Low output impedance preamp needed.

Dxvideo · 12th April 2007, 11:14 AM

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

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;

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

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;

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.

12th April 2007, 11:14 AM	#1
Dxvideo diyAudio Member Join Date: Nov 2003 Location: Turkey	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. __________________ Best regards, Ozgur

13th April 2007, 07:18 AM	#4
Dxvideo diyAudio Member Join Date: Nov 2003 Location: Turkey	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. __________________ Best regards, Ozgur

13th April 2007, 07:21 AM	#5
Dxvideo diyAudio Member Join Date: Nov 2003 Location: Turkey	And for Andrew; Do you mean a circuit like that? __________________ Best regards, Ozgur

13th April 2007, 07:49 AM	#9
Dxvideo diyAudio Member Join Date: Nov 2003 Location: Turkey	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; Is that the same kind circuit? __________________ Best regards, Ozgur

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12th April 2007, 10:18 PM	#3
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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

13th April 2007, 07:44 AM	#6
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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.

13th April 2007, 07:47 AM	#8
juma diyAudio Member Join Date: Nov 2005 Location: Zemun	Sorry Andrew, you were quicker !

13th April 2007, 08:12 AM	#10
juma diyAudio Member Join Date: Nov 2005 Location: Zemun	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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