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Old 16th November 2003, 08:31 PM   #1
jwb is offline jwb  United States
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Default jwb's ultimate aleph headphone amp

Continued from this thread. Thanks to Nelson Pass for Aleph design documents, and diyaudio.com members for feedback on my design from this summer.

First, the reference materials:

The schematic in PDF
The layout, with part number references
The bill of materials. Only $150 worth!

This is a dual mono Aleph amplifier designed for driving headphones from 32-300Ω impedance. It will drive a Grado can up to about ½W, and Sony DJ headphones up to 3W. It can also put modest, but audible, power into 4Ω speakers.

If you understand the Pass Labs Aleph designs, you'll notice several differences in mine:
  • The current source for the differential pair uses LEDs and a JFET current diode for the bias voltage, and the JFET is referenced to the negative rail instead of ground. This improves PSRR.
  • The diff pair uses a monolithic dual FET instead of matched MOSFET pair. This is mainly because matching MOSFETs is tiresome, and the ZIP-7 package is much smaller than two TO-220s.
  • The idle current is only 165mA; more than enough to drive headphones, but not enough to heat your winter cabin.
  • I have included two jumpers: one shorts the DC feedback capacitor, for my own experimentation. The other disconnects the crucial part of the current source that distinguishes an Aleph from a Zen. By removing this jumper it will be easy to compare the two designs.

Each channel is separately powered, all the way back to two primaries. Schottky rectifiers are practical in this low-voltage, low-current application (50V 3A specimens were used). Following the rectifier are improved Jung regulators, one for each of the four rails. The improvements are in the form of more JFET current diodes, which are used everywhere Jung used a resistor to bias a voltage reference. This improves the PSRR of the current source and cuts down on noise into the non-inverting terminal of the error amp. The outputs are ±13.8V.

The final embellishment is a turn-on delay circuit using a 555 timer and relay. The relay normally shorts the output to ground. After the power rails stabilize, the 555 drives the relay, breaking the short. At turn-off the relay loses power immediately, shorting the output to ground again. A zener & diode protect the output of the 555 from overvoltage when the relay disengages. It would be possible to also use this as a mute function but I didn't bother.

Hopefully you will find this design useful. The circuit board is particular to my chassis & heatsinking, but if you have any comments on the layout, please post them here. After agonizing over the layout for months, I'd hate to have any obvious mistakes.

Cheers,
jwb
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Old 16th November 2003, 10:15 PM   #2
jwb is offline jwb  United States
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Here's a slightly better idea of what the manufactured board will look like, from above.

A 34KB PNG file
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Old 17th November 2003, 04:49 AM   #3
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More pictures!
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Old 17th November 2003, 01:20 PM   #4
protos is offline protos  Greece
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Can I make a request for an Aleph -X version? Currently I have a hybrid balanced line driver chip driving a Pass X bal. line stage(why the chip? well it sounds much better this way) which has been modified to run on batteries.
It seems to me that with balanced operation headphones sound better with a wider deeper soundstage.So I am looking now for balanced headphone amps.
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Old 17th November 2003, 01:59 PM   #5
jam is offline jam  United States
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jwb,

Why would you want to use the Aleph circuit for a headphone amplifier. Efficiency is not a problem here, just run the output stage with a fixed current source biased correctly.

Regards,
Jam
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Old 17th November 2003, 03:35 PM   #6
jwb is offline jwb  United States
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Jam,

That's why I included the jumper to enable/disable the Aleph current source. I wanted to compare both topologies.
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Old 17th November 2003, 04:42 PM   #7
jwb is offline jwb  United States
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Quote:
Originally posted by protos
Can I make a request for an Aleph -X version? Currently I have a hybrid balanced line driver chip driving a Pass X bal. line stage(why the chip? well it sounds much better this way) which has been modified to run on batteries.
It seems to me that with balanced operation headphones sound better with a wider deeper soundstage.So I am looking now for balanced headphone amps.
I didn't bother with the balanced/bridged Aleph-X topology because most headphones provide only three wires, where you would need four to do the job. The modification is straight-forward if you care to undertake it, but you'll about double the cost.
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Old 21st November 2003, 04:45 PM   #8
jwb is offline jwb  United States
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I love PCB Express dearly.
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Old 22nd November 2003, 10:16 PM   #9
jwb is offline jwb  United States
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Default Almost ready to listen

That thing it's attached to also doubles as the heat sink for 16W. I'm >*< this close to listening to it
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Old 24th November 2003, 08:20 PM   #10
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Default Look Geat

JWB

The pcb look great, so how does it sound. and what H-phones are you driving with it.
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