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Old 13th November 2002, 05:26 PM   #11
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Quote:
Originally posted by chris ma
From the picture if I guess correctly that the yellow wire from the 230AC plug is the Earth, correct? But I can not tell whether it is connected to the chassis or to the cap. If it is connected to the cap then I think that is the wrong place(-Ve).
Regards,
Chris
Yep, thats earth. It is connected to a point the metal chassis is mounted at, not at to cap.
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Old 13th November 2002, 05:48 PM   #12
UrSv is offline UrSv  Sweden
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Default Coarse ground..

The grounding scheme looks a little weird. It seems you run the speaker grounds individually back to the PCB and then to the cap center. This, AFAIK, means, among other things, that your input ground is shared with the speaker return ground and all of it would be causing a big ground loop.

I would probably do/try the following:

1. Put speaker return at the new star ground point which seems to fit at the cap center connections.

2. Return the PCB ground to the star point.

3. Connect chassis ground to power suppy ground via 10 Ohm resistor.

4. Return input grounds individually to the star point if needed.

Unless somebody tells me I am completely off then I think that would help.

/UrSv
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Old 13th November 2002, 07:00 PM   #13
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Hi,

I would concure with UrSv 's suggestions. To reduce the RFI, I would suggest to connect a 0.1uF ceramic cap in parrallel with the 10 ohm resistor as in the 3rd suggestion.

Regards,
The Seven.
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Old 13th November 2002, 07:44 PM   #14
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Default Re: Coarse ground..

Quote:
Originally posted by UrSv
The grounding scheme looks a little weird. It seems you run the speaker grounds individually back to the PCB and then to the cap center. This, AFAIK, means, among other things, that your input ground is shared with the speaker return ground and all of it would be causing a big ground loop.

I would probably do/try the following:

1. Put speaker return at the new star ground point which seems to fit at the cap center connections.

2. Return the PCB ground to the star point.

3. Connect chassis ground to power suppy ground via 10 Ohm resistor.

4. Return input grounds individually to the star point if needed.

Unless somebody tells me I am completely off then I think that would help.

/UrSv
1. Tried that, didn't help at all.
2. What do you mean? The PCB's ground is connected to the star point (which the caps center i suppose).
3. Tried that.
4. Will give it a shot.
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Old 13th November 2002, 07:52 PM   #15
UrSv is offline UrSv  Sweden
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For 2. I guess that is correct and I just pointed out all connections. For 4. I would even try to connect the grounds at the input to each other and run one ground back to star. That might also work.

/UrSv
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Old 13th November 2002, 08:06 PM   #16
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Quote:
Originally posted by UrSv
For 4. I would even try to connect the grounds at the input to each other and run one ground back to star. That might also work.

/UrSv
Just came back from trying that.
That gave me loud negative spikes on the right output, and an entirely clean left output when i connected the inputs to ground.
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Old 13th November 2002, 08:22 PM   #17
UrSv is offline UrSv  Sweden
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Then there is definitely something fishy going on. Hard to say when I can't see up close.

Check everything. Component orientation, component values, solder bridges, solder joints etc...

What I can see that could be suggestions:

1.
Are the screws holding the PCBs really not touching the track on the bottom side? Lower fixing points look close. Measure and verify?

2.
Heatsink properly isolated from all transistors? Measure and verify?

3.
I have had occasional problems when not isolating the bridges from chassis. Try lifting the bridge from the chassis in case it is attached to metal. Don't ask me why.

4.
Oscillations? Check with a scope during the test.

Other than that I am out of ideas.

Good Luck/UrSv
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Old 13th November 2002, 08:32 PM   #18
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Thanks for all the help so far.
The chassis is plastic, the screws are not touching the tracks, but even if they were it wouldn't be an issue.
The trannies are isolated from the heat sink, i just measured them (did this before first start up too ofcourse, i think things would have gotten a bit hot else :-).

I'm thinking of replacing all bc546's and all the caps on both boards, will borrow some desoldering stuff from school tomorrow i think. I have checked component orientation etc a million times. :-)
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Old 13th November 2002, 08:34 PM   #19
hifiZen is offline hifiZen  Canada
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Quote:
Even though I don't argue with you about the amount of filtering caps, I don't think he'll ever sort out a ground loop by just adding more caps.
No no, of course not! I wasn't commenting on the ground loop problem, just the cap size. The two don't have anything to do with each other.

paulb: 4700 may be OK for operation of a class-B or AB amp at low signal levels, but don't expect to get great performance out of it when you crank it up. Just remember your ripple will increase as you draw more current... and the P3A as I recall, is good for 60-100W/8R. If you start pushing that kind of voltage into a speaker load with an impedance dip, you'll find 4700 is not sufficient, especially if you want clean bass. At that kind of power output, your peak current draw from the PSU will rival that of a push-pull class-A unit, and furthermore will contain a lot more noise due to the class-B circuit effectively "rectifying" the current waveforms going back to the PSU. Bigger is better, even though you might get away with only 4700uF here.
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Old 13th November 2002, 08:48 PM   #20
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Quote:
Originally posted by hifiZen
No no, of course not! I wasn't commenting on the ground loop problem, just the cap size. The two don't have anything to do with each other.

paulb: 4700 may be OK for operation of a class-B or AB amp at low signal levels, but don't expect to get great performance out of it when you crank it up. Just remember your ripple will increase as you draw more current... and the P3A as I recall, is good for 60-100W/8R. If you start pushing that kind of voltage into a speaker load with an impedance dip, you'll find 4700 is not sufficient, especially if you want clean bass. At that kind of power output, your peak current draw from the PSU will rival that of a push-pull class-A unit, and furthermore will contain a lot more noise due to the class-B circuit effectively "rectifying" the current waveforms going back to the PSU. Bigger is better, even though you might get away with only 4700uF here.
I know. I was going to go for a second pair of caps and another bridge (thus making indivudual power supplies except for the toroid). However, when i started building it i could barely afford a single psu, and since i never got it working i never bought a second pair of caps.

Speaking of which, could splitting the psu acrually solve the problem? Would be an ugly fix though, something is appearantly wrong somewhere.
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