Author Topic: Bipolar power rails from two supplies  (Read 452 times)

Tube2stomp

Bipolar power rails from two supplies
« on: March 06, 2018, 11:52:11 AM »
Hi,

Trying to put into use stuff I have around and not sure I can do the following:
I have two power supplies from HP printers, both supply positive 16VDC and 32VDC rails... I assume they are of the switched kind (?) and regulated (?)... you know those brick looking ones like laptop power supplies :-)
From my description It's obvious I'm not swimming in familiar waters here lol
Anyway, my question is can I connect them with one supply reversed polarity to make a +32V and -32V rails (to get 64V in total for low voltage tubes projects)?

Its easy and simple to do with regular AC Xformers + diodes + filtering .... but I have a hunch those brick supplies designed for +VDC probably need to reference real ground and earth?
I know how to get zapped from iron with +450V but i'm lost around newer supplies.

Please advice.


Rixen

Re: Bipolar power rails from two supplies
« Reply #1 on: March 06, 2018, 03:24:18 PM »
If the outputs are isolated from mains ground, yes
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antonis

Re: Bipolar power rails from two supplies
« Reply #2 on: March 07, 2018, 07:29:14 AM »
I assume they are of the switched kind (?)
If so, you may call them "noise makers"..
 
I assume they are regulated (?)...
It depends on what "regulation" you consider acceptable..
(they usually exibit a quite high ripple voltage factor, at least not satisfactory for semiconductors audio use - I'm not experienced with that on tube circuits..)  :icon_redface:
Can't follow your signature changes, Stephen..!!

duck_arse

Re: Bipolar power rails from two supplies
« Reply #3 on: March 07, 2018, 09:31:54 AM »
Quote
Anyway, my question is can I connect them with one supply reversed polarity to make a +32V and -32V rails (to get 64V in total for low voltage tubes projects)?

you'd stack them, erm, I can't ascii diagram it without a blizzard of (-) signs, but connect the "upper" supply 0V to the "lower" supply (+) volts. then the upper is V+, mid point 0V, lower is V- ...... or take upper-est (+) and lower-est (-) as single 64V.

and all isolated from everything.
winter. booo.

Tube2stomp

Re: Bipolar power rails from two supplies
« Reply #4 on: March 07, 2018, 01:14:11 PM »
Hi Antonis. I assume they are switched... and to honest I don't really  really know how that works lol. I know how a Marshall 100w power iron works... those black boxes are enigma to me.
I did use one supply with 32v on a circuit with opamp input stage driving tubes and no weird noises were happening. maybe cause preamp tubes pull little current its enough to use a bit of resistance and  capacitance to smooth them?

Hi duck.
[and all isolated from everything.]
Do you mean they are isolated by design so its not a problem to stack them up? Or you mean I need to use them in an isolated manner... which I'm not sure what that means. 

Tube2stomp

Re: Bipolar power rails from two supplies
« Reply #5 on: March 07, 2018, 02:58:13 PM »
Here are pictures of the two supplies. The one with the white label has 2 prong AC connector and the other one has 3 prong AC connector and it does have the 0V out connected to earth. 





Rob Strand

Re: Bipolar power rails from two supplies
« Reply #6 on: March 07, 2018, 05:58:20 PM »
Quote
Anyway, my question is can I connect them with one supply reversed polarity to make a +32V and -32V rails (to get 64V in total for low voltage tubes projects)?
You can normally get away with that.  *However* there's many caveats.
If you can't be certain of these you can create a hazard.
- The assumption is there's no connections between the mains ground and the DC outputs
- More difficult to determine:  there's no weird behaviour caused by caps which connect between
the mains side and the DC side (often present for EMC purposes).

One thing you usually *can't* do is take a 0V / +16VDC / +32VDC switch-mode and used it as -16VDC / 0V / +16VDC.  When you put a load between +32VDC and +16VDC current flows *into* the  +16VDC rail and most switch-modes will not handle that.
The mind often distorts without gain.

amptramp

Re: Bipolar power rails from two supplies
« Reply #7 on: March 07, 2018, 06:59:31 PM »
If you have power supplies in series like this, one problem that can occur is caused by current limiting circuitry operating on one supply but not the other, especially if loads are strung across the maximum positive to maximum negative.  You can get a foldback of voltage where the current limit reduces the voltage then causes a current limit oscillation.  Not usually a problem but you should be aware of it and possibly test for it.

duck_arse

Re: Bipolar power rails from two supplies
« Reply #8 on: March 08, 2018, 08:18:23 AM »
^ listen to those guys.

also a popular question arises with these powering valves schemes - how will you power the heaters?
winter. booo.

Tube2stomp

Re: Bipolar power rails from two supplies
« Reply #9 on: March 08, 2018, 03:07:30 PM »
I have a third dc power supply fir the heaters, courtesy of a Cisco router :-)

Regarding ground I confirmed at least one of them is connected to earth prong, which probably was obvious since it's AC connection uses 3 prong cable.
They are obviously not the same as the other one uses two prongs.
I have right now an old reel to reel I'm converting to guitar amp so I'm not short on volts around me :icon_mrgreen:
I guess its kinda a challenge for my self to do something not obvious with just what i have in boxes collecting dust. That's how I learn every thing... jump into something and try to swim.
« Last Edit: March 08, 2018, 03:16:12 PM by Tube2stomp »

Tube2stomp

Re: Bipolar power rails from two supplies
« Reply #10 on: March 08, 2018, 03:33:42 PM »
Regarding load, the supply with the earth connection is rated for 32V at almost 1A (about 950mA). the other supply is rated for 650mA on its 32V rail.
With 2-4 preamp tubes and 1 opamp I'll aay total load of 10mA is in the ballpark.