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Old 15th June 2005, 12:22 PM   #1
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Default Active cross speakers and the tweeter protection cap

How important is it to have a 1st order crossover to protect the tweeter from low frequencies in an active crossed system? I would like to avoid introducing a 1st order crossover onto the tweeter because of the 90 degree phase-shift involved, is this even a valid concern? If it matters, I'll be using one of BrianGT's LM3886 gainclones with the "snubberized PS" to power it.
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Old 17th June 2005, 02:37 AM   #2
mike.e is offline mike.e  New Zealand
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Youl want a capacitor value that doesnt cause any changes near the Fc while providing protection from DC signals.Sorry,A typical value escapes me.


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Old 17th June 2005, 12:16 PM   #3
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Default Re: Active cross speakers and the tweeter protection cap

Quote:
Originally posted by m0tion
How important is it to have a 1st order crossover to protect the tweeter from low frequencies in an active crossed system?
I'd say it depends on the crossover and amps used. I've been using a Marchand XM44 active crossover and upgraded Meitner amps for 8 months. I don't use anything between the amps and the tweeters, as per Phil Marchand's advice. I've had no trouble and no concerns. I can even turn the Marchand or the amps off and on, and I don't hear a click or pop or any kind of transient sound at all.

My original tweeters were crossed over at 1.5k and now I have Usher 9950 tweeters crossed at 1.8K. Putting a cap in between the amp and driver would seem to me to be a definite compromise.
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Old 17th June 2005, 05:03 PM   #4
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Hi,

Iíve been using active systems for a while and donít use a cap on the tweeter.
That said, I did loose a tweeter a while back when I crossed some wires in the amp.

Personally I think having a cap in line is defeating part of the benifits of the active systems, but if you have an amp that has a ďBumpĒ when you turn it on or off, or are using true ribbons, you might consider a cap.

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Old 17th June 2005, 06:43 PM   #5
AJ is offline AJ  United States
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Hi m0tion


This question comes up periodically and I had to ask it myself at one point. The best answer I got was from Bill Fitzmaurice who recommended 1/2 (an octave below) the corner frequency of the tweeter. I assumed Bill meant the crossover frequency for the "corner" but I went ahead and used the driver's Fs as the corner and wound up using a 27uf cap. The tweeter is a Fountek JP2 ribbon.

The reason I had for asking the question was because of some pretty nasty experiences with a bare bones Marchand XM1 and a pair of B&K EX4420 amps, which have a sizable thump-thump on their own when they shut down. Since then, I've gone to a different amp (Outlaw 7 channel) and a different, better behaved active XO (Ashly XR2001). The new combination is silent on normal shutdowns, which makes me think that problems with thumps are a combination of amp and XO together and it'll vary depending on what kind of muting circuitry is available in each. I also took the solution one step further and I run the Ashly thru a high quality UPS. If a power outage or brown-out occurs, the XO stays on until the amp has a chance to discharge. Maybe it's overkill, but ribbons are expensive and I don't like the downtime to get them fixed.

As to the phase problems of the cap, I personally haven't heard any, but I'm usually crossing the tweeters pretty high (~4khz-8khz) so my ability to hear issues like that is pretty anemic. I am wondering how it all fits in the end, having a cap that far below the XO point where phase has probably changed on its own without any help from the cap anyway. Subjectively, the cap doesn't seem to hurt anything, but that's only subjective for me. You may have bat ears and might hear things I don't.

Bottom line is probably the combination of amp, XO, and type of tweeter, and how much you're willing to risk.
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Old 17th June 2005, 06:54 PM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
I have been running active for a few weeks now.
The power amp has no relay in the speaker output and I have it connected direct to the tweeter. Xover is set to 3kHz.
No mishaps yet!
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Old 4th November 2010, 02:32 AM   #7
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With a valve amp on the tweeter there is no risk of DC blowing the tweeter and a transistor amp would be safe with something like ESP's Project 33 to mute the tweeter output on LF and DC detection. I use a valve amp for the tweeter in an active set-up (24dB/octave LW 2.88kHz) but still installed a metallized polypropylene cap of 25uF in series with the 8 Ohm tweeter for two reasons. One is to provide protection for accidental connection of the wrong amplifier or active crossover output, and another is that being tuned several octaves below the active filter xover point, any unwanted phase shift at the cap/tweeter F3 frequency was not a concern because it is at a frequency already filtered way down by the sharp slope of the active filter and the very slight phase shift up at the active crossover point was considered to be of possible benefit because the tweeter and midrange are on the same vertical panel (i.e. acoustic centre of tweeter is closer to the listener). I figured that a slight phase lag might align these a little better.
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Old 4th November 2010, 09:52 PM   #8
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The use of a capacitor for a tweeter in an active set up is essential, and is standard practice in large PA systems. A small DC offset from a transistor amp will decenter a dynamic tweeter, and a small switch on thump can do much damage. When using ribbon tweeters that have an impedance matching transformer such as Aurum Cantus G1 as I use, a small dc offset can easily saturate the matching transformer. Generally I would suggest a good quality capacitor in series for protection, dimensioned to provide a roll off around 1 or 2 octaves below active crossover frequency. For a 2khz active crossover at 8 ohms I would probably use a capacitor 20-40uF. I can not detect the difference with or without a capacitor by listening-but then my ears are getting old!:
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Old 4th November 2010, 10:32 PM   #9
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Quote:
Originally Posted by nandababaji View Post
Generally I would suggest a good quality capacitor in series for protection, dimensioned to provide a roll off around 1 or 2 octaves below active crossover frequency.
In order that the cap does interfere as little as possible with the highpass transfer function of the active crossover it should be dimensioned preferable that it rolls off rather >3 octaves below the crossover.

Depending on the setup of the active crossover, alternatively the series cap after the amp might also be an integral part of the overall crossover design.
This way You could possibly spare one cap in the active crossover part.
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Old 4th November 2010, 11:13 PM   #10
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That sounds about right, I am crossing over at 2.5khz active, with the capacitor crossing over at around 660hz, assuming the ribbon is around 8ohms at that frequency. I also fit a first order filter to the tweeter amp input around 100hz-I tend to be cautious as I have fried a few ribbons over the years! When I am first setting up, I will also wire a 4mh inductor (temporarily) in parallel with the ribbon until I am certain I have actually connected to the right amplifier. I think there are tables that describe 1st, 2nd pole etc. for the Linkwitz-Riley filters, I guess I could eliminate one pole and use third order together with the capacitor forming a fourth order filter. Does anyone here know which would be the best pole to eliminate and how to calculate it? I seem to remember the Linkwitz-Riley is a pair of cascaded 2nd order Butterworth filters.
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