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Old 30th January 2007, 01:55 AM   #1261
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HI
Me again , now I did adjust all 3 trim pot to the another end and that lower the power around 6A .
But I have another problem to .I have 2.7V offset when the amp cold , and when it warm up it goes down until 1.8V .
Al the mosfets were matched , transistors were matched .
I don't know what else I can do .
Tomorrow I will use a 12mF Sollen capacitor on the speakers terminal to see at least if it works . Now is to late to try .
Please HELP me so I can finish the amp .It cost me at least $1000CA the parts , and twice that much the work to put it together .
It look nice but that s it unfortunately .
Regards
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Old 30th January 2007, 08:59 AM   #1262
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Hi,

wait unti the amp is warm. Then adjust the center pot feeding the current source for 0V absolute offset.
Then the other two pots for equal bias in both sides.

2.7V at startup is ok and nothing to worry about.

How much is your relative offset (between +out and -out)

William
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Old 30th January 2007, 10:32 AM   #1263
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Hi Gabor,

This is a follow up on William's post. If the 2.7V to 1.8V
offset you're getting is from out+ to ground or out- to ground
(ie. absolute offset) then there's nothing to worry about.
(In fact, it seems reasonably good.)

The offset that your speakers will see is the relative offset,
ie, the voltage you measure between the out+ and out-.
What are you getting there?

Cheers,
Dennis
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Old 30th January 2007, 11:48 AM   #1264
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HI
Thanks for the answer .
When I turn on the amplifier I have 2.7V offset between the speaker terminal , between the + & -
When the amplifier warms up the offset goes done to 1.8V .
These problem exist at both channel.
I dont know what I can do , all the power mosfets were matched by Kristijan , but I double matche them . All 8 mosfet mesure inside100mV.
All the drives mosfets and transisors were mathed by me .They has the same value .
Would you please help.Can I try to connect the amplifier to the speakers using a 10uF capacitor .
At least to see if it works .
Regards
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Old 30th January 2007, 12:04 PM   #1265
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HI
I'm sorry , I forget to let you know when first I powered up the amp I had over 6V offset .
Now I adjust the trim pot in the middle one (200R) to another end and the offset now 2.7V when the amp cold .
Or 1.8V when the amp warms up.
The problem now I can't adjust more to bring done the offset close to 0V .
I adjusted the another two trim pots to to another end because the bias was very high .
At first the amp draw 7A but now after I adjust the 50K trim pots to another end it draws 5.5A.
I think that is OK , my problem right now the high offset .

Regards
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Old 30th January 2007, 02:58 PM   #1266
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HI
Unfortunately more bad news . Today morning I turned on the amp and after a couple minutes the transformer started to buzz .
It give a sound like when is over loaded . Only one channel connected to the power supply . But with both channel does the same .
I use 1.5KVA 2X18V transformer from Plitron .
I still don't know if the 6A is it to much measured between the power supply and the pc board on the positive .
Can some one please let me know .
If that is correct I have to call Plitron to discus the transformer .

Please help !!!!
Regards
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Old 30th January 2007, 03:35 PM   #1267
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Hi,

if the offset you see is between plus and minus. What offset do you have between +out and ground and -out and ground?

You have to set this absolute offset to zero first. Before doing anything else.

Then match the bias for both halves by measuring the voltage drop over the power fet source resistors and make it equal

YouŽll have to adjust the absolute offset everytime you change the bias!

If the diff offset stays that big your input dif pair isnŽt matched well enough. Are they thermally coupled?

A buzzing transformer is not unusual (but not nice).

William
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Old 30th January 2007, 03:59 PM   #1268
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HI
I tried to adjust the bias but the best I can do is 1.8V after the amp warms up .
When is cold it is around 2.7V .
I want to know how much power draw each side , I afraid because the way to high power it will blow up.
Now the transformer buzz , unfortunately so loud I can't leave the power on .
It give the same sound like when you turn on the amp and charges the capacitors for a couple second .
These happen after 5 minutes and the noise is continuous , and I tested bot side one by one and it the same problem .
I will go over on the board second time if I can find some problem .
All the power mosfet were double matched ,by Kristijan and by me .
I think the 6A is way to much , measured between the board and the power supply on the positive .
IF I'm right it supposed to be 3A at the + and 3A at the - .So all together 6A .
If the power way to high it destroy everithig .
Please somebody let me know what is the correct power ...
Regards
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Old 30th January 2007, 04:43 PM   #1269
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HI
I tested the offset between the ground and + it is over 5V .
Between the ground and the -2.7V .All these when the after a couple minutes when the amp is still cold .
No no longer possible to leave the power on to warm up the amp because the transformer start buzz very loud .
Now I'm going to remove the pc boards and look for possibility of mistakes .
If I find that good news , if not unfortunately that will be the end of the AX .
Regards
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Old 30th January 2007, 07:07 PM   #1270
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Location: austria, kottingbrunn
Thanks Mr. Pass!

So that means the maximum output power can not be increased beyond the 50% ac current gain setting?

Robert
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