P3A Problems please help!!

[TedPolzin]

Hi all I am having some trouble with a p3a. I am not sure what is causing it. I have a series of pictures from my scope I was hoping you could help me with.

First of all let me tell you about my amp. It is a 4 channel amp. 2 p3a boards with 2 channels on the left and right side of the amp. It is powered by a plitron 750VA transformer 25 volt secondaries (more like 28) with +-38 or so going to the amp. I have 30000 uF of capacitance per rail. Each amp has it's own bridge rectifier. I have look at the DC going to each amp both underload as well as unloaded. There is no difference on any channels an it is clean.

Now on to my problem

First the signal from the offending amp with no load. The division on my scope is .01v on DC.

An externally hosted image should be here but it was not working when we last tested it.

Not a huge bump but I can hear it.

With a signal line plugged in .. device on but no real load.

An externally hosted image should be here but it was not working when we last tested it.

This one is much more audible. I am not sure if it gets higher with a real signal on. I do hear it though.

For comparison here is the signal on my right side with the same 2 pictures.


No signal at all

An externally hosted image should be here but it was not working when we last tested it.

Nice and clean

Signal line plugged in

An externally hosted image should be here but it was not working when we last tested it.

Getting a bump. I can hear it but not much.


Here is my wiring. I know it is not the prettiest but I was planning on cleaning it up before using the amp. I have played with the wires while watching the scope and no difference.

Left

An externally hosted image should be here but it was not working when we last tested it.

Right

An externally hosted image should be here but it was not working when we last tested it.

The ground

An externally hosted image should be here but it was not working when we last tested it.

As I said I know this is not the best wiring. I have a much better case on the way so this is my prototype. As well I have better heat sinks.

I am just trying to get it working right now so when I get the rest of my stuff in it is a easy transfer.


I am not even sure what I am seeing. I would have thought I have all but eliminated the chances of the groung loop. I do not even think it is a ground look I am looking at.

Thanks for all your help

Ted

 

[PMA]

Most likely there is a problem of ground (0V) wiring. Is not there any ground loop? May any ground loop be closed through signal wires (shields)? It would be helpful if you could attach ground wiring diagram with all 0V wires shown (also for preamp stages).

 

[TedPolzin]

So you do figure it is a ground loop? My ground as it sits. All 0 v grounds go back to a single point lifted from the chasis. It is a single point floating ground. The pic of the ground is were they all meet.

This is my first amp and it is sure taking longer than expected. I am just not sure what else I can do with it. Nothing I do will remove that signal. It is not present on the right side and the ground is exacally the same over there.

Thanks

Ted

 

[carlmart]

So you do figure it is a ground loop? My ground as it sits. All 0 v grounds go back to a single point lifted from the chasis. It is a single point floating ground. The pic of the ground is were they all meet.

This is my first amp and it is sure taking longer than expected. I am just not sure what else I can do with it. Nothing I do will remove that signal. It is not present on the right side and the ground is exacally the same over there.

Ted,


Don't give up yet. You picked an ambitious project to be your first amp, so don't complain too much.

My first guess is there might be a loop somewhere too.

First of all let's try to be a bit methodic about this. Can you take each amp out of the case and test them individually?

Second: try sending separate wires for each ground block in the pcb to the star point. Maybe you already did that. If you did, maybe there's something wrong in the ground arrangement in the pcb. There should be at least three different and separate grounding points on the pcb.

Try that and let's see what you get.


Carlos

 

[TedPolzin]

There should be at least three different and separate grounding points on the pcb.

Hmm I only have one. The main power one. I brought back the grounds for the input and output directly to the star ground. I have separated the ground between the amps on the individual boards though, so I guess I have 2 different and separate grounds per pcb.

I think I will test each amp out tonight. I'll just disconnect the fuses on the amps I do not wish to test. I am just do not understand why it would affect one side of the case and not the other.

At least you are confirming it is a ground problem and not something else. I was not sure exactly what I was looking at.

Thanks a lot!!

Ted

 

[TedPolzin]

Ok then where should I start. I removed the fuses from the other amps and the loop, extra signal was about 1/4 of it's previous strength with a signal on it.

Where should I go from here? I was hoping removing the other amps from the equation would have eliminated it.

Thanks

Ted

 

[TedPolzin]

Any ideas where I should start. Here is what I have found out and done.

I split the ground between the two boards and that helps quite a bit so there are now 2 seperate grounds one to each board. If I combine them the loop increases a lot !! like 5 fold.

If I turn off the power it dissapears totally. I am still drawing from the caps.

Please let me know where I should proceed from here.

Thanks

Ted

 

[TedPolzin]

Ok here is what I have done

Each ground comes off it's direct source, EG input, output, amp and capacator grounds directly meet. This is rather than input -> pcb -> out.

Each amp is diretly connected to it's own ground points .... which connects to a star point which connects to the transformer ground.

EG. Amp 1 input, output, amp and cap ground collect which then collect with all the other amp grounds and connect to the transformer.

What I get .. a soft ground signal when I have signal going to more than 2 or more amps. If they are on the same physical board it is a little softer than if it going to a diffrent physical board.

If the signal is going to only one board it is virtually with out ground noise.

Please can someone give me some direction as to what to do. I really need the help!!!!! I am lost as where to go next!!! Please!!

Thanks

Ted

 

[dwjj]

Probe the power rails?

What do the power rails look like (at the output transistors)? Are they the right voltage? Without noise?

Probe the base on the output transistors.