Amplifier Test Equipment Recommendations

Hi there! I'm looking for some recommendations for bench/test equipment. Minimally I would like to improve my ability to test amplifiers that I build and repair. Stretch goal, I would like to learn more about the measurable electrical signatures of euphonic details in systems I like. Also, I may venture into reactive and acoustic measurement. Currently, the relevant stuff I have collected:
  • A few Fluke multimeters of various capability
  • Tektronix Scope (pretty decent one, though more complicated than my older one that i miss dearly...)
  • 24/192 multichannel audio interface
  • Small collection of resistive loads
  • Calibrated acoustic microphone
  • A nice old hickock tube tester
I know for sure I would like a Signal Generator. I'm usually fine with used gear, but there is a wide range of devices and prices. Are the older analog devices trouble? Any recommendations for signal generators suitable for our hobby? (I'm not expecting to go RF on purpose any time soon :) ). I have gotten by using a breadboarded sin thing with my scope for a while, but I'd like to do more accurate and reproducible work. I have less time to restore or calibrate equipment these days fwiw.

My second question is whether distortion analyzers (like the old HP units) are worth it. That is, given a good audio interface and analyzing in software (FFT), do they offer any advantage? I guess I'm trying to work out if they are worth the money and TLC to keep running well. They look super cool either way, heh :)

Last question is if I'm missing anything that has proven useful to you?

May your manifolds be smooth,
Joined 2009
Paid Member
The HP 3312A signal generator and its modern equivalent, the 33112A, are very handy. Get one that's calibrated if you buy a used one.

The HP 8903 is an excellent instrument as well. It comes in an A and a B version with the 'B' being a bit more desirable. That said, the difference in practice is subtle. I wrote quite a bit on that here: The 8903 will get you down to -80 dBc THD (0.01%), though the handful of samples I measured reached -95 dBc at 1 V, 1 kHz, 20 kHz bandwidth. For the vast majority of equipment that's plenty for fundamental "is it broken?" testing and may even be enough for performance testing.

The next step up is Audio Precision's analyzers. The older System One requires a DOS PC to work. You may be able to get a System Two 20xx, 22xx, and 23xx to work if you have an older PC as well. They require a special interface card as I recall. These System Two should work with AP's USB-to-APIB interface:
  • 2422
  • 2522 (96k)
  • 2500 (96k)
Cascade Plus:
  • 2122
  • 2622
  • 2722 (96k)
  • 2700 (96k)
2700 Series:
  • 2702
  • 2712
  • 2722-192k
  • 2720-192k
At least that's what AP told me before I bought my APx525. The current APx5xx lineup talks to the PC by USB and the software will work with Windoze 11 (assuming you get the latest version).

You'll notice that even the used AP analyzers tend to be pretty pricey, but the hardware is amazing and the software is a joy to use (and I don't say that often about software). For a lower cost alternative, the offerings by QuantAsylum look very capable:

That said. Before you rush out and buy an audio analyzer I suggest that you play with your current audio interface. You may find that a few attenuators and some free software (REW, ARTA, etc.) will cover your needs.

I designed and built an audio signal generator a long time ago. The signal generator serves two needs:

1) Low-distortion sine waves. This requires a sine-wave oscillator with a very slow feedback loop to control amplitude. Inexpensive generators have high distortion.

2) Fast rise time square waves. This is needed to test response to >>20KHz harmonics. DAC-based generators have slow rise times.

I never got around to the hardware distortion analyzer, but I have done a lot of work with software FFTs. People on this board are designing and building amplifiers with distortion below the noise floor. Analysis requires a FFT to separate the harmonics from the noise.
The older System One requires a DOS PC to work.
Hi Tom
All of the System One's will run on S1 DOS or ApWin (APIB, APIB USB) some systems will run on SERIAL or GPIB also. New PC's don't have ISA or PCI slots and only have USB. ApWin 2.14 or 2.24 need a special APIB USB ADAPTER (Non AP) to run on USB. I have stock of the APIB USB ADAPTERS.
Joined 2009
Paid Member
All of the System One's will run on S1 DOS or ApWin (APIB, APIB USB)
Interesting. What I stated above is what the AP rep told me back in 2014 when I was looking at getting a System Two. That was based on the AP USB adapter. Now that you mention it I do recall that there was an effort here to get USB going on the older analyzers. Sounds like you have a solution. Do you have a website set up for this? I'd love to write a page about audio analyzers and would link to your page.