XA.8 single-ended current sources - diyAudio
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Old 29th October 2014, 09:10 PM   #1
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Default XA.8 single-ended current sources

The XA.8 has a new single-ended current source. See the pictures of the XA30.5 and the XA30.8. On the picture of the XA30.8 I see a Cap and JFET or transistor.

XA30.5.jpg
XA30.8.jpg

It looks like the resistors have been replaced with a JFET current source, see image 15 of the following link:
https://www.passdiy.com/project/amplifiers/diy-op-amps

I somehow miss the clue how this works. Is there a Pass Labs article that describes the resistors vs JFET current source?
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Old 29th October 2014, 10:21 PM   #2
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Not really, but you can pretty much get the idea of how a current source
works. A power resistor is a current source of sorts, and we have used
that sort of approach in some earlier products, but we wanted to step up
the amount of single-ended bias used, so in the Xs and .8 products you
will see constant current sources instead, giving more current at higher
impedance.

We have been doing that sort of thing in one form or another since the
Aleph 0 in 1992, and you can read some about it in the article "Leaving
Class A" on the First Watt website.

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Old 31st October 2014, 03:02 PM   #3
hm4nine is offline hm4nine  Europe
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Besides Nelson's comment I cannot believe to get enough current with a TO92-device to make an effective CCS.
Resistors in their earlier passive CCS were at least 3W-types.
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Old 31st October 2014, 03:42 PM   #4
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Is there anybody that has the Aleph 0 service manual and that is willing to send it to me? The links to the Aleph 0 documents on this forum don't work anymore.
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Old 31st October 2014, 03:43 PM   #5
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Quote:
Originally Posted by hm4nine View Post
Besides Nelson's comment I cannot believe to get enough current with a TO92-device to make an effective CCS.
Resistors in their earlier passive CCS were at least 3W-types.
TO92 can govern bigger critter

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Last edited by Zen Mod; 31st October 2014 at 03:48 PM.
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Old 31st October 2014, 03:47 PM   #6
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Quote:
Originally Posted by MEnsing View Post
Is there anybody that has the Aleph 0 service manual and that is willing to send it to me? The links to the Aleph 0 documents on this forum don't work anymore.
shoot me at >sasica5@gmail.com<
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Old 31st October 2014, 03:49 PM   #7
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Quote:
Originally Posted by hm4nine View Post
Besides Nelson's comment I cannot believe to get enough current with a TO92-device to make an effective CCS.
Resistors in their earlier passive CCS were at least 3W-types.
100% agree with the comment of hm4nine!!!
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Old 31st October 2014, 04:10 PM   #8
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According to the article 'leaving class A' and my calculations the single ended bias is about 100mA. Should not be a problem for a to-92.
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Old 31st October 2014, 05:21 PM   #9
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Quote:
Originally Posted by MEnsing View Post
According to the article 'leaving class A' and my calculations the single ended bias is about 100mA. Should not be a problem for a to-92.
you want to review your calculations
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Old 31st October 2014, 05:46 PM   #10
hm4nine is offline hm4nine  Europe
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If you assume 2x25VDC for supply, you will have 2,5W on a TO92 for 100mA.
You will have SE only a few moments....
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