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Old 29th April 2003, 01:09 PM   #1
red is offline red  Romania
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Default Is this schematic wrong?

Hello As you can see I have a problem and I hope someone will help me.
I've build this amp (see the schematic)but It doesn't work properly.The "original" schematic
used BC107 and BC177 and as power transistors the old 2N3055 and 2N2955.
I am asking you please tell me if you can see anything wrong with it...It does amplify but it has a huge hum and on my heatsink you can fry eggs and believe me it's big enough for 4 transistors not 2 as I am using it.Besides what makes me think it is something wrong with it is that my BD235 is as hot *** hell while the others are cool.I've changed it twice and still didn't work.The schematic runs from +45 0 -45 power supply but for testing i've run it from a +22 0 -22 power supply.Did anyone,by anychance,build someting like this?PLEASE HELP ME.
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Old 29th April 2003, 01:26 PM   #2
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a quick observation:
the 4 1n4007 may not be correctly polarized. the two 1n4007 for Q3 (constantly current source) would not be conducting at any time). Maybe the V+A should have been V-B. same for the two 1n4007 for Q4.

If you switch V+A and V-B, it should be fine.
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Old 29th April 2003, 01:39 PM   #3
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Default Wrong?

Agree with the above. Realizing that the schematic may be wrong but the amp not, have you checked the quiescent current? What is the DC across the 0.16 emitter resistors? These are quite small anyway; I would use at least the double value here. Try to adjust the bias for no more than 50mA through these resistors.

Jan Didden
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Old 29th April 2003, 02:01 PM   #4
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Default Re: Is this schematic wrong?

Quote:
Originally posted by red
Hello As you can see I have a problem and I hope someone will help me.
I've build this amp (see the schematic)but It doesn't work properly.The "original" schematic
used BC107 and BC177 and as power transistors the old 2N3055 and 2N2955.
I am asking you please tell me if you can see anything wrong with it...It does amplify but it has a huge hum and on my heatsink you can fry eggs and believe me it's big enough for 4 transistors not 2 as I am using it.Besides what makes me think it is something wrong with it is that my BD235 is as hot *** hell while the others are cool.I've changed it twice and still didn't work.The schematic runs from +45 0 -45 power supply but for testing i've run it from a +22 0 -22 power supply.Did anyone,by anychance,build someting like this?PLEASE HELP ME.

Hi red,
I think you simply switched V+ and V- on the current sources of the input dfifferential pairs.
Yes I concur with Janneman 0.16 Ohm is a bit low for output-emittorresistors. Try something like 0.39 or 0.47 Ohm.
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Old 29th April 2003, 03:45 PM   #5
red is offline red  Romania
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Thanks alot for replying.
I have to admit you were right in the matter of V +A and V-B they were reversed.At least now it sings but the problem still remains even after changing the 0.16 R to 0.33 Ohms.My Bd 235 is now almost on Fire....Ouch.
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Old 29th April 2003, 04:15 PM   #6
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red, take out a multimeter and take voltages on those .16ohm resistors and report back to us.
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Old 29th April 2003, 05:08 PM   #7
red is offline red  Romania
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Across the mj15022 I have 14.99 volts and on it's resistor 1.20 volts on the mj15023 I have 14.98 volts and on it's resistor 1.19 volts while across the supply I read 18.15 volts.It's a 180 VA transformer.
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Old 29th April 2003, 05:08 PM   #8
moamps is offline moamps  Croatia
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Quote:
Originally posted by red
Thanks alot for replying.
I have to admit you were right in the matter of V +A and V-B they were reversed.At least now it sings but the problem still remains even after changing the 0.16 R to 0.33 Ohms.My Bd 235 is now almost on Fire....Ouch.

Hi,

Please make this measuring:

- Voltage between collectors BD139,140, -------- if V is greater then ca 3V or smaller try adjust this with trimmer (quiescent current), if this is not possible something is wrong with BD135
- voltage between emitters BD235,236 ---------- must be about 2V or smaller,
- voltage on 0.16E resistors -------- must be in first moment (small quiescent current) about 10mV
- voltage on output (speaker terminal) ------ must be about zero

Don't connect any speaker on output for first moment (in setup measuring); connect one resistor (for example 100E/2W).

My first mistake with BD 139,140 etc, was pining which is E-C-B.

Regards
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Old 29th April 2003, 05:12 PM   #9
moamps is offline moamps  Croatia
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Quote:
Originally posted by red
Across the mj15022 I have 14.99 volts and on it's resistor 1.20 volts on the mj15023 I have 14.98 volts and on it's resistor 1.19 volts while across the supply I read 18.15 volts.It's a 180 VA transformer.
Current is toooo high (about 10amps?), is with or without speaker?
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Old 29th April 2003, 05:24 PM   #10
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red, you got about 7-8amps of bias going through q5/q6. That is a little too excessive. idealy, both q5/q6 should be just conducting.

Can you measure how voltage drop on the amplified diode? and play with the 10K pot to adjust bias for q5/q6 down.

BTW, both q5/q6 should about equally hot, if they are wired correctly.
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