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Old 1st May 2003, 08:20 PM   #11
edm is offline edm  Netherlands
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Quote:
Originally posted by moamps



Hi,
did you try with 1K and pot (or trimmer for test purpose) on +input?

Regards
Nope I will try to do so tomorrow...


Quote:
Originally posted by pinkmouse
Where is your 'scope grounded? is it the same outlet or a different one?
same outlet

Quote:
Originally posted by pinkmouse
Is your chassis grounded?
[/B]
Currently everything is placed in large wood box, which is there to prevent my cats from being electrocuted (they have some fetish for wires)

Quote:
Originally posted by pinkmouse
Is your amp circuitry grounded to the chassis?
[/B]
I have tried to attach safety earth to the star point and the earths of the preamp and amp are connected through a 100 Ohm resistor bypassed by a 100nF cap (have also tried without in all possible permutations).


Quote:
Originally posted by pinkmouse
If so, have you checked the ground in your lead/supply/socket? [/B]
For what exactly should I be looking?

I also found out that on the amp side at the input I have some noise, which is not there on the preamp side. If the preamp is disconnected the noise worsens. Could this also be related to the grounding scheme?
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Old 1st May 2003, 09:06 PM   #12
Rob M is offline Rob M  United States
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Quote:
Originally posted by edm

What I mean by slow oscillation is that ... I see the conus of the speaker moving rather slowly back and forth, it takes a couple of seconds for a complete cycle.
I had this happen once when I was running an amp from a bench supply and had the current limit set too low.

Does that give you any ideas?
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Old 1st May 2003, 09:30 PM   #13
sam9 is offline sam9  United States
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Default 2 suggestions

This is only a shot in the dark:

A- It's thermal. The chip heats up, which for some unknown reason gain and current draw fall off until it cools off. The chip is supossed to have thermal protection but I didn't think it worked like this - just off-on-off-on.

B- 30KuF seems like a lot for a stereo pair. Maybe the chip doesn't draw enough to keeps the caps charged, especially if the bleed resistor is to low a value.

Both sound hairbrained to me - B worse than A, but it's all that I can think of that might cause a SLOOOOW oscilation.
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Old 2nd May 2003, 08:08 AM   #14
moamps is offline moamps  Croatia
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Hi, edm

here is my wiring schematic. Maybe help.
Check also ground connection between input terminal, speaker terminal and ground point on PS because your problem look to me like floating ground on PS.

Regards
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Old 2nd May 2003, 09:26 AM   #15
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Ok, I've had a think about this and I think I've got a hunch what is causing this. What happens if you disconnect the source from the amp, leave the speaker connected to the output and short the input?
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Old 2nd May 2003, 02:06 PM   #16
edm is offline edm  Netherlands
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Audiofreak,

I already tried that no oscillations... I need some stimilus of some sort (noise for that matter) in order to get the slow oscillation going on (without the damping resistors). So what is your hunch what is causing this

Moamps, this is my grounding scheme (fear my paint skills )

The red lines are the ground lines, mostly consting of 2.5mm^2 massive copper wires using in between the elcos as a star ground point. The blue lines the power supply lines and I also sketched the 'damping resistor'.


Oh and I don't have a 100nF near the elco's yet nor the 22k 2w (bleeder??) resistor. I think i will put a 100nF near the elco's (but first i need to etch/solder some pcbs for a passive filter for a friend of mine)

emiel
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Old 2nd May 2003, 02:28 PM   #17
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I'd say that the input ground reference is shifting relative to the power ground reference. Because your amp is not referenced to earth, the absolute DC values are shifting around the place relative to the source which I suspect has it's ground tied to earth. If you connect the safety earth to the ground point of your circuit it will almost certainly fix your problem.
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Old 2nd May 2003, 02:40 PM   #18
edm is offline edm  Netherlands
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Well there is only one way to find out, i'll remove the damping resistors attach the safety ground to the star ground and see what happens... I left the safety ground floating to get rid of the motorboating, but with the introduction of the 100Ohm bypassed with the 100nF i hope i can attach it again..
I'll post the results, when i'm done etching.
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Old 2nd May 2003, 03:09 PM   #19
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As an alternative to attaching earth to the circuit ground. You could do the following:

1) Leave the source ground disconnected from the amp so that the interconnect is only connected to ground at the source and use the amplifiers ground point for the amp.

2) AC couple the input to the amp

This should prove equally effective at fixing your problem and is probably less likely to cause motorboating. Of course, you also need to consider the safety aspect if you plan to leave the earth pin disconnected.
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Old 2nd May 2003, 04:56 PM   #20
moamps is offline moamps  Croatia
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Hi, edm

I think that your ground scheme is slightly incorect. IMHO
Please read my posts on Gainclone trouble

Regards
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