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Old 6th April 2005, 08:26 PM   #621
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Within a lot code, matching Vgs is perfectly adequate.

The bottom half are common-Source connection. The
top half would probably be considered an unusual
application of Common Drain.

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Old 7th April 2005, 02:29 PM   #622
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Thanks Nelson,
However I am using the group buy boards and the output is loaded to ground- everything according to the schematic.The diff mosfets are heatsinked together.
I find I get huge deviations with temperature.Seems like almost a volt for each 1 degree centigrade.This is the same for both monoblocks.
In fact the offset is so huge as I saw today when the amp is cold -up to 10v!! -that sometimes it doesn´t bias properly and turn on.I have to adjust the offset all the time to get it to work properly after two hours or so when the temp is stable then the offset stabilizes.But who can wait around for two hours or leave the amps on all the time?
Anybody else out there notice these variations?
As I said the main problem I think is with the CCS for the diff pair.That is the circuit that is thermally unstable.An extra simple circuit to stabilize it would be necessary.Or a kind of servo linked to the output.
I am sure Pass labs has solved this problem with the ax series.
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Old 7th April 2005, 02:36 PM   #623
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What I was always doing to add stability to differencial pair, was mounting devices directly to the chassis (main heatsinks). This provides more stability and when the amp reaches constant temperature, even opening the top don't change things much.

The best way for adjusting trimpots is throught the holes in a chassis, that way you don't upset termal stability (by removing the cover) and it provides convenient way for adjustement. That requires special layout though.

I believe Aleph 0 had it done this way

IIRC, in my Aleph X the absolute offset was starting hgh as well (5V or so), but after time it went down to 0V and stayed this way (no more that 100mV deviation). The differencial offset was suprisingly low after adjustement (less than 10mV).
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Old 7th April 2005, 04:12 PM   #624
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I guess I will try and current sink the zener regulator on the ccs for better stability and see how that works out.At the moment it has only a resistor to ground and perhaps theres too much current going thru the zener creating further thermal drifts.
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Old 8th April 2005, 06:33 AM   #625
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Hi Protos,

maybe you´ve seen my thread "one Aleph-X working, one to go"? Here I´ve described exactly the same problem you´ve got now. Mine also start at -8V, go down to -2V quite fast and then take forever to get to zero.
I also had to adjust the current source through a hole in the top cause otherwise absolute offset changes (just blowing into the amp changes it!)

Until now I did two things:

changed the output to ground resistors from 100 to 80 Ohms. This made the abs. offset go down faster.

changed R46/47 from 4k7 to 3k9 to 3k3.
This lowered the startup value(but not very much).

What I will try next is to make a thermal connection between the heatsinks and the diff pair as suggested by Peter, lower the output resistors to 60 Ohms and R46/47 to 2k7 (Netlist even uses 2k)

William

oh and I also tried changing the current through the zener (doubled it from 3 to 6mA) without any effect
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Old 8th April 2005, 03:51 PM   #626
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I would hate to waste more power with the output resistors.
I still think stabilising the current source is the way to go.I don´t think it is the diff pair temp.The diff pair affect more relative offset while the ccs affects absolute offset to ground.I will test this theory.Actually it is easy.
When the amp is warm with the top open touch the ccs with a frozen object and observe the offset change.Then do the same for the diff pair.If one affects the abs offset more than the other then ypu know which part needs more stability.
I think a slight variation in vgs of the current source is the cause.That´s why I suggested the current sink option which is included anyway with the boards.However nobody has commented on this aspect i.e. simple zener reg vs zener + current sink.
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Old 8th April 2005, 04:17 PM   #627
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Hi Protos,

you´re right about the waste of power with the output resistors but 60 Ohms is still 15 to 20 times as high as my Loudspeakers are so it won´t be that much waste......

I connected the zener to the negative power supply for a better imitation of a current source but I will have a look at the other option and see what parts I will need to test this.

Let me know when you´ve got results with the frozen object (or take ice spray as an alternative)

William
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Old 8th April 2005, 06:07 PM   #628
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My theory is that the zener is never going to be very stable and will change with temp.This might affect vgs by some mv and so make the ccs drift.This effect is amplified by the ccs and the diff pair subsequently.
The trick is to find which of all the destabilising factors is more responsible for the abs offset drift and try and minimize it as simply as possible.
I don´t know if I can find the recommended parts for the current sink but a small jfet can do the same job.
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Old 8th April 2005, 06:16 PM   #629
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HI,

I just looked at the schematic. Is the idea to use an SST505 (1,00mA constant current diode) and then replace the zener with a resistor and jumper R17?


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Old 8th April 2005, 08:02 PM   #630
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I guess either /or.I don't have the schematic right here.But I remember it is a very simple idea.Both a zener and a cc sink would be a tighter regulation than either alone.But I am not an EE.I know the main purpose of the zener is to isolate the gate of the ccs from rail fluctuations noise.
I have to apply the Mr.Ice zapping technique to the fets to see if I am going in the right direction or I am spouting bs.
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