Another Aleph-X coming up!

[Peter Daniel]

Originally posted by Edwin Dorre



One is 4.63V and one is 4.58V ... not a big difference...

My old 'test' 25W Aleph-X had 4.77V and 4.85V and only 0.048V of offset...

Edwin

Regular Aleph is different than Aleph X in that respect. Get as close as possible with voltage drop on those resistors by matching input mosfet in a circuit. You can adjust the offset by playing with a trimpot in CCS on input differential (which changes the current).
At least it worked for me.

[Peter Daniel]

100ohms to ground at the output doesn't influence the obolute DC offste in any way.

Your chassiss will be pretty deep

[grataku]

Edwin,
that's fine no problem. What you are describing is caused by output mosfets that have different transconductance. Nelson has suggested to match the semis at two different current levels to ascertain that the transconductance is similar between mosfets.

[Edwin Dorre]

Originally posted by Peter Daniel


Regular Aleph is different than Aleph X in that respect. Get as close as possible with voltage drop on those resistors by matching input mosfet in a circuit. You can adjust the offset by playing with a trimpot in CCS on input differential (which changes the current).
At least it worked for me.

I first build a test Aleph-X !!! With this Aleph-X all is well...

Do you mean to change R17 to a trimpot?

Edwin

[grataku]

Originally posted by Peter Daniel
100ohms to ground at the output doesn't influence the obolute DC offste in any way.

Your chassiss will be pretty deep

It doesn't at 15 V rail, at 25 I am not so sure. There are different things that happen at higher voltage as I found out. Nelson uses them.

[grataku]

R17 doesn't do anything to the DC, it just control how much current flows through the voltage reference.

[Netlist]

Hi, Edwin,
Very nice amp!!!

The nicest way to reduce offset is what Peter says in post #11.
Second, I used the resistors from common input sources to outputs and I set the value at 4.7k. Bringing it further down to 2.2k increased stability of the offset.
Coupling caps at the input also helps a lot, not only for the zero offset, but also for offset drift.
I never used 100ohm resistors at the output and it works great: 1 to 2mV after 10 to 15 minutes, and very stable.
Look at http://www.diyaudio.com/forums/showt...696#post173696 and you will see how I mounted the input fets; very handy to change them till they match perfectly.
There's also a post somewhere that says to first adjust V1/V3, then V2. V2 has a great infuence on offset.
These are my findings when building the amp.
What value did you use for R19/29 and for feedback and input R's?
Did you use C2/4?

/Hugo - can't wait to see pics of the finished amps.

[grataku]

Netlist,
yes, post 11 is all fine and dandy and that is what edwin is doing.
The problem he is having is that he can 0 the dc offset (practically speaking) but that brings produces a somewhat different current flow between the two sides as measured by the voltage drops on the source resistor, that is assuming that all the resistor are exactly equal of course.

[Peter Daniel]

Originally posted by Edwin Dorre


I first build a test Aleph-X !!! With this Aleph-X all is well...

Do you mean to change R17 to a trimpot?

Edwin

I don't know your particular setup, but you need a trimpot to adjust current flowing through differential pair. It so happens that by changing that current, you can also adjust DC offset.

[grataku]

Originally posted by Peter Daniel


I don't know your particular setup, but you need a trimpot to adjust current flowing through differential pair. It so happens that by changing that current, you can also adjust DC offset.

The guy's got 5 trimpots, as I can see from the pic. 1 appears to be for the ccs adjustment and an extra pair for r12 and 34 for the AC current gain adjustment.