Hi,
Quote:
Originally Posted by mhennessy
Looking at your numbers, they don't quite add up. About 6mA might come from R58, but 15mA is "missing" somewhere. Did you have a load connected when you made these? Perhaps these are just the errors caused by the tolerance of the 0.47 resistors...
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I think the errors may just be down to the time between each measurement. Both the DC offset and voltage on the top end of R64 drift about, never quite settling down.
Quote:
Originally Posted by mhennessy
I don't know if they would have gone to the trouble of matching the output transistors for this low-cost model.
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Probably not and neither did I.
Apart from them being unmatched, each device has its own heatsink and they all end up operating at different temperatures. I'm wondering if this is connected somehow to the drift I'm seeing.
Anyway, cutting a long story short, I've now replaced all eight output transistors and experimented with different quiescent currents.
This is how I have it operating at present, these are the voltages in mV across the 0R47 resistors, 'upper' and 'lower' as per the schematic, measured with no load connected.
It'd been left on for at least 30 minutes and had vaguely stabilised, not that it ever really does...
L: upper: 21, 24; lower: 41, 4
R: upper: 27, 13; lower: 36, 7
About 5 minutes later...
L: upper: 20, 23; lower: 40, 4
R: upper: 25, 13; lower: 35, 8
This works out as roughly 80-100mA total current for each channel, shared rather unevenly.
Distortion for both channels measured at 1kHz is 0.03% at 1W and 0.12% at 3W and 10W into 10R (the closest to 8R I could find.)
The specification (attached) states "less than 0.5%". That seems high for a solid-state amplifier but specifications never tell the whole story. Music is a bit more complicated than a 1kHz sine wave...
It's now connected to Celestion Ditton 44s and... well, I don't want to end up sounding like a magazine hi-fi review but 'detailed', 'open', 'transparent', 'clean' and 'tight bass' might be mentioned if I let myself get carried away.
"It works", "it runs a bit warm but hasn't caught fire yet" and "I'm thoroughly enjoying listening to a wide variety of music" will do
Cheers, Kat