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Old 15th December 2006, 07:09 AM   #1
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Default Filter cap not draining smoothly...?

Hello,
I'm not sure if this is the right place to post this question, I apologize if it isn't.

I have a power supply filter cap that does not drain smoothly after power down. Instead of dropping to 0V smoothly, it is erratic.

My question is, would the cap be at fault? Or would it be elsewhere. Before I replace it, I just wanted to get some advice.

Thanks,
ElectroAcoustic
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Old 17th December 2006, 10:06 PM   #2
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A capacitor discharges either internally (usually at a very, very low current) or through an external leakage path. If there is no bleeder resistor to intentionally discharge the capacitor, then I would say that it is normal for there to be some residual charge.

Can you measure the working and 'off' voltages?
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Old 18th December 2006, 12:43 AM   #3
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How do you know it's not draining smoothly?
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Old 18th December 2006, 01:57 AM   #4
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Damon,
I'm not sure of what you mean by working and 'off' voltages.

pinkmouse,
I hooked up my simpson 260 analog alternately to each rail.
When I power off, the positive raildrops smoothly to around a few volts, hangs there and drops slowly (>5 min) to 0V.

The negative rail will drop about 50%, then the voltage increases about 6V, then it drops suddenly to about 25% of the rail, then takes about 5 min to drop to a couple volts.

I don't understand why the voltage would jump back up.

When the voltage jumps up, I get a screech through the input op-amp, and the speakers. I believe this is due to the uneven collapse of the rails.

I just switched out the cap and the bridge rectifiers, and the condition remains, so I suspect the problem is elsewhere.

I am just trying to figure out what could make the rails collapse unevenly.
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Old 18th December 2006, 04:20 AM   #5
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I had the same problem with a modified version of a Leach amplifier; I was able to minimize the behavior with bleeder resistors. It did not solve the problem of the amplifier's oscillation, which was otherwise stable, but the problem in both your case and mine is in the amplifier itself and not the power supply.

I never resolved that problem other than to note that when I upgraded the amplifier to the latest version, the problem followed the old driver boards when I used them in a backup amplifier. I have a theory I made a mistake with the modified boards, possibly in not properly grounding the input stages.

At any rate, I'm convinced your capacitors are not in any way at fault. Large electrolytic capacitors may partially rebuild a charge when the amplifier has been turned off; that's part of the reason for bleeder resistors.

In addition to the oscillation, I think you will also measure a substantial DC offset on the output if the design is fully DC-coupled.
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Old 18th December 2006, 04:55 AM   #6
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Damon,
Thank you so much for the insight.

I was caught up in the problem originating from the PS, that I had tunnel vision in regards to the rest of the circuitry. I completely overlooked checking for offset at the outputs.

Indeed, when the power switch is initially turned off, I get a 'thump' at the output, which is about -3.5VDC. The transient drops to 0V, then increases to a steady -1.5VDC, and hangs for about 2 sec or so. As the DC drops back to 0V, that is when the whine is present.

When the amp is powered on, everything is stable, no offset, steady rails, no oscillations, etc. I have run it for many non-stop hours (>8 hrs at a time) with no problems.

Do you think that this is critical enough not to use the amp? In your opinion, will the problem worsen? Or is it just something I should be aware of and keep on eye on?
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Old 18th December 2006, 05:37 AM   #7
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I think the oscillation is indicative of a problem with the amplifier circuit, either in the design or the implementation. One of these daze I'm going to properly troubleshoot my amplifier down to checking all of the grounds and that my attempts to update the circuit board were correctly done. (Ver. 2.something to kinda Ver4.something of the Leach amplifier) A new circuit board with the current 4.5 version design is totally well-behaved during shut down.

It probably won't get worse with your amplifier, but clearly something is not optimum. Try some bleeder resistors on your power supply with some five or ten watt resistors (resistance=voltage squared/power in watts). That won't solve the problem but it should at least make the shutdown a bit more orderly.

What design is your amplifier based on?
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