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Some additional thoughts on a separate set of PSU's for your cMP... (Of course, long!)

First, I think AMB's Sigma regulated PSU looks like a very good choice as the main building block of a home-built cMP power supply. I think that it will be an upgrade over the garden variety LM317-type regulators (I use the better & higher current LT1085 in some places and the better LT3080 in others) and may approach a good shunt supply like the Salas high-current shunts and possibly even Paul Hynes regs.

That brings up a question... You said you'd be doing "an external box with the 3 PSUs... 3 ext. cables... 3 internal cables for P24, P4 and Peripherals."

There are at a minimum 6 different voltages for a cMP using the Zalman case:

- ATX-20/24 (P24) 3.3v/5v (both power and control voltages)/12v

- P4 12v

- peripherals HDD & USB 5v / screen 12v

Are you planning to use a separate transformer for each voltage, or combine some using transformers with multiple secondaries? Also are you planning to use a Sigma for each voltage or use lesser regulators for the less-critical voltages?

My setup uses separate power supplies with separate transformers for each voltage, only sharing one between the 12v screen and the 5v USB. If I were trying to minimize the number of tranformers while maximizing the sonic quality & benefits, from my experience, I'd suggest the following combination of PSU's & regulators:

8v-11v PSU -> 5v Sigma regulator -> ATX-20/24 (P24) 5v (both power & control voltages)

14v-19v PSU -> 12v LT1085 regulator -> ATX-20/24 (P24) 12v -> 3.3v LT1085 regulator -> ATX-20/24 (P24) 3.3v (large voltage drop here, need a significant heatsink)

14v-19v PSU -> 12v Sigma regulator -> P4 12v

8v-11v PSU -> 5v LT1085 regulator -> HDD

14v-19v PSU -> 12v LT1085 regulator -> Zalman screen 12v -> 5v LT1085 regulator -> USB header power (for mouse & touchscreen)

If you wanted to use multiple-secondary transformers for the supplies, the minimum number I'd suggest you consider is three... one for the ATX-20/24 (P24), one for the P4, and another for the peripherals. But I believe that using 5 transformers as I suggested above will sound better (and using 7 as I currently do will be even better!).

Then to your questions about grounding and wiring, each PSU will have a ground point and that should be taken to the output connector for that power line so that the motherboard becomes the star ground point for everything. (I did just realized that I deviated from that in my 5v HDD supply... It reaches ground through the data connection for the HDD. I'll have to try grounding it to the motherboard & seeing if that makes a sonic difference!)

That also provides a method to help 'reject various interferences'. One common way to do this is by pairing a voltage connection with it's corresponding ground in a twisted pair or a braid. I did this as follows with my power supplies (note that I only used the 20-pin version of the ATX-20/24 connector and don't see any reason or recommend connecting up the other 4 pins):





ATX-20 5v - One set of 5v lines from the regulator to pins 19 & 20 in a 4-cross braid with ground line pins 16 & 17, another set of 5v lines from the regulator to pins 4 & 6 in a separate 4-cross braid with ground line pins 5 & 15

ATX-20 12v - 12v line from the regulator to pin 10 in a twisted pair with ground line pin 7

ATX-20 3.3v - 3.3v line from the regulator to pins 1, 2, & 11 (tied together at the plug) in a twisted pair with ground line pin 3

ATX-20 5vSB & PWR_OK - 5vSB directly from the 5v regulator, PWR_OK from a switch in a 3-braid with ground pin 13

Note that on the ATX-20, I did not connect pins 12 (-12v) 14 (5v PWR_ON#), and 18 (-5v)





P4 - 4-cross braid using the two 12v lines from the regulator and the two ground lines

On the 5v HDD, 12v screen, and 5v USB, I just used a twisted pair on each.

This will increase the number of connecting wires you have to use, but provide for better suppression and rejection of interference

Another thing to do to reduce interference transmission is to add bypass capacitors at each plug. I have them currently at the ATX-20 & the P4 and need to add them to the 5v HDD, 12v screen, and 5v USB plugs... see the picture of my supply above.

Then see here for additional suggestions for wire and connectors: http://www.altavistaaudio.com/Umbilical.html . While you wouldn't have to use the wires he recommends, these are two examples of wire that would be better than the garden-variety copper in vinyl insulation. Also, the Hirose connectors he uses look very good and are readily available at both Digikey and Mouser in a number of pin configurations. His comments also echo those of some others on this thread who have found sound quality improvements by using good connecting wires. I haven't done this in mine yet, but plan to with my next big-rebuild.

Then to your questions about the 5v PWR_OK, 5v PS_ON#, 5vSB, turn on sequencing & relays. As I said above, you can connect the 5vSB to the output of your 5v regulator and the 5v PWR_OK to the same place through an on-off toggle switch. You don't need to connect the PS_ON# at all. And you didn't ask this, but to make it all work, you do need a power-on switch connected to the power header on the motherboard. This is nothing fancy, I just use a momentary-on switch for this.

Then to turn on the computer, do the following:

1st - plug-in or turn-on the dirty supplies (P4 12v, HDD 5v, Screen 12v, USB 5v)

2nd - plug-in or turn-on the remaining supplies (ATX-20/24/P24 3.3v, 5v, 12v, & 5vSB)

3rd - press the momentary-contact power-on switch connected to the power header on the motherboard

4th - flip the switch to provide 5v to the PWR_OK connection

This will work for the GA-G31M-S2L motherboard.

If you have a GA-G31M-ES2L board, you also need to provide -12v at startup. This can be done with a 9v battery and can be turned off after the motherboard starts booting.

If you have a GA-EG45M-UD2H board, you have to wait 8-12 seconds after flipping the switch for the PWR_OK, then switch off the PWR_OK for 1-2 seconds, then switch it back on.

I am not sure what works for the GA-EG55M-UD2H board as I don't have one yet, but based on other's comments, I suspect that the sequence for the GA-G31M-S2L will work ok on it.

As you can see from this, no relays are needed at all.

Finally, a few more comments...

First, I use a separate 5v supply with a small transformer to provide power to the 5vSB & PWR_ON connections. This was a small, but noticable improvement over taking it from the main 5v connection.

Second, I'd strongly recommend using John Swenson's raw DC supply for computers as mentioned here: http://www.audioasylum.com/cgi/t.mpl?f=pcaudio&m=82140

He uses a choke filter, a snubber, and careful design to reduce the amount of noise generated in the supply and also fed back to the power line. I have not done this on my supply yet as I don't have room because it's designed to fit inside of the Zalman case, but when I re-do it in a larger enclosure, I will do it then.

Finally, cics had repeated his advice to use separate AC cords & circuits for the base ATX-20/24 (P24) supplies and the dirty supplys... P4, HDD, USB, and screen. This does make a difference and I strongly echo his advice.

All I can think to add for now!

Greg in Mississippi
Everything matters!

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