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Old 23rd February 2006, 08:01 AM   #1
beppe61 is offline beppe61  Italy
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Default Sziklai pair - 1fet+1bjt.

Dear Sirs,

I am very intrigued about the topology called SZIKLAI PAIR for using it in a future line preamp.
I have found schematics of pairs made out of two bjts.
I wonder if it is possible to build a Sziklai pair out of a fet and a bjt.
I understand that with a fet at the input I could do without an input coupling cap.
I would maintain the output coupling cap to be safer (my current power amp does not have an input DC blocking cap).
I am pretty sure this very simple topology has the possibility to sound extremely nice.

Suggestion would be very much valuable, welcome and aprreciated here.

Kind regards,

beppe
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Old 23rd February 2006, 08:09 AM   #2
beppe61 is offline beppe61  Italy
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At the cost to turn out monotonous I am attaching an example of
sziklai pair.
I would like to replace the input bjt with a nice fet and so avoiding the input cap.

Thank you for your patience.
Kind regards,

beppe
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File Type: jpg line_preamp.jpg (56.8 KB, 971 views)
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Old 23rd February 2006, 08:21 AM   #3
lineup is offline lineup  Sweden
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you can replace input transistor with a JFET
but to remove input cap is not a good idea
not in this circuit

it is theoretically possible to use a JFET input with only a resistor connected to GND
because Jfet SOURCE (emitter) is at higher potential than GATE (base)

but to make this
you would need to build a different circuit
different resistor values and voltages

It is always good to have input cap ( high quality FILM Capacitor = MKT or MKP )
at a preamp input

Not all sources have Cap at output.
If you are sure you will only use source(s) with output Cap
you should remove input Cap.

But if you use a potentiometer before preamp, you have to use cap
for this single ended supply circuit.
Even if your CD-player have output Cap.
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Old 23rd February 2006, 08:24 AM   #4
beppe61 is offline beppe61  Italy
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Quote:
Originally posted by lineup
you can replace input transistor with a JFET
but to remove input cap is not a good idea
not in this circuit
it is theoretically possible to use a JFET input with only a resistor connected to GND because Jfet SOURCE (emitter) is at higher potential than GATE (base)
but to make this you would need to build a different circuit
different resistor values and voltages
It is always good to have input cap ( high quality FILM Capacitor = MKT or MKP ) at a preamp input
Not all sources have Cap at output.
If you are sure you will only use source(s) with output Cap
you should remove input Cap.
But if you use a potentiometer before preamp, you have to use cap for this single ended supply circuit.
Even if your CD-player have output Cap.
Dear Sir,

you have answered kindly and completely to my question.
I abandon the idea.
Thank you so much.

Kind regards,

beppe
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Old 23rd February 2006, 09:15 AM   #5
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Hi beppe,

However you can read this:
http://www.diyaudio.com/forums/showt...652#post423652

I worked on it (CFP) a few years ago, not yet finished, I got sometimes very, very good results, I say sometimes because this circuit is very prone to oscillations and I think that from time to time it oscillates, and then the sound becomes harsh.
Today, I do not really know if the circuit is worthwhile vs fet only, taking in account instabilities problems, maybe the circuit needs to be "best tuned", I have not much time for it but one of this days... I can send you the latest design, more detailed, in a few days if you are interested.
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Old 23rd February 2006, 09:31 AM   #6
beppe61 is offline beppe61  Italy
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Quote:
Originally posted by Justcallmedad
Hi beppe,
However you can read this:
http://www.diyaudio.com/forums/showt...652#post423652
I worked on it (CFP) a few years ago, not yet finished, I got sometimes very, very good results,
I say sometimes because this circuit is very prone to oscillations and I think that from time to time it oscillates, and then the sound becomes harsh.
Today, I do not really know if the circuit is worthwhile vs fet only, taking in account instabilities problems, maybe the circuit needs to be "best tuned",
I have not much time for it but one of this days...
I can send you the latest design, more detailed, in a few days if you are interested.
Dear Sir,
thank you so much for your extremely kind and valuable reply.
As I said in a very old thread the attached schematic is part of an old line preamp I had the opportunity to listen and appreciate very much indeed.
I would like to stick with a 2 active devices solution to build a line preamp and make all the efforts to set-up a really nice power supply.
The original voltage rails are 0-60V.
I did build a prototype with point-to-point wiring and all in all I found it very promising (really).
Much better than the expensive commercial preamp I normally use.
I intend to use the original BC107 and a TO126 pnp for practical reason of heatsink, soldering the device directly on their pins.
The schematic you are referring at are very much more complex.
I would like to keep it extremely simple and basic.

Thank you very much again.
Kind regards,

beppe
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Old 23rd February 2006, 10:03 AM   #7
MikeB is offline MikeB  Germany
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Hi beppe, it's impossible to remove inputcap in this circuit, no matter if you use fets or not. This is because of the single rail voltage supply, the input needs to be biased above 0v. Without inputcap you would short this bias through the volumepot or signalsource.

Mike
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Old 23rd February 2006, 10:13 AM   #8
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Quote:
Originally posted by beppe61

I would like to stick with a 2 active devices solution to build a line preamp and make all the efforts to set-up a really nice power supply...

I did build a prototype with point-to-point wiring and all in all I found it very promising (really)...

I would like to keep it extremely simple and basic...

Yes my schematic is for a power amp.

Why donít you base your preamp on a fet source follower / White follower / complementary follower ? 2 fets, 2 resistors and a good power supply (even batteries). Very easy, no caps, no offset correction, and very good!
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Old 23rd February 2006, 10:34 AM   #9
beppe61 is offline beppe61  Italy
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Quote:
Originally posted by Justcallmedad

Yes my schematic is for a power amp.
Why donít you base your preamp on a fet source follower / White follower / complementary follower ?
2 fets, 2 resistors and a good power supply (even batteries). Very easy, no caps, no offset correction, and very good!
Dear Mr. Perez,

thank you again.
Please could you explain me a little your proposal?
It sounds extremely interesting but I am afraid I have not understood very well.
I have to declare my almost complete ignorance in electronics.
The only schematic I have seen with only 2 fets are those nice buffers designed by E.Borberly.
Could you kindly provided me any link to an actual schematic?

Thank you immensely.
Kind regards,
beppe
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Old 23rd February 2006, 10:43 AM   #10
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beppe,

"The only schematic I have seen with only 2 fets are those nice buffers designed by E.Borberly."

Yes you can read on his site:

http://www.borbelyaudio.com/adobe/ae599bor.pdf

http://www.borbelyaudio.com/adobe/ae699bor.pdf

All is in Pages 18/19 (second part) of the Audio Electronics paper.
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