Active crossover with Jfet buffers?

Fuling · 29th August 2006, 10:27 PM

Is this circuit a good substitute for opamp based buffers in active crossovers?

I´m going to build a 90Hz 12dB crossover fo a biamp setup and discrete buffers would "feel" better than ICs.
This article ( http://www.tkhifi.com/div/Erno_Borbe...%20follower%22 )
mentions an output impedance of 2,3 ohms, but also that the input capacitance is high enough to cause trouble in filter applications. Should I be worried about that?

parsecaudio · 29th August 2006, 10:48 PM

Hello Fuling
no takes care to the input capacitance of the 2SK170 J-Fet:
the parasitic capacitance of the PCB tracks is higher than J-Fet.
For a crossover frequency of 90 Hz you must not have problems whit adequate resistors value(not more high than 20-30 K Ohms).
I agree with you for discrete output buffer .

Nelson Pass · 29th August 2006, 10:57 PM

Inevitably you want to have some input and output resistance
to prevent issues when attached to external equipment.

parsecaudio · 29th August 2006, 11:16 PM

....and must calculate this when designing the filter.

zenmasterbrian · 30th August 2006, 12:33 AM

This is a most interesting idea. Monolithic opamps may not be the best in every situation. I'm going to read your links.

Fuling · 30th August 2006, 09:01 AM

Thanks.

Perhaps this (Simple Low Distortion JFET Buffer) would be even better, assuming we change to a bipolar supply?

zenmasterbrian · 2nd September 2006, 04:36 AM

What do you anticipate as the benefits of doing this with a JFET circuit, instead of a bipolar discrete circuit or a monolithic opamp?

poobah · 2nd September 2006, 06:29 AM

The desire to use a Jfet is about noise. This type of buffer can be used without a feedback loop around it.

Feedback is maligned for a number of reasons... many of them valid.... some BS.

john curl · 2nd September 2006, 11:46 PM

Fuling, very good buffer. This is a fet derived 'White follower' developed in the '30'A or 40's for tubes.
One small addition can make this design almost perfect. Just add an other 22 ohm resistor between the OUTPUT source follower and the DRAIN of the lower fet. This will give you virtually ZERO DC offset from input to output, allowing you to DIRECT COUPLE. This is very important, as good caps are more expensive and space consuming than good fets. This change PRESUMES the matching of the two output fets. If you do not or can not match the fets, then put a trim pot or resistor in series or parallel to the bottom 22 ohm resistor and change its value to 10 ohms or so. Then you can adjust for ZERO DC OFFSET.

wrenchone · 3rd September 2006, 04:17 AM

This is one of the projects I'm working on also in time-sharing mode. Since my preamp is all single-ended JFET class A, it didn't make sense to me to give up and use opamps for the crossover.... My first effort uses simple JFET buffers with current source loading. I may try something more sophisticated when the rest of my projects cool down a bit.

29th August 2006, 10:48 PM	#2
parsecaudio diyAudio Member Join Date: Apr 2004 Location: ventimiglia	. Hello Fuling no takes care to the input capacitance of the 2SK170 J-Fet: the parasitic capacitance of the PCB tracks is higher than J-Fet. For a crossover frequency of 90 Hz you must not have problems whit adequate resistors value(not more high than 20-30 K Ohms). I agree with you for discrete output buffer .

29th August 2006, 11:16 PM	#4
parsecaudio diyAudio Member Join Date: Apr 2004 Location: ventimiglia	. ....and must calculate this when designing the filter.

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29th August 2006, 10:57 PM	#3
Nelson Pass The one and only Join Date: Mar 2001	Inevitably you want to have some input and output resistance to prevent issues when attached to external equipment.

30th August 2006, 12:33 AM	#5
zenmasterbrian diyAudio Member Join Date: Jul 2006	This is a most interesting idea. Monolithic opamps may not be the best in every situation. I'm going to read your links.

30th August 2006, 09:01 AM	#6
Fuling diyAudio Member Join Date: Aug 2001 Location: Sweden (Mora)	Thanks. Perhaps this (Simple Low Distortion JFET Buffer) would be even better, assuming we change to a bipolar supply?

2nd September 2006, 04:36 AM	#7
zenmasterbrian diyAudio Member Join Date: Jul 2006	What do you anticipate as the benefits of doing this with a JFET circuit, instead of a bipolar discrete circuit or a monolithic opamp?

2nd September 2006, 06:29 AM	#8
poobah diyAudio Member Join Date: Nov 2005	The desire to use a Jfet is about noise. This type of buffer can be used without a feedback loop around it. Feedback is maligned for a number of reasons... many of them valid.... some BS.

2nd September 2006, 11:46 PM	#9
john curl diyAudio Member Join Date: Jul 2003 Location: berkeley ca	Fuling, very good buffer. This is a fet derived 'White follower' developed in the '30'A or 40's for tubes. One small addition can make this design almost perfect. Just add an other 22 ohm resistor between the OUTPUT source follower and the DRAIN of the lower fet. This will give you virtually ZERO DC offset from input to output, allowing you to DIRECT COUPLE. This is very important, as good caps are more expensive and space consuming than good fets. This change PRESUMES the matching of the two output fets. If you do not or can not match the fets, then put a trim pot or resistor in series or parallel to the bottom 22 ohm resistor and change its value to 10 ohms or so. Then you can adjust for ZERO DC OFFSET.

3rd September 2006, 04:17 AM	#10
wrenchone diyAudio Member Join Date: Feb 2004 Location: Silicon Valley	This is one of the projects I'm working on also in time-sharing mode. Since my preamp is all single-ended JFET class A, it didn't make sense to me to give up and use opamps for the crossover.... My first effort uses simple JFET buffers with current source loading. I may try something more sophisticated when the rest of my projects cool down a bit.