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Old 11th February 2017, 04:24 PM   #1
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Default SE class A hybrid amp

Can a simple single ended class A monoblock be done with with a opamp and lateral MOSFET? Thinking of having the opamp run on 15v rails and the output device running on maybe 35v. After looking at opamp based headphone amps it seems like it isn't a far fetched concept however I have never seen one like this done. I would like to keep it as simple and clean as possible and the VOut of the opamp is higher then what's needed for the gate voltage so I am guessing a driver wouldn't be needed? Any help is appreciated.
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Old 12th February 2017, 04:00 AM   #2
PRR is offline PRR  United States
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One output device?

Takes two to tango. The other device can be a MOSFET etc, or a resistor, or a coil (choke, transformer).

Coil-loaded is 50% efficient in theory.

Resistor loaded can't be better than about 6% efficient. 1W of idle heat to get 0.050W in headphones is often acceptable. 100 Watts of idle heat to get 6 Watts for speaker seems much less attractive.

Ask in the Pass section, Nelson had some light-bulb (pretty resistor) loaded SE amp ideas. He probably didn't use opamp but that's not a big deal.
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Old 12th February 2017, 05:48 AM   #3
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Originally Posted by PRR View Post
One output device?

Takes two to tango. The other device can be a MOSFET etc, or a resistor, or a coil (choke, transformer).

Coil-loaded is 50% efficient in theory.

Resistor loaded can't be better than about 6% efficient. 1W of idle heat to get 0.050W in headphones is often acceptable. 100 Watts of idle heat to get 6 Watts for speaker seems much less attractive.

Ask in the Pass section, Nelson had some light-bulb (pretty resistor) loaded SE amp ideas. He probably didn't use opamp but that's not a big deal.
So you think coil loading the output device is the way to go? Can you elaborate? I only saw designs w resistive loading. I have a MOSFET that would be great driver if needed. I'm figuring since the opamp I have in mind vout is enough for the gates a simple bias scheme may be all that's really needed?
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Old 12th February 2017, 12:34 PM   #4
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by JaredC79 View Post
Can a simple single ended class A monoblock be done with with a opamp and lateral MOSFET? Thinking of having the opamp run on 15v rails and the output device running on maybe 35v. After looking at opamp based headphone amps it seems like it isn't a far fetched concept however I have never seen one like this done. I would like to keep it as simple and clean as possible and the VOut of the opamp is higher then what's needed for the gate voltage so I am guessing a driver wouldn't be needed? Any help is appreciated.
I'd suggest you build an output stage like this: SEWA - Seven Watt Amplifier

And you can put your op-amp on the front-end of it.
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Old 12th February 2017, 01:01 PM   #5
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Originally Posted by JaredC79 View Post
So you think coil loading the output device is the way to go? Can you elaborate? I only saw designs w resistive loading.
You must have missed this thread:
Zero Feedback Impedance Amplifiers
It deserves, but never had, at least the same attention that guru NP's designs enjoy here.
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