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Old 10th February 2013, 02:42 PM   #21
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Thaks for digging up that Acoustic Reality schematic Steve. I have on file as well, but was too lazy to go find it and scan it, etc. :-)
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Old 10th February 2013, 02:46 PM   #22
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There is a difference between squabling and figuring out how things work in an interactive way.

Cutting-and-pasting some examples of xovers dedicated to specific driver/enclosure combinations does not add much to understanding as such.
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Old 10th February 2013, 03:02 PM   #23
system7 is offline system7  United Kingdom
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That's quite alright. The penny will drop eventually...

Sreten & Speakerman go at series XOs

I'd listen to wolf-teeth too. He has built these things.
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Old 10th February 2013, 03:11 PM   #24
gfiandy is offline gfiandy  United Kingdom
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Hi Majerjack, if you tell us the specific drivers you are planning to use we may be able to give better guidance on the effects of both a parallel resistor and the possibility of using a first order crossover.
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Old 10th February 2013, 04:39 PM   #25
Dave R is offline Dave R  United States
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Quote:
Originally Posted by majerjack View Post
My question is: will a parallel resistor on the tweeter alter the characteristics of a first-order series crossover?

If anyone has specific knowledge on the subject, I would appreciate your reply. If you have experience with series crossovers, I would be interested in hearing about that also. Thanks to all.
Attached is a sketch of the basic 1st order 2-way series, with an L-pad on the tweeter.

In the context of your question, there is no series resistor in the L-pad (R1), only the parallel resistor (R2).

A parallel resistor (R2) will tend to attenuate the tweeter. One point of caution is to make sure that the combined impedance doesn’t go too low.

I’ve found that there is a lower limit to the size of R2 where sound quality is affected, and of course it depends on the particular tweeter. I usually try not to use R2 by itself to attenuate more than 1 or 2 dB. In general, my preference is to use the series resistor (R1) as a more effective way to attenuate the tweeter. Sometimes using 30 to 50 ohms for R2 takes out just a little of the high frequency hash, if present. Removing too much can eliminate valuable details from the sound quality.

I use series crossovers most of the time, perhaps if for no other reason than for my own amusement. However, I have yet to find a combination of drivers that allows only 1st order.

Typically, I use 2nd order on the tweeter, where a capacitor is located in-line with R1. (One end is attached to the inductor L1). That gives a steeper roll-off, allowing for a crossover frequency that’s closer to the tweeter resonance.

Along with that, 1st order on the woofer is usually adequate. An impedance compensation (Zobel) on the woofer helps flatten the woofer’s impedance. This can help to make a smoother transition to the tweeter’s impedance.

Your thought of flattening the impedance of the tweeter could be accomplished with its own “Zobel”, but there’s really little benefit in doing so.

With the right combination of drivers and circuit components, a series crossover can yield a relatively flat impedance, which is sometimes desirable.

The down side of higher orders (or additional components in the network) is the complexity, and the difficulty for mere mortals to set up and perform calculations. Without a lot of trial and error, the best approach is to use some kind of simulation software.
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File Type: jpg Ser_2way_1st.jpg (26.8 KB, 151 views)
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Old 10th February 2013, 07:09 PM   #26
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I like to use series 1st order when i can. It requires a well thot out design & drivers with with extension well beyond the XO. I also choose a more sensible XO point where i can keep the drivers within 1/4 wl.

Here is an actual impedance plot of a 2-way with 100uF + 5mH (nominal 8R tweeterm 4R woofer) reflecting the changes by adding a shunt R of 15, 25, and 15||25R.

Tweeter is in an aperiodic TL that does a good job of significantly reducing the tweeter impedance peak.

The above is still a work in progress.

dave
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File Type: gif uFonken+woofT-impedance.gif (23.1 KB, 124 views)
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Old 10th February 2013, 07:13 PM   #27
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Quote:
Originally Posted by Dave R View Post
Sometimes using 30 to 50 ohms for R2 takes out just a little of the high frequency hash
Our listening bears this out. Levels are no attentuated much at all.

dave
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Old 10th February 2013, 08:48 PM   #28
system7 is offline system7  United Kingdom
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I haven't been able to do any simming with smooth polycones for series crossovers, but it always seems to come down to drivers that play nicely together with a series XO work even better with a parallel crossover.

