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Old 24th December 2009, 12:41 AM   #101
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Quote:
Which puts you in the market for an Audio Precision 2722
Not me. The average purchase price for the test equipment on my (Tubelab) workbench is about $50 each.

My bench at work is a different story. There is an easy $250K worth of test equipment on my bench. It is mostly all RF stuff though. I do RF and a little digital or DC distribution work, no audio at all. The 8903A on my bench doesn't get used.

Quote:
I think I'm reading a little less than 2% THD on my Simple SE with some JJ 6L6s and Edcor CXSE transformers...
I used to get under 1% at 1 watt rising gently to about 2% as you approach clipping. Then the distortion goes up rather quickly. You get about 40% when grossly overdriven. The distortion and the power output is lower in triode mode. It is also lower with CFB. I haven't measured a Simple SE in about 2 years. One of my amps gets experimented on and is never in stock condition. It is currently non - functional. The other is hooked up to my speakers and sound card and is used for listening only. I am going to swap it out for a Simple P-P soon.

I have an older HP 8903A which measures THD, and audio voltage level. Built in arithmetic functions make frequency response measurements easy. I also used an Audiophile 2496 sound card and a rather crude home made interface. It was in a dedicated computer that ran Win MLS software. The measurements from these two systems matched pretty good. Unfortunately I robbed that computer for parts about a year ago.

I bought one of Petes boards, grabbed an Audiophile 192 off of Ebay, and got a $99 bare bones computer from Newegg, so I will be back in business when I get a chance to build it all.
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Old 24th December 2009, 01:15 AM   #102
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Quote:
Originally Posted by tubelab.com View Post
Not me. The average purchase price for the test equipment on my (Tubelab) workbench is about $50 each.

I bought one of Petes boards, grabbed an Audiophile 192 off of Ebay, and got a $99 bare bones computer from Newegg, so I will be back in business when I get a chance to build it all.
I recently purchased Pete's kit, and still have to order the various parts. I probably will skip the rms voltmeter as I have both a Keithley 2002 (2MHz true rms converter) and a Fluke which are good enough.. I purchased an antec aria case with Athlon 64 3000, and an Abit KV-80 mobo with a bad psu. I am familiar with both the case and the processor as I use both in other systems. (The seller indicated that the mobo tested ok with another psu, and examining the failed psu indicated it failed benignly - I'm debating whether to fix it or not.) I've ordered a replacement supply, dvd-rw drive, and have an almost unused 160GB HD.. The Aria/mobo/processor ran me $44.. I also scored an Audiophile 24192 on sale at amazon recently.

At one point my old setup functioned satisfactorily, but the new(er) laptop doesn't seem to play nicely with the m-audio transit I used previously. The attraction of 192kHz sampling rate and a PC able to run large FFTs without choking was too hard to resist once I saw Pete's board.

It's nice to be able to make consistent and repeatable measurements, something that my current FFT setup no longer seems capable of doing..
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Old 24th December 2009, 01:49 AM   #103
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The attraction of 192kHz sampling rate and a PC able to run large FFTs without choking was too hard to resist once I saw Pete's board
.

Some of my amplifiers have had enough bandwidth to need a 192 KHz card for frequency response measurement. The old 2496 card was within 0.1 db out to about 42KHz. If the Audiophile 192 will go to 80KHz I'll be happy. The 8903A goes from 20 Hz to 100 KHz, although the AC voltmeter seems accurate down to 5Hz.

Big FFT's need big processing power. No problem with the Athlon, but the dedicated PC that I used to use was P3 based with 1 G of memory and a bit on the wimpy side. The new box uses a core II duo with 4 G, so I should be able to process some 1 M point FFT's in less than a week!
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Old 24th December 2009, 02:37 PM   #104
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Quote:
Originally Posted by tubelab.com View Post
.

Some of my amplifiers have had enough bandwidth to need a 192 KHz card for frequency response measurement. The old 2496 card was within 0.1 db out to about 42KHz. If the Audiophile 192 will go to 80KHz I'll be happy. The 8903A goes from 20 Hz to 100 KHz, although the AC voltmeter seems accurate down to 5Hz.

