Electrolytic Capacitor as Audio Test Load

I was wondering what would happen if I wired the output of a 100W audio amplifier directly across a sizable (4700uF) electrolytic capacitor.

It could be assumed the amp is push-pull and the cap is polarized.

How would it react in terms of a test load?

Steve Morris

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Steve Morris
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try sci.electronics.basics

Reply to
tabbypurr

Do you post there too?

Steve Morris

Reply to
Steve Morris

Lots of us do. No stigma attached, it's just not a design question. (Cross-posted to s.e.basics)

The answer depends entirely on whether the amp itself is stable with a pathological load like that, and (assuming that it is) what its input is and what the characteristics of the cap are.

If the amp output is just sitting at zero volts, it isn't going to do anything dramatic to the cap. If it oscillates, or if you put in a large AC signal, and the cap is a smallish polarized electrolytic (as opposed to a nonpolarized one), the cap probably isn't long for this world, unless the amp dies or current-limits first.

The reactance of such a capacitor at (say) 10 kHz is

Xc = 1/( 2 pi * 10kHz * 4.7e-6 F) = 3.3 milliohms,

i.e. very nearly a short circuit. The equivalent series resistance (ESR) is probably more than that, so the cap looks like a very low resistance.

The net is that depending on the characteristics of the amp and what you put into it, your cap could be destroyed or could be completely unharmed.

Actual audio people will probably have a better handle on how your average 100W amp will respond to such abuse.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Reply to
Phil Hobbs

Merry Christmas!

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

Why do that?

At some optimum signal level and frequency, you can probably explode the cap.

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John Larkin   Highland Technology, Inc   trk 

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Reply to
John Larkin

And the amp. ...but I thought that was the point of the question.

Reply to
krw

Search for 'electrolytic rectifier'.

Reply to
Michael A Terrell

** At very low audio frequencies, the 4700uF cap could be driven into reverse breakdown, then overheat and explode.

At higher frequencies, the amplifier would see the cap as a short circuit and behave accordingly - either by blowing fuses, rapidly overheating, hard current limiting or just turning itself off.

FYI:

If you ever need to test your car's air bags, driving into a solid brick wall at 50mph is one way.

..... Phil

Reply to
Phil Allison

How Apt !!!

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Reply to
TTman

There's no such thing as solid brick wall for purposes of crash testing.

Reply to
bloggs.fredbloggs.fred

One of the UK car shows constructed a stack of solid concrete blocks to slam a car into at 70mph just to see what happens.

They hooked the car up to a winch to get it moving because there weren't many volunteers to drive it.

Reply to
Ian Field

I'm guessing this was Top Gear?

Perhaps not, then.

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Reply to
Cursitor Doom

Based up the replies so far, I could use a bipolar cap (two electros back-to-back), as well as series resistance to ground to protect the amp. I am thinking 22 Ohms to be safe.

Or, connect a resistor from the positive terminal to ground as a parallel load.

If I need to use a polarised cap, then I assume a single-ended amp would work.

Steve Morris

Reply to
Steve Morris

"Back to back" means inverse series connection, so that plus a series resistor would make a safe load for the amp at least. Whether it was safe for the caps would depend on their voltage and ripple current ratings.

So why do you want to do this?

Cheers

Phil Hobbs

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Principal Consultant 
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Reply to
Phil Hobbs

(and the resistor, amplifier swing)

That's the $64K question. It seems like a really bad idea, all around.

Reply to
krw

-------------------------

** OK, the 22ohms needs to have a 50W power rating.

** Makes no difference to any of the negative issues.

** GIANT huh ???

Load coupling with large electro caps is standard practice with single DC rail amplifiers.

Just to isolate the DC half tail voltage from the poor loudspeaker you see.

.... Phil

Reply to
Phil Allison

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