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Old 29th June 2013, 05:03 AM   #141
richiem is offline richiem  United States
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Hi, all -- just found this forum. A note on comparing software THD analyzers with analog analyzers like the HP 8903B, HP 339A, etc. The measurement floor of the 8903 series is about 0.0009% at 1kHz if in good cal, due to spuriae from the notch filter. The results for the HP 339A are similar and for the same reasons. Upgrades to opamps and other parts can significantly improve these analyzers, but using a software analyzer is so much easier if it works. If a software analyzer shows lower THD, especially measuring the 8903 or 339 oscillators, which have *very* low THD, then the software analyzer is more nearly correct.

The VA 2012 Beta link seems to be dead now. I've used ARTA regularly, but am going to look at the 2011 version of VA. If and when that gets to the bench, I'll post back here.
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Old 29th June 2013, 08:13 AM   #142
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It would be good to see a comparison of a hardware bsed THD analyzer to the software versions.

Of course a quality soundcard should be used and the best amps amps installed if possible.
I will take a look at the site, But i think you have to sign up to the forum inoder to get to the latest bets versions.

I have been using VA11 and VA12 and sometimes they seem to be a bit flakey in Win7 and seems to be more stable favoring WinXP, But this is an intermittent issue so I can't make a solid judgement on this.
For the most part the work pretty good.
Wave Spectra is another great piece software that displays THD as well.
It is a faster running FFT and runs with no glitch's.

If you can get a hold REWv5 Beta 17 it works great as well but only the Beta versions have the THD function unlocked.

FWIW

jer

P.S. I guess it is down, I hope that it comes back sometime soon !!

Last edited by geraldfryjr; 29th June 2013 at 08:24 AM.
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Old 29th June 2013, 08:28 PM   #143
richiem is offline richiem  United States
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Software THD analyzers are limited to the performance of the ADC used, as well as the residual of the source -- in the case of the EMU-0204 running at 24bits, for example, with the HP 339A as source, the 1kHz full-scale referenced THD will be in the 0.0002 - to 0.0004 area as shown by ARTA. Notching out the fundamental by even 20dB will then result in THD below 1 part per million. Almost no hardawre analyzers can get anywhere near that. Nevertheless, I still use an HP 339A for quick surveys and for most amp testing.

For more detail, see my web pages on modding Herath IG-18 oscillators and building/using an active Twin-T notch filter at Home -- article index
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Old 30th June 2013, 09:08 AM   #144
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Those are some really neat articles !!
I may one day build a hardware version THD analyzer sometime with the new opamps that are now available.

TI's latest greatest audio opamp the OPA1664 has a rated THD of .00006%!!
This is 100 times better than what my current sound cards are capable of.

http://www.ti.com/lit/ds/sbos489/sbos489.pdf

One problem I have been having is getting VA to run in 24bit mode reliably.
Sometimes I can get it to work only for a little while and then it crashes.
Sometimes it won't run at all!!

Eventually, I will look into building the actual interface that was designed for it.

Most of the other programs are limited by their sample rate and the good ones that aren't limited cost a bit of money.

jer

Last edited by geraldfryjr; 30th June 2013 at 09:11 AM.
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Old 30th June 2013, 10:33 PM   #145
richiem is offline richiem  United States
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You can use ARTA for free but not be able to save setups, etc. But in free mode nearly everything works, and works beautifully. It can handle 192kHz and 24 bits. After using it for a few days, I happily sent the registration fee -- I remember it as less than $120 US -- for the full program. As I mentioned, I use it with an EMU 0204.

I also like the QuantAsylum 400 system a lot. But it only works with it's own HW & SW as a complete package. ARTA will work with nearly any sound card/USB box.

What I would like is to have that functionality in a Macintosh app that runs 192kHz/24 bits.
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Old 17th October 2013, 10:47 AM   #146
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Quote:
Originally Posted by geraldfryjr View Post
Those are some really neat articles !!
I may one day build a hardware version THD analyzer sometime with the new opamps that are now available.

