"Light Touch" Class A Amp

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Official Court Jester
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Too bad, but the Tannoys probably occupy a heck of a lot less cubic.

As promised, here's a simulation with the values of feedback resistors R1 and R3 whacked in half - R3 goes from 1.8k to 910R, and R1 from 100R to 47R. The gain remains the same at 18dB, or 8X. Current in the p-channel rises to 16 mA. Dissipation in the p-channel is a whopping 60mW, totally ok even for a plain TO-92, cruising for the Zetex/Diodes, Inc. E-line package, which is good for about 0.5W for a sane temperature rise.

As can be seen, distortion goes down compared to the previous example and is still dominated by 2nd and 3rd harmonics. However, it doesn't go down that much, leading me to believe that I'm approaching the point of diminishing returns - time to declare victory and go home..
 

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The IRFP360 in the simulation is a placeholder for the IRFP260, my go-to monster fet. I have some IRFP150s, but would rather use the 260 to get the better junction-to-case thermal resistance and higher transconductance (one size larger die). The 150 would work, but I imagine the bias point would be a little different. Adding the variable bias setting in the bottom of the input resistor would likely make switching between output devices relatively painless.
 
I just finished running the latest circuit at 16 ohms and 4 ohms load - the default so far has been 8 ohms. At 16 ohms, the gain goes from X8 to X10, and the THD goes from ~0.2% to 0.48%. At 4 ohms load, the gain goes down to X6 from X8, and the distortion goes up to 1.14%, still dominated by 2nd and 3rd harmonic.
 
I managed to wedge an adjustment circuit into my current layout for the Light Touch, which should make it easier to use different devices, say, IRFP240 through 260. I would still be inclined to match the bottom drivers (p and n channel) to make both channels consistent.

I also added adjustment pots to the "Amp Cramp" to enable adjustment of both bias current and output centering.
 
Official Court Jester
Joined 2003
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Speaking of not holding your breath, here's
my 3 year old 2SK77B amp board.

Gotta stuff it up one of these days.....

Need some help? :)

well , I'm always available for Summer-camp (speaking of ...... Summer , Autumn, Spring , Winter , Whatever,Pronto ) ...... but The Funny One is pretty successful in ignoring me

:clown:
 
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