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.
 
m0tion said:
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.
 
Hi,

I’ve been using active systems for a while and don’t 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 “Bump” when you turn it on or off, or are using true ribbons, you might consider a cap.

Branwell
 

AJ

Member
2003-08-01 7:06 am
Oregon
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.
 
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.
 
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!::D
 
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.
 
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.
 
I guess the series capacitor does negate some of the advantages of an active set up, but is better than a fried ribbon! I have tried switch on delays, anti-thump circuits but everything in life is a compromise so I am playing it safe. I have destroyed Decca ribbons in the past several times at around £20 a pop! I have spare ribbons for the Aurum Cantus but I want to save them for an emergency. I would love to measure frequency response and phase angle with and without to get sound kind of visual grip on the effect of series capacitors.
 
Brian GT's chipamps seem pretty thump free. You do need to be careful on shutdown - the amp will run longer than the crossover. If your crossover doesn't mute its output, you could get a spike. Learned that the hard way.

I've been using 24 uf to block DC. Higher values would get the filter effect furhter out of the picture, but my slopes are 4th order or higher, so it doesn't really impact me much even only 1.5 octaves away from XO. I don't hear a difference with or without the caps - SCRs

Bottom line - IMHO, if someone else is going to use your system, you want a cap. otherwise you run the risk of a turnoff mistake blowing your tweeter.
 
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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.
If this is a true ribbon (with a transformer) You should check impedance at the aimed frequency (range) and not assume that it`s constant from DC to
light. Some ribbon transformers impedance decrease rapidly below their dedicated lower frequency range (approaching almost zero at DC).
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.
Two cascaded 2nd order Butterworth filters are 24db LR, that`s correct.
There are also 2nd order LR which are two cascaded 1st order filters.
LR filters poles are always equal Fc and Q, yielding -6dB at Fc.
If You aim for a 24dB LR, splitting in third order (active) and first order (passive) isn`t possible if You want a very textbook LR fourth order filter response.
But this is theory, in practice almost always variations of the electric filter parts are required to achieve the desired acoustic target response.
Hence this kind of 3rd/1st order splitting might work in practice though, maybe even better, maybe not, experimentation required.
For tables: just google, there must be tons out there in any flavours including online calculators.
 
There is no need for a cap, if equipment of good standard is used (ie. amps with no thumps).

Tweeters are stronger than they look. They normally blow because of clipping which is a non-issue with active crossover.

In fact I had made mistakes in programming my xo before and my tweeters were highpassed at 120hz instead of 1.2khz (doh) ... and they were fine !!
 

JamesTRexx

Member
2009-11-12 12:56 am
In fact I had made mistakes in programming my xo before and my tweeters were highpassed at 120hz instead of 1.2khz (doh) ... and they were fine !!

I've once reversed the connection sequence on the crossover causing the subwoofers to play 3KHz and up and the tweeters got 70Hz and lower (from a 2KW amp). No damage done (luckily). :)
The tweeters are rated at 80W/2KHZ, 180W/4KHz with 12dB crossovers so they can take a punch.
 
Hello,

I know this is an old thread, but I've recently read something really interesting about the tweeter protection in active systems (extracted from the "Audiolist" forum, from Brazil):

Few people realize that tweeters can be damaged not only by turn on/off thumps or DC offset, but also by another cause: the distortion of a complex musical signal. It is well known that when a simple sine wave clips, it generates higher frequency harmonics. However, when two or more sine waves (complex signal) are submitted to clipping, they generate a whole spectrum of harmonics, including frequencies lower than the signals that generated them. Those low frequencies are sent to the tweeter.

What I mean is that having a silent turn on/off amplifier will not 100% protect your tweeters.
 
Interesting thread ! Wish I'd seen this a while back....

I'm using an unusual filter for my active xo. I use a passive butterworth on the input to the buffer/line amp and then a 12dB pllxo with the resistor values set apart by a factor of 20 because I read this would give something close to LR response. The xo freq is 2260 and uses Mundorf silver/oil 1%. I measured their values to get matching sets so the XO is very precise despite being pllxo.

I use a protection cap (actually a pair of Mundorfs) that also dips the response at 2260 a little, followed by a bridged-t attenuator because I read this is the best to avoid any possible ringing. This gave the flattest xo response using Speaker Workshop to test.

However, I don't know how to test/calculate phase response, or if I really need to given the flat response. Any tips for me ? :)

Thanks,

Tom
 
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.

I've been running my Linkwitz Orions off Adcom amplifiers with a minor turn on transient without issue for the last 7 years with thousands of power cycles. We've also had power outages (in a new home the Orions got connected to an switched outlet so the cross-over was going off and on at the same time). There have been no issues.

When I built them I hung my scope on the tweeter amp terminals, found the transient less than what I got using a 1.5V batter to check, and wasn't too worried.

I double checked with Siegfried Linkwitz and he said not to worry.

In theory an output device could also fail; although nice film capacitors that give me a high-pass pole at 700Hz cost more than Seas Millenium replacement voice coil/dome assemblies so I took my chances.