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:
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
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
Here's a slightly better idea of what the manufactured board will look like, from above.
A 34KB PNG file
A 34KB PNG file
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.
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 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.
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.
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.
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.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.