Project files: A bal. driver with the THAT1646

Still busy at work, but being home for (most of) the weekend and I have time to dig in the back catalogue a bit – hope this is useful for someone 🙂

What is it?
A simple SE/BAL line driver board using the THAT1646 line driver IC. This should make it compatible with the DRV134 from TI and the – now obsolete – SSM2142 from AD as well. The board can be used as a pure line driver to feed a balanced input or of course also to bridge two suitable power amps.
The THAT-chip is combined with an on-board opamp, partly to ensure that it is driven by a low-impedance source as per the datasheet recommendation, partly to increase the versatility as the opamp can provide more gain if required.

How big are the boards?
The board measures just 1.6” x 1.9” (app. 41 x 48 mm.) The boards can be placed side-by-side or stacked. I originally had this as a stereo board with two channels on the same board, but decided that the mono-version was probably more versatile overall. If you disagree feel free to let me know 😀

What is the status of the boards?
The board is v1.0. I’ve built a single prototype and tested it (I needed one channel for a test setup) and it sounds fine as far as I can tell. No further sound impressions yet I am afraid.

Does it use any special/expensive/hard-to-find parts?
None. I am not actually aware of a source for the THAT-chip in Europe, but Mouser has them and that should work for most people I guess 🙂

Anything else I need to know?
A few things:

  • Protection circuitry: I’ve omitted the protection circuitry described in the data sheet for the THAT1646, mainly because I only expect to use it in home applications where there is no risk of a phantom power supply being present. If you are using it with PA gear that (potentially) has a phantom power supply on the inputs then you might need to look into this.
  • Grounding: I have connected the ground pin of the output connector on the PCB to GND on the board, which is actually a no-no. Connecting all three pins to the XLR would (potentially) give you “pin 1-problem”.
    As I understand it, the proper way of wiring an XLR is therefore to only connect “hot” and “cold” from the PCB connector to the XLR out connector and then connect  PIN 1 on the XLR connector to the chassis ground via as short a wire as possible.
  • Preamp-mode: Given that the THAT1646 already has an opamp onboard to drive it, if you use 100k-220k input impedance (R1) it should be possible to put a 10k-20k log pot in front of the input capacitor and convert the board (well, two of them…) to a stand-alone preamp with SE in and Balanced out. I haven’t tested this, but I see a couple of potential applications here 🙂
  • Chip substitution: The DRV134 has the same pin connections as the THAT1646. The only thing I can see that makes them different is that the DRV134 data sheet specifies 1uF decoupling caps on the supply pins rather than the 100nF for the THAT1646.
    Note that the DRV134 also has a reputation (at least in some DIY-circles) for sounding pretty unspectacular. I have no personal experience to offer here, so try for yourself if you want 😀

Downloads:
Download design files here

Edit 14th april 2016: Link has been updated to point to the correct file 🙂

Related information:
As usual, RTFD! (= read the f’ing datasheets :D)

Note: Always read the “intro post” for additional important information about my designs.

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3 Responses to Project files: A bal. driver with the THAT1646

  1. Evenslowerdiyer says:

    Hi Slowdiyer,

    when trying to download the design files for the balanced line driver from the link above, I get design files for your GP-PSUs v2 (gp-psuv2_v1-0.zip). Could you have a look into that?

    Thanks a lot for documenting all your projects in great detail.

    Best wishes,
    Evenslowerdiyer

  2. Pingback: Project files: THAT1646 in stereo… | theslowdiyer

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