active crossover - need tweeter block cap if using LM3886 ?

[linuxworks]

my question is whether adding a series protection cap on an active crossover system (lm3886 based) is necessary or not.

what's the common thinking, here?

crossover is behringer dcx2496 and I was going to run its analog outs into a stepped atten and then into a chipamp directly feeding the speaker driver.

the running dc offset is not what I care about (it will be low enough to not matter) but I wonder if turn on/off thumps, however small they are, would best be avoided for a direct wired tweeter?

I'm very open to having relays do turn on/off delays. maybe that's my solution right there (?)

have not built the speaker system yet, but just at the planning stage right now. I do want to use that crossover and a group of 2 or 3 chipamps to drive each speaker element.

go capless or not?

[richie00boy]

Try the amps on a speaker first to see if they have any turn on/off movement or noises. I'd be tempted to use a muting relay rather than cap as high value polypropylene caps are both large and expensive.

[nac134]

If you're using the behringer, you want to avoid and filter components - thats the whole point of the thing. The behringer itself has a little thump when powering up or down itself. A good idea might be to use the muting circuit on the lm3886 chip, that way you're not adding anything to the signal path.

[linuxworks]

forgot about the muting circuit! I wired it in (using the chipamp.com boards for now) and forgot about it.

that might be the thing to do. if you can avoid the relays AND the caps its a double win.

still, part of me still likes the ease and insurance of the relay method. and its also amp-agnostic; if I switch to an amp module that does not have muting, I'll have to do my own thing then anyway.

would an opto coupler work for the muting switch (using the 10k Rm typically)? I will have a cpu nearby (arduino thingie) and I don't want that digital supply near the analog stuff. I don't mind having the arduino logically turn on/off an opto isolator and that would mute/unmute the 10k on the lm3886. does that make sense?

[madisonears]

How much are your tweeters worth? How difficult will it be to replace them if they fry?

I think even a stepped volume control might generate DC spikes or nasty AC noise if the contacts get dirty.

A large blocking cap before the tweeter is not the end of the world, no matter how "pure" you think the signal will be without one. You need to buy them only once.

Peace,
Tom E

[abraxalito]

Originally Posted by linuxworks

my question is whether adding a series protection cap on an active crossover system (lm3886 based) is necessary or not.

In my experience with LM3886s, not. But if my tweeters were expensive ones, I'd probably change my mind.

the running dc offset is not what I care about (it will be low enough to not matter) but I wonder if turn on/off thumps, however small they are, would best be avoided for a direct wired tweeter?

Turn on/off thumps will be passed through the cap anyway. I'd say the reason for employing a cap is primarily fault protection. Keep coupling caps small in the tweeter circuit so turn on thumps will be shorter. Or run bridged so as to practically eliminate them.

[linuxworks]

Originally Posted by madisonears

How much are your tweeters worth? How difficult will it be to replace them if they fry?

not overly expensive, in the $50 range or less.

I think even a stepped volume control might generate DC spikes or nasty AC noise if the contacts get dirty.

Im using sealed telco quality relays (its a relay based atten) and so the atten system is pretty clean. I also make sure to mute between selections, so it simulates a make before break style when you run up or down the vol 'knob'.

A large blocking cap before the tweeter is not the end of the world, no matter how "pure" you think the signal will be without one. You need to buy them only once.

Peace,
Tom E

the purist stuff is what I'm trying to wrestle with. I sure do like the insurance of an 'install and forget' cap.

[AndrewT]

Hi,
an 8ohm tweeter crossed over at 3kHz needs a 6u6F (6u8Fn should be available or two //3u3F) for a single pole passive 3kHz filter.
If you adopt this as a protection cap then you must reduce the DCX treble filter by one pole to maintain the same crossover characteristic.

Alternatively, you can increase the cap value by a decade to 66uF to move the filter of the protection cap down to 300Hz. This will have virtually no audible effect on the crossover characteristic set in the DCX.
I suspect you can move the protection filter up quite a bit from 300Hz towards 1300Hz (15uF) with increasing audible effect. Experimentation will prove what you find acceptable.

You can adopt the same with a Mid driver using a passive single pole filter as your protection cap. Don't go a decade lower as it forces adoption of an electrolytic.

[linuxworks]

thanks andrew. I'll try for a 15uf or larger as a general target.

the turn on/off's on the 3886 really seem mild, to me. maybe I can get by with no cap if I don't use these speakers as 'testers' and only connect known good amps to them, or diy projects that seem known/good

[Pixo]

ever thought of using the kind of capacitors that ceiling fans have?

They are cheap, rated for high voltage, fully solid construction, durable and i guess they're available in 4uf and thereabouts. Parallel two of those and you're done.