Dual mono amp star grounding questions

[HiFiNutNut]

For stereo amps a single star ground was what I have used with good results. Now I am drilling the chasis for a dual mono amp. I am struggling with the choice of a single star ground or 2 separate star grounds to provide better isolation and minimum hum. I use 2 bridge rectifiers per Tx therefore the centre taps are not grounded.

Here is my plan: ring terminators and solder lugs are tied to (a) 4mm screw(s) connected to the chasis in the following order from top to bottom:


Star Ground One (Left Channel):

1. Common ground
2. Zobel ground
3. PCB rail decoupling ground
4. PSU ground
5. Speaker negative return ground
6. Earth
7. RAC / interconnect shield
8. Speaker shield (Cobra Shielded Speaker cable)
9. Chasis


Star Ground Two (Right Channel):

Same as above.


I thought 2 star grounds are preferrable to the single star ground like the one below:


1. Common ground (left)
2. Common ground (right)
3. Zobel ground (left)
4. Zobel ground (right)
5. PCB rail decoupling ground (left)
6. PCB rail decoupling ground (right)
7. PSU ground (left)
8. PSU ground (right)
9. Speaker negative return ground (left)
10. Speaker negative return ground (right)
11. Earth
12. RAC / interconnect shield (left)
13. RAC / interconnect shield (right)
14. Speaker shield (Cobra Shielded Speaker cable) (left)
15. Speaker shield (Cobra Shielded Speaker cable) (right)
16. Chasis

I guess the separate star grounds would enable large current returns from the speakers to return to the correct PSU and not to have much effects on the other channel. The Earth is branched out from the AC power socket. The chasis connects the two star grounds together. I do hope this does not introduce any ground loops because if the 2 star grounds are placed close to each other (up to 100mm apart) there should not be much potential between the 2 points.

Do you think they will work? Any better ideas, better topologies or grounding schemes?

Many thanks.
Regards,
Bill

[AndrewT]

Hi,
I would go for one star ground.
However I would separate the earth and chassis from the star.

If you decide to keep all the grounds together then move the safety earth down the bolt to the chassis and use it's own nut to permanently trap it there.

Try having the star floating with respect to the earth+chassis.
You can then try a resistor and or capacitor coupling between star ground and safety earth or even a direct wire link (all grounds back together again).
The decoupling need very low inductance to operate at the high frequencies that require to be attenuated. This demands very short wires from star to decoupling ground. Put some thought into how you can achieve this. This may be what changes the requirement from one star to two star grounds, simply to give short wires to each board. Two grounds would be what are inside separated monoblocks so it does work.

[HiFiNutNut]

Andrew,

Thanks for your help.

I have tried in my stereo amps floating the star by adding a resistor and a capacitor but it did not appear to have any difference. If I experience ground loops then I will most definitely use that again.

The picture below illustrates the my original plan for the layout of the dual mono amp using 2 stars. Note that the picture is not complete as some wires are missing. The black wires are the ground wires. I will use very THICK wires for decoupling. Nevertheless, with the layout some wires are not short. Would that be a problem?

Regards,
Bill

[AndrewT]

Hi Bill,
having a bit of trouble deciphering your diagram.
It looks like you have the mains safety earth going to both star grounds?

Take your mains safety earth direct to chassis on a permanent bolted fixing. You can then run a connection to the star grounds with a wire or resistor depending on which you decide.

The second thing is it appears that you are running the capacitor commons to the star earth? This introduces large pulse currents and voltages onto your star ground. It then becomes a dirty ground, not what you want.

Instead, couple the capacitor commons together for each bank. Then run a connection to the respective star ground.

I cannot see the transformer centre taps? Connect these to the cap commons for the respective channels.

Do you need any more advice?

[ChocoHolic]

....hm, I always use two stars...
But not one left, and another right....

My first star is the PSU star (transformer, rectifier, heavy E-caps, may be also earth and enclosure) and the second star is the signal star.
PSU star and signal star are connected with one single wire of sufficient gauge.
My reason for this is that I discovered some noise in an amp with heavy PSU. The enormous cap charging current peaks can cause some
µVs voltage drop within the star ground connection that carries these currents. The amp correctly amplified this "signals" and produced 4mV peaks every 10ms at its output......

[AndrewT]

Hi ChocoHolic,
I absolutely agree, but I cannot get the message across because some have been lucky and avoided noise on the signal ground and then they recommend to keep all grounds together.

Keep the BIG currents away from the signal ground and interconnect the different grounds with single wire and/or resistors.

[sklimek]

Originally posted by ChocoHolic

PSU star and signal star are connected with one single wire of sufficient gauge.

Originally posted by AndrewT


Keep the BIG currents away from the signal ground and interconnect the different grounds with single wire and/or resistors.

Thank you for those two posts, you both mentioned that the interconnection of the two star points 'power and signal' should be a sufficient wire guage. Would this be the awg of the signal wire to trace back to the power star or the opposite use power awg guage. Or does it not matter.

Its amazing you should see my 'misc grounding techniques file' for this and you two guys just cut out all the fat and simplified the explanation.

Stan

[AndrewT]

Hi,
the only ground wire that needs to be bigger than 24awg is the interconnect from PSU common to power ground.
Remember to take ALL your speaker returns and decoupling caps and Thiel(Zobel) networks direct to the power ground.

[sklimek]

Originally posted by AndrewT
Hi,
the only ground wire that needs to be bigger than 24awg is the interconnect from PSU common to power ground.
Remember to take ALL your speaker returns and decoupling caps and Thiel(Zobel) networks direct to the power ground.

Great! thank you...

[HiFiNutNut]

Thanks Andrew and ChocoHolic. Sorry for getting back to this late because I did not have internet connection over the weekend.

So does the following layout make more sense?

The changes are:

(1) One aluminimum flat is used as the PSU common.
(2) Speaker negative wires go directly to the PSU common (and the Zobel).
(3) Earth is connected directly to the chasis below the power socket.
(4) Single wire from PSU common connects to the star which is connected to the circuit common, interconnect shields and the chasis.

Since 2 bridge rectifiers are used so the centre taps of the transformers are not connected to the PSU common.

With both PSUs in the monoblocks sharing the same aluminium flat would it introduce more cross-talks? What about the circuit commons share the same star?

Regards,
Bill