Mic pre amp pcb layout and symmetry of differential op amp - Page 1
Forum section: electronics / beginners

author: jvphotog
author_rank: Contributor
timestamp: June 13, 2025, 04:35:39 pm
content: Hello all! This is my first time attempting to design a PCB, so please bear with me! I am trying to make a mic and guitar preamp, among a few other things, that will mate up with a codec board that someone has designed for a Teensy. The designer of the codec board has been very generous in helping me get this far, but Id like to stop bothering him so much.I have a DG442 switch that bypasses input resistors to change the gain on a differential op amp (OPA1632). The other side of that DG442 changes the gain on an OPA1656 - for guitar. I didn't realize until I started laying out the PCB that the OPA1632 needed to be symmetrical, with the same length traces. This seems impossible with how I have things set... So i guess my question is, how important is the symmetry in the input and output stages of a OPA1632? Can I add some bends or length to the traces so they measure the same length? Should I look for single channel switches that will live on each input of the mic pre amp?The output of the OPA1632 also needs to make its way back close to the xlr combo jack, to get into the codec board. I can move the 1/4" jack over if this should be a more direct route. Ive only started to lay out the components on the PCB, so please excuse the mess, this wasn't my final layout. Thank you!!

author: PGPG
author_rank: Frequent Contributor
timestamp: June 13, 2025, 07:21:16 pm
content: My 3 cents.It is too hard for me to reorganize such schematic in my mind to imagine each OpAmp as separate element with natural flow of signal from left to right.

author: jvphotog
author_rank: Contributor
timestamp: June 18, 2025, 08:33:46 pm
content: Sorry PGPG. I guess that does look pretty messy.Would anyone be willing to look over my PCB before I have it made? Thank you!

author: R-802
author_rank: Contributor
timestamp: June 24, 2025, 05:10:37 am
content: Don't know enough to offer any significant help, but yeah it's just a bit difficult to read without proper opamp symbols and grounds pointing upwards. I'm actually trying to design something quite similar to this also using the OPA1656.Quote from: jvphotog on June 13, 2025, 04:35:39 pmYes you can do this by adding meanders (serpentine patterns) to the differential pairs, I'm not too sure exactly how to do it in KiCad but it's called the Length Tuning tool. Do some research on that, because I'm pretty sure you don't need to worry about that at all for this kind of low frequency stuff.

author: jvphotog
author_rank: Contributor
timestamp: August 01, 2025, 09:36:54 pm
content: Thank you for the reply. I had the board made and it all worked fine. The OPA1656 sounds good and works well.I was also told that the length didn't matter too much at these audio frequencies, so I just made them as close as I could. I also didn't realize some of the rules/guidelines that drawing schematics follow. I'll work on that for the next one.Thanks again and good luck with your project!

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: August 02, 2025, 08:50:16 am
content: Hi,you got the pins of the XLR wrong.1= shield/Ground2= In-phase3= Out-of-phaseGood luck