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Old 10th June 2004, 03:30 PM   #1
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Question Anything wrong with this amp schematics?

I would be very grateful if amp experts would take a look at this schematics and check it. I have -27 volts at the output (30 volts supply over security resistors). I was told that 82 ohms resistor in current source emiter should be about 1,2khoms but it didnt work.

Collector voltage on right current source is about -25 volts,collector-emmiter voltage on bias transistor is about 4 volts.
By regulating P1 i can trim output voltage for +- 6-7 volts.
If I set current over 350 mA output voltage drops down,but this is no A class schematics?

Any suggestions?
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Old 10th June 2004, 08:42 PM   #2
sreten is offline sreten  United Kingdom
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I severely recommend D.Selfs Power Amplifier Design Handbook.

There are many things wrong (well not right) with the design.

sreten.
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Old 10th June 2004, 09:10 PM   #3
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It looks like it will work, but will be likely to oscillate without
an RC network to ground at the output, probably something
like .1 uF in series with 5 to 10 ohms (3 watt)
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Old 10th June 2004, 09:22 PM   #4
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P1 looks too big, a 1.2k resistor in series with a 1k pot will give a finer degree of adjustment. I think 1.2k in the second current source might be sufficient, 82 Ohm results in an unecessarily high current of around 14mA.
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Old 10th June 2004, 09:32 PM   #5
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OK, i (mis) calculated that current would be 7 mA for 82 ohms ,and yes BC141 is maybe opened too much and therefore BD140 is opened all the time???

I tried to check with short circuited input and output voltage is still DC 27 volts.

I agree that this is simple shematics ,but to my experience it should work well .Checked PCB and all transistors but still -27 volts.Maybe base-emiter voltage on bias transistor is too high so it is saturated???
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Old 10th June 2004, 09:45 PM   #6
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Measure the voltage between the bases of the differential pair.
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Old 10th June 2004, 09:46 PM   #7
mandat is offline mandat  Poland
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In the current sources you use two BC212 transistors and two 3k3 resistors and four diodes for setting the voltage of transistor bases. Try to exclude one 3k3 resistor and two diodes connected to the one transistor and then connect bases.
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Old 10th June 2004, 10:21 PM   #8
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input base dc voltage is about -6 volts and second base is -18 volts (after 120k resistor).
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Old 10th June 2004, 10:33 PM   #9
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might be worth checking those transistors
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Old 10th June 2004, 11:04 PM   #10
mandat is offline mandat  Poland
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I wrote about these in the WN corner of your picture.
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