Using Joachim Gerhard Filter Buffers as balanced crossover - diyAudio
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Old 9th January 2013, 10:09 AM   #1
APK is offline APK  United Kingdom
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Default Using Joachim Gerhard Filter Buffers as balanced crossover

Further to an idea from EUVL regarding passive filter network followed by JFET follower,The F5X Preamp I am seeking advice in using 4 of Joachim Gerhard Filter Buffers Joachim Gerhard Filter Buffer for ES9022 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, Rs = 12.1K, Cs = 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 FX5s?
Any help or views on the above would be appreciated.

Tony
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File Type: png High pass.png (8.5 KB, 540 views)
File Type: png Low pass.png (8.5 KB, 531 views)
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Old 9th January 2013, 01:02 PM   #2
EUVL is offline EUVL  Europe
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You guys are very innovative.
Never thought it could be used as a Sallen Key.
But of course, it works as proposed.




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Old 10th January 2013, 09:23 AM   #3
Calvin is offline Calvin  Germany
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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 Preamp-Buffers - simple idea

jauu
Calvin

Last edited by Calvin; 10th January 2013 at 09:36 AM.
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Old 10th January 2013, 09:32 AM   #4
APK is offline APK  United Kingdom
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Hi Calvin
Thanks for the comments and suggestions, am i correct about the 180 phase difference?
Tony
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Old 10th January 2013, 09:39 AM   #5
Calvin is offline Calvin  Germany
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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.
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Old 10th January 2013, 10:08 AM   #6
sreten is offline sreten  United Kingdom
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Hi,

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.

Look at the electrical transfer functions on Zaphs site.

rgds, sreten.

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Last edited by sreten; 10th January 2013 at 10:13 AM.
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Old 10th January 2013, 10:43 AM   #7
APK is offline APK  United Kingdom
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Thanks for the pointers, more reading! thought I had this partially sorted apparently not
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Old 10th January 2013, 10:48 AM   #8
EUVL is offline EUVL  Europe
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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.


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Old 10th January 2013, 11:22 AM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by APK View Post
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?
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Old 10th January 2013, 03:02 PM   #10
APK is offline APK  United Kingdom
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Patrick

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