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Old 24th August 2006, 08:05 AM   #1
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Question just what to change? AX-100

hello All!

Ok just what to do? on this Aleph X
After just finishing off soldering components in board and reading through wiki and various threads on the next step, I measured volt drop on both R23 and R25
R23 is 4.4V
R25 is 4.6V

With 200mV difference my next step was the heat sink on the IRF9610 but did nothing to change this value. Next step was to try diff combinations of IRF9610 but values got worse.

So placing a varible resistor in place of R25 i adjusted to get the same volt drop.
Measuring this 420 ohm

So should I purchase more IRF9610 to get this match.... or just stick a
420 in place of R25 and
390 for R23

Cheers!
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Old 24th August 2006, 09:15 AM   #2
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Changing the drain resistors of the diff pair would change the open loop gain. So by using different values of the 2 arms, one would degrade the symmetry to an extent. The feedback would correct this. You can decide for yourself whether you want corrected symmetry or inherent symmetry.

There is no other way but substitute diff pair transistors until they match. (I presume you already coupled them together thermally.) One alternative I know is to incorporate a trimpot at the source of the diff pair, and trim it until you get same voltage on both drain resistors. This would reduce the open loop gain somewhat, but retain symmetry. Close loop gain remains.

Patrick
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Old 25th August 2006, 09:22 PM   #3
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Thanks Patrick!
Ill purchase some more irf9610
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Old 26th August 2006, 08:52 PM   #4
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Hi,

did you match the 9610's? It can be a lot of work trying different fets in a working amp. It is much easier to match them for equal Vgs at 10mA. 0,005 volts or less is OK.
Normally I first measure them for 5 seconds and then do a second run with those that are close. What kind of DC offset did you get?

William
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Old 27th August 2006, 10:17 AM   #5
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Hello William

Yes I matched but only to .01v as my stock of irf9610 was insufficeint, really surprised just how well these fets must be matched.
So Im in the process of ordering some more, this time from the same batch.

For the DC offset, thats the output to ground?
have not got as far as that.
Thought Id get the volt drop the same first

Cheers
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Old 27th August 2006, 03:58 PM   #6
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Hi,

no voltage beween outputs. Before you measure anything you have to set the bias and absolute dc offset when fully warmed up. Otherwise you could be measuring anything.

william
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Old 2nd September 2006, 03:22 AM   #7
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hello

Finished connecting first mono-block this morning and all went well, no smoke let sit for an hour and did some measurements
Bias is sitting at .5V across source resistors R5,R6,R40 and R41 and this is with V1 and V3 set to zero (im guessing this is due to the 75k of R11 and R33 being at to high a value). So no need to adjust V1 or V3, adjusting just raised to .6V across source resistors.

Next I measured Relative offset, this is sitting at 40mV and Absolute offset about half of this.

Volt drop across R23 and R25 was 4.15 and 4.6, which is much improved.

My next step is to see if its plays music.

how do these values look?

Cheers
Dan
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Old 2nd September 2006, 03:25 AM   #8
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Correction
R25 4.515V
R23 4.560V
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Old 9th September 2006, 08:47 AM   #9
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Hi,

absolute offset 20mV. Are you shure?

William
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Old 9th September 2006, 09:59 AM   #10
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Hello William

Well no not sure,It was a week ago but I did get it to play music
so Im in the process of finishing off the chassis
then ill do some more tests

I will have to lower bais from 7A as Im using hammond 156B which are rated at 5A - rookie mistake!
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