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Old 10th Apr 2021, 4:11 pm   #1
bigfathairyvika
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Default HP 333A Distortion meter - opinions?

Hi All,

I am thinking about buying a distortion meter for setting up, adjusting and so on of audio amplifiers and pre amplifiers.

I have my eye on a HP333A.

It appears to be able to do the job that I need but is there anything I need to be aware of before purchasing this model? Eg: obsolete replaceable parts?


Thanks,

Mark
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Old 10th Apr 2021, 4:24 pm   #2
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Default Re: HP 333A Distortion meter - opinons

Nothing terrible. Plenty of standard parts. Switch wafers if damaged could be a problem.

If stored damp, the white surface of the meter scale can craze, curl up and fall off.

David
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Old 10th Apr 2021, 4:36 pm   #3
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Default Re: HP 333A Distortion meter - opinons

Like many older commercial distortion meters it only goes down to 0.1% FSD and may well have internally generated distortion and noise that limits practical measurement to maybe 0.03% if you're lucky. This may be no problem for you depending on the performance you require.
Of course you will need a low distortion oscillator as a test signal source and this must be an order of magnitude (preferably!) better than the equipment to be tested and the distortion meter itself on THD content.
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Old 10th Apr 2021, 5:22 pm   #4
bigfathairyvika
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Default Re: HP 333A Distortion meter - opinons

Thank you both for your input.

0.5% would be fine for my measurements for 70's and older equipment.
I suppose for newer equipment a much better version would be required, so would there be a not so expensive ( less than 200 quid ) model I should be looking out for?
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Old 10th Apr 2021, 6:26 pm   #5
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Default Re: HP 333A Distortion meter - opinons

Have a look at the Ferrograph RTS2
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Old 11th Apr 2021, 2:36 am   #6
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Default Re: HP 333A Distortion meter - opinons

If you see an HP339A that's affordable, grab it. Osc and distortion meter in one and goes lower than the 333A.

Otherwise an HP8903 which I have seen go for the sub 300 mark. These go low and have automatic null tuning. There's a lot of cleverness hidden behind an unremarkable panel. Source and meter in one box.

There's the Ferrograph RTS2 which is quite decent.

Then Radford did their low distortion oscillator and distortion meter pair which are quite high performance.

I have both an HP333 here in the round tuit pile, and a working HP 8903B

An alternative would be an audio spectrum analyser like the HP3580A which is an absolute classic. With one of these, you can measure intermod performance as well. Has a tracking generator for swept freq response plots. Does log sweeps too.

David
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Old 11th Apr 2021, 3:24 am   #7
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Default Re: HP 333A Distortion meter - opinons

Mark are you computer savvy, and have relatively modern PC gear (a laptop is especially beneficial as it avoids ground loops)?

There is the option of a USB audio interface like a Focusrite Solo with software like REW. The performance and functionality is miles ahead of the HP offerings imho, but you would need an AC voltmeter or two for absolute measurement of input and output signal levels from your audio gear, and some custom interface bits like a resistive voltage divider for amplifier output signal levels. The software provides a large array of automated test capabilities for equipment assessment like automated spectrum and distortion plots, and result plots just get saved with the equipment docs. I use such a system for audio assessment of any equipment on my bench.
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Old 11th Apr 2021, 8:34 am   #8
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Default Re: HP 333A Distortion meter - opinons

I have an 8903B, and compared to the Sound Technology unit I was using before, it is an absolute joy to use. No faffing around setting full-scale and nulling, (as you will have to with a 333/334) just press a button.
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Old 11th Apr 2021, 8:50 am   #9
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Default Re: HP 333A Distortion meter - opinons

The 8903 uses a multi-pole state-variable filter rather than wien-bridge null. So the notch is deep, but its mouth is not as wide so it doesn't miss as much noise near to the tone, so you get a more accurate SINAD measurement. The oscillator is another state-variable filter.

Frequency coverage is up to 100kHz and the measurement too, unless you engage some of the limiting filters to shape the 'channel' so you can look for things above the range of sound cards and below the range of many RF spectrum analysers. So it covers a bit of a forgotten gap. As Michael Flanders put it "Still, one ought to please any passing bat".

I need to find an 8901 to keep mine company, though I could use the demodulator output on my spectrum analyser.

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Old 11th Apr 2021, 10:58 am   #10
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Default Re: HP 333A Distortion meter - opinons

Ok, there appears to be a glut of 8903B's on ebay, all from china at around 600+ quid each.
Bit more than I'd like to spend and especially at that distance.

A audio spectrum analyser would be great, but probably more than I'm prepared to spend.

On the computer side, yes I have thought about using a audio input device and a bit of software, but the faffing about with the ancillaries is not what I want.

Maybe I'll change my views at some point.
But at the moment I'm just after a simple and cheap solution to a relative distortion measurement.

Thank you everyone for the input, I have a few more choices to keep my eye out for.

Mark
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Old 11th Apr 2021, 1:07 pm   #11
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Default Re: HP 333A Distortion meter - opinons

For your needs you could also consider the Analog Discovery 2 as it contains a 14 bit ADC (100MSa/s) and can accept inputs up to something like +/- 25V. It's not going to be good enough to satisfy the needs of serious HiFi enthusiasts but you get a 10MHz spectrum analyser that works quite well down to audio frequencies. Sadly, the prices of these have gone up a lot recently, they used to be about 200 new but now cost about 400. It might be possible to buy a used one at a sensible price. To prevent damage, some care would be obviously be needed if you coupled it up to the internals of high voltage valve stuff but that probably applies to alternative analyser options as well.

