Aragon 4004 power amp channel imbalance - diyAudio
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Old 11th August 2011, 12:30 AM   #1
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Default Aragon 4004 power amp channel imbalance

Hi there,

Was wondering if anyone might be able to give me some advice that might be familiar with the power amp I have. I am not that technical! I bought a used Aragon 4004 power amplifier and it has a slight but definite and annoying channel imbalance. Its about 2-3dB quieter on the right side. I was reading it could be possibly be down to a dud resistor in the negative feedback circuit or some bad soldering. I also notice there are two cermet pots (the type of pots with the tiny adjuster you use a small screwdriver with) in each of the channel circuits of the amp (so four pots in total in the stereo amp). These pots are near the output transistors (there are 8 output transistor per channel). Wanted to ask if anyone knew what these are for, - bias or something else. I have not tried adjusting them at all yet.

I'll try to take a pic of the amps board and post.

Cheers,
David
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Old 11th August 2011, 06:45 AM   #2
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Two of the pots (on each amp board) will be for bias, the other two will more than likely be for adjusting DC offset.

With regards to one side being quieter than the other, is the 2-3dB difference measured or are you just assuming? This imbalance could just be your brain tricking you (for one reason or another), slight differences in speaker efficiencies, volume control gang mismatch or as you say, a dud component (eg. gain setting resistor or DC blocking capacitor) in the feedback network.
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Old 11th August 2011, 09:51 AM   #3
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Hi Jack,

Thanks for the info. Unfortunately I already adjusted those pots bit to see if it made any difference to the imbalance and of course it didn't. I therefore now have another slight problem anyway as I lost count of the number of turns I adjusted by, Is there are a simple guide anywhere that could get me started on getting these adjusting the bias and DC offset correctly (I have a multimeter and the circuit diagram)
Any advice would be really appreciated.

As for that channel imbalance its definitely there. I am not measuring but I am comparing this amp to 4 other power amps I have that all have perfect channel balance (and one of them is in fact another Aragon 4004), so yes something is causing this slight channel imbalance in this 4004. I am suspecting therefor it must be down to a dud component like you say. I'll try to work out where the feedback circuit is and change the caps. As for the resistors in the NFB, - I assume I'd have to take them out of the circuit and measure them to see if they are faulty?

thanks again for your help,
Cheers,
Colin



Quote:
Originally Posted by jackand08 View Post
Two of the pots (on each amp board) will be for bias, the other two will more than likely be for adjusting DC offset.

With regards to one side being quieter than the other, is the 2-3dB difference measured or are you just assuming? This imbalance could just be your brain tricking you (for one reason or another), slight differences in speaker efficiencies, volume control gang mismatch or as you say, a dud component (eg. gain setting resistor or DC blocking capacitor) in the feedback network.
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Old 12th August 2011, 03:26 PM   #4
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Hi again.
I took a little internet crash course on adjusting stuff on amps and know a bit more now.
I recapped this Aragon yesterday (all the 12 smaller electrolytics, - not touched the 4 big PSU caps yet). Anyway I located the bias and DC offset pots on each channel board and adjusted them (there was a very useful guide for doing it for Aragon amps on the internet).

After the recap there is still a channel imbalance, - its slight, - maybe about 2dB but noticeable. I also notice when I measure the bias voltage on the 8 emitter resistors of the quieter side of of the amp the bias numbers vary quite a bit, - I got 10.7mV, 10.7mV, 8mV, 8mV, 8.7mV, 8.9mV, 10mV and 9mV. So therefore readings as big as 2.7mV apart.
On the other side of the amp the bias readings are within 1mV of each other. And I also have another (unrecapped so far) Aragon 4004 that also have very close bias readings and no channel imbalance or other problems.

I don't know that much but just wanted to ask, - are these bias variations also symptomatic of something that could cause a channel imbalance?.
I was told by a friend that as well as the possibility of dud components in the NFB circuit causing the imbalance there is also the possibility that I have a few problematic output transistors. If anyone has any kind advice and where I should go from here it would be really appreciated to let me know what you think.

Cheers,
Colin
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Old 12th August 2011, 03:42 PM   #5
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To help you further, it would probably be easier if you post the circuit diagram on here or send me a PM with it in. That way I can tell you which adjustment does what and where to measure for the adjustments.

It is possible to set up the amplifier completely without any values provided by the manufacturer, just now that you have adjusted one of the pots, not knowing what it does, I would advise you not to use the amplifier until things are sorted.
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Old 12th August 2011, 04:31 PM   #6
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Hi Jack,

Thanks for the reply. I have correctly identified the respective pots that adjust bias and DC offset on each channel board of the amp. I found this guide
https://docs.google.com/viewer?a=v&q...9cY9AMWgr7pP6w

I know you measure the bias from the emitter resistors (close to the output transistors) of the amp on mV scale of a multimeter. And to measure the DC offset you put the multimeter in between the speaker + and - terminals and short the respective input, and adjust the DC offset pot to get as close to 0 mV as possible.

I just wondered if my quite varying bias reading from one channel of the amp could be symptoms of the same problem that has caused the channel imbalance.

Cheers,
Colin
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Old 12th August 2011, 04:53 PM   #7
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I wouldn't imagine that wrong bias setting would cause channel imbalance-you can always just go and re-adjust the bias on both channels though to rule that possibility out.

Also, your right in the way to measure offset and bias so I would go ahead and carry out the adjustments.
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Old 12th August 2011, 05:46 PM   #8
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Thanks for the info Jack.
Yes I managed to adjust the bias well (at least on one channel, - varying a bit on the other from transistor to transistor) and have set the DC offset to 0 or very close on both sides.

As for fixing the channel imbalance, a friend recommended I change all the resistors in the suspect channel card to 1% metal film to see if that makes a difference (quite a lot of work there are a lot of resistors) and if it doesn't work then consider the possibility of some of the output transistors in that card being suspect.

The shematic for the amp can be downloaded here
Aragon 2004 & 4004 Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics

Cheers,
Colin

Last edited by David120; 12th August 2011 at 05:55 PM.
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