49600 single ended amp; I blew up the output caps! - diyAudio
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Old 16th July 2014, 03:30 PM   #1
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Default 49600 single ended amp; I blew up the output caps!

After many revisions, setbacks, and tweaks, I finally assembled my op amp front end, 49600 output, single ended headphone amp onto a permanent circuit board. I used two Nichicon 63 volt, 680 uF "high ripple current" "7K hour service life" caps in parallel per output as coupling caps. I first tested it with a 9 volt transistor battery and to my surprise it sounded super clean, and very powerful; although it did pull the battery down very fast. Then I ran it off of a 12 volt wall wart and it was noticeably "punchier." I intend to ultimately run it off a 20 volt wall wart once I get this last issue sorted.

It sounded so good - dead quiet background, super effortless power, and everything was there in the music; bass clean and tight but not overpowering, midrange super crystal clear, highs all there; every note was distinct and not jumbled together. I listened to it for over two hours (success at last!); loud and soft, acoustic jazz and classic rock, modern "rock" like Tool and Rage, and I know it's a cliché thing to say but I heard stuff I never heard before in these familiar tracks. Then while I was cranking it, it kind of fuzzed out and I looked down at the circuit board and the output caps were blowing their tops! The tops ruptured and the Magic Juice sprayed right out of them while I was watching!

I'm thinking that either these were the wrong caps for the application, or else the amp started oscillating. The output devices were barely warm and when I rigged it up with some cheapie caps on the output it still worked great.

So I'm going to do some snooping because I'm really ready to button this project up, but I was hoping that somebody would have some insight into this; maybe I'm missing something. So thanks for reading.
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Old 16th July 2014, 03:38 PM   #2
AndrewT is online now AndrewT  Scotland
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non polar caps (MKP, MKT etc) have a maximum rating.
But when you look at the datasheet you see the manufacturer telling us that they must be de-rated substantially when passing an AC current.

I have not checked a polar cap datasheet for what this de-rating amounts to. Mostly because I don't have any single ended amplifiers.
But I have recently started experimenting with polar and bi-polar DC blocking caps for driving Treble speakers. Must do more research.

The problem is the ESR heats up the capacitor in proportion to the current squared.

That could be a lot of power on the transients, even though they are of short duration.
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Old 16th July 2014, 03:59 PM   #3
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Yes Andrew, I was thinking this. Perhaps an array of "audio grade" caps would be a better choice? I did parallel one "audio grade" 4.7 uF capacitor with each pair of big boys.

The power involved really isn't that high, though. The 49600 are rated 250 mA output nominal; and I was listening to music, not driving a dummy load with a test tone.

I have a "zobel" network on the output; on the headphone side of the caps. Would putting it right on the device output make any difference? It seems like a minor consideration.

I'm thinking that maybe four or six smaller caps in parallel would be better? It's working fine now with a generic 470 uF cap on each output.

After fussing with a lot of details, this last obstacle seems silly. I was ready to stuff it all into an enclosure and use it, and move on to my next project.

It was pretty cool to see the caps blow up in real time, though. That made it almost worth the $5 that the four caps cost.
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Old 16th July 2014, 04:01 PM   #4
AndrewT is online now AndrewT  Scotland
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4u7F?
what is your turn over frequency?
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Old 16th July 2014, 04:13 PM   #5
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Quote:
Originally Posted by AndrewT View Post
4u7F?
what is your turn over frequency?
4.7 uF in parallel with 2 x 680 uF, for a nominal turnover frequency of 3.6 Hz with 32 ohm cans.

I wouldn't be boasting about "tight bass" if I used 4.7 uF caps; it would be no bass.

Anyway, an array of smaller caps might be better. What do you think, Andrew?
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Old 16th July 2014, 06:39 PM   #6
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High-frequency oscillation plus having the Zobel on the headphone side might explain why the caps went poof. It still remains a very odd fault, especially with two relatively high-grade (genuine, I hope?), high-capacitance parts. I have never heard of anyone blowing up their output coupling caps. What would these caps do in a voltage multiplier cascade? If there is oscillation, it's usually the Zobel resistor that suffers first.

An array of small caps may help if you need to get inductance down. The ESR-dominated impedance "floor" of a 680µ should already extend way past the audible range though, so I don't see a benefit there.
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Old 17th July 2014, 06:45 AM   #7
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Well, I figured out what happened. I have the caps on a separate board. When I tested the amplifier after blowing the caps I just desoldered the leads to the main board and soldered one lead of my cheap caps to the board for testing. I just desoldered the blown caps to test and replace them (I don't have replacements right now), and all six caps were in backwards.

This is not the stupidest thing I have ever done with electronics, but it is the second stupidest.

All six caps checked out A-OK. I will not reuse them.
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Old 17th July 2014, 06:13 PM   #8
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LOL. I figured you wouldn't have made such a basic mistake.

Interesting that it took a fair amount of current for the caps to actually blow up though. Or wait, what if reverse voltage handling were somewhat higher than 6 V, maybe 7-8 V? It's more than I'd have expected in any case.
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Old 17th July 2014, 07:08 PM   #9
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Quote:
Originally Posted by sgrossklass View Post
LOL. I figured you wouldn't have made such a basic mistake.
Me too. It's Stupid 101.

Quote:
Interesting that it took a fair amount of current for the caps to actually blow up though. Or wait, what if reverse voltage handling were somewhat higher than 6 V, maybe 7-8 V? It's more than I'd have expected in any case.
They were reverse biased for two hours, with a small AC current passing through them. How much could the current have been? I was listening to musical programs, not test tones. I doubt that the current exceeded 25 mA, peak.

They were expen$ive, high quality Nichicon capacitors. They still test as good - no leakage, and they held a charge overnight.
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Old 18th July 2014, 09:02 AM   #10
AndrewT is online now AndrewT  Scotland
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Polar electrolytics are not rated for AC.

Bi-polar electrolytics are rated for AC

Back to back electrolytics are often used for AC and usually survive, even at speaker voltages.

I don't know what you did to prepare your electrolytics for AC duty.
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