Help finishing Albert Kreuzer's preamp!

Eugenio70 · 3rd January 2008, 10:32 AM

Hi. I need help finishing an Albert Kreuzer's Bass preamp (http://www.albertkreuzer.com/preamp.htm).

I just need to add the two source resistors for the JFETs, and now I see I really don't understand the procedure to select them! I'm using 2N5457 JFETs, thats whats available here. I don't have access to an oscilloscope but I saw he gave instructions to do it without one, but now I just don't get it.
I get the Vgs(off) for the JFET and calculate the drain voltage,then what? Should I put the gate directly at V+ and adjust the source resistor to obtain the calculated drain voltage?

Thanks!

Eugenio

Markus2006 · 7th January 2008, 09:26 AM

Eugenio,

This is very well explained on the page you listed (at the bottom). If you know Vgs(off), you put a potentiometer in place of R5 (or R10) and change its resistance until you get on the drain of the transistor half of the voltage between V+ rail and Vgs. Then replace the potentiometer with a resistor of similar resistance. Take a look at the example calculation on this page.
Let me know whether it works because I also want to build this preamp.

Marek

Eugenio70 · 7th January 2008, 01:43 PM

Ok, I went back and took a look at it and I think I finally got it. I was confused because I did not know what the gate signal should be when looking for halfway between v+ rail and Vgs(off), but we dont need a signal at the gate I think. I'l try it.

By the way, Albert Kreuser answered a while ago an e-mail I sent him and suggested playing it by ear. Put trimmers in it and adjust for best sounding setting and then replace with fixed resistors. I'l try this too to see if it changes.

Let you know how it turns out.

Thanks Marek for making me take a second (or fourth) look!

Eugenio

Eugenio70 · 23rd January 2008, 08:03 PM

Finally finished the pre-amp, worked great! A lot of range on the tone controls. Really recommend it. Thanks for all your help!

Eugenio

unclejam · 5th February 2008, 11:47 PM

Pics of the build???

im getting ready to embark on this build myself and would like a point of reference

Eugenio70 · 7th February 2008, 08:05 PM

I did take some pics but I don't have the camera here, I'l try to get it back this weekend and post them here.

unclejam · 11th February 2008, 03:03 AM

Quote:

Originally posted by Eugenio70
I did take some pics but I don't have the camera here, I'l try to get it back this weekend and post them here.

Alright thanks im looking forward to them

Eugenio70 · 3rd March 2008, 06:46 PM

Sorry for the low quality, I´m waiting to get my camera back were I have better quality images, and to get a photo of the finished pre-amp.

The power supply is on the left, then the Mid select, and the main PCB. I used a case from a junked satelite receiver, the faceplate is aluminum, painted and silk-screened. In fact, at least 95% of everything including parts, case, etc. is recicled from junked TV's, radios, etc. I only bought the Jfet's, two knobs and the transformer.

unclejam · 4th March 2008, 03:51 AM

cool

Brett · 4th March 2008, 10:31 AM

How do you like the sonics?

3rd January 2008, 10:32 AM	#1
Eugenio70 diyAudio Member Join Date: Dec 2007	Help finishing Albert Kreuzer's preamp! Hi. I need help finishing an Albert Kreuzer's Bass preamp (http://www.albertkreuzer.com/preamp.htm). I just need to add the two source resistors for the JFETs, and now I see I really don't understand the procedure to select them! I'm using 2N5457 JFETs, thats whats available here. I don't have access to an oscilloscope but I saw he gave instructions to do it without one, but now I just don't get it. I get the Vgs(off) for the JFET and calculate the drain voltage,then what? Should I put the gate directly at V+ and adjust the source resistor to obtain the calculated drain voltage? Thanks! Eugenio

7th January 2008, 01:43 PM	#3
Eugenio70 diyAudio Member Join Date: Dec 2007	Think I got it! Ok, I went back and took a look at it and I think I finally got it. I was confused because I did not know what the gate signal should be when looking for halfway between v+ rail and Vgs(off), but we dont need a signal at the gate I think. I'l try it. By the way, Albert Kreuser answered a while ago an e-mail I sent him and suggested playing it by ear. Put trimmers in it and adjust for best sounding setting and then replace with fixed resistors. I'l try this too to see if it changes. Let you know how it turns out. Thanks Marek for making me take a second (or fourth) look! Eugenio

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7th January 2008, 09:26 AM	#2
Markus2006 diyAudio Member Join Date: Jan 2006 Location: Poland	Eugenio, This is very well explained on the page you listed (at the bottom). If you know Vgs(off), you put a potentiometer in place of R5 (or R10) and change its resistance until you get on the drain of the transistor half of the voltage between V+ rail and Vgs. Then replace the potentiometer with a resistor of similar resistance. Take a look at the example calculation on this page. Let me know whether it works because I also want to build this preamp. Marek

23rd January 2008, 08:03 PM	#4
Eugenio70 diyAudio Member Join Date: Dec 2007	Finally finished the pre-amp, worked great! A lot of range on the tone controls. Really recommend it. Thanks for all your help! Eugenio

5th February 2008, 11:47 PM	#5
unclejam diyAudio Member Join Date: May 2007	Pics of the build??? im getting ready to embark on this build myself and would like a point of reference

7th February 2008, 08:05 PM	#6
Eugenio70 diyAudio Member Join Date: Dec 2007	I did take some pics but I don't have the camera here, I'l try to get it back this weekend and post them here.

4th March 2008, 03:51 AM	#9
unclejam diyAudio Member Join Date: May 2007	cool

4th March 2008, 10:31 AM	#10
Brett diyAudio Member Join Date: Jan 2002	How do you like the sonics?