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Old 11th Apr 2021, 10:11 pm   #21
G0HZU_JMR
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Default Re: HP 333A Distortion meter - opinons

Although it won't be the highest performer here I just dug out my Analog Discovery 2 and downloaded the freebie Audio Analyser Suite program by 'TheStuffMade'. This program doesn't require an install, it just runs as an exe.

This program can do lots of things and the first test I tried was a THD sweep of a self test of the AD2 across 20Hz to 20kHz. This tests the audio generator and the ADC in the AD2. The test took a couple of minutes to complete (100 frequency steps) and it showed 0.02% THD typical for every test frequency.

This was with a test sine wave of 1Vrms generated (and measured) by the AD2 in loopback mode. I think the limit of 0.02% is partly due to the ADC and also the DAC that produces the test signal. This won't be good enough for hardcore HiFi enthusiasts but it should be good enough for anyone with ordinary ears. Just connect the DUT input and output to the AD2 and press run.

The plot below was taken in spectrum analyser mode and is a loopback self test at 5kHz at 1Vrms. It shows a 3rd harmonic at about -78dBc. I think this is a limitation of the 14bit ADC and/or the DAC used for the 5kHz waveform generation. A decent soundcard setup should perform better but it will require some sort of attenuator and system calibration and this will be a bit fiddly to set up every time. The AD2 can measure directly without any need for an attenuator or any scaling corrections. It can also operate up to much higher frequencies than a soundcard. To do the above 100 point THD sweep with a HP33x analyser would take a very long time. With the AD2 just press run and then eat a couple of biscuits whilst it does it all automatically.
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Old 12th Apr 2021, 9:19 am   #22
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Default Re: HP 333A Distortion meter - opinons

The HP33* analysers are good being able to test quite high voltages but as mentioned they use photo cells to null the signal. These are in a box surrounded by some sort of foam, after all these years the foam turns to acidic snot which can eat into the legs of said photo cells.

I started with a HP339 but because of the above issue it never worked right so I used a Marconi TF2331, another nice instrument but nulling, like with the older HP analysers can be tedious.

Now have a HP3903B thanks to a kind forum member, it's reet grand though mine only goes up to 50khz signal out wise, but can measure up to 100khz.

However if you have an old XP/W7 PC, download Soundcard scope which is free software, the THD meter on it is as good as the HP3903B, it also has FFT up to 20khz, a bit limited but still useable, oh it also has a very low THD sig OP ,half decent scope and XY plotter as well as audio recording. With a half decent AI ( i use an old soundblaster) it can do a hell of lot and costs nowt. I've tried REW, ARTA and every other free Audio Analyser software, SC scope is the easiest to use.

The only thing it can't do though is a frequency sweep plot, neither can the HP3903 without an XY plotter or Pete Millets software, this the AD2 can do. That said I've found an auto frequency sweep not that usefull for testing audio amplifiers, I much prefer doing a manual sweep at LF EG 10-400hz and 10 - 100khz. Unless an amp is badly set up,nothing much happens in the middle, an auto sweep goes too quick. To do a quick frequency test all you need is an old sig gen and scope, set sig gen to 1khz, flick the times by switch to x10 and x 100, job done.

Andy.
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Old 12th Apr 2021, 10:17 am   #23
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Default Re: HP 333A Distortion meter - opinons

If you can find one, a Tektronix AA501, ideally with a SG505 oscillator, is a good bit of kit. They are rare though and don't seem to come up for sale that often.
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Old 12th Apr 2021, 11:02 am   #24
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Default Re: HP 333A Distortion meter - opinons

The HP 33x series are nice for their age. I've got a 334A which I had to do some restoration work on: I replaced the Volex mains inlet with a standard IEC one, and had to make a new scale for the meter because it had peeled. I did it by scanning the peeled scale, touching up the image then printing it on plastic self-adhesive film which I stuck on to the scale once I'd washed off the loose old printing.

One reason I like it is that it will give useful results up to higher frequencies, 100kHz or more. I've used it for assessing current sense amplifiers in motor drive applications which need to work above the audio range.

However, there are oddities in its frequency response. If you set up the attenuator compensation according to the manual, there's a pronounced hump in its frequency response at about 12MHz in the 0.3% position and above. In the 0.1% position this becomes a rolloff above about 5MHz, if I remember correctly. This means that high-frequency distortion measurements can be hard to interpret accurately, and that the distortion can appear to magically decrease on the 0.1% range.

