Protection capacitor for tweeter in active speaker - diyAudio
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Old 30th July 2007, 04:59 AM   #1
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Default Protection capacitor for tweeter in active speaker

Could you guys tell me how should be installed a capacitor to protect a tweeter in an active speaker? Should I just solder the capacitor in the middle of the cable that connects the tweeter to the speaker terminals like the one in the picture? Also, should it be on the + or - side?

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Old 30th July 2007, 07:20 AM   #2
Dan2 is offline Dan2  South Africa
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the cap must be connected in series with the tweeter. its usually connected to the positive side but it doesn't really matter
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Old 30th July 2007, 07:51 AM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
how big should the protection capacitor be to avoid corrupting the crossover slope built into the amp/pre-amp?
One octave or two or more below the active crossover frequency?

Oceanic,
what is the resistor doing there on the tweeter terminals?
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Old 30th July 2007, 09:27 AM   #4
Svante is offline Svante  Sweden
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Quote:
Originally posted by AndrewT
Hi,
how big should the protection capacitor be to avoid corrupting the crossover slope built into the amp/pre-amp?
One octave or two or more below the active crossover frequency?

Oceanic,
what is the resistor doing there on the tweeter terminals?
Possibly preventing the capacitor and diver resonance from corrupting the crossover slope...?

Seriously, there are two ways of handling the effects of a passive filter (which is what this is) in an active design. Either you calculate/simulate the response and consider it when designing the active filter, or you just make the passive crossover frequency so low that you don't have to.

The latter may be perfectly ok if the purpose is to block DC from entering the tweeter, on the other hand the capacitor has to be bigger (=more expensive, possibly an electrolyte). Also, I like the thought of considering the summed effect of EVERYTHING, including the drivers, when designing loudspeakers. After all, it is the sum of everything we listen to.

Edit: to answer the original question, only one capacitor is needed, and it does not matter if it sits on the + or - side. The resistor can probably be removed if the capacitor is large enough.
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Old 30th July 2007, 11:27 AM   #5
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Wait a minute... The resistor shouldn't really be there anyway. All it's doing is drawing more current in parallel to the tweeter, not protecting it at all.
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Old 30th July 2007, 11:43 AM   #6
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Quote:
Wait a minute... The resistor shouldn't really be there anyway. All it's doing is drawing more current in parallel to the tweeter, not protecting it at all.
I would only agree to the more current part of that statement.

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Charles
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Old 30th July 2007, 12:10 PM   #7
ssmith is offline ssmith  France
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here is a useful calculator to determine cap size/xover frequency, applicable to tweeter protection.
For my active setup, which is 4th order crossed over at 2250hz with an 8ohm tweeter, I'm using a 20uf cap.
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Old 30th July 2007, 12:12 PM   #8
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Hi everyone,

The picture I posted is not with my tweeter. I just found it searching for pictures showing how to wire a capacitor to the tweeter to prevent any DC from damaging it. For capacitors I'd use 100uF Mundorf Mcap which would give me a high-pass filter point to about 400 Hz which is far away enough from the actual crossover point.

Andrew, the filter point introduced by the capacitor should be at least 2 octaves away from the actual crossover point. If the value of the capacitor is to small it will cause phase shift problems. So for a crossover point at 3 kHz you would need for an 8 Ohm tweet a capacitor of at least 26.5 uF and for a 4 Ohm one double that.

Anyway, concerning my question... Should I just solder the capacitor between 2 cable parts and then connect the resulting cable with the capacitor inserted in it to the speaker terminal and the tweeter?
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Old 30th July 2007, 12:48 PM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by oceanic30
For capacitors I'd use 100uF Mundorf Mcap which would give me a high-pass filter point to about 400 Hz which is far away enough from the actual crossover point.

Andrew, the filter point introduced by the capacitor should be at least 2 octaves away from the actual crossover point. If the value of the capacitor is to small it will cause phase shift problems. So for a crossover point at 3 kHz you would need for an 8 Ohm tweet a capacitor of at least 26.5 uF and for a 4 Ohm one double that.
Does anyone disagree with the two octave rule?

Svante recommends reducing the active filter slope by one pole and integrating the protection cap into the required overall filter type. I like this, except for the accuracy required and the fixed Q=0.71 that must be compensated by altering the Q of the active filter. Although LR2 can be achieved with an active single pole (q=0.71) and passive single pole (q=0.71). LR4 and Butterworth require more thought.
Quote:
Originally posted by oceanic30
.......Should I just solder the capacitor between 2 cable parts and then connect the resulting cable with the capacitor inserted in it to the speaker terminal and the tweeter?
yes and use hot melt glue or similar to secure the cap to something rigid, otherwise the solid core wire will fatigue and break.
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Old 30th July 2007, 12:58 PM   #10
AndrewT is offline AndrewT  Scotland
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What if the tweeter is a piezo type?
It is already, substantially, a capacitor. Is a protection capacitor also required after the filtered signal coming from the power amp.

What DC voltage can these piezo tweeters survive? Effectively no power is transferred and no heating occurs.

I have a pair of irreplaceable Audax HD3P that I would like to go active with.
The slight complication with the HD3P is that it is preceeded with an air-cored transformer and a 4r0 resistor to kid the amp on that it is an 8ohm load.

I fancy driving from a ClassA amp High Pass filtered at 6kHz and feeding the transformer direct. Maybe even omit the transformer.
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