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#1 2010-07-21 08:52:27
- twelvepoint
- Member
- Registered: 2009-10-25
- Posts: 37
Power Supply Noise - how much is too much?
Just a general question: if I'm measuring power supply ripple on the B+, or other points, what's the best way to do this? Should I use RMS voltage on a DMM? Any readings that would indicate an issue vs. what's acceptable?
Thanks
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#2 2010-08-17 14:02:04
- twelvepoint
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- Registered: 2009-10-25
- Posts: 37
Re: Power Supply Noise - how much is too much?
Anyone?
Basically what I'm trying to do here is test a power supply on a friend's amp where the filter caps are glued onto the PCB. I'd like to make sure the caps are bad before I unglue them and replace them.
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#3 2010-08-17 14:48:09
- teledeluxe
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- From: Chicago, IL
- Registered: 2008-09-24
- Posts: 238
Re: Power Supply Noise - how much is too much?
I think the only way to accurately test caps in circuit is to use something like the "Capacitor Wizard".
2 wrongs don't make a right, but 3 lefts do!
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#4 2010-08-17 20:25:50
- hangman
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- From: Seattle Washington
- Registered: 2006-09-03
- Posts: 882
Re: Power Supply Noise - how much is too much?
you use an oscilloscope.
you will be able to measure the peak to peak ripple.
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#5 2010-08-18 06:12:49
- twelvepoint
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- Registered: 2009-10-25
- Posts: 37
Re: Power Supply Noise - how much is too much?
hangman wrote:
you use an oscilloscope.
you will be able to measure the peak to peak ripple.
Ok, that makes sense and I can check that no prob.
But what sort of AC swing would indicate a problem? You know, as a rule of thumb? If my B+ is some arbitrary amount, say 350VDC, would an acceptable ripple be 5% of that? 1%? .01%?
I assume techs should be able to set their scopes to a specific range and "see" if the swing is out of whack
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#6 2010-08-18 07:43:44
- rob
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- Registered: 2008-10-28
- Posts: 140
Re: Power Supply Noise - how much is too much?
I believe the RMS ripple needs to be around 2v or less on the v4. Most tube amps need to be less than 3V RMS for acceptable noise floor. The ripple factor can be calculated for any given power supply if you know all of the numbers. I have seen formulas on line to do this. Filter caps over 20 years old almost always are worth changing and will improve tone and bass clarity of any amp.
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#7 2010-08-18 10:32:20
- twelvepoint
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- Registered: 2009-10-25
- Posts: 37
Re: Power Supply Noise - how much is too much?
Ok thanks guys. I was sort of looking for a general RMS value. So if I measured like 10-20 volts or whatever, I'd rebuild the power supply.
The amp in question is a noisy 30 watt Crate and I'm trying to decide to fix it, or turn the whole thing into a donor for a DC30 clone.
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#8 2010-08-18 12:42:22
- hangman
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- From: Seattle Washington
- Registered: 2006-09-03
- Posts: 882
Re: Power Supply Noise - how much is too much?
On point A of the power supply. The first filter stage, you will have a ripple voltage of about 5-10V peak to peak.
That kind of ripple is perfectly normal. because that is going to the plates of the power tubes it goes to one set in phase, and to one set out of phase and is ultimately cancelled.
After that first point you should see less than a volt peak to peak ripple and in the preamp it should be in the mv range.
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