I have read that the 24v Power one PSU's can be modified to give 17v.
Does anybody know how I do that?
I am thinking on buying 2x HD24-4.8-AG.
Any competent tech that understands the 723 regulator can do it.
You would probably be better off by working with a 15 V version like the HN15-4.5-AG, as much less power would be dissipated in the supply. There may be enough voltage adjustment in that model such that a modification not be required (Power-One guarantees 5%).
Whatever model you pick, go for at least 50% more current capability than what you actually need.
Some of them have adjustable outputs - some you have to change the voltage divider at the feedback input to the regulator.
Looks like the Power One units are +/-5%. This only gets you down to about 22.8v from 24v. It has a remote sense - but to use this to adjust voltages, you really need to buy a lower voltage output and insert diodes in the remote sense path.
Any reason not to use a 15v supply and set it as high as you can (in Power One's case, about 15.75v)?
Send an email off to tech support at Power One. Tell them what you want to do, and they'll tell you what resistors to change to get the voltage you want. And if you ask nicely they might even send you a schematic.
Note that the power dissipation will go up in your pass devices if you start with a 24V supply and knock it down to 17V. And if you start with a 15V supply and crank it up to 17V, your low-line dropout voltage will increase. There are always tradeoffs....
Something "rang a bell" with me, re. the 1600 series desks, so I just looked up the docs at the soundcraft.com website. In addition to +/- 18 VDC, that desk's power supply also provides:
+/- 7.5 VDC
+ 24 VDC
+ 48 VDC phantom
....all on a round 10-pin connector.
Thus, a replacement PSU will require multiple Power-One/International Power/Condor/whatever modules, beyond what is required for the audio rails.
I AM sort of surprised there is no fusing for the individual rails which would give a hint as to the current requirements, but a few rails can be guessed-at since they use 3-terminal regulators.
The 24V rail uses a LM-338 which has a max rating of 5 Amps.
The +/- 7.5 VDC rails use 317/337 regs, which are 1.5 Amp max parts per rail.
Phantom is a no-brainer since the abs-max that any input channel can pull is just a tad over 14 mA...IF mic input pins 1/2/3 are ALL shorted together. The smallest Power-One/whatever 48 VDC module is rated for 0.5 Amps, which means it can happily supply a 35 channel desk with EVERY mic input shorted across XLR pins 1/2/3.
As for the +/- 18 VDC rails, the required amperage for those two is unknown to me without doing some reverse engineering from the schematics. 4 Amps may or may not be enough......
Thank you for the replies. I just read in another thread that Jim from AudioUpgrades recommended to knock a 24V down to 17V.
Paul I have mailed the techs at PowerOne but haven't got a reply, maybe because of christmas. Do you know with resistors I should change in the 24V supply?
But maybe I should just get the 15V version and regulate it up.
I have got the console up and running on cheap laptop PSU's on +-18V, as I wanted to check if it was alive after the fire. It works but the laptop PSU's are noisy as hell, therefore I want to replace them with some better stuff.
It is possible to run the console on + - 17V (18V), as the other voltages only are used for logic stuff, and some lights. So I don't think it need that much power on the other rails. But please correct me if I am wrong.
'winding down' a 24 Volt supply to get to 17 is an inferior idea as you are stressing the supply unnecessarily (excess dissipation) and if they fail there is a decent chance you WILL damage the desk. OTOH a 15 Volt turned UP is very unlikely to damage the desk in the reduced chance that it might fail.
Power One type modules are not necessarily rated for the rated power output without 'assisted' cooling. Simply sticking them in a box with no ventilation or possibly a fan may well 'cook' them.
I don't know, without a schematic handy, which resistors to change. There are a couple of ways to configure the LM723, and I don't know which is used in that supply.
If you are capable of measuring a few resistances, we may be able to figure it out though.
In any case, I'm sure you'll hear back from Power One after the holidays.
p.s. for go/no-go testing purposes, you can run the audio sections of the console at +/-15V