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Simple Single-Ended Headphone Amp
Simple Single-Ended Headphone Amp
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Old 6th December 2018, 07:07 AM   #1
wrenchone is offline wrenchone  United States
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Default Simple Single-Ended Headphone Amp

Attached is a schematic for a simple single ended headphone amp that sounds nice and occupies a relatively small amount of real estate. I'll post pics of the real board when I get around to it, as it is already up and working sans enclosure.
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File Type: gif SE_HP_V1.gif (23.3 KB, 347 views)
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Old 6th December 2018, 07:09 AM   #2
wrenchone is offline wrenchone  United States
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Here's the gain-phase plots for the two channels. The response on both channels is the same, as expected.
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File Type: jpg SE_HP_CH1_G-P.jpg (178.0 KB, 351 views)
File Type: jpg SE_HP_CH2_G-P.jpg (178.8 KB, 305 views)
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Old 6th December 2018, 07:59 AM   #3
lineup is offline lineup  Sweden
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Simple and good!
All MOSFET
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Old 6th December 2018, 06:47 PM   #4
wrenchone is offline wrenchone  United States
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Here's how the simulation plays out. I set the currents in the sim to equal the way the actual amp is biased. The input ZVN3306A fets should be selected so that the biasing is the same for both channels. The current setting resistor on the BSS159 current source is also different than in the actual amp. This resistor wil be different for different lots of BSS159s, but very consistent within that lot. The input fet is biased at ~15 ma so that there is some current to drive the gate of the output mosfet.
In subsequent revs of this amp. I'll be inserting a pot so that the input mosfet bias current can be more easily adjusted. The output 75 ohm resistor needs to be (4) 300 ohm resistors in parallel to keep the temperature rise down. Even with two "2W" 150 ohm metal oxide resistors in parallel, the resistor temperature is around 80C, a bit high for my taste. The resistors are accordingly spaced off the board.

Sooner or later, I'll fire up my distortion analyzer to see how the thing really fares.
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File Type: gif SE_HP_SCH.GIF (17.4 KB, 263 views)
File Type: gif SE_HP_DIST.GIF (14.3 KB, 257 views)
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Old 7th December 2018, 12:52 AM   #5
wrenchone is offline wrenchone  United States
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I'm also exploring the possibility of using a small-signal Darlington as the input device instead of a mosfet. This results in lower simulated distortion (higher loop gain), allows me to ditch the input protection diodes, and also makes the input stage biasing much more consistent, removing the need to select and match mosfets.

I'll most likely build up an example over the holidays, as I have a couple of extra PCBs to play with.
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Old 7th December 2018, 01:37 AM   #6
wrenchone is offline wrenchone  United States
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Here"s the Darlington schematic with sim results.
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File Type: gif SE_HP_DARL_SCH.GIF (20.0 KB, 105 views)
File Type: gif SE_HP_DARL_DIS.GIF (14.1 KB, 96 views)
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Old 9th December 2018, 11:03 PM   #7
wrenchone is offline wrenchone  United States
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Here's pictures of the two boards - the top is the mosfet input implementation, the bottom has a Darlington input. Each board has two channels. I'm using a piece of sheet metal and some paper clamps for temporary heat sinking during testing, but the finished amp will be housed in a black anodized aluminum case that will double as heat sink.
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Old 10th December 2018, 09:45 AM   #8
lineup is offline lineup  Sweden
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The darlington circuit looks even better.
Nice boards, compact.

Have you found the depletion MOSFET BSS159N for sale?
You probably have some pieces at home.
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Old 10th December 2018, 05:20 PM   #9
wrenchone is offline wrenchone  United States
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The BSS159 is an Infineon part. Both Digi-Key and mouser have them. In a pinch, you could sub the Supertex/Diodes, Inc. part DN2530N3 and it would work just fine, but the Infineon parts have lower capacitance. The only downside of the Infineon part is that it's in a SOT-23 package. I wish they made them with legs...

Last edited by wrenchone; 10th December 2018 at 05:23 PM.
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Old 10th December 2018, 08:09 PM   #10
lineup is offline lineup  Sweden
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Yes, BSS159 is a very small transistor.
I always try to use TO-92.
DN2530N3 will work just as good, you say. I believe you.

BSS159 is a part of the Multisim spice. And so are the other of your setup.
If I want to sim your circuit.
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