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Old 16th November 2003, 09:31 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, 11:15 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, 05:49 AM   #3
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More pictures!
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Old 17th November 2003, 02:20 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, 02:59 PM   #5
jam is online now 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, 04:35 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, 05:42 PM   #7
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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, 05:45 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, 11:16 PM   #9
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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, 09:20 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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Old 25th November 2003, 02:45 AM   #11
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I will state for the record that by listing my changes to the circuits of Mr. Pass & Mr. Jung, I do not mean to imply any deficiency or lack in the originals. Indeed, I selected these designs because of my high regard for both of them.

So, If I say that I have improved the Jung regulator with a better current source for the pass device, I mean to say only that my current source has a higher Z (about a megohm, by my reckoning) than the original. Strictly speaking, you may not think this makes any difference in the final product, and you may not think it rises to the level of "improvement".

Furthermore if I say that I've improved the Aleph amplifier with a monolithic JFET pair and a higher-impedance current source, I only mean to say that I've calculated and measured lower DC offset and higher CMRR with my changes than without them. I do not mean to diminish the qualities of the original design.
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Old 25th November 2003, 03:24 AM   #12
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JWB

The circuit look nice and should sound good. However, you can't claim a high CMRR with out having a balanced input, maybe you meant PSRR. Anyway, I am sure it sounds nice. So what kind of phones are you using?
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Old 25th November 2003, 05:28 AM   #13
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I hope, sir, you are not implying this amp is single-ended!

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Old 25th November 2003, 05:29 AM   #14
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Default I hope you are all quite ready for more bad photography

This is her innards. That's the attenuator hanging out of the case.
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Old 25th November 2003, 05:30 AM   #15
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A little more assembled
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Old 25th November 2003, 05:32 AM   #16
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All ready to go, with my Grado SR 80 cans. Plans to upgrade to SR-1 are underway.
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Old 25th November 2003, 05:38 AM   #17
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Quote:
Originally posted by jwb
I hope, sir, you are not implying this amp is single-ended!


Well, one end looks single.

I'm admiring a new design, following how it's constructed, but not reading how good it sounds...



JF
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Old 25th November 2003, 06:10 AM   #18
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It sounds amazing of course.

Imagine combining an amplifier you can't afford (say, a Pass Aleph 5) with a speaker you can't afford (e.g. Wilson), and listening to it in a room your house doesn't have. That's what I feel listening to this little amp.

Right now I'm listening to Homeless from Paul Simon's Graceland. When those throaty African gentleman say he-EH he-EH he-EH, I might as easily be in the recording hall in South Africa. The impact is of the proverbial velvet hammer variety: a representation of the original power of the performance, rather than something injected by the equipment. The clicking of tongues, something ommitted almost entirely by the opamp headphone output of my receiver, is reproduced faithfully.

Previously I was listening to Dire Straits' Sultans of Swing. The bass sounded very nice. I don't want to say "it has good bass", for fear of invoking images of kids in Hondas with Kickers in the trunk. But this is a song with the bass turned up to 11, and I can hear it properly now. Knopfler's ending solo has great staccatto attacks.

This amp basically never runs out of power. I might even reduce the idle current to spare myself some heat problems.

But I never trust equipment reviewed by the builder. You might as well be reading Bose sales literature.

Instead I'll mention that initially the amp had a bit of a buzz. I thought the 1/2" run of .1" wide PCB trace would be low enough impedance to avoid the problem, but apparently not. Luckily I included a giant via right next to the filter caps, and by moving the main ground there, I eliminated the buzz. The real problem isn't, I don't think, that the grounding scheme was so bad, bad rather that I didn't design for so large a Vrip: 1Vpp.

Also it's running kinda hot. As long as the sink temperature stays under 140C, I'm within the specs. Right now the sink is running at about 60C. That would be good except it's making the plastic case smell.
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Old 25th November 2003, 06:15 AM   #19
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JF
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Old 25th November 2003, 11:04 AM   #20
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Nice work jwb,

With my old Niam amps, I've never had the luxury of headphone output. Very compact arrangement too.
Quote:
The bill of materials. Only $150 worth!
Very attractive!

Paul
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Old 25th November 2003, 01:00 PM   #21
protos is offline protos  Greece
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Congratulations! Some questions :
What is the total ma or power consumption of the amp?Is there a possibility to get the pcb from you? Are the regulators on the same board?
By the way I did once connect my headphones to my aleph 5 monos but the hum levels were way too high for headphone listening.
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Old 25th November 2003, 02:22 PM   #22
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Thumbs up Too cool for school

One of the nicest jobs I have seen on the forum. The design looks great and is only overkill from the attention to design details and not for parts count. For the penny pinchers I would say that one could use J202 fets for current regulators in place of the jfet two terminal current source. I would try the Zetex ZVP3310 for the current sources for the J109 jfet pairs. Zener protection for the mosfet gates is generally a good idea too. I wish I had one. When are circuit boards available?
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Old 25th November 2003, 03:10 PM   #23
jam is online now jam  United States
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Thumbs up ....Is Fred getting soft?

jwb,

Great effort. Has to be up there, since you got praise from Fred, no mean task I might add.

How did your experiment with bypassing the Aleph circuit turn out?

Regards,
Jam
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Old 25th November 2003, 03:28 PM   #24
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Default Yes that what I am saying

JWB,

According to your schematics, you have a non-inverting input and only one input. The output is single ended and not balanced. A balanced amp always has two inputs that are non inverting and a inverting. Thatís the only way you can get right of a common mode signal like 60Hz on at the outputs.

Audio amp don't have balanced input they have diff amps but that in it's self does not make it balanced. The Aleph is a diff amp with balanced input but not in the purest form.

Having said that, I think you done a nice job with the circuit and construction.

Regards
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Old 25th November 2003, 04:28 PM   #25
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Default Basic balanced amp

JWB,

This is the basic balanced amp design, but there are many way to implement a balance diff amp. So as I was trying to say, is if you do not have balanced input it easier to pickup 60 Hz common mode signals.
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