When I find some more round tuits, I mean to improve the attenuator compensation for a flatter response. The rest of the circuitry seems to be up to the job.

Chris
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Old 12th Apr 2021, 2:43 pm   #25
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Default Re: HP 333A Distortion meter - opinons

I have a Marconi TF-2331 DFM which works extremely well after going through it and doing extensive re-capping.

It makes for a very nice audio voltmeter and DFM, and it also has an RF detector for measuring audio distortion in transmitted RF signals up to 500Mhz.

It was very easy to work on as well.....YMMV...
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Old 12th Apr 2021, 3:22 pm   #26
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Default Re: HP 333A Distortion meter - opinons

The TEK AA501, SG505 & mainframe are good but are now horribly expensive (read overpriced). The AA501 is a real PITA to work on as its in quite a small form factor and is packed with unobtainable variants of the '5534' opamp (a DIL16 version ?) so some PCB rework is needed. Used with an HP3585A spectrum analyzer distortion products are clearly revealed. There is an option for IMD, as its not on the base model AA501 or SG505. It take quite a bit of effort to actually achieve the specified distortion floor.

dc
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Old 12th Apr 2021, 9:21 pm   #27
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Default Re: HP 333A Distortion meter - opinons

Hi!

I don't know how well you are at home–built pieces of gear, but by far the best distortion analyser, still buildable from reasonably plentiful parts, was the three–part design published by Bob Cordell in the July, August and September 1981 issues of "Audio" magazine, this auto–nulling and auto–levelling design will measure down to about 0.002% t.h.d. + n and needs only a simple 100uA moving coil meter!

It does, in its original form, unfortunately, require an awful lot of rotary wafer switches and quite a lot of close–tolerance E48 series resistors tho, plus miles of interconnecting wiring and cabling!

However, a great deal of the interconnecting wiring and switching can be eliminated by using small cheap d.p.d.t. 2A PCB relays, a DIY Audio Forum Member called "Gullandes" did this!

If any Member(s) are interested in a relay–operated version of Bob Cordell's Analyser, please PM me and I'll get started on some PCBs!

Chris Williams
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Old 12th Apr 2021, 9:51 pm   #28
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Default Re: HP 333A Distortion meter - opinons

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Originally Posted by Radio Wrangler View Post
I've just been up there and it's not a 333A, it's a 331A which is pretty similar, but it's a late enough one to have an IEC mains connector, so it doesn't need the unobtainable mains cable. Cosmetically, it's good. Don't know whether it works or not, it's just as I got it from a stall at an amateir radio doo

OK, if you want a distortion meter, you can have this one for free, if you can arrange transport. I've got another use for any packing materials I can accumulate.
Thanks for the offer David, I will pm you if I decide to take you up on the offer.

Thanks to everyone for all the suggestions, it will take me a while to process all the ideas.

Mark
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Old 12th Apr 2021, 10:33 pm   #29
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Default Re: HP 333A Distortion meter - opinons

There's certainly a lot of ideas to choose from!

The soundcard suggestion by trobbins is a good one even though I know you don't want the niggly requirements for attenuation and level calibration.

However, to show how well a cheap mobo soundcard can measure distortion I dug out an old image showing the spectrum of my two tone generator. This is based on the ARRL design and I made this nearly 40 years ago to aid in the testing of SSB transmitters. The plot below was probably taken with my little EeePC netbook running some old demo soundcard software from ST. You can see that the IMD3 terms are lost in the noise and only the second harmonic of the lower test tone is visible.

A two tone test is useful because it can test the PSU regulation as the AF stages will gulp current at the difference frequency. You can also do multi tone testing using a soundcard. I've not tested the distortion performance of the signal generator in the EeePC netbook and this might be a weak link for serious measurements.
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Old 12th Apr 2021, 11:16 pm   #30
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Default Re: HP 333A Distortion meter - opinons

I just spotted Andy's suggestion for Soundcard scope SW. It looks to be another good thing to try out and it is free.

Is there a safe download link for this anywhere? I'd like to try it but I'm nervous to download unless it is from the website of the person who created it.
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Old 13th Apr 2021, 7:20 am   #31
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Default Re: HP 333A Distortion meter - opinons

https://www.zeitnitz.eu/Scope_en there you go Jeremy.

