Man with one clock always know the time. Man with two clocks never certain. See Segal’s Law. This is an incomplete comparison of two measurements between several audio analysis systems. THD and THD+N will be compared at 999.023 Hz and 0dBV (1Vrms). Measurement system comparison setup: 1. PrismSound dScope III audio analyzer 2. Quantasylum QA401 audio analyzer 3. Picoscope 5243b oscilloscope / spectrum analyzer 4. Hewlett-Packard 339A audio analyzer 5. Tektronix SG502 audio oscillator 6. Atten ATF20B function generator 7. Fluke 189 DMM 8. AV Access U2EX50 USB cat5e 50M extender (galvanic isolation from laptop) 9. USB input – Audioquest Forest 0.75 meter 10. Tecnec 1505-B-B 75 ohm RG59 BNC cables 11. Shielded 16 AWG 5-15P IEC60320 C13 power cables 12. Signal sources set to 999.023 Hz, 0dBV (or as close as possible for analog signal generators) 13. 16384 bin FFT with Hann window function 14. 12 averages except where noted Pictures of test setup: Tektronix SG502 - Atten ATF20B Quantasylum QA401 - Picoscope 5243b - dScope Laptop currently displaying dScope system Quantasylum QA401 - Picoscope 5243b - dScope - HP339A - Fluke 189 Main, digital and analog signal management - Separate shielded mains for SMPS powered devices. Cat5e USB extender for galvanic isolation from laptop. Tektronix SG502 THD observation on HP 339A Closeup - SG502 THD observation minimizing parallax error The numbers: Editorial: For the most part I hate describing audio system performance with numbers. Graphs contain far more extensive and detailed information providing a better picture. Many people, not technically knowledgeable, would like a simplistic "goodness meter" and latch onto numbers as a way to compare two audio devices. This is misguided at best; often misleading, propagating the blind leading the blind. See the "obsolete" 20 bit converter discussion concerning Stereophile, DAVE and Yggdrasil on several forums for a very good example of how easy it is to fall into the trap. Hint: there are NO DACs providing analog output resolving to an equivalent of 32 bits digital representation as an example. Some observations on the numbers acquired: 1. 5243b THD+N is just wrong - not ruling out operator error yet 2. dScope THD+N at the top of the respective screenshots is a continuous measurement as opposed to the THD readings below which are averaged over 12 readings. It is possible for a given moment to indicate a LOWER THD+N than the THD. Over time the higher number is the one used. Very hard to synchronize a screen capture to a particular reading so there will be a few discrepancies between graphs and the chart of numbers. 3. The QA401 has an unusual signal generator output with signal amplitude ramping up and down. Measurement by other instruments expecting a continuous amplitude for asynchronous measurements are challenged to obtain a useful reading. Thus the dScope may indicate a number higher than that obtained by the QA401 which is synchronized to its own generator. Editorial Conclusion (warning - personal opinion ahead): The dScope offers a much greater set of measurements, pre-made measurement scripts, higher resolution, speed of measurements, etc. as well it should for the price. But given how well the QA401 performs within the constraints of what it was designed to measure, it represents an incredible value. If I were designing audio amps to retail, on a constrained budget, I would opt to own a QA401 and rent time as necessary for the larger set of measurements utilizing an AP, dScope, SRS, Rohde & Schwarz, or equivalent. I believe the QA401 should be part of any serious audio lab bench.