diyAudio - Using Joachim Gerhard Filter Buffers as balanced crossover

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Using Joachim Gerhard Filter Buffers as balanced crossover

Further to an idea from EUVL regarding passive filter network followed by JFET follower,http://www.diyaudio.com/forums/pass-...ml#post3300675 I am seeking advice in using 4 of Joachim Gerhard Filter Buffers http://www.diyaudio.com/forums/group...ml#post3101908 as a balanced crossover. The intention is to use the up coming F5X preamp into this crossover feeding 2 fX5's.
The idea is to use the filter buffers for the High and Low pass filters (using 4 of each filter to get the LR and balanced output) ditching the original input filters and replacing with a 2nd order Sallen-Key as below. I settled on Q=.5 (Linkwitz-Riley) as the Butterworth filter calculations I tried always seem to oscillate according to Okawa electronic designs calculator.

Fc 2800Hz, R’s = 12.1K, C’s = 4.7nf

As I understand the outputs from the LR2 crossover filters are 180 degrees out of phase (high pass to low pass), would I be correct in that one of the speaker drivers needs the polarity reversed or would I reverse the either the low pass or high pass balanced outputs into one of the FX5’s?
Any help or views on the above would be appreciated.

Tony

You guys are very innovative.
Never thought it could be used as a Sallen Key.
But of course, it works as proposed.

:)

Patrick

Hi,

I'm using this kind of buffer for several years now in filters and it works very fine. Sonically it leaves OPamp graveyards far behind.
The combination of 2SK389/LSK389 and 2N/MMBF/SST4391 guarantees for low offset without the need of matching devices or dc blocking caps.
I use the SMD parts, thermally coupled by a cooling fin covering all 4 devices. Temperature drift is then no problem and the whol buffer builds very compact.
If more current or low load impedance stability is required the buffer may be ´boosted´ in a Sziklay-style fashion as I showed http://www.diyaudio.com/forums/analo...mple-idea.html

jauu
Calvin

Hi Calvin
Thanks for the comments and suggestions, am i correct about the 180 phase difference?
Tony

Hi,

yes, simplest would be to reverse speaker polarity. Phase needs to be checked with the live object in the end anyway.

jauu
Calvin

btw. Wrenchone has shown a similar Buffer named the Firefly.

Thanks for the pointers, more reading! thought I had this partially sorted apparently not :(

With the circuits in post#1 above, I have actually done crossover filters from 1st to 4th order.
You just need to cascade two Sallen Keys for higher orders.

Patrick

Quote:

Originally Posted by APK (Post 3317654)

The idea is to use the filter buffers for the High and Low pass filters (using 4 of each filter to get the LR and balanced output) ditching the original input filters and replacing with a 2nd order Sallen-Key as below. I settled on Q=.5 (Linkwitz-Riley)

This is a description of an LR4 crossover.
It is 4pole roll off.

Quote:

No two drivers I know will work well in a 2 way with an electrical
2nd order x/o at ~ 3KHz, active or passive with a bassmid zobel.

Can we have a comment or comments on whether an LR4 can work with conventional drivers that have a good working passband that extends well outside the LR4 pass band limits?

Patrick

cascading two Sallen Keys, does this require a buffer in between?
Tony