Step 40 and 41

[Damster]

I can't get below 6.7V on step 40 and 5.3V on step 41. U1 is considerably hotter than U3 as well.

The pre sounds clean like it is working however there is a very large low frequency signal when I plug the pres into my apogee Rosetta 800's. The Low Frequency is about at 6Hz and is almost up to 0db in my Daw. Also the output pot has a sort of zipper noise that is primarily low frequency.

Any Thoughts? Thank you.

[tpryan]

I can't get below 6.7V on step 40 and 5.3V on step 41.

Something is wildly wrong. You should be able to easily adjust these voltages to below 10mV.

U1 is considerably hotter than U3 as well

That's normal.

[Damster]

Do you make those adjustments clockwise or counter-clockwise? I went counter clockwise until I heard clicking on every full turn.

Any Ideas on the low frequency thing or the output pot noise?

[tpryan]

Do you make those adjustments clockwise or counter-clockwise?

You turn whichever direction makes the voltage go down. It could be either direction.

Any Ideas on the low frequency thing or the output pot noise?

These are both caused by the huge DC offset, which will disappear when you make the correct adjustments at R36 and R33.

[Damster]

7V for R36 and 5.2V for R33 is the lowest I can get them.These Values Don't Change when I Turn SW1 to any other position. Could it possibly be a bad part or something?
All components seem as if they are working at a good temperature ( not too hot to touch ).The pre works and gives clear audio other than the 6Hz Offset noise. Yeowzaaa! Any thoughts?

[tpryan]

Are you absolutely certain that R17 and R18 are both 1K00? Can you post a photo?

[Damster]

Fixed. Tim, you are amazing! I had 100 ohm resistors at R17 and R19 and a 10K7 at R18! Don't know how I got so far off course.

I have now completed 2 c84's, 1 N72 and 1 A12 successfully! I am going for the SCA Noah's ark kind of thing ( a pair of each pre ) so I have 2 J99's and 1 more N72 and A12 to go. Thank you so much for creating this system! Truly a DIY pioneer.

[ckeene]

I just built two C84s. One works fine, and the other one is exhibiting a similar problem to Damster. On step 40, I can only get the voltage down to around a volt-and-a-half.

I checked the resistors all around the op amp and they look fine. Any other diagnostics I should try?

[ckeene]

I just built two C84s. One works fine, and the other one is exhibiting a similar problem to Damster. On step 40, I can only get the voltage down to around a volt-and-a-half.

I checked the resistors all around the op amp and they look fine. Any other diagnostics I should try?

*bump*

[tpryan]

If all the resistors are in the right place and all of your solder joints are good and you still can't adjust the offset to 0v I don't have any more suggestions. Q1 or Q2 is probably damaged.