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Old 4th February 2013, 11:07 AM   #1181
robertor is offline robertor  Hungary
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Jfet BOZ
this BoZ by Jims-Audio does not want to work.
powered by a 20 volt transformer, on the control points of the voltage I find 8.5 volts , and not 16v as shown on the PCB.
the Zener diode (D2 D3 D4 D5) are 3.3v.
with the multimeter between D and S there are 18.7 volts, between G and D there are 15.2 volts.
where there is the error? Thanks in advance for any help.
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Old 4th February 2013, 11:49 AM   #1182
AudioSan is offline AudioSan  Norway
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you will never get 16V With 4x3.3V zeners. but 8.5V is a little low.
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Old 4th February 2013, 12:01 PM   #1183
cjkpkg is offline cjkpkg  United States
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I have the same board however I powered it using a Dell 19V PSU. I eliminated the rectifier and used 3 6.2V zeners IIRC I remember having to string them unconventionally per the board skipping a few spaces.

All other PSU filtering remains on the board and my voltages are spot on.

This is a fine little preamp.
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Old 4th February 2013, 12:02 PM   #1184
robertor is offline robertor  Hungary
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Jfet BOZ
hi Audiosan, thanks from answer.
So how can I change the situation?
I use only one zener diode?

p.s.: the BOM of Jims-Audio suggested that value.
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Old 4th February 2013, 12:38 PM   #1185
cjkpkg is offline cjkpkg  United States
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Each zener passes that much voltage...divide you finished voltage by the value of your zeners to know how many to use.

I would shoot for 18-19V so I would use all 5 locations D2-D6 with the 3.3v zeners. This gets you a raw 16.3V so if you are dropping down to 8 you need to figure out why. Or get some 4V or 5V zeners and use less.

Doublecheck your zener orientation and your fitering "R" R3-R6 and see how much voltage is dropping over each if any.
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Old 4th February 2013, 04:31 PM   #1186
AudioSan is offline AudioSan  Norway
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Quote:
Originally Posted by robertor View Post
hi Audiosan, thanks from answer.
So how can I change the situation?
I use only one zener diode?

p.s.: the BOM of Jims-Audio suggested that value.
the BOM say 4*4V zeners. at least mine do.
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Old 4th February 2013, 05:02 PM   #1187
ramallo is offline ramallo  Europe
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Jfet BOZ
Quote:
Originally Posted by AudioSan View Post
the BOM say 4*4V zeners. at least mine do.

In this thread you have several modifications of this board and BOM

Jfet BOZ
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Old 4th February 2013, 05:27 PM   #1188
AudioSan is offline AudioSan  Norway
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i'll read some more of to night i'm going to build this one of this days as wel.
just to try it i have the board somewhere
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Old 4th February 2013, 05:55 PM   #1189
tinitus is offline tinitus  Europe
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Jfet BOZ
still working on it
jfets not mounted
actually there are 3 channels
one channel in, split into 2 channels
on board pot is just a channel balance
maybe it could have two functions, and also do tone control work
but I haven't figured that out yet
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Old 5th February 2013, 08:16 AM   #1190
robertor is offline robertor  Hungary
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Jfet BOZ
hi ramallo.
the diagram I find on page 104 of the thread you recommend, is the correct schema with the TL431 as the voltage regulator?
I'd try to build on proto board and having a transformer with 2 secondary to 18v, build it as a pseudo-mono so I can finally trashing this Chinese PCB.
with 18v the value of the resistors (resistor R21 e pot-trimmer R22) on the TL431 will be the same?
thanks for your patience.
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