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-   -   Low output impedance preamp needed. (http://www.diyaudio.com/forums/solid-state/99916-low-output-impedance-preamp-needed.html)

Dxvideo 12th April 2007 11:14 AM

Low output impedance preamp needed.
 
Hi all,

I need a low output impedance (about 1K) preamp stage with no gain. I prefer JFETs to use and if possible with one transistor on each channel.

Does anyone know that kind of preamp schematic?

Thanks a lot in advance.

Ayne 12th April 2007 03:30 PM

2 Attachment(s)
JFET input .
No gain.
Low output impedence..

I think it's all u needed.

AndrewT 12th April 2007 10:18 PM

Hi,
a source follower will give you what your after. Well nearly, the Rs <<1k0 and gain may be <1.0
It will perform better if a CCS loads the tail.
Have a look at the various schematics here and in diyhifi.org

Dxvideo 13th April 2007 07:18 AM

Thank you Ayne.
But I am a bit confused about op-amps. Because I have tried TL072, NE5532, LM833 and LM4562 also.They all for specially audio applications. But I afraid I need a warmer sound. Because my amplifier is a lateral MOSFET Class A-AB one.
I like its sound too much but when I connect it a high output impedance device then I can hear some noise and I want to reduce that noise. This preamp is needed for that. And naturally I dont want to change its sound.

Dxvideo 13th April 2007 07:21 AM

And for Andrew;

http://www.diyhifi.org/forums/files/borb3_193.jpg

Do you mean a circuit like that?

AndrewT 13th April 2007 07:44 AM

No.
that's a dual complementary LTP with cascode and then a complementary follower (or is it a VAS?). =8fets, expensive in real estate and dollars.
You asked for single jFET transistor.
I suggested a follower (on it's own).

I think NUUK's site shows exactly what you are after. His update right at the end after the opamp options.

juma 13th April 2007 07:45 AM

2 Attachment(s)
No, Andrew meant a circuit like one of these :

juma 13th April 2007 07:47 AM

Sorry Andrew, you were quicker !

Dxvideo 13th April 2007 07:49 AM

Ok I see. I saw a lot of circuit like that. But somebody says
"that kind single ended circuits very prone to noise"
What do you say about that?
And also what do you say about that circuit;
http://www.q-audio.com/images/simplefet3.jpg
Is that the same kind circuit?

juma 13th April 2007 08:12 AM

Yes it is. The lower JFET acts as a constant current source (the source resistor should be of smaller value - something like 22 Ohms). But you said that you want a circuit with one JFET.
Source follovers are not noisy, but they perform better with good power supply.


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