Aleph headphone amp

[jleaman]

see in the top left hand corner. the word sun..
well that stands for

sun microsystems hah.

www.sun.com.
or buy a older one..

www.ebay.com

 

[jwb]

hey jwb ! your amp looks great!! i want to make an aleph headphone amp. where did yo get the case, it's awesome !!! greg

The case is a Sun 411, a SCSI enclosure from about a decade back. You can snag them at swap meets or on eBay for $5-10, power supply included.

 

[Edwin Dorre]

Hello Marcello. Output offset is trimmed to zero, unmeasurable when the amp is warmed up. Why do you mention it?

By the way, your amp is my inspiration. Very nice job!

How did you trim the offset away? I build one channel of my mixture of Aleph 5 and the Mini-A . I have about 11mV of DC offset left over. I can get away with this with a 2200uf or so and a 470 ohm to ground however I do not like a Zen like capacitor in my signal if I can get away with it.

Did you use the Aleph 30 like of resistor around the 9610's? How does this work?

Many thanks,

Edwin

 

[jwb]

You can trim the DC offset by adjusting the current in the input differential pair. On my amp I replaced R11 (in the Aleph 5 Service Manual) with a pot. By adjusting the current I can get near zero DC offset. I think it helps that I also used the 2sj109 FET for the diff pair. This is a monolithic dual FET and therefore very well matched.

This amp has lots of things to fiddle with. I included a jumper to bypass the DC feedback capacitor (C5) and another jumper to disconnect the Aleph operation altogether (C10), which transforms the amp to a zen-like topology.

 

[Edwin Dorre]

You can trim the DC offset by adjusting the current in the input differential pair. On my amp I replaced R11 (in the Aleph 5 Service Manual) with a pot. By adjusting the current I can get near zero DC offset. I think it helps that I also used the 2sj109 FET for the diff pair. This is a monolithic dual FET and therefore very well matched.

This amp has lots of things to fiddle with. I included a jumper to bypass the DC feedback capacitor (C5) and another jumper to disconnect the Aleph operation altogether (C10), which transforms the amp to a zen-like topology.

Hmmm... The R11 does not do a thing, well almost, I can move the offset way up, about 3mV down but that's it...

A more important problem is noise. I have about 4mV of constant noise (measure with a Tek 2232) which you can hear very well on a el-cheapo discman headphone with the inputs grounded. I have about 2mv of ripple between plus and gnd and 2mV between gnd and minus on my power supply, and almost no noise. When I unplug the amp but let the scope run and plugged in the noise goes way down, so my scope does not make this noise up...

I am a bit worried about this noise. I use kristijan his PCB's and maybe they are a bit more subjective to external noise. The amp is not in a grounded box. It sounds ok however you will hear the noise all the time.

HELP!

Edwin

 

[jwb]

The DC offset would improve with better matched FETs on the input stage. The 2sj109 is remarkably good in this role. I'm not sure why your amp has such a small response to the front-end current. I can swing mine ±100mv by adjusting the CCS. You might also try fiddling with the value of R14.

I should also mention that the DC offset in my amp is certainly related to temperature of the power FETs. You might try heatsinking them a bit more. Eventually, I got my design nailed down so it is less sensitive to temperature, but in the beginning it could make a 20mV difference.

For the noise, you should be able to increase the PSRR by repacing the zener reference in the CCS with a buried zener, and replace the zener bias resistor (R13 in the Aleph 5) with a current regulator diode.