Measuring Distortion with a HP 339A and a Keithley 2015 THD

I have had my HP 339A for a long time now and despite only using it a small handful of times it is a really great instrument – but – its big and takes up a lot of shelf space. A few weeks back a purchased a Keithley 2015 THD which is a 6.5 digit bench meter with a built in distortion analyzer, I bought it because of the distortion analyzer function. I would like to get some more instrument shelf space back so am able to replace the HP 339 with this 2015THD I will make that much-needed space. I decided to build a simple output circuit so that I can induce some crossover distortion and compare the basic measurements on these two devices to see the results. Now I must make it clear that this is a basic 101 on measuring distortion, I am sure there is a lot more to know that I do but I believe I cover off the basis.

The simple drawing I used to explain the test and test circuit.

I will tear down the HP 339A on video to show how nicely these are built. This is one of those rare “all analog” devices that has not digital parts at all, watch out for that video if you like seeing the guts of nicely built equipment.

Thanks for watching.

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  1. I just wondering whether or not to but one of these on ebay today for £320 (2015) to test a HiFi tube amplifier I am in the process of designing…

    Then I found your video, and it answered my question, that yes it can measure an audio amps THD at the push of a button, and so I bought the meter which should arrive soon.

    Many thanks


    1. Hi Peter, glad you found the video of some use. The guy I bought mine from (kjds_stuff) has quite a few of them for sale on e-bay, I would say if you are into audio amps and want to measure distortion this is a pretty good meter to do it with. Gerry

        1. Hi Gerry,

          I’m not sure you’re correct there. I don’t think THD analysis is an option for the 2015 and 2016, it’s always present. A 2015 without THD is a 2000. Great videos, thanks for making them!


          1. Hi Anton,

            Yeah you could be right having just googled again, I was convinced I had seen it as an option – perhaps it was late 🙂 Thanks for the comment


  2. Thank you for the video of the 339a distortion analyzer. I watched the tear down video as well. It was a trip down memory lane for me since I designed the notch circuit (part of which is on that large circuit board in the back that you had some trouble removing.

    That board kept me awake many nights. By the way, both the notch circuit and the oscillator ciercuit are based upon the “bridged T” notch filter which is tuned by the resistors and capacitors chosen by the wafer switches, then fine tuned by two photodiode/LED modules.

    I spent weeks doing calculations comparing the responses of Wien bridge, twin T, phase shifter, and bridged notch circuits before I chose the bridged T configuration.

    The biggest headache was the slower speed of the auto null circuit as the frequency to be measured approached 10 Hz.

    Production started in 1977 I think and I followed the 339a into production which was a very busy and frustrating time for me. You end up reporting to several bosses at once and their objectives may differ.

    The oscillator and input circuits were designed by Fred Kitson and the power supply was designed by the project manager, John Manning.

    Design and production were done at HP inLoveland Colorado. The 339 was later produced at HP in Japan.

    Best regards, Joel Nesheim

    1. Hello Joel,

      Thank you for posting, wow its great to get a comment from one of the original HP designers and to get some insight, I was 11 years old when production started 🙂 I hope I done the 339a justice when I done the teardown, review and explanation of how it works. I love the way HP used to design and construct their equipment, pure quality done properly. Thanks for watching reading the blog.


  3. Well, What are the odds of somebody doing a teardown on some kit and one of the desighn team
    reading that blog donkys years later!!!.
    After watching the vid and reading the blog i tried googling a firm i used to work for and a little
    desighn work for ( astronic , dalston gds stanmore ) but found almost nothing despite there
    having been in production for 20 or 30 years…

  4. Purchased a 2015 some time ago and now it’s time to do some THD measurements. Nice of you to do the video. I see that the 2015 setup is simple. The manual doesn’t show the actual hookup procedure. Do I use an 8 ohm load across my tube amp output and place the 2015 leads cross the load. I’ve built a couple of nice tube amps with just a DMM, but I’m a brick short on other types of measurements. Your comments would be greatly appreciated. Thank you again for your video. Regards, Bill Pennington

    1. Hi Bill,

      That’s correct, you drive your amp into an appropriate resistive load and measure across that, the amp needs to see the right impedance in order for the distortion measurements to be meaningful. Good luck, glad you find the video of some use


  5. Hi Gerry, I have get one 2015THD and I would know if you have use serial port to get individual harmonics of a signal.
    Thank you.

  6. Would desperately love a 339A, please let me know if yours ever needs a loving home. Repair amps for charity and would love to test performance, have a free scope from rigol but that’s not much help. On loan perhaps for a annual fee?

    1. Hi Rupert,
      You can find these on e-bay, they sell for around £400, if you search ebay now you will see a few of them. Most of them come from the US so if you are in the UK you have shipping costs and import duty to contend with. The unit I have is in exceptional condition for its age so I am very reluctant to part with it. That being said I hardly use it so if you want to make me an offer and its going to go to a good home and get some real use I would probably consider it.

  7. Hi Gerry,

    thank you for comparing HP 339A and Keithley 2015 THD.

    40 years ago I worked with HP 339A and I was very pleased.

    Today I am retired, and I would like to make some amplifier developments. So my question is: What do you recomment if I am looking for s distortion measurement set (which should be not expensive). It shoul dbe able to detect distortion above 0.005% at 100 Hz, 1 kHz and 10 kHz.

    Are there and A/D converters with PC support, which I could use? or what do you recommend?

    Thank you Peter

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