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1st Order (6dB per Octave) Crossover!

MakeMineVinyl

Well-Known Member
In my quest to endlessly tweak, and remembering how good my Altec A-7-500s sounded when I had a 3rd-order electronic crossover many, many years ago, I decided to rip open my crossover and re-think the whole thing.

I decided to really go out on a limb and go ultra minimalist - a single pole, 6dB per octave crossover between the woofer (15") and the HF horn. Building something like this is simplicity in the extreme; just a single resistor and capacitor in front of the LF and HF power amplifiers for low pass and high pass respectively. The entire network can be passive, so its an electronic crossover without the active electronics (although I do use a single buffer IC before the power amp inputs to give a more predictable load impedance for the RC filters).

1st order crossovers have a big advantage in phase response - they're the only type which can successfully reproduce a square wave. All other types mangle a square wave into something almost unrecognizable. Do we actually listen to square waves? Heck yes; a clarinet's waveform is essentially a square wave (in fact square waves are used as the basis of synthesizing a clarinet voice on an old-skool analog synthesizer). Theoretically, the advantage should be audible as more accurate reproduction of transients etc.

1st order crossovers do have one dis-advantage though; a lot more low frequency information is passed to the tweeter, so one must be careful. In my case, since I'm running a maximum of 5 watts into the tweeter, and since anything over a half of a watt never happens at even extreme listening levels because of the efficiency of my horns, I'm pretty safe. However, I did take this opportunity to change the basic crossover frequency to a minimum of 1.2kHz, which was the optimum frequency arrived at by Altec Lansing for the A-7 series while I worked there (this was when they were still located in Anaheim, across the street from Disneyland).

The transformation was pretty unreal, in a very good way. Everything seemed to open up, and the soundstage widened to well beyond the physical edges of the speaker cabinets (even with program material which was not processed to give head-related transfer function cues for false surround type effects). Additionally, the speakers became much more "invisible" in that the sound never seems to be actually be coming from the speakers themselves with well recorded stereo material - that's a huge feat with speakers the size of a refrigerator.

After much experimentation, I arrived at a very unconventional location for the crossover points, 1.2kHz for the woofer and 4kHz for the HF horn. In theory, this would produce a huge suckout, but the HF horn has excess energy in the range below about 2kHz which was very effectively smoothed out by shifting its crossover point much higher. The magenta curve for the HF horn below shows the response with the crossover point raised. The green curve for the woofer is its response with no filtering at all for reference.

Here are the response curves; the magenta curve is the HF horn only, the green curve is the woofer only, and the yellow curve is the final listening position response. I have to use a link since for some reason the forum cannot directly post pictures from a Google drive account.

https://drive.google.com/file/d/0B7DJENUU7T6RMDFleW4xazlXT1E/view?usp=sharing

For those with an experimental streak, this is a very simple and potentially beneficial project. Just be very careful and keep the listening levels low and monitor the amount of energy getting to the tweeter to avoid overloading it until it can be determined that proceeding is safe. Raising the crossover frequency above stock is a way to mitigate this to a certain extent, but you still need to be careful.

Single pole crossovers can, and have been implemented in passive crossovers in manufactured speakers, including some high end ones made today, but here I'm talking about low level passive filtering before the power amps.
 
I have first order 6db/octave crossovers in my Dynaudios.....passive obviously.

Cool you can tweak your system and get big , positive results. Wish I had the skills.
 
jamhead said:
I have first order 6db/octave crossovers in my Dynaudios.....passive obviously.
I've been meaning to ask you...at what frequency do you cross to your sub?

After a TrueRTA session while using my old Swan speakers, a 90Hz XO setting showed the smoothest response so I kept that for years. When I got the Contours, I was excited to lower the setting to 70Hz but when I realized they had a 6dB slope, I didn't think it would be wise to set the XO so low. So they too are crossed at 90Hz.
 
Zing said:
jamhead said:
I have first order 6db/octave crossovers in my Dynaudios.....passive obviously.
I've been meaning to ask you...at what frequency do you cross to your sub?

After a TrueRTA session while using my old Swan speakers, a 90Hz XO setting showed the smoothest response so I kept that for years. When I got the Contours, I was excited to lower the setting to 70Hz but when I realized they had a 6dB slope, I didn't think it would be wise to set the XO so low. So they too are crossed at 90Hz.


