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Old 11th December 2009, 11:38 AM   #51
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Maybe Rod thought it better to include additional amplifier stages in the LF path due to the added distortion being less audible here?

Anyway, be that as it may, when making this dedicated filter one could as well reverse polarity at one driver.
But, why not opt for a LR4 filter? Have a look at Linkwitz' pages. And as he states, if you do not do correct for other things when running active, it is really no use. Well, have a look at the man's page!

All the best!

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Old 11th December 2009, 01:33 PM   #52
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Originally Posted by wintermute View Post
Hi Jaques, I note that Rod Elliot included an inverting buffer in his 12db/octave crossover for the LP section (which seems a bit odd as I would have thought the tweeter would be the one you invert). Anyway I guess the question is, what were you planning on doing with respect to the 180 degree phase shift of low compared to high? reverse the polarity on the speakers for one (as would be done in a passive crossover)?
Tony.
My plan is to reverse the leads to one of the drivers. I don't see an advantage to doing anything more elaborate than that. For the proper operation of the crossover it is necessary to reverse the polarity of one of the outputs -- it doesn't matter which one.
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Old 11th December 2009, 01:49 PM   #53
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Default Tone Control

Jacques,
I use full range drivers. People are always telling me to run a BSC circuit to tone down the top frequencies and then the frequency response will flatten out a little more. What I was thinking was to put this between source and amp and use it to in effect replace the BSC by dividing the frequencies, adding some gain to the bass and putting the signals back together before they hit the amp. Tone control. I could do that with this circuit right? The way I read the circuit I could increase/decrease gain on either HF or LF. Is it this easy or do all the other values change as well?
Uriah
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Old 11th December 2009, 02:01 PM   #54
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Originally Posted by R-K Rønningstad View Post
But, why not opt for a LR4 filter? Have a look at Linkwitz' pages.
There is no reason not to. It would be straightforward to implement as it essentially consists of two 2 pole filters in series. In that case it may be possible to omit the additional buffers for the feedback caps in the LP section. The stopband attenuation without them will be in the neighborhood of -100 dB.

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And as he states, if you do not do correct for other things when running active, it is really no use. Well, have a look at the man's page!
Yes, there is a lot of good information on the Linkwitz site. I believe that there is significant advantage to be achieved by active crossovers by virtue of bandwidth limiting the power amplifiers (less distortion) and by better coupling of the amplifier outputs to the individual drivers, even if you do not follow through as thoroughly as SL indicates. Bear in mind that this crossover is just one building block in a system.
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Old 11th December 2009, 02:08 PM   #55
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Originally Posted by udailey View Post
Jacques,
I use full range drivers. People are always telling me to run a BSC circuit to tone down the top frequencies and then the frequency response will flatten out a little more. What I was thinking was to put this between source and amp and use it to in effect replace the BSC by dividing the frequencies, adding some gain to the bass and putting the signals back together before they hit the amp. Tone control. I could do that with this circuit right? The way I read the circuit I could increase/decrease gain on either HF or LF. Is it this easy or do all the other values change as well?
Uriah
Uriah,

I would not implement a BSC that way at all. Linkwitz shows implementaions of active and passive shelving networks on his site for just this purpose. A passive shelving filter nestled between two B1s would probably better suit your purpose.
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Old 11th December 2009, 02:15 PM   #56
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Originally Posted by Jacques Merde View Post
There is no reason not to. It would be straightforward to implement as it essentially consists of two 2 pole filters in series. In that case it may be possible to omit the additional buffers for the feedback caps in the LP section. The stopband attenuation without them will be in the neighborhood of -100 dB.



Yes, there is a lot of good information on the Linkwitz site. I believe that there is significant advantage to be achieved by active crossovers by virtue of bandwidth limiting the power amplifiers (less distortion) and by better coupling of the amplifier outputs to the individual drivers, even if you do not follow through as thoroughly as SL indicates. Bear in mind that this crossover is just one building block in a system.

Yes. I did a LR4 with dipole compensation with SL's circuitry. It works very well. It uses opamps. But it is possible to implement with JFETs as well, and the B1 or similar is well suited. If you build a x-over with "normal" discrete transistors, you normally have lots of stages (input diff, output follower etc) and lots of transistors of course, and multiplied by four for LR4, by two for stereo and then dipole compensation etc.
But with B1 circuits it becomes less complicated.
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Old 11th December 2009, 02:35 PM   #57
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Originally Posted by R-K Rønningstad View Post
Yes. I did a LR4 with dipole compensation with SL's circuitry. It works very well. It uses opamps. But it is possible to implement with JFETs as well, and the B1 or similar is well suited. If you build a x-over with "normal" discrete transistors, you normally have lots of stages (input diff, output follower etc) and lots of transistors of course, and multiplied by four for LR4, by two for stereo and then dipole compensation etc.
But with B1 circuits it becomes less complicated.
Yes, those JFETs start to add up quickly . FWIW, the JC/EB complementary JFET buffer looks like an attractive option as well, IMO.
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Old 11th December 2009, 07:26 PM   #58
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Thanks RK and Jaques, I see SL simply says to reverse one driver too.

LR4 would be an option, but I do like the idea of using bessel filters compared to butterworth, after having seen the difference in group delay (though SL says it isn't detectable)... it looks nasty on the graphs I have seen. and the LR implementation seems to not suffer from the usual bessel tradeoff of a non flat response.

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Old 11th December 2009, 08:09 PM   #59
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Originally Posted by R-K Rønningstad View Post
Have a look at Linkwitz' pages.
SL uses the Sallen-Key topography...

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In a very clever paper, Peter Billam (JAES v26 n6, p426, 1978) showed that the positive feedback “aggravated” the distortion through the crossover region, and by a surprisingly high amount. His measurements suggest an 80-fold increase in distortion at the corner frequency.
Quote taken directly from SY's archeron crossover page. The archeron offers a pretty nifty way of doing active 2nd (and 4th) order crossovers without resorting to the sallen key or state variable methods.

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Old 11th December 2009, 08:52 PM   #60
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Quote:
Originally Posted by d to the g View Post
SL uses the Sallen-Key topography...



Quote taken directly from SY's archeron crossover page. The archeron offers a pretty nifty way of doing active 2nd (and 4th) order crossovers without resorting to the sallen key or state variable methods.

syclotron.com
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