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Old 31st October 2005, 04:10 AM   #1
Jay is offline Jay  Indonesia
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Question Speaker protection with capacitor

I have been building some amplifiers. All are prototypes (the build, not the components quality), so I don't have the gut to fire my main speaker. I don't need to listen for long, but I need to listen with good speaker (I surely need the transmision line to value it's lows).

So I have this idea, I will protect my main speaker simply by putting a 6800uF cap at the output. I have no problem with the effect of the cap to the sound. But will this work? What should be taken into account to sufficiently protect the speaker from all kind of amplifier failures?
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Old 31st October 2005, 12:33 PM   #2
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Not sure whether a cap that big will give your amp a hard time, but it should protect your speakers against DC faults. Have you worked out what the corner freq will be of the crossover that the cap will create? (ie at what point it will roll off your bass).

In order to protect against excessive power dissipation you could use some of these http://www.jaycar.com.au/productView...Max=&SUBCATID=

I hadn't thought of using them for amp testing before, but I think it is an excellent idea. just choose a trip current that is within the range you want to test and you should be safe. If the cap is ok then combined with a polyswitch, I think you should be covered.

On second thoughts if your amp puts out a lot of energy only at high freq you may still fry your tweeter, as it could get say 50W and only be rated say 15W. When I have used polyswitches for speaker protection I choose a suitable one for each driver

edit: and make sure the cap is of sufficiently high voltage too probably minimum of 50V depending on the max power of your amp, 50V should be more than enough for 100W... and I think it should be a bipolar...... can you get bipolar electros that big??

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Old 31st October 2005, 01:19 PM   #3
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The cap needs to be bipolar. You can make something out of a pair of back-to-back connected normal electrolytic caps, each should be 10,000uF giving you -3dB @ 4Hz with an 8R speaker.
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Old 8th November 2005, 04:12 AM   #4
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Thanks Tony, Richie. I will follow that link shortly. I don't know about the polyswitch, I hope it is a simple thing...

I picked 6800uF because I felt that it is just enough for 20Hz. Not quite sure with the real thing. The voltage ratings are 160V/200V/400V. And yes bipolars.

Now that richie00boy mentioned it, I have a confusion regarding these terms: BP, NP. If I connect both negative ends, what is it gonna be? If I connect both positive ends, what is it gonna be? What is the difference between them (I found that certain connection is soundwise better than the other in certain circuit)

If a NP can be made by connecting 2 polar caps, the resulting NP cannot pass DC correct? How about conventional NP like Black Gate N? Shouldn't I use it for DC coupling in pre/amps? (Hint: GC!)
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Old 8th November 2005, 04:45 AM   #5
Jay is offline Jay  Indonesia
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So that what a polyswitch is

I don't have an online purchase privilege, but it reminds me of a primitive approach. Yes, a fuse! But not sure with the proper rating for that.

And the polyswitch looks like a cap for handling a very high frequency. Now I remember the oscillation issue of a failing amplifier, may be shorting the tweeter's legs with silver mica 5nF will help? Well, I think I can rely on the coupling cap alone for protection, after running at least 1 hour on my bookshelf
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Old 8th November 2005, 04:58 AM   #6
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the polysiwtches work great for protecting against clipping... I used to take my amp and speakers to parties a lot, before I put polyswitches in I blew a LOT of tweeters, and mid ranges. After the polyswitches not a single failure many years later when doing some amp testing I found that the point above which the tweeters would blow, just happened to correspond with the point the amp started clipping

and yes it is pretty much an auto resetting fuse

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Old 8th November 2005, 11:48 AM   #7
Jay is offline Jay  Indonesia
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Tony, I think I have misunderstood you and richie00boy

I just remembered the way a cap blocks DC is by fact that DC is a very low frequency (or no phase change to allow for discharge). I thought that you mentioned BIPOLAR as having positive and negative poles to be able to protect DC. But I guess Bi-polar is not 2-pole capacitor right? Rather, it is quite similar with non-polar? So the question is, why it has to be bipolar??? I'm more confused now
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Old 8th November 2005, 12:58 PM   #8
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I believe It's basically because if the amp develops a fault (putting out DC) you won't know whether the DC will be +ve or -ve on the amp side of the cap.... so if you put in polarised there is a 50% chance it will explode if the amp goes into a dc fault state if you use bipolar it doesn't matter

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Old 9th November 2005, 07:15 AM   #9
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Oh I see So I need to use 2 caps. But to avoid problem with lows cut-off I need huge capacitance! (My PSU caps will ruin the sound. But those 6800uF/200V caps, if sufficient, are very musical )
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Old 10th November 2005, 11:13 AM   #10
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Quote:
Originally posted by Jay
Oh I see So I need to use 2 caps. But to avoid problem with lows cut-off I need huge capacitance! (My PSU caps will ruin the sound. But those 6800uF/200V caps, if sufficient, are very musical )
Stick your two 6800 caps in series, + to + with one - to amp output and the other - to speaker. That'll give a -3dB point about 6Hz with an 8 ohm speaker.
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