60Hz. That's the lowest frequency where the A-7 bass cabinets still have strong output. Musically, it sounds better too when I adjust the level with most music. Movie soundtracks are mostly synthesized noise down in those octaves, so it doesn't really matter to me.

I also use a Behringer Feedback Destroyer Pro before the subwoofer power amps. It has as many as 20 parametric equalizers per channel and is really good at taming peaks and dips in response (after you've done all that you can do acoustically with the room of course). Its also not too expensive. I wouldn't feed anything but bass frequencies through it though since its sound quality is probably crap higher up in frequency.
 
Love this stuff...

had I thought about it, I would have thought a 1st order XO would be best when you have a horn and a "traditional" speaker driver -- the assumption being that each type of driver would have a different "sound" so the more you blended the response between the two, the better.

What about just going full electric with the crossover and trying different crossover types? E.g., Bessel vs. LR vs. Butterworth? I have all the "stuff" to experiment with this, I just need to get off my derriere and do it. It's been sitting for far too long. Actually, I thought you DID have an electronic crossover... hmm...
 
TKoP said:
Love this stuff...

had I thought about it, I would have thought a 1st order XO would be best when you have a horn and a "traditional" speaker driver -- the assumption being that each type of driver would have a different "sound" so the more you blended the response between the two, the better.

What about just going full electric with the crossover and trying different crossover types? E.g., Bessel vs. LR vs. Butterworth? I have all the "stuff" to experiment with this, I just need to get off my derriere and do it. It's been sitting for far too long. Actually, I thought you DID have an electronic crossover... hmm...

I've tried pretty much every type of crossover alignment over the years with my A-7s, except for a single pole type. Its ironic that its taken so long to get to a place where I think I've finally dialed them in for every type of music I listen to. Massed violins have always been difficult for the A-7s (and indeed for all horns), but even those are really good now with a good recording. I say a good recording because there are so many recordings which have BAD violin section sound (just the violins; the violas, cellos, and contra-basses are generally always good on most recordings). It doesn't help that general recording practice, even with classical in many instances, is to stick the microphones all-too-close to the violin section and above them. This is a position an actual listener would never have their ear. More natural recording methods with co-incident pairs generally avoid this strident sound.

I've found the audible differences between Butterworth, Bessel and other alignments are pretty much inaudible except in instances where the frequency response at crossover point is altered enough for their slopes to be audible. A single pole does average the two drivers very well. In the A-7, the bass down to about 200Hz is also horn loaded by the short wooden horn in front of the woofer, so the drivers are not as dissimilar as they normally would be.

On any speaker with a single pole crossover, lobing is more of a problem, so steps have to be taken to address that. In the A-7, the horn loading defines the dispersion pattern for the HF and LF drivers quite a bit, so lobing is not as much of an issue as it could be.
 
Zing said:
jamhead said:
I have first order 6db/octave crossovers in my Dynaudios.....passive obviously.
I've been meaning to ask you...at what frequency do you cross to your sub?

After a TrueRTA session while using my old Swan speakers, a 90Hz XO setting showed the smoothest response so I kept that for years. When I got the Contours, I was excited to lower the setting to 70Hz but when I realized they had a 6dB slope, I didn't think it would be wise to set the XO so low. So they too are crossed at 90Hz.

I have mine at 80hz. I've tried 60 and 100, but settled on 80. Overall, I had a smoother response at 80 then either. Honestly couldn't tell a difference in a blind test....
 
One trick I've used and had good results with is to completely bypass the high pass crossover in the mains and only use a low pass in the subwoofer. So the woofer and the subwoofer play together over some range.

This may or may not work in every circumstance and room, but its a free tweak. Just be careful when playing movie soundtracks at +11. :D
 
Zing said:
jamhead said:
I have first order 6db/octave crossovers in my Dynaudios.....passive obviously.
I've been meaning to ask you...at what frequency do you cross to your sub?

After a TrueRTA session while using my old Swan speakers, a 90Hz XO setting showed the smoothest response so I kept that for years. When I got the Contours, I was excited to lower the setting to 70Hz but when I realized they had a 6dB slope, I didn't think it would be wise to set the XO so low. So they too are crossed at 90Hz.

