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A question for the experts

post #1 of 7
9/2/11 at 9:29am
Thread Starter 

I'm building a desktop amp.

Power will be a center tap transformer.

The power supply is a dual rail 317/337 in the standard arrangement

with filter caps that go from each rail to the center tap of the transformer.

The ground return from the headphone jack also goes directly to the

center tap of the transformer.

Now the question is, since the amp board has only a ground reference for the

input and feedback resistors, should the local filter caps go from rail to rail or

rail to ground? Which would be more effective?

 

Here is a picture of the power supply for clarity.

 

Power Supply Dual.jpg

 

Thanks

 

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post #2 of 7
9/2/11 at 9:56am

for TRS common gnd I think the "star" point should be the TRS gnd - signal gnd can be a gnd plane but I would put power, ps gnd return close to the output devices, return their local supply caps to the pwr gnd as near the TRS gnd as possible

 

the local HF bypass should be close to the chip's ps pins to the plane since it is low inductance

 

drawing shows amp output star - ignore the active splitter since you have a dual supply

 

star.jpg

 

 

 

 

another opion is Bridged output with no gnd return from the "balanced" wired headphone


Edited by jcx - 9/2/11 at 9:59am
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post #3 of 7
9/2/11 at 9:58am

Connect your local bypass caps from rail to ground.

 

se

 

 

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post #4 of 7
9/2/11 at 10:02am
Thread Starter 

Thanks for the reply JCX, but that is the topology I am using and that wasn't really the question...

 

The "Star" point will be at the center tap of the transformer, where impedance is lowest.

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post #5 of 7
9/2/11 at 10:17am
Thread Starter 

Here is the transformer.

 

x-former.jpg

 

The power supply ground, amplifier board ground and TRS jack ground

will all return to the center tap.

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post #6 of 7
9/2/11 at 10:18am
Thread Starter 

Thanks Steve, thats one vote for ground plane board and bypass caps to the ground plane.

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post #7 of 7
9/2/11 at 10:52am



 

Quote:
Originally Posted by Avro_Arrow View Post...

The "Star" point will be at the center tap of the transformer, where impedance is lowest.

 

 

the "lowest impedance" heuristic isn't helpfull in this instance - you want a "hierarchical" gnd - the run from the TRS gnd "Star" to the xfmr CT is a "dirty gnd" - the xmfr secondary happily floats the amount of the Vdrop in the wire from the CT to amp TRS gnd

 

with two separate amp boards and a panel mount TRS their is some unavoidable compromise with one good solution being differential amps in a "gnd breaker" configuration sensing/subtracing out the TRS gnd "gnd contamination"

 

of course at most headphone's current levels you can just use heavy wire and never noitice small differences from "best practice"

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