Ï have just finished the Aleph30.THD is about 0.9% ( at 1W), which is much more than Mr. Pass states in the manual. Bias ( input pair, output MOSFets) , voltages is almost the same as manual shows. Has anybody an idea why?
Mr. Pass raises an important point: the frequency at which you take the THD is crucial. Standard is 1 kHz, but measuring across the audio band is typical; just report a given THD at the specified frequency, along with the RMS voltage (approx 2.8 V rms with 8 ohm load = 1 W).
Also, you need to state the test load. 4 ohm measurements will be greater than 8 ohms. Further, was a capacitor added in parallel with the load for test purposes and then left during THD measurements?
Finally, if the distortion is truly greater, it could be related to your layout (grounding issues, power supply design, etc) and not inherent in the overall Aleph design.
RlJones, Mr. Pass: You know something about electronics?! Dont you? )) Thank you. Now I realised the trouble. THS is overall value. Counting only with 2nd ( 10mV) and 3rd ( 5.5mV) harmonics we get much better values ( 3.8V ef on output , 4 ohm load ( cca 3.6W) at 1 kHz - ie cca 0.3%.) More than you wrote but better than previous one. Is it much or not? I forgot hum form the power supply ( measurement were done in a little hurry - lunch- you know on old device. Ordinary device - Audio precision is on regullary maintaince service in Switzerland).
The AudioPrecision turned back but still is not avaible for me -the guy lives more than 300km away - (in Europe it is more than in US ). So I took PC with sound card, soft and tried to do it my self / of course with known limitations of this solution).
The first is output from the spectrum analysis - at 1kHz, cca 8V ( ef) ( the amplitude you must multiply by 10) to the 5 ohm. Prominent is hum form the suplly ( 100Hz) and harmonics - 2nd and 3rd. THD about 0.06%.
Second picture - similar power, load, only with current mirror in the input differential pair. THD is lower, ( lower 2nd, a little higher 3rd).
What do you think about it? Is it right? How behaves the "factory" Aleph?
Sorry , i did not noticed the picture size limit. Picture no2 will follow.