuators and it would be dead handy for quick power amp testing.

Elvee 25th February 2012 10:17 AM

Quote:

Originally Posted by pinkmouse (Post 2921882)
Interesting. Add a couple of switchable attenuators and it would be dead handy for quick power amp testing.

The input potentiometer acts as a variable attenuator, the input level can range from less than 1.5V to 100V, that is sufficient for 99% of power amplifiers.

pinkmouse 25th February 2012 01:25 PM

Doh!

What about input protection for incorrect settings. Diodes to the rails or suchlike.

Elvee 25th February 2012 04:27 PM

Quote:

Originally Posted by pinkmouse (Post 2922020)
Doh!

What about input protection for incorrect settings. Diodes to the rails or suchlike.

With R1 and C1, it could probably survive momentarily a direct connection of the input to the 230V mains.
Not something I'd recommend, but rugged enough to survive "normal" mishaps.

Some instructions for those unfamiliar with manual THD meters:

-Connect input to the A.U.T. and output to a DVM, range 2V, select "Set level" and adjust the level to read ~50% (0.5V)
-With the main tuning pot P2, search for the maximum amplitude. When done, adjust level to read exactly 1.000V.
-Select "100%" and adjust P2 for a minimum reading
-Switch to "10%" and adjust fine tune P3 for minimum
-If necessary do the same in 1% range

The output is linear, and 1V=100%, thus a reading of 110mV in the 10% range is equivalent to 1.1% distortion.

Note that the rectifier is not a true rms, and for high levels of multiple harmonics, the reading will be slightly underestimated, but at "normal" distortion levels, this effect is negligible.

For example, a pure triangle wave reads 11.2% instead of the actual ~12% it should, but this shouldn't be a problem as this instrument is not destined to be used in metrological applications

Frex 25th February 2012 05:59 PM

Quote:

With R1 and C1, it could probably survive momentarily a direct connection of the input to the 230V mains.
Not something I'd recommend, but rugged enough to survive "normal" mishaps.
I'm not sure that your 22k input potentiometer survive on a 230V main direct connection... (it must dissipate ~2.4W).

Frex

Elvee 25th February 2012 07:44 PM

Quote:

Originally Posted by Frex (Post 2922367)
I'm not sure that your 22k input potentiometer survive on a 230V main direct connection... (it must dissipate ~2.4W).

Frex

Certainly not for a long time, in this sentence:
Quote:

With R1 and C1, it could probably survive momentarily a direct connection of the input to the 230V mains.
the important word is momentarily: the time to realize there is a funny smell around, and dive to pull the plug....

Frex 26th February 2012 06:31 AM

Hello,

You are right Elvee, but you probably need to unplug it very faster ! ;)
So, about your design, did you have done some Bode plots to show notch depth and width in it's frequency setting range ?
Often, graphs speak more than words.
Regards.

FRex

Elvee 26th February 2012 07:57 AM

4 Attachment(s)
Here you are:
First a global outlook with the frequency adjusted at 1KHz, marker at 2KHz (gain error = 0.4dB).
Then a zoom at the two frequency extremes.
And finally, the impact of random component deviations on the depth of the null.

Surprisingly, the fundamental rejection performance of the physical prototype is significantly better than predicted by the sim.
Perhaps caused by parasitic capacitance of the potentiometers, I don't know exactly

Frex 26th February 2012 08:23 AM

Hi,

Interesting, but when i had speak about Bode plots, it was real world measurements (done with sound card for example), not simulations.

Frex


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