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Old 4th May 2011, 04:24 PM   #391
Art M is offline Art M  United States
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Default Biasing output

Quote:
Originally Posted by jprco View Post
With the changes in various fixed resistors on the MKII supercharged, what are the accepted values for quiescent current (60-100ma ?) .
Jean
I strongly suggest you you look at the advice for adjusting quiescent levels in the book by Douglas Self (i.e we don't adjust for quiescent current ). This amplifier is a HIJACKED version of his design with trivial modifications.A lot of the advice given here regarding it's operation is just plain incorrect
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Old 8th May 2011, 09:15 PM   #392
jprco is offline jprco  Canada
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Default Voila!

Rudi, Carlos: Completed 2 boards, MKII Supercharged and as expected, they sound greatttttt. I attach a photo and hope it comes through ok.
A couple of assembly notes:
1- mounted the damping coils underneath just for looks,
2- had some problem like Rudi had with shorting to heat sink but with the MJE's not the BD - poor quality shoulder washer or tightened to much!
3- must remember to mount the board MJE with correct pin allignment- board shows BD139 layout.
4- Opted for a trimpot and the MKII lineup for C19/C20 in the end. Also I might get just away using the 10000uf/63V so far so good. If not I'll add a pair off board.
Great project, now I wonder what design to go with for the other 2 boards.
Thanks again for all your help.
Jean
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File Type: jpg DXMKIIS.jpg (274.6 KB, 299 views)
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Old 8th May 2011, 10:01 PM   #393
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Default Congratulations and thanks Jprco

Thank you by the nice picture and your opinion about.

regards,

Carlos
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Old 9th May 2011, 05:38 AM   #394
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Jean,

nice job! Thank you for the picture.

Sorry. I forgot to mention in my Builder's Manual that you have to flip Q7 by 180 degrees if you do not install an ECB transistor (like BD139) in this place.
You will receive strange results else!

Enjoy the music now.

Best regards - Rudi_Ratlos
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Old 12th May 2011, 08:00 PM   #395
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Jean,

I have adjusted the quiescent current, I still have to solder some cables, and then, tomorrow, I will have a big whisky and listen to my DX Blame MKII
(left channel only) for the 1.st time.

Click the image to open in full size.URL]

Best regards - Rudi_Ratlos
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Old 15th May 2011, 12:21 PM   #396
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Default Strange things happening

Dear DX Blame friends,

after adjustment of the quiescent current (to about 50mA) and measurement of the DC voltage on the speaker's output (between 3 and 6 mV),
I powered up the 1st channel of my DX Blame MKII:

Click the image to open in full size.

and - Aaarrrgh: Smoke coming from the PCB.

Click the image to open in full size.

I forgot to unsolder the 100 Ohm resistor, that I used during the adjustment procedure, and this resistor melted away .

The fuse, that I inserted into the fuse-holder after my adjustments have been completed, is 2A. I measured it; it is o.k.

This should have never happened, because the current is flowing through the fuse now!

I am using 15.000F power caps, and the power to the PCB is currently delimited to <60 Watts by means of my light bulk tester.

Click the image to open in full size.

These things are very strange.
Has anybody an explanation for what is happening?

Best regards - Rudi_Ratlos
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Old 15th May 2011, 04:12 PM   #397
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Rudi,

I think it's the current that those powersupply caps are drawing when you fire up the amp the first time.Insert a 5watt 0.1 ohm resistor inline with the DC rails

Last edited by meanman1964; 15th May 2011 at 04:14 PM.
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Old 15th May 2011, 05:09 PM   #398
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Patrick,

I have been able to adjust the quiescent current according to Carlos recommendation, I measured very little (2-7mV) DC on the output, ...,
the AMP worked as designed during the adjustment phase.

When I connected a source and a speaker to it, something happened.

Moreover: I am currently no more able to adjust the quiescent current to "low values" (about 50mA).
The minimal quiescent current, that I can achieve when I turn the BIAS trim poti to its maximal value (500 Ohm), is about 80mA.
This is strange as well.

I did a mistake somewhere. But currently I have no idea, where.

Best regards - Rudi_Ratlos
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Old 16th May 2011, 07:52 AM   #399
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Default DIY picture of the month ...

... or re-definition of Ohm's Law.

The rail-current is not flowing through the fuse (which is intact) - that would be too easy -, but through the 100 Ohm resistor,
that I used during the setup procedure, and melts the resistor.

I measured the fuse afterwards: it is o.k.

Click the image to open in full size.

http://www.abload.de/img/fusexey2.jpg

Best regards - Rudi_Ratlos
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Old 16th May 2011, 10:28 AM   #400
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Default Soon you will find the bug...check AC voltage in the output terminals

Maybe you gonna need more compensation because of different transistors and different board.... if you obtain AC in the output, without signal entering, then increase the Miller capacitor till this stops.

Good luck.

regards,

Carlos
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