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Old 18th August 2010, 03:02 PM   #1
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Default My amp makes a warble noise! :I

ok it's a guitar amp but I still feel this is the appropriate category to ask under. It seems after the amp has been on for some time and gets good and warm, I start to get this warbling that sounds like a chorus effect...an effect I absolutely hate the sound of.

I had noticed it ever so faintly in the past but it rarely ever occurred when it did surface. So I woke up this morning, turned it on and while cool, no problems but I'm sure once it's heated up for an hour, this warble will come back.

So far myself and another suspect a power cap is going bad and I'm picking up some power wave modulation and need to replace the caps which I dread the job of based on the internal architecture of the amp.
Here's the power amp schem:
Click the image to open in full size.

I was hoping someone on the pedal forum would say something easy like "oh you might have a shorted op amp in your pre" but no luck lol. I suppose I should be happy it's not the pedal I designed since it's been 4 years of grueling work to achieve the perfection it is.
So is it likely one of these power caps going bad and giving me warble?

Also, can I replace these caps with ones of higher voltage and value?

I'm assuming this will mean cleaner, more reliable power but I'm not experienced with these high voltage circuits. I have heard increasing farad value will improve power quality and figured upping the voltage handling wouldn't hurt either but am unsure if it's safe.
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Old 18th August 2010, 04:02 PM   #2
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If C39 and C40 are increased too much, the diode bridge will blow. I don't know the calculations for this, but it is likely the manufacturer picked the minimum value for these caps, in which case you should be able to increase them a lot. If I replaced them I would upgrade to 63V or larger caps, for an extra 10V of margin (just me though).

C43 and C44 can be increased, but if you use physically larger caps, I would add another set of C45-47 of the same type and value. I'm guessing by now you have checked the +-16V rails; if D26 and D25 blow C43 and C44 will likely be pushed past their voltage ratings and begin to die. Might also check R89/90 since they are high dissipation and more likely to fail (though unlikely).

Lastly before upgrading take a good look at the caps, especially if they are fancy colored, and check if they are a special part. Sometimes low-ESR and/or low-ESL parts are used.

- keantoken
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Old 18th August 2010, 07:24 PM   #3
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It's unclear what you mean by warble. It could be an instability in the amplifier. Your diagram only shows the power supply, and certainly that isn't in the high voltage category.

The best would be to use an oscilloscope and see just what you have going there.
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Old 18th August 2010, 09:27 PM   #4
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By warble I was referring to modulation, I was assuming the term was pretty self explanatory. Anyway, left the amp on for about 5-6 hours and it hasn't started doing it yet but I also wasn't playing through it during most those hours like I normally do. By high voltage bob, I'm talking about LETHAL. I'm sure if a 4700mfd 100v cap discharges into my arm, I may or may not live to regret it. My suspicion is one of the power caps is dying and I'm picking some low frequency modulation.

Keantoken I appreciate the information and the clarification. I think the safest option is to replace them all with the stock values but "higher quality" in the sense of what I've used in preamp circuits that sounded/performed best to my ears. I will check to see if there are any special codes on them just as you mentioned. I hope some day to spend more time studying the power grid of high voltage amps but for now I'm dedicated to preamp design and development.
I think some -40c to +105c caps should survive a few decades of daily use. I always wanted to replace the stock with better grade anyway so I suppose there's no time like the present. The downside being Xicon has discontinued their neon green high performance caps and I don't think I can replace all the values with this model line due to the now limited availability of values
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