Class A, 4 Watt, No Feedback, Simple Circuit, Great Sound

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#101
Thinking long and hard about how to lower the noise on my compression driver (minidsp...) I realized I have a good solution in my drawer.
A high quality SE amp with reduced gain (15-16dB)!

Finally built this and stripped the chassis clean from 2xLM3886 and a big transformer.
Now it is a tiny and light SE-amp at ~1.6W max. Biased at ~600mA.
Power dissipation is 15W/channel.
Perfect for compression drivers!

Thanks lineup!

PS.: Also have a P089ZB from Mark Johnson in line with the amps. Thought it couldn´t hurt keeping eventual noise from the Vicor SMPS´s away.
Thanks to Mark too!
 

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#103
Sure, here they are. (Kicad-files, schematic, gerber and LTSpice file if you want to simulate with different supply voltages, bias currents, RC-filters etc.)
 

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#107
I wouldn´t call that a high price but it seems JLCPCB extends the PCB by the looks of it.

Prasi´s PCB sure is a better choice if your heatsinks are higher than 50mm and you want to mount it vertical/directly on the heatsink.

R6 (1k) in lineup´s schematic (post 4) has about 350mW power dissipation with the values shown.
If you want to go any higher with the supply voltage (like I did with 24V),
power dissipation is ~500mW and too high in my opinion for a 0.6W resistor.
I used two 2k in parallel.
 
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#113
bear said:
The amp is essentially a Nelson Pass Zen but with the output config made more complex, and the input transistors made more complex. The same but different? Ok the Zen had gain in the output, iirc. But still...
Click to expand...

Iirc the Zen is open-drain, not unity gain at the output stage, so it's configured as a current source / high output impedance, not a voltage source. A current source may have distortion benefits for the speakers.

OTOH, linearising the output stage with feedback means that non-linearities and phase shifts from the speaker are also picked up (cone break-up, box reverb etc) and fed back to the input, which I think is not a good thing.

But many commercial class-D chip amps also have an output stage without feedback, so I think it's safe to say they tested it out.
 
#114
MEGA_amp said:
thank you, works perfectly. My version was a bit noisy as well, was using a smps, probably don't need as much gain as well, as it's a desktop amp
Click to expand...
Check if it could be a grounding issue, if not maybe try a RC-filter.

Not sure how to best lower the gain but it seems that lowering R2 does the trick. If you change R2 from 1k8 to 1k gain halves to 8db IIRC.
 
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#119
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