Need a power supply for Quad Eight pre-amp project...

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SimonB

Active member
Joined
Jul 22, 2009
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41
I'm looking for a power supply kit for a Quad Eight pre-amp kit I'm getting from Orphan Audio. Ken from Orphan Audio says "run on Bi-Polar 28 volts (plus and minus rails) at 100ma max, and 48v phantom at 15ma."

I was looking at the 2448 kit from Five Fish Studios but that says it outputs a max of 24V on two rails and 48V on one rail. I looked at the Five Fish Studios forums and I found a brief post about using a 22-0-22 transformer to achieve 28 volts but I'm not sure if that would be pushing things with that kit because of overheating. Anyway, any ideas for kits?

Also total ripple and noise is supposed to be less than 1 mV with 300-400uV recommended, do any kits provide a stable output like that?

I'm sort of a newb when it comes to this DIY electronics stuff so cut me a break if something doesn't sound right. I've done a fair amount of soldering but haven't built anything besides some guitar pedal kits.
 
hi simon,
you could go with the JLM PSU kits or Owel's Five Fish.  both are adjustable.
since the Five Fish PSU is rated up to +/-25V you could just up the value of the capacitor voltage rating to be safe (50V) if the size allows on the PCB.  double check the O.D. of the caps you intend to upgrade to to find the ones which will fit.  if space is limited Panasonic NHG types are small, so are Nichicon VZ types (both are smaller O.D., but sometimes taller depending on the value of the cap).
also check the height if going in a 1U rack chassis.  the regulators will be able to handle the +/-28V voltage as they are the LM317/LM337 types.

you'll need to do some research on how to figure out how much voltage a full bridge rectifier will end up putting out to the V-regulators from the start VAC from your transformer.  look here and on the web.  lots of info.
this will tell you how much V you need from your PSU transformer to start.
remember that the more regulation (voltage drop your regulators need to accomplish the more heat - make sure to add appropriate sinks).  also the regulators need a bit of headroom to work so a couple of volts over the required +/-28V should be coming out of after rectification.

a pair of your channels should be less than 250mA of current drawn which is not going to pull the voltage down by even a volt most likely (unless you have something shorting or severely wrong in your build).

your psu transformer should be able to supply about 2-1/2 times the current of the expected current draw of the unit.  so look at the VA rating of the transformer.  toroidal is always a good idea to reduce noise, especially if the PSU is in the same box as the amplifier.

it's good to question the person you're getting the kit from about your specific needs and any modifications you'll need to do.  i'm sure they'd be happy to help you get what you need from their kits.

hope this helps.
kind regards,
grant
 
Alright thanks for the answers.

I'm going with the 2448 kit from five fish studios. All the capacitors are rated at least 50V so I think it will be fine. The kit uses two LM317 regulators and a single LM337 regulator.

I need to get a low profile toroidal transformer that can fit into a 1U rack case. I saw the Amveco transformers on digikey's site and I'm considering one of those. I'm not sure what load rating I need though. Can anyone help me out with that?
 
SimonB said:
Ken from Orphan Audio says "run on Bi-Polar 28 volts (plus and minus rails) at 100ma max, and 48v phantom at 15ma."

dissonantstring said:
a pair of your channels should be less than 250mA of current drawn which is not going to pull the voltage down by even a volt most likely (unless you have something shorting or severely wrong in your build).

your psu transformer should be able to supply about 2-1/2 times the current of the expected current draw of the unit.  so look at the VA rating of the transformer.

calculate the current of the two channels.  look at the max current output of the transformer wired in parallel and that will give you the VA rating you should get.
i would get the max possible VA rating that can possibly fit into your chassis comfortably with some clearance at the top so the retaining bolt doesn't touch the chassis cover.

hmm...a 22V transformer after FW rectification would be around 31VDC.  i guess that is enough to get your 28V if you're not taxing the transformer (which you shouldn't if you get the proper VA rating with enough headroom at max current draw of the two circuits).  if you want to be safe you could get a 24V-0V-24V transformer which will give you a little more input voltage into the regulators.
remember that the regulators need at least a couple of volts difference to maintain regulation / work.  the regulators would need to dissipate a little more voltage with the voltage difference, but should do fine with sufficient heatsinking. 

-grant
 

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