Driving transformers to play nice with audio interface line outs & line ins

[user 133392]

On the THAT1646 using the "ground" pin as a "Din-" balanced input is a bit risky owing to the fact that the 10 pF capacitor in the OutSmarts® network needs to be grounded. Any inductance to ground in that line - we're talking nH here - will make it oscillate around 17 MHz. The 1606 brings out the ground pin separately.

Another thing to note about the 1606/1646 is that the 5KΩ input resistors have ±20% absolute tolerance. People get the idea they can add series resistors to attenuate. Not only does that degrade noise and output balance but you can't depend on the value either.

The only time I've used the 1646 ground "input" as an input was to bias it to 1/2 supply for single-supply applications.

The additional inputs which can be driven are the Sense lines which is how I achieved common mode drive of the outputs for the 1646 transformer driver. Bob Katz just reviewed and earlier build of the 1646 used as a headphone amp. Dual Class-A Line and Headphone Output Board Documents - Page 13 - Pro Audio Design Forum

Brian's 1646 hybrid replacement looks to be perfect for the OP's need.

 

[ruffrecords]

The 1606 still needs a "0Ω" driving source impedance. Any external source impedance is in series with the internal input resistors and becomes part of the OutSmarts® bridge network degrading noise and signal balance.

I can understand how it might degrade output balance but not noise. The internal resistors are 5K and 10K respectively so adding a hundred or so ohms in series will have a negligible effect on noise.

A design that demands are zero ohm source is not Smart in my book.

Cheers

Ian

 

[user 133392]

I can understand how it might degrade output balance but not noise. The internal resistors are 5K and 10K respectively so adding a hundred or so ohms in series will have a negligible effect on noise. A design that demands are zero ohm source is not Smart in my book.

Cheers

Ian

"0Ω" meaning an op amp output.

Rsource upsets bridge balance as can be seen in the sim below. The 1646 and the 2142/DRV134 all have high common mode gain in order to provide their transformer-like behavior. It's not the Johnson noise of the resistor Ian it's the bridge balance upset that reduces the noise reduction of the common mode noise when source impedance is added. Things begin to head north above about 50Ω.

The DRV134 and SSM2142 also have a low driving impedance requirement.


Another THAT1646 "don't" is to take a single ended output from one side without grounding the other.

The reason for that is that the noise floor will be about -60 dBu if the opposing leg is left floating or <-100 dBu if grounded. This is also true of the SSM2132/DRV134.

 

[Script]

Brian's 1646 hybrid replacement looks to be perfect for the OP's need.

+1

 

[Cqwet Dbdfte]

Hand soldering small SMT parts is a blast, if you have a decent microscope with appropriate lighting, an assortment of solder tips, hot air blower, thinnest available roll solder (or paste), good tweezers requiring low force to hold parts, a can of paste or liquid flux, de-soldering braid, and you will be in good shape,
I'm not designing in anything smaller than 0402, unless as a desperate band-aid. Design your PCB's with 0603 parts or larger. Low noise resistors should be metal film, not thick film, availability is poor for thin film 0603, 0805 etc, MELF types could work, more choices. Clean up with a small hard brush, acetone, and thin paper towel. Isopropanol does not clean well. Some use aqueous wash and 24 hour dry-out in an oven. Nobody likes to look at flux residues even if harmless.
Hakko makes good soldering irons with long, very easy to exchange tips. Avoid lead free solder unless someone holds a gun to your head.
Rotate work piece as you add parts, not your hands, keep those stable with elbows on desk. A silicone work surface is good.
A PCB video camera and a monitor is easy to use once you get used to not having stereoscopic imaging.
Finding thru hole parts can be a challenge, more choices for SMT parts.