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Beast with 1000 JFETs redux ?
Beast with 1000 JFETs redux ?
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Old 16th January 2021, 09:47 AM   #161
EUVL is offline EUVL  Europe
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I have been swapping drain & source of JFETs for years.
Absolutely no issue at all.
Applies to all small signal JFETs I know of, not only Toshibas.
Although I have not tried with LU1014 or Semisouth.
No real need in those cases.

It helps to understand some semiconductor physics.


Cheers,
Patrick
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Last edited by EUVL; 16th January 2021 at 10:04 AM.
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Old 16th January 2021, 09:49 AM   #162
EUVL is offline EUVL  Europe
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> The idea is to make a B1 R1, not R2.

The purpose of his drain resistors is to measure current / for debug.
For the lower half (CCS), it does not matter whether it is connected to the drain or the source.


Patrick
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Old 16th January 2021, 10:03 AM   #163
EUVL is offline EUVL  Europe
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250x 2SK2145GR wired as source follower at 4mA Idss per FET would make :

Idss ~1A
Transconductance 3S (Zout 0.3R)
Crss ~750pF

Compared with say 2SK1530 at 1A bias
Transconductance ~2S (Zout 1R)
Crss ~100pF

Compared with SJEP120R100 at 1A bias
Transconductance ~5S (Zout 0.2R)
Crss ~100pF

None of them has low enough Zout without NFB.
Both Toshiba's have positive tempco which needs to be dealth with.
And you will need a low-Z gain stage to drive the 2SK2145's.


Cheers,
Patrick
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Old 16th January 2021, 01:44 PM   #164
woofertester is offline woofertester  United States
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Thank you, Patrick. I appreciate your advice based on experience.

The attraction is a completely JFET power amp using devices in production. The output impedance appears to be low enough for Mr. Pass to drive various speakers and he approves of the sound. That is enough reason for me to try one or more variants.

The versions I am working on are using devices in stock at the large reputable distributors.

I will look into adding driver buffer(s) on board similar to what Mr. Pass did.

At the moment, the device count for the massive B1 is 500 pcs of 2SK2145 per board. I may increase that to 1,000. The board size will be within allowable dimensions at the fab I use.

I may try a 100mm x 100mm SMT version as a trial run.
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Old 16th January 2021, 07:13 PM   #165
woofertester is offline woofertester  United States
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Default Swiss Army knife PCB for buffer building

I have wanted one of these for a long time. This is a good reason to create one.
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File Type: jpg VeroBoard.jpg (693.4 KB, 445 views)
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Old 18th January 2021, 04:53 PM   #166
woofertester is offline woofertester  United States
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Here is the final Parallel Buffer Proto Board. Small-scale and universal. The intention is to try out a J113 B1 on this board.
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File Type: jpg BufferProtoBoard.jpg (746.7 KB, 389 views)
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Old 19th January 2021, 06:24 PM   #167
woofertester is offline woofertester  United States
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Default J176 JFET

J176 Vgs-Id curve and transconductance plot
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Old 20th January 2021, 11:51 AM   #168
woofertester is offline woofertester  United States
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Default J175 Vgs-Id

This is J175 Vgs-Id and transconductance curve.

This appears to be a good complement to the J113 for an original NP Beast.

The Idss is such that 50 buffers in parallel will get you 1A bias.
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File Type: jpg J175_transconductance.jpg (89.0 KB, 203 views)
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Old 20th January 2021, 12:00 PM   #169
vitorio is offline vitorio  Spain
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Then, can I use J113+J175 instead of 2sk170+j74?

Same resistors as original Beast?

Matching needed?



J113 and J175 are stock and cheap parts. Good!
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Old 20th January 2021, 02:51 PM   #170
woofertester is offline woofertester  United States
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Quote:
Originally Posted by vitorio View Post
Then, can I use J113+J175 instead of 2sk170+j74?

Same resistors as original Beast?

Matching needed?



J113 and J175 are stock and cheap parts. Good!
The pin-out for J113/J175 is different than K170/J74.

Idss of J113/J175 is 2x or more than K170/J74.

You do not need to match pairs. You can match the total groups. For example, you may need 50 pcs of J175 and 40 pcs of J113 to achieve minimum output offset voltage. That is why I am designing with drain resistors. You can remove a resistor to disable a JFET.

The output impedance of the amplifier will be larger than K170/J74 Beast.

You can sort the JFETs easily with a plug in proto board and a power supply and a DMM. That is what I do. I make a row of receptacles with labels 21mA, 22mA, 23 mA, etc.

You can make a spreadsheet with quantities of each Idss container. Then group them all for a close match of Idss for N and for P.

I allow the JFETs to warm up until they have a stable Idss reading. The temp coefficient makes the parts change Idss 10% to 20% from cold to hot.
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