Phase Linear 400

[jamesltaylor]

I have a repair question and yours is the first site I have approached. If you cannot or will not respond to my question please direct me, if you can, to a site where I can field this question.

Background: I thought I heard distortion in one of the channels of my final amp. I got documentation for the amp and started to check it out. I checked the performance and then I replaced all of the capacitors in the amp since it is old. Then I fired it back up and checked the offset and bias. The bias was just a little off on one channel offset was fine. From the start I noticed that when I input a sine wave when the output went over 1 volt p-p noise started to appear on the peaks of the sine wave. At 1.5 volts the noise was a significant part of the output waveform but as volume was increased the noise amplitude remains the same and becomes a small part of the output waveform.

The noise is present on the ground bus of the preamp, which also shows up in the output waveform, and appears to be generated by reverse EMF from the speakers getting into the ground plane. By all outward signs the amp sounds normal.

Question: Is this normal behavior for a Phase Linear 400 type 2 final amplifier? If not what may be wrong

 

[(*steve*)]

I have a repair question and yours is the first site I have approached. If you cannot or will not respond to my question please direct me, if you can, to a site where I can field this question.

That is one of the most weird openings to a query I've read yet. I'm not sure what the point is of asking for a response if we "will not" respond...

Background: I thought I heard distortion in one of the channels of my final amp. I got documentation for the amp and started to check it out. I checked the performance and then I replaced all of the capacitors in the amp since it is old.

When you say "all the capacitors", do you really mean that -- you replaced every single capacitor? Or did you replace just the large electrolytics? Or was it so old that it had paper capacitors?

When was it manufactured?

Did the documentation tell you exactly what sort of capacitors to use, else how did you determine this?

Was it working correctly before you replaced the capacitors?

What is your level of experience? (Do I really need to ask the preceding questions?)

Then I fired it back up and checked the offset and bias. The bias was just a little off on one channel offset was fine. From the start I noticed that when I input a sine wave when the output went over 1 volt p-p noise started to appear on the peaks of the sine wave. At 1.5 volts the noise was a significant part of the output waveform but as volume was increased the noise amplitude remains the same and becomes a small part of the output waveform.

DO you get distortion on just the one channel?

Have you carefully checked that none of your replacement caps are badly soldered, or perhaps that you have accidentally replaced one capacitor incorrectly?

Can you trace the signal through the stage and determine where the noise starts to appear?

The noise is present on the ground bus of the preamp, which also shows up in the output waveform, and appears to be generated by reverse EMF from the speakers getting into the ground plane. By all outward signs the amp sounds normal.

When you say it's present on the ground bus of the preamp, what are these measurements with respect to?

Is the ground not common?

Are you sure that it's not noise on the power supply for the preamp?

What sort of power supply is it? It's not switchmode is it?

Question: Is this normal behavior for a Phase Linear 400 type 2 final amplifier? If not what may be wrong

I doubt it is normal. I think you have a fault.

 

[jamesltaylor]

That is one of the most weird openings to a query I've read yet. I'm not sure what the point is of asking for a response if we "will not" respond...

Sorry, I guess I am good for wierd openings. Actually it was written for a manufacturer site so I was begging.



When you say "all the capacitors", do you really mean that -- you replaced every single capacitor? Or did you replace just the large electrolytics? Or was it so old that it had paper capacitors? Yes all of the capicitors. No they were not paper.

When was it manufactured? 1970s

Did the documentation tell you exactly what sort of capacitors to use, else how did you determine this? Yes and I ID the part markings for value and voltage ratings.

Was it working correctly before you replaced the capacitors? No it appeared to have minor distortion in left channel.

What is your level of experience? (Do I really need to ask the preceding questions?)
30 years in electronics mostly in power, digital controls and automation. Audio is a hobby that I do not have lots of experience in.


DO you get distortion on just the one channel? Yes Left.

Have you carefully checked that none of your replacement caps are badly soldered, or perhaps that you have accidentally replaced one capacitor incorrectly? Yes inspected carfully

Can you trace the signal through the stage and determine where the noise starts to appear? I can trace it but it travels through the ground buss and into the preamp and onto the output signal in a loop.



When you say it's present on the ground bus of the preamp, what are these measurements with respect to? Well in reference to input ground ground. This unit has input ground isolated by.2 ohms above ground potential. With the scope ground on chassis ground I am reading the signal ground when looking for noise in the front end.

Is the ground not common? Chassis ground is the common, input ground is differant.

Are you sure that it's not noise on the power supply for the preamp? The power supply +/- is quiet at no signal. When signal amplitude is increased a very small, by comparison to ground, noise component is present on the +/- power busses.

What sort of power supply is it? It's not switchmode is it? Simple full wave bridge onto a filter cap on each side +/-. No regulatotion or nothin.



I doubt it is normal. I think you have a fault.

Please see picture. The conditions of this photo are output at 1 KHz & .5 volts per division:

 

[jamesltaylor]

Thanks for your help.

 

[(*steve*)]

OK, if chassis ground and signal ground are different, and if the noise appears between them, and it does not appear on the power supply, then something is causing weird currents to flow across that resistor linking the 2 grounds.

I'd be looking for something oscillating.

The image of the oscilloscope display was too small for me to see anything. Can you confirm that the noise appears across the 2 grounds, and also between signal ground and the signal output.

If it's a discrete amplifier, then I'd be looking at each of the stages, trying to determine if one of them is oscillating.

Are there any biasing differences between the left and right pre-amp?

If you remove the preamps and inject a signal directly into the main amp, do you see any distortion?