J Fet Buffer problem - help !

AndrewGM · 22nd October 2009, 07:33 PM

Hi

Just put together a cheap J fet buffer with regulated +/- 15 v supply.
All off E Bay from the usual suspects in the far east and less than £ 30 for the lot - including traffo - great !
Lashed it together in a hurry - placed it between CD player and Alps Black Beauty and hey presto....much better in bass, extra mid info and slam department.

I then read about putting it between the pre and power amp and found that in doing so the protection relay in the power amp clicked in and out.
No hum, music in between clicks from both speakers but could not find the problem.
Reversed it back to original config and it was fine again !!

My power amp has input impedance of 1V / 50 Kohms and the black beauty is 50 K.

Don't know whether this is relevant but if it is what value should I ideally have
please?

What have I done ?

Andrew

Steve Eddy · 22nd October 2009, 07:43 PM

Perhaps some DC offset on the output of the buffer?

Is the buffer cap coupled on the output?

se

wrenchone · 22nd October 2009, 07:44 PM

Did you put DC into the power amp input?

AndrewGM · 22nd October 2009, 08:21 PM

Truth is I really don't know.

If I have put DC on the input why does it work after the CD player and not after the pot ?

I think I've attached a pic of the buffer - you'll have seen it before
no doubt.

Can you tell me what to do ?

Hey, maybe there'll be no difference having it before or after the passive but I want to find out if poss.

Really appreciate the help

Andrew

grufti · 22nd October 2009, 08:42 PM

disconnect your buffer from all the other pieces of equipment - short the buffer inputs - power up your buffer - measure DC on your outputs - anything more than a couple of mV is no good.

AndrewGM · 22nd October 2009, 09:32 PM

Grufti, thanks.

I'll give it a try and see.
Any idea why it's OK after the CD ?
It really is good, much better than when passive.
Thanks to all who have thought about this for me.

Andrew

kannan_s · 22nd October 2009, 10:14 PM

Quote:

Originally Posted by AndrewGM

Grufti, thanks.

I'll give it a try and see.
Any idea why it's OK after the CD ?
It really is good, much better than when passive.
Thanks to all who have thought about this for me.

Andrew

Since it looks like a small circuit with few components - make the schematics ( or ask the Hong Kong seller for the schematics) - measure the DC voltages/post it here so- people can tell you what is wrong - if any?
it will also help you understand this in relation to many buffers
sk

22nd October 2009, 07:33 PM	#1
AndrewGM diyAudio Member Join Date: Jun 2008	J Fet Buffer problem - help ! Hi Just put together a cheap J fet buffer with regulated +/- 15 v supply. All off E Bay from the usual suspects in the far east and less than £ 30 for the lot - including traffo - great ! Lashed it together in a hurry - placed it between CD player and Alps Black Beauty and hey presto....much better in bass, extra mid info and slam department. I then read about putting it between the pre and power amp and found that in doing so the protection relay in the power amp clicked in and out. No hum, music in between clicks from both speakers but could not find the problem. Reversed it back to original config and it was fine again !! My power amp has input impedance of 1V / 50 Kohms and the black beauty is 50 K. Don't know whether this is relevant but if it is what value should I ideally have please? What have I done ? Andrew

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22nd October 2009, 07:43 PM	#2
Steve Eddy diyAudio Member Join Date: Sep 2002 Location: Sacramento, CA	Perhaps some DC offset on the output of the buffer? Is the buffer cap coupled on the output? se

22nd October 2009, 07:44 PM	#3
wrenchone diyAudio Member Join Date: Feb 2004 Location: Silicon Valley	Did you put DC into the power amp input?

22nd October 2009, 08:42 PM	#5
grufti diyAudio Member Join Date: Jun 2004 Location: Hillside	disconnect your buffer from all the other pieces of equipment - short the buffer inputs - power up your buffer - measure DC on your outputs - anything more than a couple of mV is no good.

22nd October 2009, 09:32 PM	#6
AndrewGM diyAudio Member Join Date: Jun 2008	Grufti, thanks. I'll give it a try and see. Any idea why it's OK after the CD ? It really is good, much better than when passive. Thanks to all who have thought about this for me. Andrew

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