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Old 5th January 2008, 05:34 PM   #11
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This may be the biggest idiot question of the year but what would happen if I were to put a 3 ohm resistor (impedance of the drivers are 3 each) on the crossover output for the respective driver as a "dummy" load on the XO and leave everything there?



BTW:
Dont know why youre having probs with the pic....works fine for me.
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Old 6th January 2008, 12:55 AM   #12
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I’m committed now, here’s No 2, bit more than I wanted to spend but you dont see these everyday!

http://cgi.ebay.com/ws/eBayISAPI.dll...0%3D%26fvi%3D1
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Old 6th January 2008, 04:51 PM   #13
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Quote:
what would happen if I were to put a 3 ohm resistor (impedance of the drivers are 3 each) on the crossover output
It would really just be a waste of power. Effectively the amp would still be delivering full power output in the unwanted frequency band, only to be dissipated by the resistor instead of converted to audio by a speaker. It would also take a resistor capable of dissapating that amount of power. Large, expensive, and hot at 70W.

The biggest advantage IMHO of using two amps over one along with passive XO's would be in dynamic range and clarity at higher volumes. That's simply because of having separate power supplies for each set of drivers.

I got the picture to load, and it looks to be a 12dB on HP and LP with Lpad for tweeter and no impedance compensation. Pretty simple. If you can post a shot of the underside and the terminal labels, we can easily figure it out exactly.

The big Solen polypropylene cap is in the tweeter circuit, and the yellow electrolytic cap is in the woofer circuit. Chances are you can remove the cap from the unwanted circuit and the remaining components would be out-of-circuit if nothing was connected to the corresonding output terminals. Again, a couple more pictures would reveal all.

An electronic crossover would be more effective because the amplifiers' work would be entirely dedicated to the desired frequency band. Given what you already have invested, it shouldn't be a major expense. If you consider the option, be sure to look for something with resistor/chip programmable filter frequency instead of pots for more precise control.
With that kind of power on those tweeters, you want to be dang sure your HP frequency isn't 1kHz lower than what you think you've dialed in.

Tim
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Old 6th January 2008, 07:48 PM   #14
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It looks like an active XO would be a better way to go, I didnt even consider going active (I need an active XO anyway).
I have never used active filtration without using the supplied XOs, always felt that it was somehow bad for tweeters.
It has been a million years sense I was looking for an outboard XO….is there any you would recommend????

BTW: Good suggestion with the chip and not pots….but I always loose them and live to tweek pots.
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Old 6th January 2008, 08:51 PM   #15
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Quote:
I have never used active filtration without using the supplied XOs, always felt that it was somehow bad for tweeters.
Kinda feels like there's no safety net?

The most immediate concern is that there is no series capacitor to block DC. And while that could kill any driver, tweeters are more sensitive with their little voice coil wires.

You can always add a cap inline with the tweeter to remedy this. Choosing the value isn't critical, but some thought should go into it.

Otherwise, the concern is having a variable crossover frequency. With premade passives, you can't run them out of their safe range.

As far as playing with the pots go, I'm right there with you. The fun really begins after everything is installed and you get to tune it. However, I don't trust that the silkscreened writing is position-accurate enough on the crossover cases to risk expensive tweeters.

More on particular units later...

Tim
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Old 7th January 2008, 12:59 AM   #16
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As the mids are much easier to dial in requiring the Hz and are more sturdy than the tweeters couldn’t I use the supplied passives in conjunction with the actives and use the active filtration for the mid without having to purchase an additional set of XOs?

This way I could take advantage of the Dynas tweeter attenuation option.


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Old 7th January 2008, 04:02 AM   #17
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Quote:
Originally posted by Jonny Hotnuts
As the mids are much easier to dial in requiring the Hz and are more sturdy than the tweeters couldn’t I use the supplied passives in conjunction with the actives and use the active filtration for the mid without having to purchase an additional set of XOs?

This way I could take advantage of the Dynas tweeter attenuation option.


[/IMG]
I have a pair of ADS crossovers that that have a very similar layout to this at least for the woofer circuit.

You will be affecting the sound as active crossovers and your passive are shaping the response differently. I haven't followed car audio for a while but 24dB/oct linkwitz-riley and 12db/octave butterworths were most common, which probably wont match the passives electrical response. It may even sound better using the active filter but I would use an RTA and mic to measure the affect.

