Question about Expat Audio uber power supply (+/-18V)

[JW]

Tried sending to  Rochey, but his inbox is full.

Just a quick embarrassing question. I have one of Expat's uber power supplies I bought from a forum member, powering 4 channels of S800 eq's.

Anyway, I want +/-18V to the eq's. I think they draw around 100mA or less each (So, X4)
The power transformer has 18V secondaries.
I'm trying to figure out the value of R1 and R2 which adjust the DC output voltages . Here's a link to the uber page with the excel calculator:  http://expataudio.myshopify.com/products/uber-power-supply

Anyway, with the variables above,  the calculator give me around 8 ohms. I've tried everything from 6.8ohm to 68 ohm and I'm not seeing any difference in the DC rail outputs (loaded or not, as a matter of fact) It's always outputting  +/-18.2V. Which is almost perfect, but it's bugging me that they're a little high. I don't want to stress the chips in the long run.

Should I be going higher or lower?

 

[PRR]

> R1 and R2 which adjust the DC output voltages

No.

"optimized value of R1 and R2 ... to help ease the voltage drop required by the linear regulators."

> 100mA or less each (So, X4) ....18V secondaries. .... around 8 ohms. I've tried everything from 6.8ohm to 68 ohm

18V AC (exactly?) should give 24.4V DC. So there is about 6V drop across the regulators. Assuming 0.4 Amps, 2.4 Watts of heat in the regulator. Naked, they will shut-down. With a couple square inches of heat-sink, they will be fine forever.

You *need* 21V DC into the regulators on the worst bad day, or the ripple dips will pass-through. Taking a modest 10% allowance for wall voltage sag, you can hardly afford to lose one more Volt. I would not use more than 2 or 3 Ohms. Considering the large PSRR of these regs, I might just jumper it.

68 Ohms at guesstimated 0.4 Amps is a 27V drop. There is no way you have 4*100mA load and still have 18V at the output. Best re-figure your figures. Put a 10 Ohm resistor in at R1 and R2, measure the voltage drop. V/R=I, of course.

> It's always outputting  +/-18.2V.

Then your "18V" regulators are 18.2V.

I would not worry too much.

 

[Rochey]

I'll clear my inbox once I'm done here. You can always find us directly on the expat audio facebook page. no inbox limits there.

18.2 is virtually textbook. the 0.2 excess is well within the 1.5% minimum tolerance for some of those 7818 regulators.

Looking at the schematic for the s800 online, the ABS-Max for each rail is:
NE5532: 22V per rail
TL071: 18V per rail.

So... the TL071 might be a little stressed, but frankly,  (and speaking from the perspective of a daytime job in semiconductors) there is a smidge of overhead even after the Abs-Max.

Where you'd normally be concerned is if you had additional high frequency noise that on the input of the regulators that might fly right on through the regulator, but the uber has a low pass filter as part of it's RCR input. Lovely.

/R