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Old 16th October 2011, 07:26 PM   #1
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Default Using stupidly large caps for a headphone amp?

I have access to some 300,000uF can-style capacitors. I was wondering if I could use them instead of 680uFs for input power caps. I know that the purpose of the caps are to reduce ripple, and wouldn't a stupidly large cap eliminate the ripple?


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Old 16th October 2011, 10:11 PM   #2
jcx is offline jcx  United States
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it can be bad for your transformer, diodes, fuse

big caps draw all of the required current from the xfmr sec in increasingly narrow, higher current pulses as the C goes up

you can add some series R, make a "pi" filter with more reasonable caps next to the bridge

Last edited by jcx; 16th October 2011 at 10:13 PM.
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Old 16th October 2011, 10:27 PM   #3
Pano is offline Pano  United States
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Well those would indeed be stupid, but in a fun way.

As JCX says, use a pi filter. Put a more sane cap value after the rectifier, then a resistor, then your stupid caps. That resistor will slow down the big current inrush. But you'll have the stupid big caps as the reservoir. The ripple won't get thru the resistor.

Sounds like fun.
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Old 17th October 2011, 03:56 PM   #4
qusp is offline qusp  Australia
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hahahahaha...... thanks you made my day!! good form I say. its all been said above, but i just had to register my approval of the scale of your supremely stupid capacitance =)

when you finish we demand pics!!
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Old 17th October 2011, 04:35 PM   #5
Mooly is offline Mooly  United Kingdom
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Add a solar panel and charge the caps.

Design a micro power headphone amp and let the caps power it
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Old 17th October 2011, 07:22 PM   #6
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Quote:
Originally Posted by Pano View Post
Well those would indeed be stupid, but in a fun way.

As JCX says, use a pi filter. Put a more sane cap value after the rectifier, then a resistor, then your stupid caps. That resistor will slow down the big current inrush. But you'll have the stupid big caps as the reservoir. The ripple won't get thru the resistor.

Sounds like fun.

So...
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?
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Old 17th October 2011, 08:48 PM   #7
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Just a thought, what if the enormous caps discharge unevenly upon power down?
That can put quite a bit on DC on the headphones for a good amount of time if there is no relay protection or coupling cap on the output
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Old 17th October 2011, 10:04 PM   #8
Atilla is offline Atilla  Norway
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Um. How would they discharge unevenly, given that they're connected at the ground? They'll try to keep at the same potential, even with different ESRs. Am I mistaken here?

But - two things:
1) 680uF might be too small for the first C. Try something around an order of magnitude better.
2)2k resistor is a LOT. That'll waste I^2 * R watts of power. And with 330k of capacitance you don't need a lot to eliminate ripple. You'll need to lower that quite a bit and you'll still need to get resistors with high wattage.

It sounds like a lot of fun tho. I want to see pics of those beasts.
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Old 17th October 2011, 10:08 PM   #9
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Originally Posted by Atilla View Post
Um. How would they discharge unevenly, given that they're connected at the ground? They'll try to keep at the same potential, even with different ESRs. Am I mistaken here?

But - two things:
1) 680uF might be too small for the first C. Try something around an order of magnitude better.
2)2k resistor is a LOT. That'll waste I^2 * R watts of power. And with 330k of capacitance you don't need a lot to eliminate ripple. You'll need to lower that quite a bit and you'll still need to get resistors with high wattage.

It sounds like a lot of fun tho. I want to see pics of those beasts.
What about ~6800uF and 20 Ohm?
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Old 17th October 2011, 10:27 PM   #10
Atilla is offline Atilla  Norway
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6800 is fine. You can drive a headphone amp with that alone

The resistor you can size based on how much power you can afford to lose. That depends on how much current will pass trough it. Let's say, for the sake of it, that your two channels will draw about 250mA each. That means you'd be drawing 500mA trough the resistor, which for 20 ohm will have to dissipate 10W. I'd be using 20W rated, heatsinked resistors for this, maybe more. Sounds a bit too wasteful, but on the other hand - you are putting 330k caps in this, so I'm guessing that nothing can be considered overboard
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