AMB Laboratories DIY Audio

[Shaman]

Hey all,

Regarding balanced use, I was checking out Ti's connection diagram..



I'm going to omit the pot in my application but I was also wondering:
Is it necessary to connect XLR's pin 1 to the input G pad of the modules?

I'd rather only connect Pin 1 to chassis.
Would that cause any trouble?

[amb]

Yes, you should connect pin 1 to the α20 input G pads. This is almost always necessary when you're using two ground-based amps to implement the hot and cold sides.

[Shaman]

Aren't they joined anyway at the common G of the output?

[amb]

Aren't they joined anyway at the common G of the output?

How would the pin 1s be connected to the common G at either the input or the output, unless you do so explicitly?

[Shaman]

I should have clarified this is for my "input buffer application".
The "output G's" of the A20s will be tied together but not on an XLR but on the input G pin of the power amp (which in turn has an electrical connection to chassis).

[amb]

OK, but the inputs are not being referenced to the source's ground (pin 1)...

[Shaman]

Could this cause a problem (aka destroy something)? The source is also balanced, if that helps.
If there is not such danger I can try it out and see how it goes.

I'm mostly trying to avoid multiple connections of G to the chassis. If I follow the recommended wiring I have "audio G" to chassis connections at the A20 inputs AND on the power amp's PCB.
To be safe perhaps I should first try wiring it the way you recommend and, if I have any issues, then try to disconnect pin1.

I'm also trying to have my gear (which is mostly balanced) follow the "recommended" XLR wiring which is very specific about Pin 1.
Most sources say Pin 1 should not be a part of the "audio" chain. It should be connected to chassis and only there.
Here is such a reference.

[amb]

Could this cause a problem (aka destroy something)? The source is also balanced, if that helps.
If there is not such danger I can try it out and see how it goes.

Without an input reference to ground (when your "balanced α20" is comprised of two unbalanced, ground-based α20 modules) it'd be akin to connecting an unbalanced source RCA cable partially (the center pin makes contact but not the ground). The result is a big hum.

I'm mostly trying to avoid multiple connections of G to the chassis. If I follow the recommended wiring I have "audio G" to chassis connections at the A20 inputs AND on the power amp's PCB.
To be safe perhaps I should first try wiring it the way you recommend and, if I have any issues, then try to disconnect pin1.

There only needs to be one place for chassis grounding. If your power transformer is external to the amp chassis, then connect pin 1 to the amp's case (But nowhere else). If your power transformer is also in the same chassis, then the AC ground from the IEC module should be connected to the chassis, and the input pin 1s should connect to the α20 signal grounds, but isolated from the chassis through a ground loop breaker.

Why do you need to connect the power amp PCB to the chassis?

I'm also trying to have my gear (which is mostly balanced) follow the "recommended" XLR wiring which is very specific about Pin 1.
Most sources say Pin 1 should not be a part of the "audio" chain. It should be connected to chassis and only there.
Here is such a reference.

The Rane notes apply to "balanced" electronics that are not comprised of two ground-referenced modules. The pro gear world typically either convert the balanced signal to unbalanced internally (using a balanced line receiver chip or a signal transformer), or more rarely, use a fully-differential design that isn't ground-based.

[Shaman]

There only needs to be one place for chassis grounding. If your power transformer is external to the amp chassis, then connect pin 1 to the amp's case (But nowhere else). If your power transformer is also in the same chassis, then the AC ground from the IEC module should be connected to the chassis, and the input pin 1s should connect to the α20 signal grounds, but isolated from the chassis through a ground loop breaker.

I'll try the latter, thanks Ti!

Why do you need to connect the power amp PCB to the chassis?

It's recommended in the application note of the amp module.
To be specific, one of the mounting holes (which I tested and also has an electrical connection to all the input signal/speaker out/aux supply "G"s) is supposed to be connected to chassis.
Perhaps I could use a ground loop breaker there?

Btw, it is an ICEPower Class D amp so SMPS and amp section are on the same PCB.

[amb]

The power amp wasn't designed with the α20s ahead of it. So you should isolate it from the chassis (use plastic standoff and screw on that "special" mounting hole). You can connect the ground loop breaker between signal ground and the chassis anywhere, but often I find it works best at the input.