FET Buffer Help

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colin.hepburn
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FET Buffer Help

Post by colin.hepburn » Thu Dec 13, 2012 5:08 pm

Hi All
Well I going to build this buffer for the new DAC WM8741 as you can see from the schematic it’s a
Pedja Rogic Buffer for his Gainclone Amp so has an output filter to suite the GC amp would I need to keep the filter the same or can I just remove it and add a 1k resistor after the 4.7/10uf cap and say a 520K to ground and disregard C1 0.47uf
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Buffer 1A.jpg
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colin.hepburn
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Post by colin.hepburn » Fri Dec 14, 2012 10:30 am

OK as the buffer starts at the input and ends at the output of the 4.7uf cap
is it ok to take the output strait of the 4.7uf cap and do away with the filter part
or do I have to add a resistor to ground for the buffer to feed a valve pre amp I could just try it i suppose but it makes life easer if I don’t need to add this filter as per the schematic
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Paul Barker
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Post by Paul Barker » Fri Dec 14, 2012 7:52 pm

It isn't really my area of knowledge but you could just try coming off the 4.7uF cap and replace the 1uF shunt cap with a 100k resistor.
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Post by colin.hepburn » Sun Dec 16, 2012 3:26 pm

Paul Barker wrote:It isn't really my area of knowledge but you could just try coming off the 4.7uF cap and replace the 1uF shunt cap with a 100k resistor.
Thanks Paul
I will have a play around with it just an experiment anyway just to see how it sounds
Got some 6922 and 12ax7 6SN7s which I may try out as well if I can find a good buffer schematic
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Mike H
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Post by Mike H » Sun Dec 16, 2012 4:34 pm

Yes what Paul said. Image

That's very like my stepper's buffer, doesn't have the bootstrap bit though.
 
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Post by colin.hepburn » Sun Dec 16, 2012 4:45 pm

Mike H wrote:Yes what Paul said. Image

That's very like my stepper's buffer, doesn't have the bootstrap bit though.
Yep I have your schematic for that buffer mike will likely try it as well but without the stepper :)
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Post by Mike H » Sun Dec 16, 2012 4:51 pm

Noticed lately jFET's getting hard to find now, mind you found a load at Mouser

http://uk.mouser.com/Semiconductors/Tra ... et&FS=True


2SK170BLF

http://uk.mouser.com/ProductDetail/Tosh ... Db1ds9w%3d
 
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Post by colin.hepburn » Sun Dec 16, 2012 5:05 pm

Yep I got some 2SK170BL/j310 from crickelwood I see thay have increased the prices for then now and got some BF245B which came from rapid now discontinued
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Post by Mike H » Sun Dec 16, 2012 5:10 pm

Yes I know! Having established that was a useful one!

Image
 
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Post by colin.hepburn » Sun Jun 02, 2013 4:28 pm

Hi All
Well
Have built this Sold state discrete buffer with Jfets using it with my EBay AK4396 Dac
As advised here I took the single off the 4.7uf cap and replaced the 1uf cap with a 100k resistor to ground see moded schematic I also set the Source leg of the J2 FET which is a j310 too 270R as the Start point as recommended

It seems recommended to adjusting the trimmer for 6mA as this is suggested to be the sweet spot for this buffer? But for the moment the trimmer pot is left at the start point 270R and it sounds very good running in to the Nad Preamp and the JE Labs EL34/5881 the NAD pre is also a FET input stage a 2SK369

I will need to check the voltage across the trimmer on the J310 to get the current but not sure of the spec for the J310 FET as cant find the data sheet any advice on the current limits on the J310 would Be a help if anyone has the spec sheet

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Post by colin.hepburn » Mon Jun 03, 2013 4:13 pm

Right found the Data sheet for the J310 so its 25v 10mA
setting the preset @ 271R gives me a voltage of 2.29v
2.29v/271R=0.008mA would this be a tad to high for the J310 limit of 10-mA
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Post by colin.hepburn » Sat Jun 08, 2013 3:28 pm

Well not a lot of interest in this Buffer them :)
I have backed off the current to about 6mA 2.45v/373R= 0.006mA
Also would there be any benefit in adding a 220uf with 0.1uf bypass caps on the power +/-rails of the buffer
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Post by colin.hepburn » Sun Jun 30, 2013 3:23 pm

Hi all
Well so far this pedja Rogic Buffer is sounding very good on the end of the EBay AK4396 Dac inputted to a Pioneer SA-500A pre-amp into a Pioneer SA6200 power amp used as my PC Headphone system with a pair of sennihizer HD650 Cans it makes a killer headphone system indeed

However I have come across the same buffer circuit but using a 6922 valve I would welcome anyone views on this valve circuit version would it work or is it just another wrong internet posted idea as many seen to be see schematic
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6922 Pre Buffer.jpg
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Post by JamesD » Sun Jun 30, 2013 6:49 pm

Its interesting to see these circuits as when I do I wonder why people don't just use a pentode cathode follower and be done with it?

High performance pentodes have Gm up to 55mA/V so are competitive with these FETs and miles better than a 6922 CF even with the silicon help... and pentodes like the E280F can be had for a handful of euros still...

I suppose the price you pay is anode current - its going to be between 20 - 50 mA depending on the pentode you choose but distortion would be even lower and it would sound better than the sand or the sand/triode options... Some would say noise performance would be worse but actually in this application they would probably be wrong - the high Gm pentodes can be quieter than triodes as cathode followers when implemented properly.

Maybe its just me - I admit that I like pentodes :D

James

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Post by JamesD » Sun Jun 30, 2013 10:39 pm

Thinking about it, I suppose the main reason for using the sand solution is the lack of high voltage B+ :D Thats rather a big consideration!

I'm not anti-sand per se - designed and built a lot of sand amps and I'm listening to a Toshiba separates system at present but the glass amps are ultimately more open and have better tone - at least to my ears.

James

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