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Old 16th November 2001, 08:32 PM   #1
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I have measured the voltage over r120-123.
R122, 123 showed 0,5v both.

But R120 showed 0.61v and R121 showed 0,39v

What should i do to make those values right`?

thank you in advance!
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Old 16th November 2001, 10:18 PM   #2
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You need a little better matching on Q106 and Q107.

On the other hand, this is only slightly outside
the spec, and will still work, although with slightly
less maximum power.
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Old 17th November 2001, 01:07 AM   #3
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Thank you for the reply MR. Pass

Before i changed R111 and R113 all the values was spread from 0,2-0,39. So when i made the change q108 and q109 matched suddenly. So i thought perhaps it was possible to something to make the other qs to get same value.

Im not so good at these things but is it OK for the transistor to run @ 0.6?

best regards - kasra
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Old 17th November 2001, 12:24 PM   #4
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I have just finished two ALEPH3 channel's (had a small problem, see aleph3 problem thread) and found the following:
Meticulous matching pays off. I have my output FETS matched within 0.01 Volt. Try to keep conditions as similar as possible when doing the matching. When the FET is in the testcircuit it tends to heat up and Vgs starts drifting. You can see this if you measure a FET for 5 minutes. Write down the measured Vgs every minute.
If you decide to swap FETS, swap them in pairs. So Q106 and Q107 in one swap or Q108 and Q109 in another.

I have also matched the 5W source resistors using a milliohm meter. I found the spread in these quite large.
The ones I use are UTM 0.47 Ohm +/- 10% tolerance. When I measured these it looked more like +/- 20% tolerance

Voltage across R120..R123 in my first channel is now
0.49, 0.50, 0.49, 0.49 V respectivly. In the second channel voltages are 0.53, 0.53, 0.54, 0.53 V. All measurement taken after an hour playtime for the channel, so speaker and CD connected.
I also checked the temperature for each transistor: 51C (degrees centigrade) where sink temperature was 39C and ambient temperature was 22C.
It goes without saying that proper mounting of the FETS is essential, specially in any class A design with paralled output devices. When there is imperfect mounting differences like you describe could occur. But then this is easily verified with a temperature meter. Fixing it will probably be a little more difficult.

Please do not take your measurements when the ALEPH is cold. You will get funny results.

Hope this helps a little
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Old 18th November 2001, 01:23 PM   #5
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Thank you for the reply rtirion
!

I changed r111 and r113 on my other channel which by the way has matched transistors, But there the values moved up to 0.6 v all of them.

Any ideas what resistors to use to get this channel down to the other channles specs - 0.5v?

thank you in advance!
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Old 18th November 2001, 09:24 PM   #6
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Is the 0.6V over R120..123 each? Do you take the measurements after an hour or so? What are the new values for R111 and R113?

I would not worry to much about 0.6V as long as all resistors, R120..R123 that is, show the same value. That case you know for sure each IRFP has the same amount of work to do.

If the temperature of your heatsinks remain reasonable i.e. less than 50 degrees centigrade, I would say enjoy the music.
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Old 18th November 2001, 10:24 PM   #7
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The measuring was taken over the 0.47 resistors. I had changed R111 and R113 to 4.75kohm and 221kohm, as you described.

On the first channel i got vales around 0.50v on the second i got values around 0.6v

For the first channel i was using bc550 and for the second channel ZTX... Could this be the problem?

So if one channel runs @ 0.5 and the other @ 0.6 thats no problems?


Best regards - kasra
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Old 19th November 2001, 02:33 PM   #8
Koy is offline Koy  Czech Republic
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Matching the output MOSFETs: I am not able to match ( avaible FETs) them very closely. The goal is the same current throught MOSFETs. So I can adjust the current by different values of source resistors. Is it true? Does it affect the sound?
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Old 19th November 2001, 07:13 PM   #9
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This doesn't work that well, as the value you
get will wander with signal. On the other hand
it's better than nothing.
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Old 19th November 2001, 09:02 PM   #10
Koy is offline Koy  Czech Republic
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Thank you Mr. Pass. Not very encouraging but of worth.
Do you thing could this solution affect the sound?
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