Hello there,
hope this is the right section (I thought maybe the IC amp area would also work).
I have built my first prototype on an double stage phono pre (double LT1007 for MC / OPA 2604 for MM and OP249 as 2nd stage), following an existing design from the German analog-forum.
I built the schematics in Eagle and created a PCB from it (so I know that the board matches the schematics).
Unfortunately, I just got a bug hum when I tested on MC, with a slight and very distorted idea of music.
So I checked everything with an oscilloscope, and while the MC and MM stage looks good, the 2nd Stage shows a very wired behaviour.
Even if the Jumper between the stages is open and all other ICs are removed, the OP249 got -12,6V DC on both output pins (1&7).
When I measure the socket, without the OP249, there is no voltage on the pins.
Power supply looks good (+14,8V on pin 8 and -14,8V on pin 4), and I can not measure any strange connection from any of the pins, all looks good.
Another thing that is strange: I pulled the input pins (3 & 5) to GND to ensure that I do not amplify any strange input.
While one output pin (7) stays at -12,6DC, the other one (1) went to +14,3V DC.
I already swapped the OP249 to another one, but it shows the same behaviour.
Any idea, where I should look?
Here is the schematics of the stage /don’t mind the wired dots, they are just for multi-raster for the foil caps):
Thank you already and with kind regards,
Felix
hope this is the right section (I thought maybe the IC amp area would also work).
I have built my first prototype on an double stage phono pre (double LT1007 for MC / OPA 2604 for MM and OP249 as 2nd stage), following an existing design from the German analog-forum.
I built the schematics in Eagle and created a PCB from it (so I know that the board matches the schematics).
Unfortunately, I just got a bug hum when I tested on MC, with a slight and very distorted idea of music.
So I checked everything with an oscilloscope, and while the MC and MM stage looks good, the 2nd Stage shows a very wired behaviour.
Even if the Jumper between the stages is open and all other ICs are removed, the OP249 got -12,6V DC on both output pins (1&7).
When I measure the socket, without the OP249, there is no voltage on the pins.
Power supply looks good (+14,8V on pin 8 and -14,8V on pin 4), and I can not measure any strange connection from any of the pins, all looks good.
Another thing that is strange: I pulled the input pins (3 & 5) to GND to ensure that I do not amplify any strange input.
While one output pin (7) stays at -12,6DC, the other one (1) went to +14,3V DC.
I already swapped the OP249 to another one, but it shows the same behaviour.
Any idea, where I should look?
Here is the schematics of the stage /don’t mind the wired dots, they are just for multi-raster for the foil caps):
Thank you already and with kind regards,
Felix
You have a Schmitt trigger there. 🙂
Do as Marcel mentioned, and also make sure you understand the operation of JP15/16. Those are problematic as well.
Dave.
Do as Marcel mentioned, and also make sure you understand the operation of JP15/16. Those are problematic as well.
Dave.
Hey there,
yes, you where right.
I checked the original schematics, and I missed the red traces:
They where easy to do with jumper wires, and now it works like it should!
Just need to test the MM stage now^^
Thanks and regards,
Felix
yes, you where right.
I checked the original schematics, and I missed the red traces:
They where easy to do with jumper wires, and now it works like it should!
Just need to test the MM stage now^^
Thanks and regards,
Felix