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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

BJD BUFFER stage for passive preamps
BJD BUFFER stage for passive preamps
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Old 6th February 2018, 06:32 PM   #11
Calvin is offline Calvin  Germany
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Hi,

my Buffer only relies on local feedback.
The JFET Master and the bipolar Slave in Sziklai configuration form a highly linear 'supertransistor' due to the very short, direct local feedback.
But since the circuit topology consists of only these two transistors the local feedback could be regarded as global feedback loop at the same

You can use the BF862 in that configuration too ..... there's already been a thread showing a small PCB layout for the Buffer utilizing the BFs.

jauu
Calvin

Last edited by Calvin; 6th February 2018 at 06:35 PM.
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Old 6th February 2018, 06:34 PM   #12
kinku is offline kinku  United States
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BJD BUFFER stage for passive preamps
Quote:
Originally Posted by Calvin View Post
Hi,



If You can't reduce the supply rails then cascoding is a option -also reducing THD considerably-

Calvin I guess I understand what you are saying now.
Cascode is the way ,SST4391 is obsolete, so I used MMBF4391.
Can we use same JFET in all 4 positions? When I did it gave me significant THD figures.
And this one is considered local feed back?
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Last edited by kinku; 6th February 2018 at 06:48 PM.
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Old 6th February 2018, 06:46 PM   #13
kinku is offline kinku  United States
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BJD BUFFER stage for passive preamps
Quote:
Originally Posted by Calvin View Post



You can use the BF862 in that configuration too ..... there's already been a thread showing a small PCB layout for the Buffer utilizing the BFs.

This part I did not understand well.
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Old 6th February 2018, 08:24 PM   #14
kinku is offline kinku  United States
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BJD BUFFER stage for passive preamps
While waiting for Calvin to reply ,I made the changes. Not sure matching the lower resistors with upper, like Feucht recommended still apply to cascode.
Also provided pads for 1206 resistors for shaky hands
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File Type: jpg cascoded PCB2.jpg (137.2 KB, 112 views)
File Type: jpg cascoded SCHEM.jpg (139.5 KB, 136 views)
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Old 7th February 2018, 08:49 AM   #15
AndrewT is offline AndrewT  Scotland
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the cascode transistor should be a high Vpinchoff device.
And to get a high voltage across the follower it also needs to be high Idss.
If you are using 12mA bf862, then I suggest you look at Idss for the cascode >24mA and when this cascode is passing 10mA (~90% of 12mA Idss) the Vgs should be >2* Vpinchoff of the 862.
You can measure your cascode to achieve this. Pass 10mA through it with a VR in it's source and measure the Vgs. You want at least 1.5Vgs, preferably >2V since this becomes the Vds of the Follower.
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Old 7th February 2018, 12:36 PM   #16
kinku is offline kinku  United States
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BJD BUFFER stage for passive preamps
Thanks Andrew, I think Calvin recommended 4391 JFET and meets all this requirements. MMBF,PMBF ,SST Suffixes only indicate different manufacturers.
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Old 8th February 2018, 12:37 AM   #17
kinku is offline kinku  United States
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BJD BUFFER stage for passive preamps
Well , I went ahead and ordered my first idea PCB. B1 version of Feucht buffer. If any one interested in testing them out let me know.
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Old 8th February 2018, 02:13 AM   #18
ammel68 is offline ammel68  United States
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Originally Posted by stellavox View Post
Excuse me Ammel - but what is "SQ"?
Sorry...Sound Quality.
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Old 8th February 2018, 02:16 AM   #19
ammel68 is offline ammel68  United States
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BJD BUFFER stage for passive preamps
Quote:
Originally Posted by AndrewT View Post
Could you explain?
Andrew, as long as you've been on this site and as many posts you've replied to, you would be one of the last members I expect to not understand such a simple statement.
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Old 8th February 2018, 02:32 AM   #20
ammel68 is offline ammel68  United States
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BJD BUFFER stage for passive preamps
Quote:
Originally Posted by Calvin View Post
my Buffer only relies on local feedback.
The JFET Master and the bipolar Slave in Sziklai configuration form a highly linear 'supertransistor' due to the very short, direct local feedback.
But since the circuit topology consists of only these two transistors the local feedback could be regarded as global feedback loop at the same
Having built several examples of Calvin's Buffer, I can attest to the fact that it sounds really good.
In fact, it's probably my favorite discrete buffer even though it uses many more components than Nelson's B1.
I like it so well that I designed my own PCB for it, which uses only through-hole components.
I use PF5102 and J107 JFETs which I match myself.

Sorry kinku for the slight thread derailment.

When you get your PCBs back for your B1 version of Feucht buffer, please keep us posted on how it turns out.
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