The question mark in your layout should be a poly switch for protection of the tweeter.
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Old 7th January 2008, 04:45 AM   #18
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Hi Jonny,

Damn. I was gonna buy that one...

Dynaudio used to recommend using the high pass part of their crossovers for the tweeters to protect them from dc offset... basically confirming what tsmith1315 typed above. You can just hook the PQ-20 to the Dyn crossover and hook it up to the tweeters with no problems. If you'd prefer to change the xover point, you can just use a capacitor to block the DC offset rather than the whole crossover thing.

AudioControl makes very nice external preamp level crossovers, but you should remember that the crossover slope on them is 18 or 24 dB per octave. You can find a 24xs at a very attractive price on eBay. In fact, I was going to do *exactly* what you're doing right now when I originally started collecting equipment to do my ultimate system. I've long since gotten rid of both PQ-20s, but I still have the Dynaudios and the AudioControl crossovers... I ended up going with a McIntosh MC440 for the amp and not bi-amping the front stage.

These days, I've gone to JL Audio Slash series amps for mo' power... but I find it terribly amusing to see someone else coming up with the same solution for some serious sound.
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Old 7th January 2008, 11:46 PM   #19
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jpruden, make no mistake it was my original intent to get a Mtosh. and made a few bids in the 7 range and came close a few times but decided a PQ would be an acceptable substitute after prices were going in the 1K+. After getting a PQ the idea of 2 seemed like it would take things to the next level (and hopefully take some of the sting out of having to settle).

Still wish I had McIntosh amps though….but a very clean 300 watts per side of ADS power seems like it shouldn’t be too hard to live with.

BTW:

There is currently another PQ for sale on eBay now, and is being sold by a member of this forum.

http://cgi.ebay.com/ebaymotors/ADS-P...QQcmdZViewItem



I am looking very hard at the Audio Control 6XS.
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Old 11th January 2008, 06:25 PM   #20
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Jonny,

Here’s a redraw of the schematic in a more “standard” XO look, using the part numbers you designated. The sections are very recognizable.

The 27uF and 2R2 are there to flatten the woofer’s rising impedance, and L1 is the actual first order filter.

First order filters are Butterworth filters with a Q of 0.707. This can’t be varied.

“therm” is the protection device.

C2 (10uF) and L2 are the tweeter’s actual filter while R2, R3, and R4 form the attenuator for the tweeter.

Being a 12dB/2800 Hz filter, using C2 of 10uF, L2 will be about 0.3mH. The Q, or response shape of the filter in the crossover region will be determined by the impedance of the tweeter in that frequency range. Looks like the MD100 is pretty close to 3 ohms around 2800Hz. That puts the filter Q around 0.53 with these component values. A Linkwitz-Riley filter is represented by a Q value of 0.5. A Bessel filter is represented by a Q of .577. You’re within a few percent tolerance of either.

So within a reason given standard value components, you have a 2nd order filter with a conservative Q in the mid 0.5 range at 2.8kHz on the tweeter and a 1st order Butterworth at 2.8kHz on the woofer. Assuming the specified 2.8kHz crossover frequency is close, that is.

All that said, I wouldn’t beat my brains out over choice of crossover slopes or trying to duplicate Dynaudio’s exact filter response. Looking at the x250 info on their website, it’s a bit generic so as to work with any one of 4 woofers. In other words, it would be pointless for the passives to have been designed/tuned with an exact target response in mind, with the infinite possibilities between woofer choice and installation variables.

Yet Another option:
The 7” woofer rolls off pretty smooth above 3kHz on its own. Let it roll off naturally, and only use an active section on the tweeter.

IMHO, a good quality active crossover really should have a bipolar power supply, the control features that you want, and be quiet. That’s about it.

That’s not to say it must have a bipolar supply, the ADS 642CSi was a well-liked 12V unit. It was a 4-channel two way but IIRC, had independently selectable F/R HP points and Q factors by DIP resistor pack. If that’s the case, you could HP the 7's at the sub LP frequency and let them roll off naturally on the top end, and the HP the tweeters at 2.8kHz & Q of your choice with one of these.


Tim
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