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Mosfet class A headphone amplifier
Mosfet class A headphone amplifier
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Old 10th December 2017, 05:12 PM   #11
Buzzy is offline Buzzy  Singapore
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The lt1086 and lm317 are positive voltage regulators. Will it work when the out pin is connected to negative supply ?
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Old 10th December 2017, 05:24 PM   #12
sevenup2278 is offline sevenup2278  Netherlands
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Sure, it is only relative. Output should be lower then input.
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Old 10th December 2017, 05:40 PM   #13
Mooly is offline Mooly  United Kingdom
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Mosfet class A headphone amplifier
The RF filter looks OK. 470pF is suitable to use with a 1k. Any decent small film cap is fine, but not a ceramic disc type.
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Old 10th December 2017, 06:08 PM   #14
tomchr is offline tomchr  Canada
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I'd put 470 kΩ to ground on the "outside" of the input coupling cap to prevent the cap from charging when the input is disconnected.

Tom
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Old 10th December 2017, 06:35 PM   #15
BesPav is offline BesPav  Russian Federation
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Quote:
Originally Posted by sevenup2278 View Post
Schematic

First, change connection to inverting.

Second, read this thoroughly:
http://www.ti.com/lit/an/slyt174/slyt174.pdf

Use circuit B with something like AD8067, LT1222, LM7171, THS4021.

Third, drop linear regs, use passive CRC with 7-10 Ohms inline. Decouple opamp locally closer than 5 mm, higher than 10 nF NP0.

Fourth, drop MOSFET, use medium power BJT this will shift opampís output stage slightly with A-class guarantee and decrease load capacitance.

Fifth, do not use toroid trafo for supply, only sectioned El.

Eight, put little common mode choke at the input.

Seventh, put common mode choke at the output.
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Old 10th December 2017, 06:43 PM   #16
sevenup2278 is offline sevenup2278  Netherlands
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Quote:
Originally Posted by tomchr View Post
I'd put 470 kΩ to ground on the "outside" of the input coupling cap to prevent the cap from charging when the input is disconnected.

Tom
Hi Tom,

Is this resistor still needed whene there is a potmeter on the input?
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Old 10th December 2017, 07:38 PM   #17
sevenup2278 is offline sevenup2278  Netherlands
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The idea was to make a mosfet output stage and use fet input opamp. I will stick with that idea for the moment.
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Last edited by sevenup2278; 10th December 2017 at 07:52 PM.
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Old 10th December 2017, 10:01 PM   #18
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by sevenup2278 View Post
Is this resistor still needed whene there is a potmeter on the input?
If you have a volume pot soldered to the input of the amp board, the volume pot will drain the cap.

Tom
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Old 10th December 2017, 10:04 PM   #19
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by BesPav View Post
Third, drop linear regs, use passive CRC with 7-10 Ohms inline. Decouple opamp locally closer than 5 mm, higher than 10 nF NP0.
That will greatly increase the supply impedance compared to the regulated supply. It will also result in worse filtering, in particular when the parasitics of the supply caps are factored in.

As for inverting vs non-inverting: I'm assuming you're concerned about common-mode distortion. That's not really a thing anymore with modern opamps. I'm sure you can find one opamp out there somewhere that shows common-mode distortion, but amps like the LME49710, OPA1611, etc. perform better non-inverting than inverting, at least for buffer applications, due to the 6 dB higher loop gain of the non-inverting configuration.

Tom
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Old 10th December 2017, 11:54 PM   #20
PRR is offline PRR  United States
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Is Tori still around??

I don't know why you would want +/-15V and 200mA for any headphone. Well, maybe this week, we are in for a cold spell.

When I looked at this, a long time ago, I could not avoid a ~~10MHz instability without the added C5-R4 network to decouple the NFB above 150KHz.
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