simple pre with offset problem

kp93300 · 19th March 2010, 05:33 AM

I build this circuit contributed by a fellow forum member.
I am unable to trim the dc offset to zero.
Both channels show 0.1 V dc output.
It sound very good with a output cap .
What is the solution?
Do i need to match the vgs ? can a 50R trimmer solve the problem and change the sound character?
Thanks

kp93300

Bonsai · 19th March 2010, 06:00 AM

1. You would benefit from matching
2. I dont think a 50 Ohm pot will change the sound character much
3. If you have a scope, make sure it s not oscillating.
4. Note that this circuit will b e susceptible to changes in load, or capacitive loading. Ideally you would want to buffer it.

DigitalJunkie · 19th March 2010, 06:08 AM

Trimming/matching the regulators might help.I've noticed they tend to vary a bit,and are rarely 12.00V exactly.

davidsrsb · 19th March 2010, 12:56 PM

This circuit should have OK supply rejection, so the regulators are not critical
The p and n channel jfets are not going to be well matched - that is your problem

Ipanema · 19th March 2010, 01:56 PM

Try increase the pot resistance e.g. 100ohms. Higher degeneration reduces mismatch between Pfet and nfet.

Good luck.

juma · 19th March 2010, 09:44 PM

Quote:

Originally Posted by kp93300

I build this circuit contributed by a fellow forum member.
I am unable to trim the dc offset to zero.
Both channels show 0.1 V dc output.
It sound very good with a output cap .
What is the solution?
Do i need to match the vgs ? can a 50R trimmer solve the problem and change the sound character?
Thanks

kp93300

You misslabeled JFETs - Drains and Sources are upside down on your schematic. Sources should be connected to 20R pot and Drains should be connected to power rails.
More than 10 forum member built this preamp and all report offset of couple of mV at most.

godfrey · 19th March 2010, 10:00 PM

Quote:

Originally Posted by juma

Drains and Sources are upside down on your schematic.

Perhaps they are connected upside down in the circuit too?
That might be causing the problem...

juma · 19th March 2010, 11:47 PM

Quote:

Originally Posted by Ipanema

Try increase the pot resistance e.g. 100ohms. Higher degeneration reduces mismatch between Pfet and nfet.
Good luck.

Pot value determines the Id through JFETs. Increasing its' value will lower the Id and that will ruin the preamp as it is.

Quote:

Originally Posted by godfrey

Perhaps they are connected upside down in the circuit too?
That might be causing the problem...

Off course. JFET has the property that Source and Drain are somewhat interchangeable - but not absolutely and not all JFETs. They will work that way, but poorly...

kp93300 · 20th March 2010, 01:53 AM

thanks very much for the pointers.
I will troubleshoot based on these lines.
The existing build sounds very good with an Bosch motor run 5 uf output cap.
I use a LDR pot in front and F5 after the pre .
kp93300

kp93300 · 20th March 2010, 02:02 AM

By the way, I also removed 1 UF C5 as there is no dc offset from the DAC befroe the pre.
Will this be an issue ?
kp93300

19th March 2010, 06:00 AM	#2
Bonsai diyAudio Member Join Date: Aug 2006 Location: Taipei, Taiwan	1. You would benefit from matching 2. I dont think a 50 Ohm pot will change the sound character much 3. If you have a scope, make sure it s not oscillating. 4. Note that this circuit will b e susceptible to changes in load, or capacitive loading. Ideally you would want to buffer it. __________________ bonsai http://www.hifisonix.com/

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19th March 2010, 06:08 AM	#3
DigitalJunkie diyAudio Member Join Date: Apr 2003 Location: Portland,Oregon Blog Entries: 3	Trimming/matching the regulators might help.I've noticed they tend to vary a bit,and are rarely 12.00V exactly.

19th March 2010, 12:56 PM	#4
davidsrsb diyAudio Member Join Date: Dec 2005 Location: Kuala Lumpur	This circuit should have OK supply rejection, so the regulators are not critical The p and n channel jfets are not going to be well matched - that is your problem

19th March 2010, 01:56 PM	#5
Ipanema diyAudio Member Join Date: Jun 2004 Location: KL	Try increase the pot resistance e.g. 100ohms. Higher degeneration reduces mismatch between Pfet and nfet. Good luck.

20th March 2010, 01:53 AM	#9
kp93300 diyAudio Member Join Date: Mar 2007	thanks very much for the pointers. I will troubleshoot based on these lines. The existing build sounds very good with an Bosch motor run 5 uf output cap. I use a LDR pot in front and F5 after the pre . kp93300

20th March 2010, 02:02 AM	#10
kp93300 diyAudio Member Join Date: Mar 2007	By the way, I also removed 1 UF C5 as there is no dc offset from the DAC befroe the pre. Will this be an issue ? kp93300

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