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Old 30th September 2014, 05:27 PM   #1
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Default Which one of these wiring schemes will have lower noise?

Hi everyone, I am trying my best to understand ground loops and send and return paths to achieve the lowest noise possible but I feel like I am retarded sometimes because I still don't fully understand. Is one of these figures better than the other or are they both wrong?

Also, I am planning on using 18 gauge wire for all of the internal connections shown. Is this too heavy of a gauge because it will have too much capacitance? My project box is 16"(41cm) x 8"(20cm) to give you an idea of the lengths of wire for connecting the PCBs.
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Old 30th September 2014, 05:36 PM   #2
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I'd put the rings/shafts of the input jacks to ground near the power supply of the op amps or transistors receiving the signal. there should be some sort of common mode rejection mechanism there to reduce hum. then let the power supply of the preamp board go to the star ground.
Put a steel bulkhead around the power transformer and keep the 120 VAc stuff entirely out of the case of the preamp . Like a dynaco PAS2 is two boxes, a 120 vac box and a signal box, all inside one case. I spent months chasing hum in a badly packaged mixer, finally going to a wall transformer outside the case, and no AC switch.
If you've got a turntable, the star ground point is the headshell ground terminal on the back of the case. The one for the green wire from the turntable.
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Old 30th September 2014, 05:43 PM   #3
jcx is online now jcx  United States
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transformer gnd? - if you mean secondary Center Tap - that should just go to the PS reservoir Caps gnd junction


busy sim but maybe helpful

left side sims xfmr secondary - see that I put the gnd on the other side of the reservoir C junction

Quote:
Originally Posted by jcx View Post
yes a "star", by itself, is exactly a "too simple" idea

hierarchical grounding, "dirty"/"clean" branch separation are important ideas too

and like DF says - CT to reservoir cap junction is "dirty" - has rectified charging current pulsing in the wire, creating nasty nonlinear Vdrop in its R,L

the mains xfmr secondary is floating, the isolation should be taken advantage of by letting it be an "open" end of the dirty current branch

Click the image to open in full size.

at the very least the location of the "star" point can be chosen for better or worse performance - and worse would be the xfmr CT
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Old 30th September 2014, 07:54 PM   #4
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Originally Posted by indianajo View Post
I'd put the rings/shafts of the input jacks to ground near the power supply of the op amps or transistors receiving the signal. there should be some sort of common mode rejection mechanism there to reduce hum.
You mean common mode rejection from the op amps on the preamp board and that they won't be effective if the in's ground goes straight to star?
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Old 30th September 2014, 07:59 PM   #5
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Originally Posted by jcx View Post
transformer gnd? - if you mean secondary Center Tap - that should just go to the PS reservoir Caps gnd junction
The first image is what I have ready to go now (the one on the left). Is this proposed image (on right) going to be better for noise?

"the mains xfmr secondary is floating, the isolation should be taken advantage of by letting it be an "open" end of the dirty current branch"

I don't understand this. Does it mean leave the center tapped disconnected?
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Old 30th September 2014, 08:16 PM   #6
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The center tap of the transformer, if you're using op amps, is connected to the regulator board center, then the input board signal ground. The rings of the input jacks refer to that. You want that isolated from the safety ground of the wall socket. If you're not using a turntable and phono cartridge, you'll put something like a 10k ohm resistor parallel .047 cap between op amp power supply ground and safety ground. That will prevent static buildup if any component has any leakage current .
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Old 30th September 2014, 08:23 PM   #7
rayma is offline rayma  United States
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Originally Posted by shredhead View Post
I don't understand this. Does it mean leave the center tapped disconnected?
You must connect the center tap or the supply won't work.
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Old 30th September 2014, 08:46 PM   #8
Vasquo is offline Vasquo  United States
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I work with mic level input signals all the time. This is how I do my wiring.

IEC AC Ground - this is the safety ground, connect to metal case immediately, right near the IEC AC inlet.

Transformer Center-tap - connect to PSU PCB.

PSU output ground - connect to preamp board

Audio Input and Output XLR jacks - connect ground (pin1) via wire to preamp PCB.
Audio Input and Output XLR jacks - also, short pin1 to metal body case of XLR jacks, which is then screwed to the metal case, and making electrical contact with the case... effectively grounding the case

Any noise coming externally via the input or output XLR jacks are immediately shunted to ground/metal case right at the entrance.

The only "wire" I have connected to the metal case is the safety ground from the IEC AC inlet connector.

This scheme has proven to be very quiet, even with the input signal amplified to 72dB gain (4000x amplification).

Bonus: Use a toroid transformer, and rotate toroid a few degrees left or right to find the sweet spot where hum/noise is the minimum. (of course, unit is powered on and you're listening to the output noise via a connected speaker). Once the sweet spot is found, turn off power and tighten bolt on toroid transformer.
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Old 30th September 2014, 09:28 PM   #9
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Thank you for your info. I am not using a safety ground. I also don't have a ground plane on my preamp PCB, that is why I was sending all grounds to the star point which is a copper plane attached to chassis.

Is the star ground not the way to go for preamp signals or something like that?
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Old 30th September 2014, 09:55 PM   #10
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<< I am not using a safety ground. >>

I very strongly recommend that you do. Or don't let the kids within a mile of the thing.

The post below is intended to show the possible result of not having both the circuit and the housing grounded (the safety ground). The green line is not present as an actual wire, but it does exist electrically, since ultimately all grounds are the same ground.

If your local supply voltage is 120 volts, that voltage will knock you across the room but might not kill you. In countries where the supply voltage is 220, results can be grim.

<< I am planning on using 18 gauge wire for all of the internal connections >>

On a happier note, using #22 or #24 wire is fine until you get to the output. Output wires must be sized to carry the expected wattage, and so must power supply wires, of course. #18 is probably fine.
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