That 1646 balanced line driver PCB

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Balanced impedance connections require extreme balance of impedance to work as intended.
That includes the capacitances as well as the resistances.
0.1% for the resistances is an absolute minimum. 1% is no good for interference rejection: unbalanced with good attenuation measures can better that.

If you can match to better than 0.02%, then do that.
Capacitance becomes an issue at higher frequencies.
I think you can get away with 0.5% matching in the capacitors.
The traces and wires also need attention to minimise the effects of differences.
Worth reading B.Putzeys' article in EDN,
It is linked in this Forum. Linear Audio charge for that article.
 
I made a pcb design for the That 1646 balanced line driver. I posted it on the dutch Zelfbouwaudio forum and Kostas alias TheShaman found it and contacted me if I had some spare pcb's.

I will post here some pictures, the schematic and gerber files so everybody can order the pcb's at for example Itead.
Is anything needed to condition the inputs? I don't think so, but I thought it worth asking. You never know what someone might plug in, although the 1646 should not draw any current via its input.

The example schematics from THAT Corp do not show any input conditioning, resistive or otherwise. I just wanted to confirm.
 
Is anything needed to condition the inputs? I don't think so, but I thought it worth asking. You never know what someone might plug in, although the 1646 should not draw any current via its input.

The example schematics from THAT Corp do not show any input conditioning, resistive or otherwise. I just wanted to confirm.
Further research tells me that there are a few reasons for input conditioning:

1) THAT 1600 data sheet, page 6, says "devices must be driven from a low-impedance source, preferably directly from opamp outputs, to maintain the specified performance." So, if you want to connect this board to a variety of sources, you might want to add a unity buffer with really low-impedance output. Cost of the added stage can be anywhere from $0.20 to $22.35, depending upon the quality desired.

2) If the input will ever be disconnected while powered, then the floating pin could cause the THAT 1600 output to hit the supply rails. To protect against this, 100 kΩ to ground would be helpful. However, if the input is always connected to another circuit, then you can just move the 100 kΩ further ahead in the signal chain to where your input jacks might be.

3) The input impedance of the THAT 1600 chips is 5 kΩ. This may not be problematic in all situations, but some audio sources might distort or otherwise perform poorly with a 5 kΩ load. In these cases, the unity buffer mentioned above could solve two problems with one fix.
 
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