I would not worry about the Dynaudios having a 1st order crossover on the tweeter when determining the ideal subwoofer crossover. If the tweeter's crossover is centered at 1.9kHz, it will not be getting any significant energy at 100Hz or lower - heck, it won't be getting anything of substance below 800Hz. Look at it this way, if the F3 is at 1.9KHz (as it is according to the spec sheet), then it is filering 3dB at 1.9KHz, 9dB at 950kHz, 15dB at 425Hz, and so on. So, at 800Hz the power to the tweeter is already cut at least 10dB, which is a 10x cut in power, so a 200W amp will be limited to 20W at 800Hz into the tweeter, but real signals won't demand that. More likely a 20W RMS is going on (at most) and that is less than 2W RMS into the tweeter, which it can handle with that killer voicecoil. Even 2W is likely more power than you'll be pumping into that thing. At 100Hz it will be fraction of that.

Remember, Dynaudio and Morel (who share tweeter voicecoil and motor assembly patents) love to brag that their dome tweeters can take 1000W at 10mS peaks with ease.
 
Flint said:
I would not worry about the Dynaudios having a 1st order crossover on the tweeter when determining the ideal subwoofer crossover. If the tweeter's crossover is centered at 1.9kHz, it will not be getting any significant energy at 100Hz or lower - heck, it won't be getting anything of substance below 800Hz. Look at it this way, if the F3 is at 1.9KHz (as it is according to the spec sheet), then it is filering 3dB at 1.9KHz, 9dB at 950kHz, 15dB at 425Hz, and so on. So, at 800Hz the power to the tweeter is already cut at least 10dB, which is a 10x cut in power, so a 200W amp will be limited to 20W at 800Hz into the tweeter, but real signals won't demand that. More likely a 20W RMS is going on (at most) and that is less than 2W RMS into the tweeter, which it can handle with that killer voicecoil. Even 2W is likely more power than you'll be pumping into that thing. At 100Hz it will be fraction of that.

Remember, Dynaudio and Morel (who share tweeter voicecoil and motor assembly patents) love to brag that their dome tweeters can take 1000W at 10mS peaks with ease.
That does indeed make me feel a little better but I do have another concern about lowering the XO point and that is I think I'm a little light on power to be pushing serious SPLs through an 86dB speaker 9 feet away. I take comfort in the little bit of protection and headroom a 90Hz XO point offers me.
 
The Dyn's first order crossover should not be one of the reasons you are fearing a lower crossover point for the subwoofer.

You may be concerned about blowing the woofer, or simply pushing your amp into clipping with too much deep bass content, but I would not worry about the tweeter due to its crossover being 6db per octave.
 
In my quest to endlessly tweak, and remembering how good my Altec A-7-500s sounded when I had a 3rd-order electronic crossover many, many years ago, I decided to rip open my crossover and re-think the whole thing.

I decided to really go out on a limb and go ultra minimalist - a single pole, 6dB per octave crossover between the woofer (15") and the HF horn. Building something like this is simplicity in the extreme; just a single resistor and capacitor in front of the LF and HF power amplifiers for low pass and high pass respectively. The entire network can be passive, so its an electronic crossover without the active electronics (although I do use a single buffer IC before the power amp inputs to give a more predictable load impedance for the RC filters).

1st order crossovers have a big advantage in phase response - they're the only type which can successfully reproduce a square wave. All other types mangle a square wave into something almost unrecognizable. Do we actually listen to square waves? Heck yes; a clarinet's waveform is essentially a square wave (in fact square waves are used as the basis of synthesizing a clarinet voice on an old-skool analog synthesizer). Theoretically, the advantage should be audible as more accurate reproduction of transients etc.

1st order crossovers do have one dis-advantage though; a lot more low frequency information is passed to the tweeter, so one must be careful. In my case, since I'm running a maximum of 5 watts into the tweeter, and since anything over a half of a watt never happens at even extreme listening levels because of the efficiency of my horns, I'm pretty safe. However, I did take this opportunity to change the basic crossover frequency to a minimum of 1.2kHz, which was the optimum frequency arrived at by Altec Lansing for the A-7 series while I worked there (this was when they were still located in Anaheim, across the street from Disneyland).

