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My MOSFET amplifier designed for music.
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Old 19th February 2020, 01:41 PM   #1051
prasi is offline prasi  India
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My MOSFET amplifier designed for music.
Quote:
Originally Posted by Mooly View Post
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.
i want to make a pcb for this pre. could you tell me, whats connected at x and y ? , other inputs? if so, can we make them in proper phase and not anti phase?
and also what the psu requirements?
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Old 19th February 2020, 02:05 PM   #1052
Mooly is offline Mooly  United Kingdom
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My MOSFET amplifier designed for music.
X and Y are logic voltage inputs using non standard levels. When X is at 0.3V and Y at -8 the selected input is off. Reverse the logic and it is on. All line level inputs (as many as you want) were summed at the opamp inverting input.

+0.3V which is right at the upper level actually was chosen (you can use zero volts) as that gives minimum on resistance of the FET while not causing the gate channel to conduct and pass the voltage as a DC offset. I used germanium diodes to clamp the levels.

The decoder was discrete and based on this diagram although the output sections were a bit different in the final build. I was going to use opto isolators but didn't in the end.

The circuit lent itself to adding remote control at a later date and that is what I did a few years later with the remote inputs simply paralleling the logic inputs. So the amp can all be used manually but if you select a different input via remote control it simply overrides the knob on the front.

An H bridge driver drove the motorised volume control.

There was also a reset generator to allow the amp control knob to regain control as soon as it was moved. I see it shows the germanium diode clamps and drive circuit.

Power supply was -/+18 volts for the opamps.

There are some more pictures here although these might be in this thread somewhere as well:

Post your Solid State pics here.
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Old 23rd February 2020, 11:56 AM   #1053
Ultima Thule is offline Ultima Thule  Europe
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Quote:
Originally Posted by Mooly View Post
The original 22pF is mission critical and an essential component for reliable and repeatable stability margins.

Also remember that the basic amp can be tested without the FET's fitted. The output should settle to within a couple of millivolts of zero.
Hi Mooly,

am curious wouldn't it work as well moving the 22pF to between Q4 base and Q5 collector?
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Old 23rd February 2020, 03:17 PM   #1054
Mooly is offline Mooly  United Kingdom
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My MOSFET amplifier designed for music.
Interesting idea...

So yes, it would work but it looks like the the phase response could be a little lumpy in comparison.
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Old Yesterday, 12:46 AM   #1055
Ultima Thule is offline Ultima Thule  Europe
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Thanks Mooly for running a quicky on ltspice, I was merely curious whether there were some design criteria behind C7 (22pF) location with regards to single device input stages vs LTP etc..
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