How do I measure Audio Amplifier THD using my gear ? - Page 1
Forum section: electronics / beginners

author: RichardM
author_rank: Frequent Contributor
timestamp: March 10, 2022, 10:36:36 am
content: Hi all,I have finished making an Audio Amplifier (Jean Hiraga Super Class A 30W) and it works and sounds great to my ears.For no particular reason, apart from the learning experience, I would like to measure the THD of the amplifier. I was hoping for some guidance/suggestions of how to best set it up using the test gear I have. This includes:- The Amplifier obviously and 2 x 100W 8ohm resistors.- HP35660A Dynamic Signal Analyser for THD measurement +-4V P-P (RMS ?)- Several Audio sources, the cleanest sine wave comes from my HP-239A oscillator. Another very clean sine wave comes from my Data Precision Model 2020 waveform synthesizer and the HP-35660A itself is pretty good. I have just restore 2 x HP-200CD and could use these also for fun.- Siglent 1104X-E 4 channel scopeI also have a 10db and 20db BNC attenuator and a home made one with 600ohm termination.I don't really understand the inner working of these BNC attenuators, namely why they add to THD compared to a simple voltage divider ? Maybe the quality of the BNC attenuator makes a difference to the additional distortion added ? I don't want to blow up my 35660A which is my main concern. I also don't appreciate what "termination" does for this type of testing ?So I guess I was thinking:- Amplifier outputs to 8 ohm resistors- Sine wave input to Amplifier from Data Precision source into a BNC T-connector to get 2 sine waves for left & right amplifier inputs.- Alligator clips from each side of each resistor to a Banana/BNC adapter on each channel of the 35660A, do I also put the attenuators inline with each 35660A input ? Effect on distortion ?- Set a very small amplitude 1kHz sine wave on the Data Precision source. What are amps normally specified at ? i.e. 1V P-P RMS @ 1kHz ?- Use 35660A to estimate THD as normal.I guess you can also manually do this with the Siglent scope instead of the 35660A.Anyway, welcome a sanity check on the above or any other suggestions.CheersRichard

author: srb1954
author_rank: Super Contributor
timestamp: March 10, 2022, 01:12:15 pm
content: Quote from: RichardM on March 10, 2022, 10:36:36 amStick with the HP239A as your signal source. The 2020 doesn't have good enough signal purity to be able to measure really low distortion levels < 0.1%.QuoteThe attenuators shouldn't add any measurable distortion since they are purely passive devices. It should be remembered that BNC attenuators are mostly designed for use in 50systems and can't withstand too high a input voltage. Applying the output of a 30W amplifier to a 50attenuator will probably burn out the attenuator elements in a typical BNC attenauator.Most likely the excess distortion readings you are seeing are because you are attenuating the input signal to your analyser too much. If the input signal is too small, relative to the analyser's full scale input rating, the quantisation error produced by the analyser's digitiser will contaminate the digitised signal and appear as excess distortion.For a 30W amplifier it should be possible to directly connect the amplifier output to the analyser input without additional attenuation. Just be sure to set the analyser to its minimum sensitivity first and slowly increase the sensitivity until just before input overload is indicated.Even with the optimum input signal level the 35660A won't be able to measure the lowest distortion levels (<0.03%) because of its limited dynamic range.QuoteThe Siglent scope is less suited for measuring low distortion levels than the 35660A due to its even more limited dynamic range.

author: TimFox
author_rank: Super Contributor
timestamp: March 10, 2022, 10:29:41 pm
content: Note that THD analyzers actually measure THD+N (including noise), since they null out the fundamental and measure whatever crap is left.A 50 ohm attenuator might increase the THD of a generator if it overloads an output designed for higher impedance; usually, 600 ohm outputs like a 239A can tolerate the 50 ohm load since the output has already gone through a 600 ohm attenuator, so the load on the source doesn't change much even though the output decreases with the stronger load.When measuring THD+N of a "normal" amplifier, at a reasonable frequency, as a function of output voltage, one expects 3rd-order to dominate at high levels (% THD proportional toV2), with 2nd-order at medium levels (% THD proportional toV). However, at very low output levels the harmonic distortion should be negligible (absent crossover) and what is left is a relatively constant noise voltage, which will appear as a % THD increasing with decreasing level (% THD proportional toV-1). To get a reasonable estimate of the amplifier distortion, do the measurement up to the clipping level (where the distortion suddenly rises precipitously).If your signal analyzer is only good up to 4 V, you may need to add a reasonable voltage divider across your 8 ohm load (such as 180 and 20 ohms in series) to decrease the input to the analyzer.

author: srb1954
author_rank: Super Contributor
timestamp: March 11, 2022, 03:36:25 am
content: Quote from: TimFox on March 10, 2022, 10:29:41 pmAccording to the 35660A spec sheet its input range is up to 31.7V peak. This is sufficient to directly handle the output of a 30W amplifier into 8without additional external attenuation. Testing an amplifier higher than 60W output into 8would require additional external attenuation.

author: TimFox
author_rank: Super Contributor
timestamp: March 11, 2022, 03:41:30 am
content: What was your reference to +/- 4V for the analyzer?

author: RichardM
author_rank: Frequent Contributor
timestamp: March 11, 2022, 10:54:37 am
content: Quote from: TimFox on March 11, 2022, 03:41:30 amIt says +-4v max on the front panel near the inputs ? Perhaps that is referring to DC ?

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: March 11, 2022, 12:52:45 pm
content: I would also add a multimeter to check and recheck everything.Jeff