P3A problem - DC at output - diyAudio
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Old 19th June 2012, 01:15 PM   #1
denry12 is offline denry12  Estonia
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Default P3A problem - DC at output

Hi there!

I tried building an ESP P3A (for general amplification, I am not really chasing the "crystal clear sound") since I needed a 50-100W amplifier.
My current test runs involve 12V 21W incandescent bulbs in places of fuses and +-12V power supply (in practice this means 11.8V @ each supply rail). When the amp is powered up this way, the lamps do not even start glowing, therefore consumed current can't be too high. I have not measured that.
However, the output is constantly at 2.34V, therefore I have not risked hooking a speaker there. With 100 ohm dummy load it dropped to 2.28V, but am I mistaken if I believe it should not be more than 0.1-0.2V there without any load?
I have made some modifications:
1) readily-available transistors MJE3055 & MJE2955
2) currently 1 ohm resistors for output transistors (originally I tried with 0.1 ohm but replaced them with 1 ohm just to be sure this is not related to my issues)
3) BC547 instead of BC546
I have desoldered and measured every transistor individually, they all seem fine, therefore I am out of ideas where to look for the issues. I was hoping some kind person would check out my measurements and give me some directions where to look at to find my problems.
In the attached picture I marked any differences with original schematic with red and measured voltages in dark green.
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File Type: png p3a-f1_volts.PNG (22.1 KB, 214 views)
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Old 19th June 2012, 02:37 PM   #2
superR is offline superR  Netherlands
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Weird, voltage across c1.... Make new measurements when you disconnect any input and short the point between r1 and r3 to gnd. BC547 should be fine. Have you used the bd139 and bd140's? Check the pinout of those parts. They might be different from the ones used in your pcb-layout-program. What is your bias "voltage"?

Last edited by superR; 19th June 2012 at 02:45 PM.
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Old 19th June 2012, 02:43 PM   #3
denry12 is offline denry12  Estonia
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BD139 and BD140's certainly have the correct pinout. Since base is not the middle pin, I can easily identify pins with multimeter.
I will now proceed to short the point between R1 and R3 to ground and take new measurements.
EDIT: The results are in and I still can't make any sense of it.
EDIT2: I noticed you asked about my bias voltage. My trimpot is set to middle, so VR1 = 1k at the moment. Collector of Q8 is at 1.27V and collector of Q7 is 1.76V.
p3a-f1_volts_shorted.PNG
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File Type: png p3a-f1_volts_shorted.PNG (23.9 KB, 208 views)

Last edited by denry12; 19th June 2012 at 03:02 PM. Reason: Added information, for everyone to enjoy (oh boy!)
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Old 19th June 2012, 03:25 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
the voltages around R1 to 3 should all be -ve. The emitters of q1&2 should also be -ve. Something weird.
Vled ~1.9V Vr7 ~1.2V, Ir8 9.94V
ILED ~0.45mA, far too low, Ir7 ~2.1mA, Ir6 ~1.25mA, leaves ~0.9mA through Q2
Ir10~2mA, too low
Vbeq4~700mV, too high for a 2mA Ic.
It could be that the artificially low supply rails are not biasing the VAS to design current that the voltages don't appear right. That's one of the downsides to VAS fed with resistors. Very intolerant of varying rail voltages.
But, that does not explain the +ve voltages around the input???
I wonder if Q1 is dead?
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Old 19th June 2012, 03:27 PM   #5
denry12 is offline denry12  Estonia
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I now marked the collector voltages on schematic and apparently I had mismarked the output voltage. It's ~1.5V instead of 1.25V.
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File Type: png p3a-f1_volts_shorted.PNG (24.1 KB, 198 views)
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Old 19th June 2012, 04:11 PM   #6
Vostro is online now Vostro  South Africa
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Yeah agree with AndrewT, never seen him to be wrong.
Q4 looks saturated, either is blown shorted, or more likely
like AndrewT wonders, Q1 is Shorted (dead).

Reduce R9,10 to increase Vas current.

Hope you get satisfactory results from discrete amp at low rails.
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Old 19th June 2012, 04:14 PM   #7
Did it Himself
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Check the value of R6.
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Old 19th June 2012, 04:34 PM   #8
denry12 is offline denry12  Estonia
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Gah, AndrewT was right indeed. Replaced Q1 with some BC546 I found lying around, currently nicely 0V at output. However, the one I removed was brand new when installed and when I measure it right now with multimeter it looks perfect - it's like two diodes with ~800mV voltage drop. Seriously confused. (anyone have ideas how to perform more tests to see what the devil was up with that?)
I will continue with testing and will keep you posted.
Thanks a lot for all your help, guys!
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Old 19th June 2012, 05:13 PM   #9
AndrewT is offline AndrewT  Scotland
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Is it NPN?
Can you measure hFE?
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Old 19th June 2012, 05:24 PM   #10
denry12 is offline denry12  Estonia
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Well, there's your problem! hFE = 4. However, if I reverse the leads hFE=245.
According to all datasheets I have seen, BC547 should have emitter on the right side when facing labels. I measured all of the transistors I had in this batch (12 of them) and 11 had emitter on the LEFT and hFE around 245-255. One of them had the emitter on the right and hFE=435!!!. They are all Fairchild's BC547B's. Possibly fake?
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