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
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
Attachments
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.
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.
Trimming/matching the regulators might help.I've noticed they tend to vary a bit,and are rarely 12.00V exactly.
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
The p and n channel jfets are not going to be well matched - that is your problem
Try increase the pot resistance e.g. 100ohms. Higher degeneration reduces mismatch between Pfet and nfet.
Good luck.
Good luck.
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.
Perhaps they are connected upside down in the circuit too?
That might be causing the problem... 😕
Pot value determines the Id through JFETs. Increasing its' value will lower the Id and that will ruin the preamp as it is.
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...
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
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
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
Will this be an issue ?
kp93300
I remove the 2sk 170 ,2sj74 and replace with matched set with IDSS of 10 and 10.05 ma.
I rewire the drain source orientation.
The input voltage is + 12.07 V and -12.09 V . The dc offset could be trimmed up to 0.1 to 0.14 V only for both channels ---no change.
BTW, i use LM 313 and 337 as regulators .
The pre runs very well without any undue heat in the regulator or the transisitor. I have left the thing on for 1 to hours without problem.
Do i need to match the BC 550 and 560 ?
I do not own a scope, how to check for oscillation as suggested in one of the previous post?
Thanks to all for the suggestions.
Will do further checking later.
Let me enjoy some music ! I am still very satisfied despite the dc offset.
kp93300
I rewire the drain source orientation.
The input voltage is + 12.07 V and -12.09 V . The dc offset could be trimmed up to 0.1 to 0.14 V only for both channels ---no change.
BTW, i use LM 313 and 337 as regulators .
The pre runs very well without any undue heat in the regulator or the transisitor. I have left the thing on for 1 to hours without problem.
Do i need to match the BC 550 and 560 ?
I do not own a scope, how to check for oscillation as suggested in one of the previous post?
Thanks to all for the suggestions.
Will do further checking later.
Let me enjoy some music ! I am still very satisfied despite the dc offset.
kp93300
kp93300, you are doing something the wrong way - please check everything carefully,it must be something with wiring or components' values.
BJTs don't have to be matched.
BJTs don't have to be matched.
Interchanging drain and source with 2SK170 and 2SJ74 is posible without problem.
Try to trimm R6, R7.
Try to trimm R6, R7.
Why is dc voltage of 0.1V a problem , is this stage going into a power amp.
Regards
AR
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