Is this schematic wrong?

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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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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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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
 
red said:
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.:idea:
Yes I concur with Janneman 0.16 Ohm is a bit low for output-emittorresistors. Try something like 0.39 or 0.47 Ohm.;)
 
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.
 
diyAudio Member
Joined 2002
red said:
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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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.
 
The 47 pF should be connected to the collectors of either BD139 or BD140. This capacitor is for local feedback at high frequencies.

The 470 nF at the Vbe multiplier is quite unneccesary. The capacitor across the transistor will do the job, which also is the normal design.

The 4 pcs. of 680R at the diff stage are quite unneccessary. Never understood what they were for? Can anyone tell me? I have never used this kind of resistors and my diff stages have allways worked swell.

BTW: How have you come this design? Have you calculated all component values?
 
Quote from Peranders:
"The 4 pcs. of 680R at the diff stage are quite unneccessary. Never understood what they were for? Can anyone tell me? I have never used this kind of resistors and my diff stages have allways worked swell."

They look like damping resistors for parasitic oscillations related to base wire inductance. If you have an emitter follower which has a (small) capacitive load and some wiring inductance in series with its base, together with the base-emitter capacitance of the transistor itself, you get a nice parasitic Colpitt oscillator, which usually oscillates somewhere in the neighbourhood of the fT of the transistor. A resistor in series with the base, mounted as close to the base as possible, can usually solve this; the same applies to a dissipative (low-Q) ferrite bead around the base lead.

A differential pair with a somewhat capacitive tail current source behaves just the same as the emitter follower, at least for common-mode signals.

My approach is usually to start without such resistors and put them in in a later stage if this turns out to be necessary.
 
Thanks
I found this schematic in two difrent books regarding a powerfull amplifier.The two desinges were quite very much the same.
I tested the Bd 135 and it is working perfectly but what concerns me is that the trim pot is at 0 and the curent is too high.If I try to modify the resistance of the pot the Mj's get even hoter then already are...and I don't want to burn them.And the problem is that I'am only testing it because the power supply I was planing to use has a 400 VA transformer that provides 2x30 V AC...
 
red said:
Thanks
I found this schematic in two difrent books regarding a powerfull amplifier.The two desinges were quite very much the same.
I tested the Bd 135 and it is working perfectly but what concerns me is that the trim pot is at 0 and the curent is too high.If I try to modify the resistance of the pot the Mj's get even hoter then already are...and I don't want to burn them.And the problem is that I'am only testing it because the power supply I was planing to use has a 400 VA transformer that provides 2x30 V AC...


Hi,

If BD135 is OK, and trimmer is on 0ohm, than current thru drivers and output transistor can't be high (will be zero). Transformer isn't now your problem.

Regards
 
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moamps said:



Hi,

If BD135 is OK, and trimmer is on 0ohm, than current thru drivers and output transistor can't be high (will be zero). Transformer isn't now your problem.

Regards

I agree. In this case, the output transistors shouldn't be conducting at all. Something else must be wrong, and seriously wrong. Maybe the wiring isn't incorrect? or the transistors not seated correctly? - believe it or not, it happened to me more than a couple of times, :(
 
Hy and thanks to you all.I finally figure it out.It was a simple mistake.I reversed the polarization of the output MJ15023.It works almost perfect.It is still a bit hot but I think it's the 4 ohms load I am using.[:)].It seems to be a good amplifier but if you have suggestions for a better one please inform me.And thanks again. RED
 
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