Big FFT's need big processing power. No problem with the Athlon, but the dedicated PC that I used to use was P3 based with 1 G of memory and a bit on the wimpy side. The new box uses a core II duo with 4 G, so I should be able to process some 1 M point FFT's in less than a week!
We think alike.. I also have a 2496 which lives in the media server and provides analog i/o for speaker measurement, as well as sound for the home theater set up.. The digital spdif output goes to my dac and is actually supporting the primary purpose of the media server to serve high quality digital to my stereo system. (I spent a lot of time getting it "bit perfect." Strangely enough these days I use my Shigaclone a lot more. I like selecting and handling the disks I guess, the difference in sound quality is probably non-existent.)
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Old 24th December 2009, 05:04 PM   #105
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Originally Posted by TubeMack View Post
That said, I was doing some browsing. That always gets me into big trouble! And I have to admit i'm really tempted by the super cheap Rigol 50MHZ 1052E, and 100MHZ 1102E DSO's.Lots to read on the net about these. Some seem to like them just as well or better than the expensive TEK TDS starter DSO scopes. How would something like that compare to Audio Tester on the PC? How about the FFT function? Either of these would suppliment a new BK 30MHZ analog scope, and mainly be used for tube equipment. I'm getting alot of pleasure from that new analog scope BTW. Super Geek thrills!
Precede the 'scope with an analogue distortion meter and take the oscilloscope output to the 'scope for FFT. That way, you have the dynamic range of the analogue meter with the detail of the FFT. Or just use Pete's card and a good sound card.
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Old 24th December 2009, 06:09 PM   #106
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Pete,

Thank you very much - I just received my board.

Merry Christmas and happy new 2010 to you and yours!

-- josť k.
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Old 24th December 2009, 06:15 PM   #107
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Precede the 'scope with an analogue distortion meter and take the oscilloscope output to the 'scope for FFT
.

To make this statement a bit more clear one needs to understand how an analog THD analyzer works:

A signal from a pure sine wave source is applied to the unit under test. Its output is connected to a suitable load and also connected to the THD meter.

First the meter takes a reading of the total signal voltage.

Then a notch filter is engaged to remove as much of the original sine wave signal as possible.

The meter then takes a reading of the remaining signal voltage after the original fundamental tone is removed. The remaining voltage is assumed to be THD but may also be hum and noise.

The meter will display the ratio of the original signal to the signal remaining after the original fundamental tone is removed by the notch filter. These results can be displayed as % distortion, or as db's.

These functions can be completely manual as with the old HP 331A's or automated as in the HP 8903's.

Most analog distortion analyzers have an output that is taken after the notch filter so that the signal remaining after the fundamental is removed can be looked at with a scope, or analyzed by FFT. It is also possible to chain the output from one THD analyzer to another for IMD measurements. I keep a pair of old HP331A's around for this purpose.

The 331A does not have a built in audio source. Any imperfections in the source signal will show up as THD in the measurements. I use an old HP204D for a source. I have two for IMD measurements. Some of these are better than others in the THD department. One of mine measures .05% distortion which is good enough for tube amps. The other one is .12%.

There are software audio generators for use with a sound card. There are also test CD's that can run in a CD player. Both are good alternatives for an audio test source.

The output of the THD analyzer can be connected to the scope or sound card. Set the THD analyser up for level measurements and measure its output on the scope or sound card. Absolute accuracy is not needed, only a relative level. Then engage the filter in the analyzer to remove the fundamental tone. The output of the analyzer will drop by 10's of db because the main signal has been removed. The individual harmonics remaining can then be measured and compared to the original fundamental tone. The additional dynamic range boost afforded by this measurement technique will depend on the THD present in the amplifier under test, but 30 to 50 db is certainly possible.
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Old 24th December 2009, 08:52 PM   #108
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To some extent we here at DIYAUDIO do think alike but not to the extent that we have nothing to share. Perhaps in this thread we have similar tools.
So here is a particular question. How do we use our dedicated test PC and Millett interface to measure an amplifier’s output impedance?
And a more general question. What amplifier performance parameters can we measure with this set of tools? (dedicated test PC and Millett interface)
BTW; it is interesting that the tools cost more than the $0.63 6BQ6’s that I have on the breadboard to test.
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Old 25th December 2009, 01:48 AM   #109
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Wow that was some good info! I do happen to have a 331A, but never attempted using it as I heard what a PITA it was. Is it really that bad for infrequent usage? Does a PC setup offer a better alternative, or any alternative at all for THD measurements?

I'm still eyeballing that Rigol for a toy fix! Perhaps just the cheaper 50Mhz version. Reading suggest that the FFT funtion would not be as good as a Soundcard. Somethong about 8 bit resolution only offering a -70 db bottom.
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Old 25th December 2009, 08:08 AM   #110
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The children are all tucked snugly into their beds and I am having visions of oscillations in my head, Merry Christmas.
To visualize oscillation imagine that shopping cart we all have had the occasion to push through the produce section, just as we reach critical velocity that pesky front wheel begins flopping all about making the cart hard to push and creating a terrible noise. Magic? No.
It is not just high frequency current feedback op-amps and shopping cart wheels that oscillate. Even vacuum tubes will oscillate well into the Radio Frequency range. Check out the data sheets for the 807 and similar tubes. They can operate and oscillate well past 125 M Hz. A feedback loop can shift from negative to positive feedback due to phase shift. The wheel can fall off the cart and we can destroy the radio reception for two blocks away. We may not even know it at the time.
This is the thinking that makes me want to maximize the bandwidth of my next O-scope, one that can resolve RF oscillation.
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