DI idea looking for critiques. TI DRV134 Content.

[Bill Mountain]

I didn't want to further derail another thread so I started this one.

I'm trying to learn about building active DI circuits with balanced driver IC's.

The attached circuit is mostly for discussion purposes but I will be building something similar here in the near future.

What I'm wondering is if the DI section will work as designed.  I followed the DRV134 and THAT 1646 datasheets but wasn't sure about a few things.

I'm still learning Eagle too but I would build it on vero to confirm it worked before doing PCB's any way.

Am I missing anything major on the balanced output?  Is a ground lift necessary with these balanced driver chips?  The datasheets talk so much about noise cancellation but do not specifically mention ground loops so I wasn't sure.

The DRV134 says it adds 6dB of signal so maybe I should pad the mosfet down somewhat.

Thanks for looking!

[LiLFX]

The DRV134 calls for larger power supply bypass caps than your schem has, but that shouldn't be a show stopper. The biggest mistake I've seen in DIY boards that have differential audio pairs is those pairs not being routed together. The main premise of differential pairs is that the differential amplifier will cancel out the noise that is common on both sides. If the pair isn't routed tightly you can have noise show up on only one side which makes it's way through the amplifier. So when you build this on a PCB route the output traces tight to each other. When you build on vero twist your wires together.

One of the best demonstrations I've ever seen is to take an XLR connector and solder one end of an 18" piece of wire to pin 2 and the other end of the wire to pin 3. Plug that XLR connector into a mixer, mic pre, audio interface...whatever. Now take a degaussing coil and place it near that wire. You'll hear all sort of noise. Next twist that wire really tight and repeat the test. Dead silence. If you don't have a degaussing coil (I don't) you can use the untwisted wire as a noise sniffer around your electronics.

[anotherjim]

I noticed that Sansamp dual op-amp drive uses 10k output pull-downs. Also 100R in series to the XLR. Might be a bit more protection against accidental phantom power? Also, 10uF output caps should perhaps be non-polar, again with P48 in mind, rated 63v?
I can't see anything in the data for the DRV134 to suggest what happens if a high voltage pulse appears on the outputs (which could also occur if it has no power when the P48 arrives). I wonder if protection diodes should be added? That said, Sansamp don't fit diodes either.

[LiLFX]

I wouldn't worry so much about phantom power unless you are actually planning  on plugging this in to a mic pre. Yes, I realize that is what a DI is meant to do, but most interfaces and mixers nowadays have separate line inputs which this would driver beautifully. Couldn't you protect from phantom by using a pair of diodes with the anodes connected to the outs and the cathodes connected to the positive supply? That way anything higher than the supply voltage is kept off of the output?

[Bill Mountain]

Thanks gang.  I do intend to stick this on my pedal board.  I decided to forgo the boost design and just use an opamp buffer at the input.

My current idea is to do a buffered 1/4" isolated output for my amp and the tap the output of the buffer with the DRV134 for the balanced out to the mixer.

So it sounds like I need to look at some phantom power protection schemes just in case.

Thanks!