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Multi-Way Conventional loudspeakers with crossovers

Introduction to designing crossovers without measurement
Introduction to designing crossovers without measurement
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Old 4th May 2019, 03:48 PM   #681
phivates is offline phivates  United States
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Eyes glazed over after re-reading this whole thread...I am refurbishing a friends old Scott 188T three ways. 10"/5" CTS, 1" Phillips. My question is about the tweeter leg connection point in the crossover. Some say it should come after the mid cap and others have all three legs in common. Amplifier will be ordinary receiver. Posts 178 and 342 show variations on the first approach. Discuss? Irrelevant?
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Old 5th May 2019, 01:06 AM   #682
AllenB is offline AllenB  Australia
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I assume this is one of those that cascades the tweeter after part of the midrange network, maybe after one capacitor? The main question probably is does the midrange capacitor have an effect on the tweeter in the band where it is audible.

Another question is does the presence of the tweeter and its circuit affect the midrange, and does the tap out of the middle of the midrange circuit involve it enough to have an effect on the tweeter? Since the reactance of the first midrange capacitor is likely low over the midrange and the tweeter bands, the answer might be not enough to cause a significant effect, unless it is proven otherwise with measurements, where it is still likely to be small.

Another issue is the effect of the extra capacitor on the tweeter at low frequencies, below its audible band. Maybe this is the appeal of this kind of circuit. The idea that unnecessary excursion can be limited, reducing heating of the voice coil, also reducing the potential for non-linear distortion as the coil ventures beyond design recommendations.

Harmonic distortion has been brought into question, especially recently. There is no doubt that we can hear it under certain circumstances. However, I think that it isn't responsible for most of the errors that we can hear in a speaker but can't seem to identify the reason for. Furthermore it has been shown that other specific things are responsible for a significant proportion of these faults.

With regards to voice coil thermal characteristics speakers have limitations, and 1" dome tweeters are no exception. There may be a couple of types of thermal concern. One that depends on the overall heat buildup, and one that may cause a minor effect regardless of how hot the voice coil environment is. How this can be addressed, and whether extra high passing has an effect or not can make interesting research. At one time it must have been thought worthwhile.

Back to the primary concern. Will changing the tweeter network input point from after the midrange capacitor, to before, bring a result that is significantly similar? Probably yes. Will calculating the value of the first tweeter cap and the first midrange cap in series, to get the effective capacitance, address much of the remaining discrepancy? Probably yes. Will it be perfect, maybe not yet .. but could you achieve the original intended response after bypassing the midrange cap, and after further work to determine the differences? Yes.
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Old 5th May 2019, 01:09 AM   #683
AllenB is offline AllenB  Australia
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Edit: I just noticed you referred to post 342 in this thread. Is this the crossover in question? ... Most of the above will still apply, with some extra notes.
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Old 5th May 2019, 03:44 AM   #684
phivates is offline phivates  United States
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Post 342 has a simple common feed option, and another which has the tweeter paralleled with the midrange, sort of. So I was looking for a reason to cascade the tweeter after the midrange cap, an arrangement more often seen in older texts like the Phillips book on loudspeakers from the 70's, which is not exactly option two in the cited post. I'm inclined to feed the tweeter directly unless there's a reason not to. No electrolytics involved by the way. Thanks for the response and for this great tutorial.
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Old 16th May 2019, 12:15 AM   #685
brooks is offline brooks  United States
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Just got thru reading this great thread. Thanks Allen and all for the great information. Hope I could ask a question about an old Realistic Nova 8 crossover. My dad bought me a pair of these for me in the mid 70s, and a few years later I replaced all the drivers with top-end Philips drivers, with the intent of redoing the XO, which I never did (ah, the 70s...).

I recently got them out after 20+ years and recapped them (with original values), and was planning on just restoring the cabinets, but then thought about checking out the XO first.

Have you seen an XO design like this before? The schematic is right, but I cant verify the driver connections. One thing I would like to verify is the polarity/phasing of the drivers. It looks to me like the woofer should be reversed so the mids are reversed and woofer/tweeter in phase?

At some point I want to take all this good info and redo the XO, but should I scrap this and build a new one, or could I modify this old design? FYI - these guys actually sound quite good as is, but I know they can sound better.
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File Type: jpg Nova 8 Crossover Schematic XSim.JPG (218.9 KB, 140 views)
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Old 16th May 2019, 04:59 AM   #686
AllenB is offline AllenB  Australia
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This crossover does look a little unusual. This is because in part it is a series crossover.

With a series crossover (eg the image below), a second order filter can be made with an inductor and a capacitor. Then one driver, say a woofer connects to the filter in the normal way, and the tweeter uses the filter upside down at the same time.

In your case the woofer and midrange(s) are in this configuration, meaning L1 and C1 are also responsible for cutting the lows from reaching the midrange drivers. The tweeter takes its signal from the same place as the mids, but its response is likely more the result of R1 and C3 than on C1 and L1 because it covers frequencies away from their effective band.

The series crossover has certain benefits. Naturally it saves parts but, it also has the quality of reducing the crossover sensitivity to impedance variations in some speakers.

