JFET input SuSy Chipamp

Russ White · 6th May 2007, 09:21 PM

Hello folks,

Some of you may find this interesting. Some possibly revolting...

Since I was reworking My (or rather Terry Aben's) XBOSOZ for JFET inputs I though this might be worth a go too.

Its the basis for a CCS, Cascoded, JFET input, Super Symmetrical ChipAmp (LM3875,LM3886,LM4780,OPA546, etc...) ala the schematic posted some time ago by Nelson Pass. Besides being a mouthful it should sound pretty good too, but I have not built it yet. I have however built a few conventional SuSy chipamps and think this should work just fine. If I ever get the time I will build a pair.

Q1 and Q2 can be ZTX450 or whatever you like but should have fairly high beta.

As shown with LM3886 it should be able to easily do 200W into 8-ohms.

The schematic is not complete, you would need to add bypass caps etc.

Input impedance as shown is ~4.7K.

Gain is 26db with balanced input and 20db with SE input which requires -in to be tied to GND.

Cheers!
Russ

Nelson Pass · 6th May 2007, 11:23 PM

Very nice.

You should realize that it's even easier than that. I put together
a quickie which simply used the uncascoded diff pair and
capacitively coupled the Drains of the JFETs to the chip amps,
which made the absolute offset a non-issue. And of course
the input impedance can be much higher if you like.

Russ White · 6th May 2007, 11:36 PM

Thank you Nelson.

I like your idea even better.

Something like this?

Nelson Pass · 7th May 2007, 03:00 AM

Worth a try...

lovechild · 7th May 2007, 08:12 AM

I was just reading through the SuSy Gainclone thread this weekend, wondering if I should give it a try and if the JFETs (2SJ109BL) I have sitting here could be used for the input stage ... and now that!

You Guys Rock.
martin

7th May 2007, 08:12 AM	#5
lovechild diyAudio Member Join Date: Oct 2005	Telepathy? I was just reading through the SuSy Gainclone thread this weekend, wondering if I should give it a try and if the JFETs (2SJ109BL) I have sitting here could be used for the input stage ... and now that! You Guys Rock. martin

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6th May 2007, 11:23 PM	#2
Nelson Pass The one and only Join Date: Mar 2001	Very nice. You should realize that it's even easier than that. I put together a quickie which simply used the uncascoded diff pair and capacitively coupled the Drains of the JFETs to the chip amps, which made the absolute offset a non-issue. And of course the input impedance can be much higher if you like.

7th May 2007, 03:00 AM	#4
Nelson Pass The one and only Join Date: Mar 2001	Worth a try...