Strange oscillation with poor THD in 306 and 606

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10-Jan-2020 06:46 PM

Mark B

Mark B

Posts: 12

I have an interesting issue or finding which I hope someone may have encountered before. I have a 306 and a 606 ii both of which I’ve rebuilt as per the dada instructions,


they are sounding ok at least but yesterday I was fiddling with my new picoscope pc  oscilloscope and decided to test the distortion of these and a 405/2. On both the 606 channels and one channel of the 306, which interestingly was shorted briefly some time ago, injecting a 1khz test signal under no load gives a really good thd of less than 0.008% but on loading with a 8 ohm dummy load (8.05 ohms 1.4uh inductance) and injecting a 1khz clean sine wave signal to give Around 10w or more out, these three channels emit a noticeable oscillation at about the fundamental ***from the electronics in the amp itself*** and the thd figures worsen noticeably to say 0.015-0.018%. The remaining channel on the 306 seems unaffected.


it seems odd to me that two physically separate circuits in the 606 behave in a almost identical way (making an identical fault rather unlikely to say the least) and yet only one channel in the 306, and the one that was briefly shorted at that, does so whilst the other seems unaffected.

I suspect some internal oscillation but I’d be really interested if anyone else had come across this before, partly from an intellectual view but also because it would be nice to know if this actually needs fixing or is just a feature of these amps. 

Any thoughts?

Mark

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11-Jan-2020 10:34 AM

apper

apper

Posts: 9

I get the same THD with my two 303's, around 0.017-0.015 and THD+N 0.045%.

Although the two 303 have different driver boards.

Think I need to check the connection from 303 LS output to soundcard (use Arta), as there is way too much 50Hz and it's Harmonics.

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