Audio test equipment

Joseph G

Joseph G

Active Member
Hello, does anybody have a recommendation for a distortion analyzer/meter with a built in oscillator that isn’t extremely expensive and that is easy to use. Google searching has become futile, and I want to know from real people what they are using. Thank you
 
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I use an old Sound Technologies 1700a, its not too hard to use but its quite elderly at this point and probably not super cheap to find or deal with if it needs any sort of repair. If you trip over one for small money its a nice piece but pretty hard to recommend someone search out one of these dinosaurs.
 
Look around for a pre-owned Sound technology unit from the 80's and 90s. They measured down to .005 as I remember and thats good enough for any one. maybe it was .01% but thaTs good too for most folks. Just make sure you have a good AC supply and a good ground to get the best consistent readings. Oh and will need to make either your own oil cooled load resistors or buy the really expensive ones. Youwill need loads for pre-amps and equalizers of the appropriate values. Old tube stuff the loads should be over 100, 000 ohms to test preamps. And newer pre-amps about 10,0000 ohms. Hook cables need to be well made too. And should a great true RMS/peak volt meter the is calibrated back to the Bureau of standards to make sure your sound technology is up to snuff. Some had average and peak value switches. Neither of which are true RMS voltages for when specking high fi equipment, so get your self a calculator, too. Audio Classics has had some for sale in the past.
 
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Another item to consider is the HP 339A. This will generate and measure harmonic distortion down to 0.01% full scale which allows consistent and accurate measurements in the range of 0.002%. I had (on my bench) both the Sound Technology 1700 and the 339A - I gave away ST1700 since I found it just took up too much space and provided no real advantage over the 339A. Now, if you get the 1700 with the IM measuring option, that would tilt the favor towards it ... if you have the space. If you're able to do so, both of these are frequently available "broken" and can be repaired (manuals are available on the Internet). That's what I did in both cases here.
 
Keep in mind that almost all "vintage" test equipment will have issues, and you can easily spend a great deal time and effort working on an old piece of gear trying to get it stable, reliable and trustworthy. While many older units can and still are performing as well as they did when new, it's usually because we've put a lot of hard work and effort into them.

All of the older machines have regular failure points; relays, LDRs, switches & semiconductors, many of which are custom or are no longer easily available, if at all. And that's in addition to all of the usual failing caps, especially Tantalum types...

And while spending big money is no guarantee of getting a good machine, spending peanuts is almost a guarantee of frustration.

My own bench is equipped with Amber and Rohde & Schwarz analyzers, and it took a long time to get them all "just right"

If I ever win the lottery I'll be getting me an Audio Precision system:)
 
Look up TrueRTA online. I found it very useful when I was using it quite a few years back. It is, or was, a small program that was pretty powerful. Not sure if this is what you are needing, but give it a look.
 
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I have a Krohn-Hite 6880A . It has an accurate 1khz oscillator. I got it for less than $150, but when it arrived, it was not functioning properly. The seller offered to pay shipping back and fully refund all money I sent him. I asked if he would mind if I opened it. The service manual was easy to find online. It is designed with a main board that several ckt cards plug into. I was hoping one of those cards was loose. No such luck. I was able to identify a broken leg on a transistor. It was using the back panel as a heatsink and I think it took a hit during shipping which cracked the leg on the transistor. Again the seller, offered to order a replacement part, but I had already ordered a couple at <$1. After reacing that part, it worked perfectly. The service manual has a detailed calibration procedure. I’ve had it for about 3 years and it has been reliable. They aren’t as common, but the price may be right if one comes along.
 
Thanks for all the advice, it drives me crazy when I know an FM discriminator inductor has drifted, and relying on my ears to adjust it without a generator and analyzer just won’t cut it. I am impressed by all the dedication and skill of the people in “repair land”, as it has almost become a lost art. Thank you
 
arts said:
All of the older machines have regular failure points; relays, LDRs, switches & semiconductors, many of which are custom or are no longer easily available, if at all.
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thats the big issue. I had a bad LDR in the ST1700a and managed to find a cross of a cross. It did work but it was fussy to get adjusted right. There are a lot of parts inside that thing that would need a 40 year old parts donor to replace though.

the other thing with this old stuff is the measurement process is totally manual. To do response and distortion plots I end up using Excel to record a bunch of measurements by hand. Its tedious and irritating. Modern stuff you can just push a button and it goes. Things with an FFT plot function will actually show you the distortion plot instead of just giving a number.
 
Joseph G said:
Thanks for all the advice, it drives me crazy when I know an FM discriminator inductor has drifted, and relying on my ears to adjust it without a generator and analyzer just won’t cut it. I am impressed by all the dedication and skill of the people in “repair land”, as it has almost become a lost art. Thank you
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There’s a guy on youtube, xraytonyb, that makes videos of vintage audio repairs and restorations. He has a method for adjusting discriminator using an FM generator and a scope. He has a lot of videos and covers what he is doing very thoroughly.

Here’s a link for a tuner he is aligning and shows the visual method he uses.
 
Keithley 2015. They are expensive but they have multiple functions so you are really getting several pieces of equipment for one price: High quality DMM, frequency counter, distortion analyzer, sine wave generator. The DMM has flat frequency response out to 100KHz, and can measure in dB, so you can use it to set up tape decks or check frequency response.
 
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