In your crossover, C2 and L2 make a notch filter (a bandstop filter) for the midrange. This is designed to suppress a region around 8.5kHz. With new drivers it becomes uncertain whether this is still needed. You could connect a clip lead across them to see if you hear a difference. You could disconnect the tweeter first to make the difference more obvious. Possibly you'll want to consider a larger inductor in place of the two components, low-passing the upper midrange.

The midrange level is easily adjustable. The tweeter level can be changed through the resistor. When you increase this value, you also lower the tweeter crossover point. This can be countered by changing the capacitor value, but for small changes it's up to you.

Another option would be to use an L-pad on the tweeter. Normally these are set to a certain point and left alone, they are not meant as treble tone controls. However as you have new drivers you will need to find the correct level for the tweeter and mids.
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File Type: jpg series.jpg (26.8 KB, 134 views)

Last edited by AllenB; 16th May 2019 at 05:02 AM.
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Old 16th May 2019, 05:13 AM   #687
Nag is offline Nag  India
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Hi anyone has passive crossover near equal to lxmini.
Thanks.
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Old 16th May 2019, 11:59 AM   #688
brooks is offline brooks  United States
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Quote:
Originally Posted by AllenB View Post
and the tweeter uses the filter upside down at the same time.

The tweeter takes its signal from the same place as the mids, but its response is likely more the result of R1 and C3 than on C1 and L1 because it covers frequencies away from their effective band.

In your crossover, C2 and L2 make a notch filter (a bandstop filter) for the midrange. This is designed to suppress a region around 8.5kHz. With new drivers it becomes uncertain whether this is still needed. You could connect a clip lead across them to see if you hear a difference. You could disconnect the tweeter first to make the difference more obvious. Possibly you'll want to consider a larger inductor in place of the two components, low-passing the upper midrange.

The tweeter level can be changed through the resistor. When you increase this value, you also lower the tweeter crossover point. This can be countered by changing the capacitor value, but for small changes it's up to you.

Another option would be to use an L-pad on the tweeter. Normally these are set to a certain point and left alone, they are not meant as treble tone controls. However as you have new drivers you will need to find the correct level for the tweeter and mids.
Many thanks for the reply.

So if the tweeter uses the filter 'upside down', does that mean the current polarity/phasing of the connection is correct? Also, R1 in my diagram is actually a 10 ohm pot (same as R2/R3) used as a variable resistor (side/middle pole). So varying this changes the XO point and not just the level to the tweeter?

So is the phasing for woofer-mids-tweeter ( - - +) correct?

I'll test out removing the notch today and see how it sounds removed.

BTW - the original XO points for this were 800 and 5K.

The other mystery here that I thought I had bought 8 ohm woofers when I replaced them, and they are stamped on the magnets "/W8" Philips 8 ohm designation, but both these measure 3.6 ohms. (Total box DCR is 4.6 ohms). The woofer has a fairly broad range (30-1500hz) - with this series filter, do I now need to change C1-L1 to compensate?

Thanks again, I lost all notes to what I originally intended on doing with these - that was a long time ago!
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Old 17th May 2019, 02:59 AM   #689
AllenB is offline AllenB  Australia
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Quote:
Originally Posted by brooks View Post
So if the tweeter uses the filter 'upside down', does that mean the current polarity/phasing of the connection is correct?
Changing the polarity of one driver will flip the phase by 180 degrees in any case. It is fair to assume it was set right originally.

The series connection should give a result approaching second order, which more often likes a reversed polarity. I can see the woofer has its positive connection attached to the positive output of the amplifier, and vice-versa for the midranges.

Beyond this, the phase may wander. If you are dissatisfied trying to get the drivers to blend and have tried both polarities, your next step is to try to blend the response slopes. Can I assume you have them from the manufacturer?
Quote:
So is the phasing for woofer-mids-tweeter ( - - +) correct?
Presently, it is (+ - +).
Quote:
Also, R1 in my diagram is actually a 10 ohm pot (same as R2/R3) used as a variable resistor (side/middle pole). So varying this changes the XO point and not just the level to the tweeter?
Yes. The reason is more to do with the first order crossover. It may not sound obvious and may not be a problem.

The following is only a suggestion. It may be of interest to use an L-pad instead. This way you could vary the level without the frequency change, and there would be a reduced response variation in general, as you do. This would also reduce any response bump introduced by the tweeter resonance impedance peak.

So, how would you decide which setting of the potentiometer works best (so that you can decide this value works well with the fixed capacitance, when you remove and replace the pot..), because when changing the pot it also upsets the level and makes it hard to decide? Maybe it would be easier to vary the capacitance after removing the pot from its original location and configuring an L-pad.
Quote:
woofers ... measure 3.6 ohms ... do I now need to change C1-L1 to compensate?
In your case, the output of the woofers is likely to be reduced at its top end around the crossover. Reducing the value of L1, or increasing C1 are also going to affect the mids. Maybe the sympathetic nature of a series crossover will be partially self correcting. Maybe your ability to change the mid level is fortunate?
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Old 17th May 2019, 03:18 AM   #690
brooks is offline brooks  United States
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Thanks. What I couldnt understand was the midrange bandstop (I had to look that one up).
I didn't get a chance to test a bypass of that ckt, but I think I may take your advice and remove that cap and figure a higher value for that low pass L2.

Really appreciate your help!
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