help: Aleph x temperature sensitive

[Berry]

I have a problem with my Aleph X. It is build with the original schematic of Grey Rollins (15 v, 4 irfp240). When the temperature increase (decrease), there is a change on the DC ouput voltage.
This voltage changes with the irf9610 (Q6) temperature from 1 to -1 volt. A heatsink on the irf9610 doesn't solve the problem.

[rtirion]

If it is the absolute offset ( One output to ground) there is no
problem.

If it is the relative offset ( +ouput to - output ) there could be
a problem.

Which one are you talking about?

[Berry]

It is the relative offset ( +ouput to - output )

[wuffwaff]

Hi,

if it really is the relative offset then something must be wrong. Both my A-X have almost the same offset cold as warm (only a few mV difference)

Is there anything else that changes during warm up? Did you check the bias in both halves?

William

[rtirion]

If bias settings are correct, (hope you measure them without a
preamp connected), like William suggests than there are a few
other possibilities.

Poorly matched diff pair. You could try to swap these with a better
matched set.

Before even looking at this:

How do you drive the Aleph-X? Balanced or unbalanced?
In both cases input caps are mandatory.
These are NOT in Grey's schematic.

Are you sure your source has no dc-level on it's output?
Especially since you do not specify how you measure.

Maybe you can make a diagram showing your test setup.
Or a photo (less than 100k) or two might help.

Regards

[Berry]

Thanx,

I changed the 2.2 k for 4.7k for absolute DC offset. When 2.2k is used, changing V2 has no influense. Put a capacitor from the - input to the ground. Now the offset is 20 mv, but the resistors over the output and the ground are hot (9v over both).

[rtirion]

I am afraid you're not going the right way around.

Try to answer the questions in the previous post.

Either way 9V absolute offset is too much.
That's why the resistors are getting hot.

This absolute offset is partially adjustable using V1 and V3. (Bias)
Set these half way first. Adjust V2 for minimum absolute offset.

(Re)adjusment must be made when the amp is warm.
Keep an eye on temperature on the outputs when doing all this.

Take a full set of relevant DC measurements and report back
using a diagram. Otherwise it will be difficult to help further.

Regards

[GRollins]

I vote for either a mis-matched input differential or DC at the input(s), both mentioned above. I use a tube preamp, so I'm cap-coupled already. Barring parts failure, my preamp will not pass DC. That doesn't mean that other preamps don't do it. Measure and be sure.
If you can get away with it, don't use heatsinks on the front end differential devices. It makes things worse. They increase the coupling of the device to the air. Usually this is a good thing, but in this case it allows vagrant air currents to drift by and upset the balance. I'm comfortable with operating a TO-220 device at .5 to .75W with no heatsink at all. Nelson would no doubt go for a full Watt or more. I doubt the man has any fingerprints left--no doubt sizzled off years ago.
Or you can do what Nelson does in the production units and place the differential devices on their backs (but with heat sinks), and let the bulk of the board shield them from air currents. They're also more sturdy mechanically if you're inclined to move the amp around much.

Grey

[Berry]

If bias settings are correct, (hope you measure them without a
preamp connected), like William suggests than there are a few
other possibilities.

0,5 v over r5,r6,r41,r40

Poorly matched diff pair. You could try to swap these with a better
matched set.

4,8v over r23 and r25

Before even looking at this:

How do you drive the Aleph-X? Balanced or unbalanced?
In both cases input caps are mandatory.
These are NOT in Grey's schematic.

Unbalanced

Are you sure your source has no dc-level on it's output?
Especially since you do not specify how you measure.

No source conected( input caps connected to the ground

[Berry]

Is it possible that a broke some parts when the input was connected to te ground without an input cap ?

When adjusting v1,v2,v3, what is the best thing to do.
V1=?ohm
V2=?ohm
v3=?ohm