The transformation was pretty unreal, in a very good way. Everything seemed to open up, and the soundstage widened to well beyond the physical edges of the speaker cabinets (even with program material which was not processed to give head-related transfer function cues for false surround type effects). Additionally, the speakers became much more "invisible" in that the sound never seems to be actually be coming from the speakers themselves with well recorded stereo material - that's a huge feat with speakers the size of a refrigerator.

After much experimentation, I arrived at a very unconventional location for the crossover points, 1.2kHz for the woofer and 4kHz for the HF horn. In theory, this would produce a huge suckout, but the HF horn has excess energy in the range below about 2kHz which was very effectively smoothed out by shifting its crossover point much higher. The magenta curve for the HF horn below shows the response with the crossover point raised. The green curve for the woofer is its response with no filtering at all for reference.

Here are the response curves; the magenta curve is the HF horn only, the green curve is the woofer only, and the yellow curve is the final listening position response. I have to use a link since for some reason the forum cannot directly post pictures from a Google drive account.

https://drive.google.com/file/d/0B7DJENUU7T6RMDFleW4xazlXT1E/view?usp=sharing

For those with an experimental streak, this is a very simple and potentially beneficial project. Just be very careful and keep the listening levels low and monitor the amount of energy getting to the tweeter to avoid overloading it until it can be determined that proceeding is safe. Raising the crossover frequency above stock is a way to mitigate this to a certain extent, but you still need to be careful.

Single pole crossovers can, and have been implemented in passive crossovers in manufactured speakers, including some high end ones made today, but here I'm talking about low level passive filtering before the power amps.
Hello there!
My friend and I have Altec A7 speakers, and your information on the optimal crossover is very significant to us. The link above to show response curves no longer works. Is it possible to see them now? Do you still like the 1st order crossover with your A7s?
We are using 803b/416B woofers (16Ohms) and 802D compression drivers (16Ohms), with 811 horns. We are trying to build new crossovers for our speakers, and anything you could offer here would be greatly appreciated. Currently, we are using original N-800E and N-800F crossovers…which sound good, but the caps in them have to be going on 60+ years now. Also, you mention that Altec later determined the 1200 Hz was better for the A7 than the 500Hz or 800Hz? That sounds promising too.
 
Hello there!
My friend and I have Altec A7 speakers, and your information on the optimal crossover is very significant to us. The link above to show response curves no longer works. Is it possible to see them now? Do you still like the 1st order crossover with your A7s?
We are using 803b/416B woofers (16Ohms) and 802D compression drivers (16Ohms), with 811 horns. We are trying to build new crossovers for our speakers, and anything you could offer here would be greatly appreciated. Currently, we are using original N-800E and N-800F crossovers…which sound good, but the caps in them have to be going on 60+ years now. Also, you mention that Altec later determined the 1200 Hz was better for the A7 than the 500Hz or 800Hz? That sounds promising too.
I am not longer using 6dB crossover points - I went back to the original 500Hz 12dB per octave crossover. I was an engineer at Altec in the 80s when they were still in Anaheim, CA - I have had my A7-500s since around 1969.

One thing which I would certainly do is to go fully active with crossovers and use separate power amplifiers for the HF/LF. I use a single ended triode 2A3 amplifier for the HF horn for two reasons; the single ended nature of the amplifier avoids crossover distortion and the low power of 2.5 watts assures that I cannot damage the diaphragms. I use different tube power amps for the LF, a McIntosh MC240, Dynaco MKIIIs, or Audio Research D-76A. Any of these work, but vacuum tube amplification is best with these speakers.

I have braced the LF cabinet with 2x4s on the side interior and a double thickness panel on the back behind the woofer. Inside the walls of the LF horn, these have been lined with Bondo to damp out resonances. The HF horn has a thick 1/2" - 3/4" coating of Aquaplas on its entire exterior to damp out the severe resonances of the aluminum horn. Aquaplas is no longer available so Bondo does just as well - Bondo body putty is available from auto parts stores.

If you have any other questions, please ask.
 
I am not longer using 6dB crossover points - I went back to the original 500Hz 12dB per octave crossover. I was an engineer at Altec in the 80s when they were still in Anaheim, CA - I have had my A7-500s since around 1969.

