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

[joensd]

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!

 

[fazildiken]

looks like a very nice circuit. Will you share your files?

 

[joensd]

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.)

 

[fazildiken]

Thank you for sharing.

 

[prasi]

lineup 4W amp

Hi fazildiken,
here is as requested.

 

[fazildiken]

Thanks Prasi. When I want to order pcbs to jlcpcb, it includes transistors as in the picture and the price is high. Can you remove the transistors from the pcb?

 

[joensd]

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.

 

[prasi]

I think its a problem with the jlcpcb online rendering tool/ software. anyway here are files without the trannies.

 

[fazildiken]

Thank you for your interest.

 

[MEGA_amp]

Hi Joensd,

You wouldn't happen to have the missing .txt file, possibly in your LTSpice sub-folder? I'm getting an error when I run your .asc file.

thanks
John

 

[joensd]

here you go!

Amp is sounding very sweet.
Thinking of further lowering gain and lowering supply voltage.
(asking it to do 1.6W but supplying it with 24V is just raising power dissipation)
the minidsp´s noise is still audible with ~16dB gain.

 

[MEGA_amp]

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

 

[abstract]

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...

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.

 

[joensd]

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

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.

 

[joensd]

Actually I didn´t remember correctly:
R2=1k gives ~11dB gain, 680R gives ~8dB gain.

 

[fazildiken]

PCB's assembly has been completed. He has a very high quality sound. Thank you everyone.

 

[manniraj]

Quick question as the mosfets MJL4281 being obsolete with fewer options to buy genuine, I was reviewing if we can replace this with 2SC3281 as datasheet wise they seem to be exact replacement?

Thanks

 

[thimios]

Quick question as the mosfets MJL4281 being obsolete with fewer options to buy genuine, I was reviewing if we can replace this with 2SC3281 as datasheet wise they seem to be exact replacement?

Thanks

MJL isn't mosfet,it is BJT and is available here.https://www.reichelt.com/de/en/transistor-pnp-to-264-350-v-15-a-230-w-mjl-4302a-p59008.html

 

[manniraj]

MJL isn't mosfet,it is BJT and is available here.https://www.reichelt.com/de/en/transistor-pnp-to-264-350-v-15-a-230-w-mjl-4302a-p59008.html

My bad I meant BJTs only but in India these become very expensive to get them shipped with the landed price.
So I was looking at local Indian sites and did find few genuine 2SC/2SA equivalents - https://www.electroncomponents.com/2SA1943-A1943-Power-Amplifier-Transistor?search=MJL4281

 

[thimios]

My bad I meant BJTs only but in India these become very expensive to get them shipped with the landed price.
So I was looking at local Indian sites and did find few genuine 2SC/2SA equivalents - https://www.electroncomponents.com/2SA1943-A1943-Power-Amplifier-Transistor?search=MJL4281

Be sure that you will get genuine parts!