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Mosfet class A headphone amplifier
Mosfet class A headphone amplifier
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Old 10th December 2017, 09:06 AM   #1
sevenup2278 is offline sevenup2278  Netherlands
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Default Mosfet class A headphone amplifier

I have been working on a mosfet buffered class A headphone amplifier.

Onboard power supply
Bridge rectifier and C-R-C smoothing
Separate voltage regulators for both channels (lm317/lm337)
Single opamp for voltage amplification
Class A Mosfet output buffer
LM317 Constant Current Source for bias

Heatsink can be mounted on the ridge of the PCB, making it one unit.

Done a test with LF356 opamp and IRF510 mosfet, works great.

Now working on the PCB layout, should fit on a 10x10cm board.
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Last edited by sevenup2278; 10th December 2017 at 09:11 AM.
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Old 10th December 2017, 01:51 PM   #2
sevenup2278 is offline sevenup2278  Netherlands
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Schematic for 1 channel
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Old 10th December 2017, 02:02 PM   #3
Mooly is offline Mooly  United Kingdom
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Mosfet class A headphone amplifier
Looks good

A couple of thoughts... a FET input opamp doesn't need R1, also you can increase R2 and decrease C3 and so use a small film cap. There is no noise penalty as the resistor is shunted by the impedance of the driving source component. 470k and 0.47uF are standard sort of values and would give an LF response way below anything useful for headphones.

An input RF filter might be a good idea.

Although there should be no offset issues with a FET opamp, you should consider returning the internal gain to unity at DC (which means a cap in the feedback return).
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Old 10th December 2017, 02:16 PM   #4
Mark Johnson is offline Mark Johnson  United States
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Mosfet class A headphone amplifier
After observing that current source bias resistor R4 will dissipate 230 mW, I would be tempted to stuff and solder a resistor rated for 2 watts continuous dissipation, such as (this) 2W resistor whose dimensions are only 3.9mm x 10mm.

I think I would also include a Zobel network on the output, which loads the amplifier and prevents oscillation, when no headphones are plugged in.

Finally, I personally prefer a 2C+1R snubber across the transformer secondary, instead of 4 capacitors across all 4 diodes in the bridge rectifier (as shown on the PCB layout). I myself prefer to control the damping of the transformer secondary circuit, and indeed I deliberately choose to overdamp the secondary (zeta >= 1.0) to guarantee the complete absence of ringing. There is a .pdf document called Quasimodo here on the DIYA website, which discusses my snubber preferences at length. But, I hasten to point out, these are my preferences. Other design engineers have their own preferred approaches, of course.
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Old 10th December 2017, 02:22 PM   #5
sevenup2278 is offline sevenup2278  Netherlands
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Quote:
Originally Posted by Mooly View Post
Looks good

A couple of thoughts... a FET input opamp doesn't need R1, also you can increase R2 and decrease C3 and so use a small film cap. There is no noise penalty as the resistor is shunted by the impedance of the driving source component. 470k and 0.47uF are standard sort of values and would give an LF response way below anything useful for headphones.

An input RF filter might be a good idea.

Although there should be no offset issues with a FET opamp, you should consider returning the internal gain to unity at DC (which means a cap in the feedback return).
Thanks for the input

I have made some changes, however not sure about the RF input filtering. Is this what you mean?
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Old 10th December 2017, 02:24 PM   #6
Keruskerfuerst is offline Keruskerfuerst  Germany
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You need a RF filter at the input.
R1=1kOhm
C=470pF FKP/MKP
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Old 10th December 2017, 03:03 PM   #7
sevenup2278 is offline sevenup2278  Netherlands
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Quote:
Originally Posted by Keruskerfuerst View Post
You need a RF filter at the input.
R1=1kOhm
C=470pF FKP/MKP
Ok, so it is a series R / parallel C lowpass filter.

Would this be a good capacitor for this purpose?

https://www.eoo-bv.nl/stabiele-hq-36...pf-630v-2.html
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Old 10th December 2017, 03:14 PM   #8
sevenup2278 is offline sevenup2278  Netherlands
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Pitch for the CCS resistor is bigger then the rest (12.5mm) so I should be able to fit this resistor MF 6,8 Ω 2W 5% MO

I have added a zobel to the output, is 100n / 10Ohm ok?

Quote:
Originally Posted by Mark Johnson View Post
After observing that current source bias resistor R4 will dissipate 230 mW, I would be tempted to stuff and solder a resistor rated for 2 watts continuous dissipation, such as (this) 2W resistor whose dimensions are only 3.9mm x 10mm.

I think I would also include a Zobel network on the output, which loads the amplifier and prevents oscillation, when no headphones are plugged in.

Finally, I personally prefer a 2C+1R snubber across the transformer secondary, instead of 4 capacitors across all 4 diodes in the bridge rectifier (as shown on the PCB layout). I myself prefer to control the damping of the transformer secondary circuit, and indeed I deliberately choose to overdamp the secondary (zeta >= 1.0) to guarantee the complete absence of ringing. There is a .pdf document called Quasimodo here on the DIYA website, which discusses my snubber preferences at length. But, I hasten to point out, these are my preferences. Other design engineers have their own preferred approaches, of course.
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Old 10th December 2017, 03:32 PM   #9
Keruskerfuerst is offline Keruskerfuerst  Germany
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MKP 470pF

Yes, this one is OK.

Do you have a power supply unit?

Last edited by Keruskerfuerst; 10th December 2017 at 03:52 PM.
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Old 10th December 2017, 05:03 PM   #10
ammel68 is offline ammel68  United States
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Mosfet class A headphone amplifier
Quote:
Originally Posted by Keruskerfuerst View Post
You need a RF filter at the input.
R1=1kOhm
C=470pF FKP/MKP
Why is a MKP capacitor necessary for a RF filter?
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