One thing which I would certainly do is to go fully active with crossovers and use separate power amplifiers for the HF/LF. I use a single ended triode 2A3 amplifier for the HF horn for two reasons; the single ended nature of the amplifier avoids crossover distortion and the low power of 2.5 watts assures that I cannot damage the diaphragms. I use different tube power amps for the LF, a McIntosh MC240, Dynaco MKIIIs, or Audio Research D-76A. Any of these work, but vacuum tube amplification is best with these speakers.

I have braced the LF cabinet with 2x4s on the side interior and a double thickness panel on the back behind the woofer. Inside the walls of the LF horn, these have been lined with Bondo to damp out resonances. The HF horn has a thick 1/2" - 3/4" coating of Aquaplas on its entire exterior to damp out the severe resonances of the aluminum horn. Aquaplas is no longer available so Bondo does just as well - Bondo body putty is available from auto parts stores.

If you have any other questions, please ask.
Thank you very much for your reply. Active crossovers are not practical for us right now.
Why did you go back to the 12db crossover?
You mentioned 1200Hz as a better crossover frequency in your older post. Would 1200Hz with a 12db crossover be better than our 800Hz, or is it better to stick with Altec’s original 800Hz design?
 
Thank you very much for your reply. Active crossovers are not practical for us right now.
Why did you go back to the 12db crossover?
You mentioned 1200Hz as a better crossover frequency in your older post. Would 1200Hz with a 12db crossover be better than our 800Hz, or is it better to stick with Altec’s original 800Hz design?
If you have a 811 horn, you have to use an 800Hz crossover. 12dB per octave just sounds best to my ears. 1200Hz allows the woofer to go higher, but it isn't particularly well behaved that far up. The horn has better projection. I would really, really recommend active crossovers as these bring the sound of these speakers up to the best of today's offerings from other brands.
 
If you have a 811 horn, you have to use an 800Hz crossover. 12dB per octave just sounds best to my ears. 1200Hz allows the woofer to go higher, but it isn't particularly well behaved that far up. The horn has better projection. I would really, really recommend active crossovers as these bring the sound of these speakers up to the best of today's offerings from other brands.
I will take to heart your active crossover recommendation, but it is not cards at this time. If you don’t mind, i would appreciate learning what you are using for an active crossover.

Regarding the passive, it seems from you feel the 800Hz is better for the woofer than 1200Hz?

In attempting our crossover rebuild, my friend and I are trying to figure out the exact type of 2nd order crossover design Altec used in their N-800D, N-800E, and N-800F 16 Ohm dividing networks. The older 1950s N-800D, and N-800E both originally used inductors of L1,L2 = 3mH, and C1,C2 = 10.5mF. Online crossover calculators seem to have different views on whether this is a “two way, 2nd order Linkwitz-Riley”, or “2nd order Chebyshev” crossover design. These values workout on these online calculators, for crossover impedance of 16.9 Ohms, and at 890Hz. In 1960 the N-800F changed L1L2 to 3.5mH, but still with 10.5mF C1C2 which works for 18.3 Ohms, at 830Hz.

It seems unusual that they would use a 2nd order Chebyshev design, which I blieve would produce a +6db peak at the crossover point. But maybe we are misunderstanding their implementation of these L and C values, and it is in fact a different 2nd order design?
 
f you don’t mind, i would appreciate learning what you are using for an active crossover.
The active crossover is my own design.

One way to experiment with crossover slopes (although it costs money) is to get a MiniDSP digital crossover. You can play around with crossover points all you want with that. I wouldn't recommend using this full time however unless you have a heavily digital system upstream.
 
Can you elaborate on this statement?

"unless you have a heavily digital system upstream."
 
Can you elaborate on this statement?

"unless you have a heavily digital system upstream."
The miniDSP is a digital device, so it makes sense to supply it with a digital input. While the miniDSP also has an analog input, using this means that the signal is run through an A/D and D/A conversion before sending to the power amplifiers. For me, this is an unnecessary conversion step which I would like to avoid in my mainly analog system. Playing from my computer using digital files or streaming would be fine to send digitally to the miniDSP, but playing vinyl would have to go through a digital step. I just don't like to do that because there are losses, however small.

I use the miniDSP for my subwoofer crossover exclusively now - I originally bought it for that purpose but before that I experimented with using it as my main crossover. It worked well in that role, but like I said above, I didn't like the unnecessary conversion steps in my analog system.
 
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