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Old 25th February 2010, 01:22 PM   #41
melorin is offline melorin  Hungary
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Thank you!
I found what the problem was with my amplifier. I used BC212 transistors as a E-B-C, but there are two kinds of BC212. My ones are C-B-E. Damn! Fortunately, after I changed the polarity, the amp got up well. Its temperature is very good, and the bias current is about 100mA (R11=4,7ohm). But you were right. I got +0,9volts at the output But I am still happy to have a well-operating amplifier There remains no more but to find out the solution how to get 0V at the output.
ps.:Sorry for my bad english, it's not my native.
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Old 25th February 2010, 04:18 PM   #42
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Hi, I gave you the solution to fix the output dc voltage.
Just use a pot for R4 . Or use a fixed resistor in parallel with a variable pot.
Like say a 10 K resistor in parallel with a 10 K preset. Set it to give 4.7K ( measured with a DMM with power off) and then adjust it with power on to give you < 1mV at the output.

Can you post any pictures ? How does it sound ? My board isn't ready yet.
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Last edited by ashok; 25th February 2010 at 04:25 PM.
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Old 25th February 2010, 04:44 PM   #43
melorin is offline melorin  Hungary
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Quote:
Originally Posted by ashok View Post
Hi, I gave you the solution to fix the output dc voltage.
Just use a pot for R4 . Or use a fixed resistor in parallel with a variable pot.
Like say a 10 K resistor in parallel with a 10 K preset. Set it to give 4.7K ( measured with a DMM with power off) and then adjust it with power on to give you < 1mV at the output.

Can you post any pictures ? How does it sound ? My board isn't ready yet.
I tried to use a 11,52K for R4 instead of 4,7K. Output DC was nearly 0V, but it was changing slowly. Starting from +13mV to -12mV, then up to about +10mV again, and back It took minutes, not seconds.
Using 12K R4 it was -40mV, with 10K it was +100, so I will really need a potmeter. The problem is that the voltage is not fix, it is changing.
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Old 26th February 2010, 01:48 PM   #44
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I don't expect the dc to stay fixed.It's referenced to base emitter voltages and diode drops. These are not in physical contact with each other and hence not varying by the same amount ( deg C ).So dc will vary from the set point. Best to set it ( several times) after 30 minutes of power on. It might have a reduced variation. Varying isn't the problem. Varying too much is the issue. I wanted the diodes to be mounted on the same heatsink as the output devices. Unfortunately I am getting no spare time to finish my pcb ! I hope this weekend is good.

Done any listening tests yet ?
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Old 26th February 2010, 07:34 PM   #45
melorin is offline melorin  Hungary
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Quote:
Originally Posted by ashok View Post
I don't expect the dc to stay fixed.It's referenced to base emitter voltages and diode drops. These are not in physical contact with each other and hence not varying by the same amount ( deg C ).So dc will vary from the set point. Best to set it ( several times) after 30 minutes of power on.
Done any listening tests yet ?
I changed the circuit to set the DC to zero. I use a 10K pot, not a 4,7k.
It seems to work. I can set the DC to be about zero, but it's changing a bit. After a long it moves slightly, max. 1-2mV. But the heatsink is very hot. It's not a small one, so I don't know if it's normal or not. And the heatsinks on the 7812/7912 are very hot, too. Maybe the 4,7ohm R11 is too low? But as I measured, the offset is 110mA, and the stab ICs are 2A ones.
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Old 27th February 2010, 02:18 PM   #46
melorin is offline melorin  Hungary
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Quote:
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Done any listening tests yet ?
Sorry, I haven't recognized this question. After I set the zero DC, I listened how it sounded. I soldered the C1 cap to the ground, plugged the headphone(a cheap one). I was surprised because I was able to hear some odd sound from it.
A sound, like hum but not just a hum, I also heard a "bzz" effect, as if I searched a channel on an old radio. Also, when I touched the ground, or grazed a metal to another one (not in the amlifier, just beside it), it confused the sound, as if there was no ground-shield (on the input cable), but there was of course. So I was afraid of listening it too long, but I connected an MP3 player to the input to see whether it sounded or not. It sounded very well, and the volume was quite enough, maybe too much The bass was very impressive, but there was noise among the music sounds.
So I still have to work on it to find out what the problem is. Any idea?
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Old 28th February 2010, 01:09 PM   #47
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I will try to rig up my board by tomorrow. Will let you know what happens then. I might add that you need to be careful how all the ground routing is done. Low level ground must not 'ever' carry/share any dc supply path. That can give you hum and buzz.
Is your amp grounded to the metal chassis ? Is the chassis grounded to mains earth. Sounds like the ground/chassis is floating.
About running very hot.
The heatsinks might not be enough. This circuit dissipates twice the amount of power than my single supply circuit. You can increase the 4.7 ohm resistor to decrease the quiescent current. Try 6.8 ohms. Any drawing or picture of the heat sink you have used ?
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Last edited by ashok; 28th February 2010 at 01:15 PM.
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Old 28th February 2010, 05:51 PM   #48
melorin is offline melorin  Hungary
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Quote:
Originally Posted by ashok View Post
I will try to rig up my board by tomorrow. Will let you know what happens then. I might add that you need to be careful how all the ground routing is done. Low level ground must not 'ever' carry/share any dc supply path. That can give you hum and buzz.
Is your amp grounded to the metal chassis ? Is the chassis grounded to mains earth. Sounds like the ground/chassis is floating.
About running very hot.
The heatsinks might not be enough. This circuit dissipates twice the amount of power than my single supply circuit. You can increase the 4.7 ohm resistor to decrease the quiescent current. Try 6.8 ohms. Any drawing or picture of the heat sink you have used ?
Here are some picture.
The chassic is wooden.
"Low level ground must not 'ever' carry/share any dc supply path." I don't understand this sentence. What do you mean? What am I supposed to do?
Anyway I tried to grounded it to the earth, but it didn't help. As you can see I connected the heatsink to the ground, no success. Not to say, since I did it, It has made a noise when I touch the heatsink with any metal.
I have recorded the sound effect from its output. I hear that when the 'Left' output cable draws close to the 'Right' cable, or if the headphone cable writhes (spiral).
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Old 1st March 2010, 01:26 AM   #49
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OK , you have oscillations !
It might be due to the way things are connected . Check any capacitive coupling ( due to close wires ) between output and input. Note that it could be RF oscillations with spurious products in the audible range. RF oscillation could be due to the circuit !
Let me rig up my board and see if I get a similar thing.
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Last edited by ashok; 1st March 2010 at 01:30 AM.
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Old 1st March 2010, 10:44 AM   #50
melorin is offline melorin  Hungary
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Quote:
Originally Posted by ashok View Post
Check any capacitive coupling ( due to close wires ) between output and input.
The input and output are far away from one another as it is seemed in the picture I attached.
But, maybe, I found the source of the problem. I bought the C5 capacitor as a 330pF, but at home I recognized that it was only a 33pF, because the text on it is '33'. I will try to get a truly 330pF and see whether it solves the oscillation or not.
I hope your amp will be all right Good luck!
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