Distortion? Let’s get analogue: HP 339A in the house!

HP 339A distortion measurement set

Through a stroke of luck and friends in the radio broadcasting business (Thanks, Mathias!), I’ve found myself with this classic bit of kit on my bench.

Basic specifications for the HP 339A

While the 339A can’t quite match the performance of a high-end computer sound card, it’s a very competent piece of measuring equipment for evaluating audio frequency power amplifiers and other things that you don’t really want to connect to your computer – at least for us poor DIYers who can’t afford proper audio scopes.

The bitter-sweet part about this particular unit is that it’s barely been used at all, and that it did come with a calibration report – from 1988. Going through the performance checking procedures in the manual and cross-referencing measurements with my computer (equipped with an ASUS Xonar DX sound card and Vitrins Multi-Instrument 3.2) indicates that it’s in decent shape. Thankfully there aren’t many “time-sensitive” components in the signal chain at all, so I still feel fairly secure in using it. However, I can’t fully trust it to be absolutely accurate without at least a check-up by a proper cal lab.

A 1.45 V, 1 kHz sine wave generated by the 339A’s internal oscillator. Click for a slightly larger picture.

Connecting the internal oscillator to my computer shows that the internal oscillator is within both the frequency (+/-2 %) and THD (<0.0018 %) specs. In fact, it performs so well that the reasonably competent ADC on my sound card can’t even resolve its noise; the measurement above roughly equals the specified noise figures for the ADC chip itself – even disregarding the output buffer.

The true RMS output voltage of a Xonar DX sound card. The monitor output doubles as a fantastic label holder.

Aside from its signal generator and distortion analyser functions, the 339A incorporates a true RMS voltmeter. This is a very useful feature, since it allows the operator to quickly measure both the signal generator’s output level as well as the input level, without the need for separate tools or messing about with the connections.

The HP 339A makes it easy to switch between measuring distortion, input and output levels

Output from monitor connector at clipping of the input

It furthermore offers a monitor output for connecting a spectrum analyser or an oscilloscope. The monitor output is essentially hooked up in parallel with the meter, i.e, the fundamental tone is heavily suppressed (by -100 dB) and the distortion products come through untouched at 1 V full-scale. A very useful feature for poor sods such as I, since it offers a convenient “computer adapter” for heavier-duty equipment.

For the uninitiated, a distortion analyser like this works by suppressing the fundamental frequency (say, 1 kHz) with a very sharp notch filter and then measuring the amplitude of the harmonics at 2 kHz, 3 kHz, 4 kHz and so forth as well as noise produced by the equipment being tested. The level of the harmonics and noise is then presented on the meter in dB-below-the-fundamental and %. The monitor output essentially provides a distortion-only output – and the distortion is what we’re after.

To finish this up, some pictures of the guts of this thing:

The top cover only reveals the bottom of the boards – very unsatisfying.

Flipping it over reveals a maze of aluminium. HP went all out with the shielding in this one.

Sadly, I couldn’t find a clear map over which “sections” do what in the manual.

The power supply section beautifully displays the gold-plated circuit boards.

3 thoughts on “Distortion? Let’s get analogue: HP 339A in the house!

  1. Question: when saying that the 339A does not match the performance of a sound card, what sound card and what analysis software do you have in mind?

    • I’m comparing it to a Xonar DX, using Virtins Multi-Instrument to extract data. The dynamic range of the digital set-up is a good 10 dB better than the 339A, at the cost of the flexible front-end.

  2. Head on over to DIYAudio, they’ve been working through the HP339A.
    Also it appears that your HP339A is better than the sound card you have too.
    Imagine that. Here is a link to the short version.
    If you ask around there is a 5800 post version that is burried in another thread.
    Hope this helps.
    Also I think it would help me and other people if you
    could share some of the spread sheets that you or other people have made.
    Very well done and look to be pretty good tools.

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