LM3875 kit grounding, optimal grounding for separate amps and psu? - diyAudio
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Old 2nd May 2005, 04:55 AM   #1
findog is offline findog  Australia
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Default LM3875 kit grounding, optimal grounding for separate amps and psu?

Hi guys,

What is the reasoning for the separate PG- and PG+ grounds on the rectifier PCBs, as they join to the same PCB trace on the amp PCB's? Is this just to give more wiring options?

Would it be OK to join this directly on the rectifier board, then take this straight to the chassic ground point, and do the same with the amp boards? What advantages/disadvantages are there?

It just seems strange to run 2 ground leads to the amps, just to join them and then run this back to the chassis ground. With the conventional wiring for running 2 amps off the single rectifier board, you get a total of 6 leads for grounding, whereas it seems to me this can be done with 3 leads?

What will be the optimal grounding arrangement for using 2 aluminium heatsink style enclosures, amps in one and toroid + rectifier in the other?
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Old 3rd May 2005, 10:24 AM   #2
waltona is offline waltona  United Kingdom
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That is something I'm struggling to get my head around. Lets hope somebody can enlighten us!
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Old 3rd May 2005, 10:36 AM   #3
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I've the same interogation !
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Old 3rd May 2005, 11:11 AM   #4
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Power+ brings the (positive) recharging current spikes for positive rail.
Power- brings the (negative) currents for negative rail.
Unsplitted PowerGnd bring back the whole currents (Pgnd+ + Pgnd-)

I think of 3 benefits:
1) Pw+/Pgnd+ twisted and Pw-/Pgnd- twisted radiate less very high frequencies than Pw+/Pw-/Pgnd braid. And it probably causes less HF noise coupling between P+ and P-
2) Unless doubling the cable section, unsplitted powergnd cable will exhibit 2x the impedance of Pg+/pg- cable pair
3) Each bridge only knows about the ground star point, and is not really impacted by what is going on at the other bridge. Namely: no common impedance coupling between bridges.

Since all of this rounds about reacharging current spikes, I guess having a pair of smoothing capacitors as close to the rectifier as possible is good practice. IMHO, dealing with half the speaker currents associated to each power rail is easier than dealing with recharging current spikes.

In case of 1 bridge topology, i'm not convinced that splitted grounds is better than unsplitted ground...
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Old 4th May 2005, 06:56 PM   #5
findog is offline findog  Australia
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Cheers for the reply Yoghourt, I didn't think I was gonna get an answer!!!

Quote:
3) Each bridge only knows about the ground star point, and is not really impacted by what is going on at the other bridge. Namely: no common impedance coupling between bridges.
Can you explain what effect this will have? Is it desirable or not (I have very basic knowledge of electronics.)

Quote:
In case of 1 bridge topology, i'm not convinced that splitted grounds is better than unsplitted ground...
By 1 bridge topology do you mean a single rectifier/PSU circuit supplying 2 amplifiers in parallel? If so I think I'm with you on this, I can't spot an advantage of having a complicated grounding arrangement as opposed to a basic one (KISS philosophy).

Any other ideas?
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Old 6th May 2005, 05:22 AM   #6
findog is offline findog  Australia
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Seriously guys, with all the advanced knowledge shown off concerning circuit variations and specialised components etc, surely there can be a bit more input on this topic?

I'm dead sure I'm not the only one who will benefit. I would highly appreciate some hints relating to my original questions, i particular this one:

Quote:
What will be the optimal grounding arrangement for using 2 aluminium heatsink style enclosures, amps in one and toroid + rectifier in the other?
Cheers guys
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Old 9th May 2005, 10:18 AM   #7
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Quote:
Originally posted by findog
Can you explain what effect this will have? Is it desirable or not (I have very basic knowledge of electronics.)
Without common impedance, Power+ bridge switching noise is not propagated to power-.

Quote:
By 1 bridge topology do you mean a single rectifier/PSU circuit supplying 2 amplifiers in parallel? If so I think I'm with you on this, I can't spot an advantage of having a complicated grounding arrangement as opposed to a basic one (KISS philosophy).
I mean 1 bridge per rail or 1 bridge for both rails. I have not talked about how many amplifiers you connect to your power supply.
Here are the corresponding schematics, from J.M.Plantefeve website.
Click the image to open in full size.
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Old 9th May 2005, 10:22 AM   #8
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Default Re: LM3875 kit grounding, optimal grounding for separate amps and psu?

Quote:
Originally posted by findog
What will be the optimal grounding arrangement for using 2 aluminium heatsink style enclosures, amps in one and toroid + rectifier in the other?
1st enclosure: toroid + rectifier(s) + part of supply capacitors.
2nd enclosure: rest of supply capacitors + amplifiers.
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Old 10th May 2005, 01:56 PM   #9
Aghead is offline Aghead  Canada
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What is the best way to set up the rectifier for independent secondaries versus centre tap?

More specifically, if (like many) of the schematics posted, they are twin bridge designs for independent sec., if you want to use a centre tap do you just split the centre lead to each side or use a more conventional single bridge across the full voltage with centre tap to ground?
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Old 16th May 2005, 09:27 PM   #10
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Heya!

If you have a transformer with independant secondaries, you can use either 1 or 2 bridges.

If you have a transformer with the 2 secondaries connected (center tap), you can only use the 1 bridge topology.

Sorry for late answer,
Yoghourt
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