3 out of 4 channels are distorted

nonprof

Member
2011-06-08 1:33 am
Hello Guys!

I'm here again soliciting for your help and opinion.

I have 4 channels generic class AB amplifier. The problem is distorted channels intermittently. I've tested the functionality of output BJT transistors, preamp, and biases everything seems normal.

I have some observation, If I crank the amp loud, the distortions on some channels went out but if the amp become cold the distortion comes back.

Do you have any opinions?

Thank you and appreciated all your responses. i've attached picture.
 

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nonprof

Member
2011-06-08 1:33 am
It could be insufficient biasing.

Post a photo of the distorted signal with a sine wave.

Hi Perry,

Here are couple of screenshots of two channels. I measured again the bias resistor, transistors and some capacitors they seems within tolerance. 3 out of 4 channels behave like this.

Thank you!
 

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nonprof

Member
2011-06-08 1:33 am
Good Day!

Voltages +-15V on differential amplifiers are present. Amp is sometimes 3 channels are working and sometimes 1 channel only :)

For this king of amp may I know what is the resistance of the shunt resistor and a zener voltage in a regulator? I've read 330 ohms 2watts and a 15V zener diodes (1N4744). I'm thinking that previous tech altered these parts.
 
I just went through an old targa 4 channel amp and an old profile 2 channel. Both had terrible notch distortion on all channels and about the same notch. It was very noticeable at low volume. I replaced the bias resistor on one of the channels with a variable pot and adjusted to clean up the notch. Then pulled the variable pot out and measured the value, then replaced all the bias resistors with the new value. After rechecking the sine wave of all channels, everything was perfect and sounds perfect.
I don't know if age or just cheap design was the problem but what I did worked.
Everything else on the amps were fine.
Note; I did not have to change the value of the resistors much to get it back to normal.
Do not know if this will help but might be worth a try.
 

nonprof

Member
2011-06-08 1:33 am
I just went through an old itearga 4 channel amp and an old profile 2 channel. Both had terrible notch distortion on all channels and about the same notch. It was very noticeable at low volume. I replaced the bias resistor on one of the channels with a variable pot and adjusted to clean up the notch. Then pulled the variable pot out and measured the value, then replaced all the bias resistors with the new value. After rechecking the sine wave of all channels, everything was perfect and sounds perfect.
I don't know if age or just cheap design was the problem but what I did worked.
Everything else on the amps were fine.
Note; I did not have to change the value of the resistors much to get it back to normal.
Do not know if this will help but might be worth a try.

Thank you 90s. Yes this is cheap designed amp. I definitely do that but where is the bias resistor you are referring? Any clue where this connected? i suspect the pre driver are under bias.
 
I downloaded a schematic and most of them are similar. So Q5 on this one is the bias and R15, R16 and VR2 make up the bias of Q5.
If you look at schematics online of different models, you should see a similarity. The ones I messed with, the resistors were on the base to emitter side that I adjusted to a different value.

This is what I do for bias adjust.... http://www.diyaudio.com/forums/car-audio/261670-bias-adjust-one-way-many-worked-me.html
 

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nonprof

Member
2011-06-08 1:33 am
Perry,

You're almost right...there's something a muting problem. I tried reducing the base voltage at the muting transistors and distortion disappears but another problem occurs the low side or high side pair of transistors were heating up past.

Right now all channels are working. What I did is pulling up the base voltage by connecting resistance between base and collector of the TO92 transistor that driving the transistor TO220 transistor. Sorry I don't know what is called.
 

nonprof

Member
2011-06-08 1:33 am
Perry,

You're almost right...there's something a muting problem. I tried reducing the base voltage at the muting transistors and distortion disappears but another problem occurs the low side or high side pair of transistors were heating up past.

Right now all channels are working but i'm not sure if this a long term solution but this refer to Car90s suggestion. What I did is pulling up the base voltage by connecting resistance between base and collector of the transistor connected to bias resistors.
 
Most amps have only one muting transistor per channel. It's typically between the preamp stage and the differential amplifier stage. One terminal is typically connected to the secondary ground. One to the audio line and one to the control circuit.

Can you post a good quality photo of the area in the attached image?
 

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nonprof

Member
2011-06-08 1:33 am
Thank you all who responded.

Perry,

You almost right! there something to do with the mute lines. One mute transistor is shorting the adjacent lead and affects the other 3 channels. They go at mute threshold why they are intermittent.

I have one more question how did you know by seeing the scope shot that there is a muting problem?
Thank you!
 
I initially thought it was a bias problem. After seeing the scope image, it seemed to be more of a power supply problem. I've seen muting circuits distort the signal so that was one of the things to check when you stated that all of the supply voltage were OK.

The opto-coupler switches the power supply for the differential amplifiers. This isn't a conventional muting circuit where the audio is clamped to ground. It disables the first stage of the amplifier.

If shorting pins 3 and 4 of the opto-coupler makes the amp work properly, the problem is that the opto-coupler isn't being driven fully on. The capacitor on the base of the transistors that drive the LED for the opto-coupler has been leaky in other amps and when leaky doesn't allow the opto-coupler to be driven on fully.