Tweeter protection for active speakers - diyAudio
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Old 6th January 2005, 11:20 AM   #1
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Default Tweeter protection for active speakers

Tweeter: Vifa D25AG
fs: 850 Hz
6 ohms

I've been looking into using a cap, but a value of 31 uf is needed to protect at fs, or 33 uf at 800 Hz which is actually below resonance but satisfies the 1.5 octaves away from the 2.4kHz xo point.

Is it acceptable to put caps in series to make up the value?
I can only find electrolytics of this value. Even if I could find polyester, they seem to be too expensive. I'd spend more on tweeter protection than on the tweeter itself.

I have considered using an electrolytic and just replacing it periodically as its value drops.

I would also like to use an active loudspeaker protection cirtuit, but I also want passive as a last line of defence.

The amp I plan to use for the tweeter will be a chip amp, which apparently has its own protection built in which has ugly performance when it starts to work. However, this chip amp has about 60w whereas I only need about 20w at most - hence it will NEVER be driven hard.
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Old 6th January 2005, 11:41 AM   #2
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It's the switch-on glitches you need to worry about, not overdriving them/protection ccts kicking in. Monitor the output of your amp with a scope and see what comes out at switch on. If you have access to a chart recorder type device that would be the best thing to use.

I didn't risk it with my active system that used a chipamp to drive a tweeter; I went for a relay.

One easy way is to use a small AC relay driven from the transformer to switch in an RC network across the DC rails. This network drives a transistor that drives a relay that switches the speaker. Or see Doug Self's site for a way that uses a feed from the AC with just one relay but an extra few components.
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Old 6th January 2005, 11:46 AM   #3
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IMHO, it is acceptable and wise. The caps should not be in series but in parallell. Use for instance 4 pcs of 10 uF polypropylene caps of some reasonable quality that suits your wallet and taste. Surely you must be able to find such standard caps around Melbourne?
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Old 6th January 2005, 11:53 AM   #4
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I can get up to 10uf with good quality caps. They cost about the same as my tweeter though to get 3 of them

I don't have access to a scope, will a software one do?

What is a chart recorder?

How about this for tweeter protection:

http://www.altronics.com.au/index.as...=item&id=K5052

Click the image to open in full size.
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Old 6th January 2005, 11:55 AM   #5
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I'm tempted to take my chances with an electrolytic, see how it sounds, but also use the protection circuit. If I'm really unhappy with the electrolytic I could consider taking it out or more expensive caps ...

It does seem a bit of a compromise where I have to put it though. I have to go just slightly below fs to get 1.5 octaves below the xo point of 2.4k ... that's 33uf cap (800Hz)

fs is 850

Thoughts?
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Old 6th January 2005, 12:00 PM   #6
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If you are building the amp, driving this tweeter, by yourself and if you are concerned about the effect of the cap on signal quality then you might like to take off the NFB partially after the cap.

Regards

Charles
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Old 6th January 2005, 12:13 PM   #7
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Quote:
Originally posted by phase_accurate
If you are building the amp, driving this tweeter, by yourself and if you are concerned about the effect of the cap on signal quality then you might like to take off the NFB partially after the cap.
NFB?
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Old 6th January 2005, 12:22 PM   #8
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Hi Paul,

These guys http://www.speakerbits.com/net/catalogs/cat19.aspx (in Melbourne) have solen 33uF caps for about $30.... probably about the same as what you were looking at for 3 10Uf caps


I've been looking for caps today too.... have been playing around trying to find the best resistor/cap combo for a zobel (somewhere around 30uF is looking about right for me too .... I think I will go buy a bunch of BP electros in various values and use them for experimenting, then when I'm happy with a value, I'll consider forking out for the more expensive polys.....

If you think standard polys are expensive just have a look at the hovland music caps on that site!!!! I think I'll stick to the solens or possibly even the generic ones from WES. BTW WES have solen caps up to 15uF and they aren't as expensive as the site I posted above (whether they are the same series or not, I don't know).

Dare I suggest that you consider my preferred form of speaker protection the polyswitch??? I know a lot of people think badly of them, but I would think they have less of an effect (if any) than a reactive component like a cap.... My current amp (soon to be replaced/complimented) has relay protection (dc sensing) and also a speaker turn on delay. The relays don't connect the speakers until about 2 or three seconds after turn on, stops the clicks and thumps nicely, also disconnects the speakers the instant the power is switched off..... probably something similar to the Altronics kit you linked to, except it was integrated into the amp (Playmaster series 200).

Tony.
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Old 6th January 2005, 12:26 PM   #9
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Quote:
NFB?
Negative Feed Back: i.e. the thing that makes you IC (like LM3886 or similar) behave like a linear amplifier rather than like a comparator.

Regards

Charles
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Old 6th January 2005, 12:27 PM   #10
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Thanks for the tips Tony. I have actually taken polyswitches out of my passive xo's as I considered them unecessary!

Different story now though ...

How do you work out the values? What exactly are polyswitches? Aren't they resistors that act at a certain temperature or current? Do they protect against DC?
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