Andy.
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Old 13th Apr 2021, 8:24 am   #32
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Default Re: HP 333A Distortion meter - opinons

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Originally Posted by Keith956 View Post
If you can find one, a Tektronix AA501, ideally with a SG505 oscillator, is a good bit of kit. They are rare though and don't seem to come up for sale that often.
You probably don't want to know this - but I picked up my AA501 for USD14 on eBay BIN around 10 years ago.

The SG505 cost rather more.

After many years of faultless operation the AA501 keeled over during a measurement session around 3 months ago. The whole autonulling and autogain is controlled using vactrol opto isolators, and after an hour or so of diagnosis, one of the vactrols had died. Fortuitously I had nearly the right sort in my box of Tek spares, so it was easy to sort out.

That combination goes down to about 6ppm THD, limited by the AA501 rather than the SG505.

But for bang for buck you can't do cheaper than a Focusrite Scarlett 2i2. There are three gain levels available with distortion in the 12 - 20ppm range. Less than £100 from Amazon. Then use Visual Analyser http://www.sillanumsoft.org/ , a phenomenal freeware set of audio analysis tools. One of the preset functions is THD and THD + noise.

Craig
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Old 13th Apr 2021, 9:34 am   #33
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Default Re: HP 333A Distortion meter - opinons

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But for bang for buck you can't do cheaper than a Focusrite Scarlett 2i2. There are three gain levels available with distortion in the 12 - 20ppm range. Less than £100 from Amazon. Then use Visual Analyser http://www.sillanumsoft.org/ , a phenomenal freeware set of audio analysis tools. One of the preset functions is THD and THD + noise.

Craig
I actually have a Scarlet 2i2 I picked up cheap on ebay, it is indeed a great bit of kit and works with my laptop (Macbook). Alas it looks like that software is Windows only.

The AA501/SG505 has been very reliable like all the Tek stuff I have, the only problems have been mechanical - the pushbutton switches need the occasional clean with a bit of IPA. And I had to replace the shaft coupler once on the SG505.
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Old 13th Apr 2021, 10:34 am   #34
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Default Re: HP 333A Distortion meter - opinons

You can still get CdS2 photocells, so you could homebrew a photo coupler. LED light output is quite linear with current. The time constant of the thing is largely determined in the photocell.

Clairex also made them. If you find any scrap HP Microwave Link Analysers, there are a couple in the receiver unit (the plug-in module if I remember)

David
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Old 13th Apr 2021, 3:33 pm   #35
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Default Re: HP 333A Distortion meter - opinons

The last ones I got were from Perkins Elmer but a quick google says that ALL CdS LDR's are banned in EU...

I actually got better results with a homebrewed version using an ORP12 and a jumbo LED in a suitable plastic tube!
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Old 13th Apr 2021, 7:10 pm   #36
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Default Re: HP 333A Distortion meter - opinons

RS still list them, most I clicked on seem to be extended range items, but the one in the link isn't.

https://uk.rs-online.com/web/p/ldr-l...stors/9146714/

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Old 13th Apr 2021, 8:04 pm   #37
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Default Re: HP 333A Distortion meter - opinons

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The last ones I got were from Perkins Elmer but a quick google says that ALL CdS LDR's are banned in EU...
Clearly not https://uk.farnell.com/c/optoelectro...t-optocouplers

And Farnell order code 327700 is a good old CdS ORP12, with no lead! 4,333 in stock.

Craig
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Old 13th Apr 2021, 8:15 pm   #38
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That's great then. First thing I saw in a google search was discussions about what a pity it was that they were now banned in the EU due to the use of cadmium... A reminder that you can't believe everything you read!
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Old 13th Apr 2021, 11:42 pm   #39
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Default Re: HP 333A Distortion meter - opinions?

Yes, there is a small amount of Cadmium in a photo-conductive cell, but then there is a small amount of arsenic in LEDs!

Oh, and mercury in compact fluorescents.

David
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Old 14th Apr 2021, 1:21 am   #40
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Default Re: HP 333A Distortion meter - opinons

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https://www.zeitnitz.eu/Scope_en there you go Jeremy.

Andy.
Thanks! I'll have a play with it over the weekend. I'm hoping to use it with the little EeePC netbook on my main workbench.
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