Here's some paper drivers with a parallel and a series. I much prefer what I'm seeing with parallel in terms of impedance and cone-breakup suppression. As DaveR says, first order bass, second order tweeter is minimum in either topology. FWIW, the LCR Fs resonance filter in the second (series) example is effectively second order.

If I was going to try a simpler series filter, I'd really want something like that famous Vifa P13WH of Lynn Olson's, or a Morel driver. Around 0.6mH anyway, to avoid needing bafflestep correction.
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File Type: jpg Parallel_versus_Series_FR.JPG (168.1 KB, 117 views)
File Type: jpg Parallel_versus_Series_Phase.JPG (144.2 KB, 114 views)
File Type: jpg Parallel_versus_Series_Network.JPG (60.7 KB, 110 views)
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Old 10th February 2013, 08:50 PM   #29
tvrgeek is offline tvrgeek  United States
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Originally Posted by vacuphile View Post
Tvrgeek, that looks like an interesting project, but if I may make some suggestions: before you make a passive filter, first do some measurements and listening tests with an active setup using something like a MiniDSP. This will show you which filter slopes and xover frequencies will work best. Don't just measure on axis, but also look at what happens off the horizontal and vertical axis. 5K from mid to tweeter is in my experience way too high, as you will note from such tests.

A very nice little speaker that would match up good with your other components is the Fountek FR88EX they have at Madisound. It is advertised as fullrange, which it is. It does have some breakup above 10KHz and ultrasonic it goes nuts, but it is a very good midrange.
Actually, I do my first tests with my DCX. And surprise! I was looking at the FE88 or 89. The 89 seems to have a slightly better motor.

The listening position for where these will go is pretty much on axis. Even a 3 inch , yea maybe that is high. 4K then? Remember, the low slope means the tweeter is still doing a lot lower down. Only testing will tell. The purpose if this build is to see if getting the crossovers out of the critical sensitive hearing range and low order crossovers is the next step. Some claim it to be so. If it fails, I just upgrade the FE85's on my computer and rebuild the Seas into another set like I have. Only building and letting my wife listen will tell.

For those who have not followed all my amp escapades, she has the most fantastic and critical hearing of anyone I know. The slightest problem in the mids is a "no". My last Seas set were my first "yes" speakers and could be tolerated on a wideband amp. ( Parasound). I found I had to use Rotels to tame whatever every other speaker was doing. They are clever in their compensation design that makes them highly suitable for less than perfect speakers, where the good Mr. Curl does not tolerate weak speakers so he designs amps expecting perfection. Two designs both perfectly suited to their market. I now have a pile of RB 951's for sale.
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Old 10th February 2013, 09:26 PM   #30
tvrgeek is offline tvrgeek  United States
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Quote:
Originally Posted by planet10 View Post
I like to use series 1st order when i can. It requires a well thot out design & drivers with with extension well beyond the XO. I also choose a more sensible XO point where i can keep the drivers within 1/4 wl.

Here is an actual impedance plot of a 2-way with 100uF + 5mH (nominal 8R tweeter 4R woofer) reflecting the changes by adding a shunt R of 15, 25, and 15||25R.

Tweeter is in an aperiodic TL that does a good job of significantly reducing the tweeter impedance peak.

The above is still a work in progress.

dave
If the primary goal was to manage the impedance, why not a Zobel? More expensive, but does a better job. Unless you are crossing the tweeter over to a super tweeter, the increasing impedance may be a good thing as conventional solid state amps generate less distortion with a higher impedance load. 16 Ohms is Nirvana for most. If you are transformer output, then the rules change completely, so for the fans of glass bottles, ignore why I have not seen the need for taming tweeter Le where you may consider it paramount.

Did Dave say he was working on a multi-way? Hark! Oh I forget. A full range, with a sub and a helper super tweeter in the same cabinet looks just like the big old floorstanders Actually, their methods for taming full range driver breakup makes them especially suitable for high quality wide range mids with low order crossovers.
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