Project files: THAT1646 in stereo…

What is it?
A stereo version of my THAT1646 balanced converter/preamp shown here. I wanted to build a small controller/pre for some active monitors and while the stacked mono boards were probably a good idea in princple, I decided to resurrect the stereo layout instead 🙂

How big are the boards?
The board measure 2.7″ x 1.9″ (app. 69 x 48 mm.).

What is the status of the boards?
There are two board versions which differ only slightly. One is 100% through-hole and basically a stereo version of the mono-board shown earlier. The other has the R4 gain resistor replaced with a 1206 SMD type and mounted on the top of the board (under the IC socket). This means the feedback loop area is much smaller and the routing is a bit neater. Both boards are otherwise the same size and electrically identical. If you want to change the gain after building the through-hole version is probably easier to work with, but otherwise the SMD-version should be the best design. Both boards are labelled as version 1.0 although I’ve only prototyped the SMD-version in stereo.

Does it use any special/expensive/hard-to-find parts?
As usual, not much. Maybe the THAT IC itself. Mouser has it though, so that should work for most people I guess.

Anything else I need to know?
A few things:

  • Gain: You can tweak the gain of the circuit as you wish using the resistors for the pampas, but remember that the THAT1646 should add 6dB gain on its own when you go from SE to BAL.
  • Opamp selection: You should be able to use pretty much any single opamp here. if you don’t have a favourite already I’d once again recommend that you start with either the OPA134 or the LME49710 and then experiment from there.
  • SMD resistor: If you are using the board version with the SMD gain resistor, remember to solder R4 on the board before you fit the IC sockets (otherwise some swearing may ensue when you discover it… :D)
  • BW limiting capacitors: There is no space on the board for BW-limiting capacitors for the opamp. Not sure why really, but with the opamp only driving a very short trace with a fixed load at the end (the THAT1646) I felt quite sure most opamps will behave. If not, soldering some small ceramics on the bottom of the board should be easy 🙂

Download design files here

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

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


4 Responses to Project files: THAT1646 in stereo…

  1. Daniel says:

    Hi! I recently built this to go from the pre-outs of my AVR to a power amp requiring balanced signals. I feed it from an 100VA torroid with a stablized PSU (+12V/-12V).
    I’ve got the gain resistors as 1K and 3.3K,

    As soon as I crank the volume up past -15 on my AVR, the output distorts badly. Tried turning the Power amps gain down, but it distorts just as badly at -15dB. So I guess my issue lies with the Balanced -> SE board, somewhere… I have a Linebox (ART cleanbox Pro) as well, there I can drive the power amp hard at the same roughly gain (measured using a dB meter) and way past -15dB without a single hint of distortion.

    Am I driving too much voltage into the OPamps causing them to clip the output fed to the THAT1646? All components on the board are as specified by you down to the opamps (LME49710).

    • theslowdiyer says:

      Hi Daniel, it’s a bit difficult to troubleshoot something like this from a distance, but let me try 🙂

      If changinging the power amp gain doesn’t change the situation, then I agree it’s the THAT-board that’s having issue. However, there is no telling how much voltage your AVR is spitting out at -15dB volume, so that could well be the problem.

      Your first step should be to reduce the gain of the THAT-board by bridging the 3k3 resistor with a 1k5 resistor. That reduces the gain from app. 4.3 to very close to 2. You can also disconnect the 3k3 and bridge the 1k with a piece of wire to turn the to opamp unity gain. If either of these tricks changes the overload margin then that is an indication that the board is being overloaded. The only other change you can do is to push the supply rails to +/-15V which should give a bit more headroom as well.

      • Daniel says:

        Thank you for an extremely helpful and quick reply!
        I know troubleshooting over distance is difficult at best, especially if it’s something you do in your free time.

        I have after some pain, sweat, swearing and grumping identified the culprit!
        A scope, multimeter and poking solved it, reducing the gain to unity showed the exact same symptoms.

        So i tried what I should have done from the beginning, I poked out the 1646 chip and replaced it with a spare I had on an unfinished prototype-board … Ye and behold, distortion gone! Dead 1646 chip, or at least defective… putting it back gave you instand head-ache!

        I can run the rails as high as needed, will the bigger voltage swing help in high gain applications? Torroid had tripple secondary windings and PSU is thankfully adjustable. (+/-12, +/-16 and +/-22V.) Figured +12V would be nice comfortable for everything that’s going to be connected to it. So now I have to order another THAT1646… They’re difficult to source!

        Thank a lot for your assistance and prompt reply!
        Also, really nice of you to make your designs available to the masses!

        Best regards from Sweden.


        • theslowdiyer says:

          Glad you got it fixed – defective IC’s are a real pain! Normally you don’t add any more gain than necessary, because you have to throw it away again which generally introduces noise. However, if everything works then I wouldn’t worry too much about it 🙂

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