DC blocker cap for tweeter, electrolytic OK?

whoaru99

whoaru99

Epic Member
I'm going to do some experimenting with converting my Behringer B2031P speakers to biamp operation. Yes, "real" biamp by pulling out the existing crossover and installing a 4-pole Neutrik SpeakON (1+/1- for LF and 2+/2- for HF) in each speaker.

I will use MiniDSP 2x4 for crossover and response tuning.

Thinking I should incorporate a DC block on the tweeter roughly 2 octaves below expected crossover point of 1500Hz, give or take. So, puts it in the range of 50-60uF.

For this purpose a small electro of sufficient voltage rating should be fine, rather than a big old film cap, yes?
 
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I believe it has to be NP. There are work-a-rounds for polarized cap, but at best a compromise (unless your using a cap coupled amp to begin with....) My 2 cents.
 
Agreed a NPE should work...assuming it doesn't have any effect on sound quality. I have no idea because I've never considered this application of a cap in a speaker or read about it. Hopefully if it's sufficiently below the range of the tweeter it will be unnoticeable.
 
If you are using the cap to set the frequency of crossover, you should not use electrolytic. But if you already have the frequency setting cap in the crossover network and you just want to add another DC blocking cap AND you are using an electrolytic cap that is way way larger than the value of the crossover cap, then I guess it's ok.

All caps has voltage distortion ( capacitance change vs voltage across the cap), electrolytic has high dC/dV. You want to operate the electrolytic at frequency where the electrolytic is basically a short so there is no voltage across it.

But usually you already have the real cap in the crossover that block the DC already. Measure with an ohm meter, if it read open, you know you already have a DC blocking cap and you don't need an extra one.

If you really want the extra cap, you should have a lower value film cap in parallel to keep the impedance of the cap low at high frequency.
 
I'd go film if you can, but NP electrolytic will get the job done. If its not really being used as a crossover, you should be well above the point at which it will really start to mess with the signal it passes. Maybe bypass with a film as a compromise though.
 
are you wanting to do this to save the tweeter in case of clipping? As in too much power in or the amp clipping?
 
It won't save the tweeter from clipping, it only block the DC, nothing more. This is where I don't understand, tweeter should already have a cap in series, you don't gain anything adding another one. Also you worry about voltage charging up between the two caps.
 
This is an active crossover application. The the speaker drivers will each be directly driven by an amplifier, so no existing capacitor. The original crossover components are not being used, hence the concern for protecting the tweeter.

I would likely rely on the DC protection of the amplifiers in use. My and it just my OCDness tells me that removing the passive crossover components may be a good thing and adding a capacitor back in like (speaker level) goes against my OCDness. I am sure not everyone agrees.

Will you take into account the change in the time domain relationship between the low frequencies and high frequencies when you add the capacitor?

Some think that this is important and some not so much.
 
Just had a thought - most DC protect circuits have an RC built into them to keep low frequency audio from tripping the protect circuit. Maybe instead of adding a DC blocker, consider changing the cap to a smaller value so that low frequencies, as well as DC would trigger protection. (cause the output relay to open) YMMV... (Inspred by the Sansui service manual misprint which had the cap off by a factor of 10, and if you put the manuals value, low frequencies cause the relay to click off...)+
 
I LIKE MUSIC said:
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I would likely rely on the DC protection of the amplifiers in use. My and it just my OCDness tells me that removing the passive crossover components may be a good thing and adding a capacitor back in like (speaker level) goes against my OCDness. I am sure not everyone agrees.

Will you take into account the change in the time domain relationship between the low frequencies and high frequencies when you add the capacitor?

Some think that this is important and some not so much.
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I think all my amps have some sort of DC protection if high DC is detected at the output as in some sort of failure. But, I know at least one of my amps has a bit of a turn-on thump. That one, however, isn't specifically being considered for this use.

As well, I don't recall if the MiniDSP 2x4, or the other possible candidate Behringer DCX2496, have any turn-on/off thumps that could be problematic if forgotten in the startup/shutdown sequence.

As well, the DC blocking cap/high pass filter might give me a little cushion if I screw up the settings for the tweeter. :)

Far as time relationship (phasing?) my plan was to try to capture that using Room EQ Wizard and incorporate that back into either the MiniDSP or DCX2496 DSP corrections and response filters.

Ordered a couple custom aluminum plates from Front Panel Express for the SpeakONs to replace the existing speaker terminal cups. The plates are smaller than they might appear here (actual ~93mm x 75mm). Black anodized finish with engraved text/numbers and white fill of the engraving.

B2031P_Biamp.png
 
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I have a pair of Bose 301 lll's that use a small light bulb to take the hit to protect the tweeters. the first 301's tended to blow the tweeters so the bulb was added. if you blow the bulb Bose will now send you a fuse holder and a fuse instead of a bulb. Dunno if thats an option for what tou wan to do. the bulb reminded me of a dim bulb tester. if over driven it gets bright, or pops. I guess that might not do for a quick burst of DC though.
 
whoaru99 said:
For this purpose a small electro of sufficient voltage rating should be fine, rather than a big old film cap, yes?
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Yes, definitely. One thing to pay attention to, is the electrolytic caps sold for speaker crossovers may have a lower ESR than the ones sold for general electronic use.

For this use you want an axial capacitor with heavy lead out wires, not a tiny radial.

For some reason I had a hard time finding speaker crossover type electrolytics at Mouser, etc and ended up ordering from a speaker vendor.
 
Was thinking how to do the cap. Probably need to use a terminal strip, or something anyway, with the cap strapped or glued to it so the leads don't have any undue stress directly on them.
 
This seems counterproductive to me. One of the principal reasons for Bi-Amping is to get caps out of the signal path. I would go with a fuse or PTC thermistor. Klipsch has used devices similar to the RXEF050, for instance.
 
BinaryMike said:
This seems counterproductive to me. One of the principal reasons for Bi-Amping is to get caps out of the signal path.
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I understand, but it seems relatively common to include a DC blocking cap/very low high pass filter on the tweeter.

How much effect does it have 2 octaves or more below the approximate actual crossover point, when the cap is 6dB slope but the working XO will likely be at least 24dB slope?
 
Well, according to Douglas Self, signal voltage over 80mV across the terminals produces measurable distortion in electrolytics. Are you able to calculate the worst case terminal voltage for your situation?
 
I received the Neutrik SpeakON sockets and mounting plates. However, I ordered the SpeakON sockets that have screw terminals and now see these variants are not sealed like the SpeakON sockets having Faston/solder tabs.

Since the speakers are ported, will the non-sealed SpeakON sockets cause a meaningful problem with box/port tuning?
 
I guess that would depend on the relative size of the 'leak' compared to the port. If you have something plugged into it, it's probably negligible. I would be a lot more worried with an acoustic suspension speaker.
 
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