TI's latest greatest audio opamp the OPA1664 has a rated THD of .00006%!!
This is 100 times better than what my current sound cards are capable of.

http://www.ti.com/lit/ds/sbos489/sbos489.pdf
Very interesting. I'm a new convert to op amp rolling, having just purchased some single and dual op amps from Dexa as replacements for the stock op-amps in my Eastern Electric DAC.

Anyway, my real reason for posting here is that I am after some advice for assembling a probe to attach to the soundcard to perform voltage ripple tests using VA.

I tried last night by stripping down a cheap microphone with a 3.5 mm jack and soldering a couple of old voltmeter probes to the exposed wires (two in total), assuming it would be that simple. But the volt readings were extremely wild (calibration attempts made no difference), leading me to think that my DIY approach was simplistic.

Apologies in advance if this is a somewhat naive question! But I can't find any guidance in this area.

FWIW, I was trying to measure a regulated 5v supply, which may be too much - I don't know - but VA does offer V as well as mV as acceptable reading parameters so I assumed it (or, more accurately, the soundcard and the PC!) would be ok with it.

I am running VA 2011 on Windows 7. I doubt the OS is the problem, nevertheless I thought I would mention it.
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Old 17th October 2013, 11:42 AM   #147
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Originally Posted by quinnpad View Post
...by stripping down a cheap microphone with a 3.5 mm jack and soldering a couple of old voltmeter probes to the exposed wires....
You need shielded cable and a good, shielded plug. NYS231BG REAN / Neutrik | Mouser Otherwise, RFI and mains hum will be picked up, rendering that great THD useless.

A real stereo sound card would be good. Then you will have two channels, too. They are expensive but much cheaper than a real scope. In fact, with VA, the soundcard is the scope, so to speak.
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Old 17th October 2013, 12:55 PM   #148
xstreme is offline xstreme  Europe
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V..A..
latest beta - V 0.4.2 - is out now


http://casa.accattatis.org:9090/Late...beta-0.4.2.rar
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Old 17th October 2013, 01:07 PM   #149
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Originally Posted by ITPhoenix View Post
You need shielded cable and a good, shielded plug. NYS231BG REAN / Neutrik | Mouser Otherwise, RFI and mains hum will be picked up, rendering that great THD useless.

A real stereo sound card would be good. Then you will have two channels, too. They are expensive but much cheaper than a real scope. In fact, with VA, the soundcard is the scope, so to speak.
Many thanks for the sound advice.
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Old 17th October 2013, 02:13 PM   #150
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Use a unity gain buffer on the input of your sound card.
Some if not all sound cards impedance varies with the input frequency and this can cause false reading especially if you are using a resistor divider between your probe and the input to the card.

I show such an interface here that I use to measure high voltages from step-up transformers and amplifier outputs as well,

A TEST JIG FOR FINDING ESL STEP-UP TRANSFORMER PARAMETERS

Below I have posted the schematics from that thread.

You can omit the dividers if you wish as long as the A.C. content of the signal is below about 1v to 2v peak and the D.C. voltage is within range of your opamps.
Your sound card will be already DC blocked by its internal DC blocking capacitor.

Some opamps are not unity gain stable in non-inverting configurations as well and may cause issues as I have had this problem occasionally.
I plan on revising this circuit with something like the LT1007 which is a high speed precision opamp and it is unity gain stable in non-inverting mode.

I used this same technique in my regulated HV supply as well here,

how can test the stator insulation and mylar coating?

This same technique can be found here in its simplest form without the divider resistors,

http://wb6dhw.com/RLC_Meter/RLC_Meter-sch.pdf

This is the link to the original website,

WB6DHW

I have used this interface a lot and with great success and I have found it is necessary to use the due to the fore-mentioned issues.

In this thread I show how well VA does with very low level signal buried in the noise floor compared to my scope and it basically puts the scope to shame in the particular test!!!!

http://www.esldiy.com/index.php?topic=176.msg310#msg310


Cheers!!


jer

P.S. Thank you xstreme, For the heads up on the latest version, I will be giving it a try shortly!!
Attached Images
File Type: jpg TRANSFORMER TESTER.jpg (303.6 KB, 133 views)
File Type: jpg Updated Test Jig Schematic.jpg (68.5 KB, 134 views)

Last edited by geraldfryjr; 17th October 2013 at 02:23 PM.
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