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Old 30th December 2012, 03:20 PM   #341
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hi!

news from my preamp...

when I shorten the input, I get +140mV on the output.
turning the pot is giving me +-15mV (from +165 to +140mV)

- the fets are matched (both have 10mA) perfectly.
- negative rail has got -11.88V
- positive rail has got +12.2V

and that's only ONE of TWO failed channels.

I don't know WHY?!

pics will follow!

best regards,
matthias
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Old 30th December 2012, 04:09 PM   #342
juma is offline juma  Germany
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Check voltage drop across R4 an R5 - it should be equal (when resistor values and current through JFETs are equal). Voltage drop across R6 and R7 should be equal too.
Check your BJTs' hfe - there shouldn't be a big difference between them.
Also check all the connections, orientation of all transistors and resistor values - it's very simple circuit, there's not much that can go wrong...
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Old 30th December 2012, 04:14 PM   #343
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Quote:
Originally Posted by Marra View Post
Spencer supplies well matched devices and gives good service with tracking number in fact some arrived this morning.Only one week from dispatch to delivered.
i would also like to second/third all the good comments about spencer.

i got 20 or so jfets from him.
i had my fingers crossed, ive read about fakes allover.
not only where they not fake, but well matched!
ive got his fets in my Xono, a preamp for my dad, and im playing with this pre too and a simple jfet riaa from salas in a breadboard rightnow.
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Old 30th December 2012, 08:12 PM   #344
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Quote:
Originally Posted by juma View Post
Check voltage drop across R4 an R5 - it should be equal (when resistor values and current through JFETs are equal). Voltage drop across R6 and R7 should be equal too.
Check your BJTs' hfe - there shouldn't be a big difference between them.
Also check all the connections, orientation of all transistors and resistor values - it's very simple circuit, there's not much that can go wrong...
hi juma and thank you for the answer!

hmmm, the only thing I can imagine is, that I have a big error in my circuit?
Are there any other mistakes I could have done? I have no 10k-Pot in the Circuit yet, but that should be no problem, I think?

the resistors aren't matched, but have the correct values.
the BC550/560 aren't matched, too.
Have all the parts in the circuit been matched to work properly?

hfe is something I heard about, but don't know how to measure it exactly?
It's such a simple circuit, and I can't find the mistake I made...

I'll build it completely new, and try again.
Sometimes I think I'm a big big idiot...

Electronic is a strange thing... It's big part of my job, but I don't understand 90% of it...

If you have any other hints for me, feel free to write them down here!

Thank you!

Best Regards,
Matthias
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Old 31st December 2012, 07:36 AM   #345
mikvous is offline mikvous  Greece
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Is the input referenced to ground in any way?Just put 100k res from input to ground.It may oscillates.Hope this helps
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Old 31st December 2012, 08:48 AM   #346
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Hi everbody!

As a rule of thumb: BC550C has got the same pin configuration as BC560C!

I thought, I have to turn 560C 180 degrees?! Reading a datasheet isn't always a bad idea...

Now I can adjust it to 0mV +/-2mV.
But If I cut the connection to ground on the input, it drifts away (sometimes +560mV, sometimes -560mV, etc...)
Is that because of the open input without any load?

Best regards,
Matthias
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Old 31st December 2012, 09:13 AM   #347
juma is offline juma  Germany
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Gates of JFETs must have reference to ground (R3).
Standard working mode of the preamp is with input connected to signal source.
Using an input cap is optional (depends on your signal source - if it puts the DC voltage out or not).
Volume pot is needed to regulate the signal level.
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Old 31st December 2012, 09:20 AM   #348
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hi juma!

R3 is installed correct. So is it normal, what my preamp is doing?

Open Input brings +560mV to -560mV on the output.
I know that normally a device is connected to the input, but if it isn't, I could get massive DC on my speakers, because of no input cap in my F5.

Best regards,
Matthias
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Old 31st December 2012, 03:12 PM   #349
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Hello!

Now it works!

I built it completely new.
The only thing is a small hum while running the F5 with preamp.
But I think its because I use loooong wires without shield (those with small clamps on every end) for testing everything.

So I can start to assemble the rest of my preamp!

THANKS TO ALL HELPING HANDS HERE!!!

Best regards,
Matthias
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Old 6th January 2013, 06:14 AM   #350
jarkaa is offline jarkaa  Finland
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Hello,

Thanks for the tip on 2sj74. I contacted Spencer and ordered the real things.

While the fets are on their way I examined the possibility to get higher voltage swing for my purposes, since my current buffer can push over 100w inro 8ohms.

Here is a prelimiray schematic for pre with higher voltage swing capacity.

Part values are chosen according to what I have in my drawer at the moment.

What do you guys think?
Attached Images
File Type: png cascoded_juma_pre.png (13.4 KB, 339 views)
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