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Old 22nd June 2001, 12:52 PM   #1
diyAudio Member
Join Date: Jun 2001
Location: Singapore
Hi everyone! This is my first post to this forum.

I'm a big Nelson Pass fan. I have so far completed a number of Bride of Zen and Bal. Zen Line Stage Preamp. They all sounded wonderful. But the best of all, I must say, is the one similar to Aleph 1.7 but with +/- 60 Volt and only constant current source on the tail side. Wow!

I have recently completed the Aleph 60 circuit, but couldn't get it to work. I have pretty much follow the parts specified except the input pair. (IRF9620 instead of 9610 and IRF240 instead of IRF244)

The following reading is what I got:

a) 5.5V across R11
b) 3.1V across R14
c) no reading across Q5 (Changed a new ZTX450, Still no reading)
d) no reading on the output arrays.

Method Tried :

1) Lowering R11 to increase the bias current. But the reading across R14 stay the same.
2) Increase/decease the value of R19
3) Decreasing the value of R21

I'm at a lost here. I know that the value of R17,R19 & R21 is rather critical. Any suggestion as to what value to try? The output Mosfet are in good working order, I've checked that.

By the way, I'm on the other side of the time zone. So Immediate respond is not possible. Any Help is much appreciated.
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Old 22nd June 2001, 10:17 PM   #2
The one and only
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Join Date: Mar 2001
Sounds like the front end is biased up properly
but the positive current source isn't being driven.

Q5 voltage = 0, so what's the voltage across R17 and
R18? You should be seeing something resembling
15 volts across these.
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Old 22nd June 2001, 10:26 PM   #3
Jens is offline Jens  Germany
diyAudio Member
Join Date: Feb 2001

Did you checked the correct placement (C/B/E) of Q4+5? The pins are E,B,C (viewing the flat side from left to right).

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Old 23rd June 2001, 04:01 PM   #4
diyAudio Member
Join Date: Jun 2001
Location: Singapore
Thanks for the responds. Especially from the Master!

There is 16 volts across R17 but couldn't recall what is across R18. I have taken them apart to test the output stage.

And yes Jens, the placement of Q4+5 should be correct as Q4 is working.

Meanwhile I'm taking a weekend brake with my family (It's Saturday here in Singapore). Will be back on Monday to report any new finding….

I will also try to revive my home page and put up the projects to share with everyone.

And thanks again Mr. Pass for sharing and being so supportive of us DIYers.
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Old 24th June 2001, 11:53 PM   #5
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Join Date: Feb 2001
Location: Columbia, SC
What voltage are you reading across Q5's base and emitter? (I.e. what are you reading across the current sensing resistor?)

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Old 25th June 2001, 11:53 AM   #6
diyAudio Member
Join Date: Jun 2001
Location: Singapore
Ok, here we go. The Readings are:

Supplied Voltage = +/- 38V
R11 - 5.5V
R14 - 3.1V
R17 - 21.2V
R18 - 16.80V
R19 - 16.14V
R16 - 0V
Q5 Vce - 0V
Q5 Vbe - 0.69V
Q4 Vce - 3.1V
Q4 Vbe - 0V

Correct me if I'm wrong, but I think the current source circuit is working. Instead voltage across R14 is only 3.1V. Thus Q12-17(average turn on Vgs is about 3.6) are not on.
What is causing this? Is it wise to lower the value of R11 thus increasing tail current and Voltage across R14? Do I need to alter R19 to balance out voltage across R17 & R 18?

Best Regards

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Old 25th June 2001, 05:28 PM   #7
The one and only
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Join Date: Mar 2001
I guess I'm assuming that the output voltage is
near zero, which I don't think you have specified.

I still don't think the current source, driven by Q5
is working. What is the voltage across R40 - R45?
Probably 0.

I suspect that the collector of Q5 is somehow been
shorted to the output, possibly a mis-wire.
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Old 27th June 2001, 04:02 PM   #8
diyAudio Member
Join Date: Jun 2001
Location: Singapore
After spending two nights staring at the circuit and doing troubleshooting. The bugs are finally resolved.

Thanks for the helps Jens, Grey and Mr. Pass.

Hope to share the results with you soon……
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