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Mooly 10th March 2008 11:05 AM

My MOSFET amplifier designed for music.
2 Attachment(s)
Hi, Well after having seen all the amazing projects and designs you guys have come up with I thought I would share my design on these pages. First and foremost this was designed for music reproduction in a domestic setting and without wanting to "hijack" another current thread, YES amplifiers do sound different, that is why I spent so much time in the development of this one. This amp is without doubt "musical" in all the best sense of that term, it's ability to recreate a believable soundstage is absolutely compelling. If anyone is interested in what it looks like and further details there are some piccys in the "Post your solid state pics here" forum. (About post 283) The thinking behind this project draws on the work of the late John Linsley Hood whose designs and thinking I much admired.


This index of the thread should help you find the relevant important notes and general information.

1. The circuit diagram in this, post #1 is correct. The only omission is a 10 ohm 2 watt carbon/metal film across the output coil.

2. The TL071 opamp must not be substituted. This type works correctly on the single negative 12 volt rail. Other device types may not do so.

3. A speaker relay incorporating a switch on delay is essential due to the action of the servo and the single ended input stage. This can be extremely simple consisting of a FET, a C-R network and relay. Post #273 in another thread shows the idea of solid state relays developed into a working example. Credit for the idea of the SS relay goes to those mentioned in that thread.

4. alex mm' brilliant PCB designs. Post #300

5. The FET pinouts, post #113

6. Squarewave testing, posts #157 to 161

7. Heatsinks, post #164

8. A glowing endorsement, post #186 and this,

9. Spicing it up... LTspice files, post #312

10. And finally some of my benchmarks and my thoughts on how it sounds. The adventures of getting the design to where it stands today, post #107


Mooly 10th March 2008 11:08 AM

2 Attachment(s)
This is the Preamp in outline form (I am only just getting to grips with a PCB Software package) all the original work was hand drawn. The series /shunt switching works brilliantly and give total isolation between inputs. Even with something ridiculous like a 10volt peak/peak squarewave as an input, there is no audible breakthrough even with volume on max.

Tube_Dude 10th March 2008 02:50 PM

Hi, Mooly

You don't need R24 and R23... ;)

darkfenriz 10th March 2008 03:24 PM


Originally posted by Tube_Dude
Hi, Mooly

You don't need R24 and R23... ;)

... and C3

Tube_Dude 10th March 2008 03:39 PM


Originally posted by darkfenriz

... and C3

Hi Darkfenriz, how are you? :)

I agree , the servo is "in charge" of taking care of the offset...

Anyway, C3 in the schematic is with the polarity inverted... ;)

Mooly 10th March 2008 06:32 PM

Hi Tube_Dude, you know, I wondered that at the time, my reason for including them was to provide more? if thats the right word isolation for the MOSFET's . They do reduce efficiency though. I would like to develop this design further, possibly running the front end (Up to Q3 and Q5) on a higher rail with shunt regulation (Power supply rejection is one area that could be improved).
Darkfenriz, wish I could do away with the electrolytic in the feedback arm, as you say the servo is in charge, but the base voltage of Q1 is around -6volts, so C3 is unfortunately needed (polarity is correct on circuit with about 5 volts across it) . That's the price you pay for using a single ended input stage but one that's well worth it. The TL071 works fine with only a single negative rail in this application, many other Op-Amps won't and would need + & - supply.
Thanks for the input

jerome69 10th March 2008 07:02 PM

Hello Karl,

Very good !


darkfenriz 10th March 2008 07:54 PM

I'm fine, Jorge
Just a little overworked, underpaid, bored and lazy last days.
How are you?

darkfenriz 10th March 2008 07:57 PM

the -6V on Q1 base will go up closer to zero if you remove the capacitor, like -1.5V or so...

mikefromaus 11th March 2008 08:00 AM

OK now we need someone to design a PCB ;).
I have tried designing a few using ExpressPCB but it seems to end up a mess!
I have many of the 2sk1058/j162 waiting for something like this!!

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