The main disadvantages of the older HP gear is that it is big and heavy and quite restrictive in what it can measure. With a modern DSP solution like the AD2 you can look at individual harmonics or spurious terms. You can also use it as a scope or a crude waveform generator.

A few years ago there was a glut of Keithley 2015 THD bench DMMs on the market and these could be purchased for under 300 each. This meter has built in DSP and can do THD or measure individual harmonics. I think the residual harmonic level is -87dB which ought to be good enough for all but the most serious applications. The distortion performance of the internal signal source isn't very good on this meter though. Prices of these meters seem to have rocketed in the last couple of years and I guess this is down to supply and demand.

A basic rule of thumb is that the internal harmonic distortion level of the test receiver has to be about 20dB better (lower) that the distortion in the device under test if you want to measure an individual harmonic to within about +/-1dB uncertainty. The signal source obviously has to be very clean too.
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Old 11th Apr 2021, 1:58 pm   #12
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Default Re: HP 333A Distortion meter - opinons

Like a lot of things you get what you pay for. If you want really cheap, I still have this looking for a new home, dunno why the thread got closed.
https://www.vintage-radio.net/forum/...d.php?t=167778

You have to set the levels and tune the null by hand, but it does measure distortion.
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Old 11th Apr 2021, 3:25 pm   #13
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Default Re: HP 333A Distortion meter - opinons

I do need to reduce the amount of stuff in the attic.

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.

It's a nice bit of classic HP sixties gear, in the system-1 style cabinet.

David
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Old 11th Apr 2021, 3:56 pm   #14
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Default Re: HP 333A Distortion meter - opinons

I have an HP 334A, many of the electrolytic capacitors were gone and also it uses light bulbs with photoelectric cells for the nulling circuit. The bulbs go and I am not aware of any replacement parts. You also need a low distortion audio signal generator which is an added cost. I replaced the capacitors and bulbs in mine, and have checked my HP334A and it is within spec. down to 0.1% accuracy. If I had to do it again I probably would have gone with a HP 339A, 8903B or ShibaSoku Audio Analyzer, but at the time and still appears they are much more expensive.

Whatever route you go, I would make sure it is in working order.
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Old 11th Apr 2021, 6:29 pm   #15
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Default Re: HP 333A Distortion meter - opinons

Quote:
Originally Posted by Dickie View Post
... dunno why the thread got closed.
https://www.vintage-radio.net/forum/...d.php?t=167778
I think threads automatically close if there is no activity for a period (maybe 6 months?), but you can ask the mods to reopen them. Clicking on the red triangle to the left of a posting will bring up a "message the mods" box.
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Old 11th Apr 2021, 7:27 pm   #16
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Default Re: HP 333A Distortion meter - opinons

https://www.vintage-radio.net/forum/...44#post1363244

Re-opened.

m0cemdave is correct hit the red triangle (report post) if you need something doing to a post or thread, or wish to report an issue with the post.
He's also correct threads auto close after 6 months of inactivity.

Cheers

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Old 11th Apr 2021, 7:51 pm   #17
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Default Re: HP 333A Distortion meter - opinons

Thanks Mike T. I must admit I hadn't noticed its closure until today.
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Old 11th Apr 2021, 8:11 pm   #18
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Default Re: HP 333A Distortion meter - opinons

Quote:
Originally Posted by Radio Wrangler View Post
I do need to reduce the amount of stuff in the attic.

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

David
I wouldn't describe the PH-163 power cord (used for 1960's HP) as unobtainable, instead I would just say that they are expensive as you would either have to import them, or wait patiently for one to turn up.

I've got a 331A & 333A here, trouble is they have been badly treated, the 333A has a smashed meter (different casing to other HP equipment) and the 331A needs a lot of parts straightening & a coupler for the large dial. One day I'll have to decide which to restore out of the two.

David
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Old 11th Apr 2021, 8:19 pm   #19
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Default Re: HP 333A Distortion meter - opinons

Quote:
Originally Posted by mark-jacobs View Post
I have an HP 334A, many of the electrolytic capacitors were gone and also it uses light bulbs with photoelectric cells for the nulling circuit. The bulbs go and I am not aware of any replacement parts.
CM/VCC may have made the 1869 lamp (10V 14mA) obsolete, but they are still available from another manufacturer.
https://www.mouser.co.uk/ProductDeta...SABEgKxN_D_BwE

David
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Old 11th Apr 2021, 8:42 pm   #20
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Default Re: HP 333A Distortion meter - opinons

Quote:
Originally Posted by Max Power View Post
I have an 8903B, and compared to the Sound Technology unit I was using before, it is an absolute joy to use ...
Another vote for the 8903. It's my favourite audio instrument. I also have a 3961 so I can see what the distortion is. These make up my workhorse kit and I use them most working days. I also have a scope. It's occasionally handy, but is probably 10-20 times less useful than the other two when it comes to checking a piece of valve audio gear.

On the very rare occasions I've had my kit in the company of other people's I have to say I've been impressed with some of the computer-based stuff. When properly implemented it can deliver very impressive performance indeed. And if you already have a suitable PC it can be relatively inexpensive.

Cheers,

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