+24V and +48V power supply from the same transformer.

Hi guys I am just knocking up a 2 channel discrete preamp and need some help finding a simple power supply schematic using only one transformer (if possible) to get +24VDC and +48VDC. I searched the META and while there is a lot of talk about different ways to do it, I couldn't find a decent schematic. My current idea is to use ONE transformer with 2 x 24VAC secondaries.

One of the 24VAC secondaries I will use to make 24VDC (single ended) for my amps. A two stage design using amps similar to the Neve BA338.

The other 24VAC secondary will be used in a voltage doubler to make 48VDC for the phantom power.

My questions are:

A) Is this ok? Hah funny question I know, but I have always used separate transformers for the phantom and B+ voltages. Although I know that the secondaries aren't connected I just wondered if there would be any other interference.

B)Is there a better way to do it? From memory these amps only really draw around 40ma max and there will only be 6 of them (2 to drive the meters) so I don't need the big heavy duty power supplies that I usually make for multi channel stuff.

Here are the schematics that I plan on using. If anyone has suggestions (especially if to simplify) then please feel free to post! I feel like I might be going over kill for just a 2 channel thing.

http://powersupply88.com/24-volts-power-supply-at-2-amperes.html/24v-power-supply-at-2-a

http://lh6.ggpht.com/_FdGFE8NBDgc/S90YNITcvnI/AAAAAAAACeE/wx3W8K-yRUw/48-v-microphone-supply-circuit-diagram.GIF

Thanks a bunch! - Benjamin

Looks good to me.

Cheers

Ian

Should be fine. Alternatively, you can just make two identical 24v supplies and reference the second one to the output of the other, to make a stacked 48v rail.

Good filtering would be paramount, of course (in either case).

MagnetoSound said:

Should be fine. Alternatively, you can just make two identical 24v supplies and reference the second one to the output of the other, to make a stacked 48v rail.

Click to expand...

Is this what you mean magneto? (see picture attached)

Thanks guys! I have always been a bit scared of merging my Phantom and B+ circuits together but I have filtering on each of the 6 opamps via a 33R resistor and a 220uF cap so I might take the easy option and do the double!

Thanks again - Ben

That's it. Extra C from +48 to 0v will help to keep it quiet.

Or just up your C4s to 470uF or even 1000uF. (Make sure your upper C1 is not grounded, of course!)

you could try it both ways and choose the least noise from ripple or ground schemes,

you can route the phantom gnd right to the XLR if it is separte from the electronics gnd, which may or may not give you less noise,

i like the flying capacitors as the ripple is easier to filter, and a phantom supply needs to be dead quiet, with the Greinacher doubler, ripple is close zero with no load, and mics are close to no load, however, diodes can be noisier with the flying caps,

the stacked 2x24V should work just fine, but you will need a transformer overrated by almost 50%.. the 48V will need some 20mA max, but this winding will have the same rating then the 'lower' 24V one. that lower one must be rated to the current you need for your Preamp - lets say 200mA just for kicks. so you will have 180mA * 24V  -> some 4.32VA on a 9.6VA transformer 'unused' on the upper leg. OK, not really a big deal since you will probably get a bigger tranny anyway....  now if your main electronic is rated at 1A this becomes more of an issue.

I looked and found this neat trick:

http://www.bunkerofdoom.com/lit/4x8/index.htm

this combines a bridge rectifier and a voltage double circuit on the same single secondary winding with common ground. the 48V will draw very little current as stated above so this would be a practical solution with some decent filtering of the 48V...

just food for thoughts....

- michael

electric_diaries said:

Thanks guys! I have always been a bit scared of merging my Phantom and B+ circuits together but I have filtering on each of the 6 opamps via a 33R resistor and a 220uF cap so I might take the easy option and do the double!

Click to expand...

I am afraid I am going to be the voice of dissent here. I think you should keep the phantom supply completely electrically separate from any others. For best noise/interference performance you should connect the zero volts of the phantom power only to the pins1 of the mic input XLRs. The XLR pins1 should be connected directly to the chassis at the XLR. The audio zero volts should be connected to the chassis at the power supply at the same point where the mains safety earth is connected to the chassis.

There are several threads around where grounding has been discussed and the detailed reasoning for doing it this way can be found there.

Cheers

Ian

ruffrecords said:

I think you should keep the phantom supply completely electrically separate from any others. For best noise/interference performance you should connect the zero volts of the phantom power only to the pins1 of the mic input XLRs. The XLR pins1 should be connected directly to the chassis at the XLR. The audio zero volts should be connected to the chassis at the power supply at the same point where the mains safety earth is connected to the chassis.

There are several threads around where grounding has been discussed and the detailed reasoning for doing it this way can be found there.

Cheers

Ian

Click to expand...

Thanks Ian, I felt a little uneasy about it and your comment was enough to discourage me from trying it. It probably works well for some people but it just feels wrong. Like having a toilet in your kitchen. I'm still keen to keep it on the same transformer though, but electrically isolated. Any recommendations on a simple 48V Greinacher doubler? I only need to power 2 mics max and the schem I posted earlier seems a little overkill.

May seem "not" in the vein of DIY, but why not go with a JLM or p-towns univeral PS.  Has +/- rail, 0v and 48v.  Or perhaps look at how they're doing it and make your own. 

electric_diaries said:

ruffrecords said:

I think you should keep the phantom supply completely electrically separate from any others. For best noise/interference performance you should connect the zero volts of the phantom power only to the pins1 of the mic input XLRs. The XLR pins1 should be connected directly to the chassis at the XLR. The audio zero volts should be connected to the chassis at the power supply at the same point where the mains safety earth is connected to the chassis.

There are several threads around where grounding has been discussed and the detailed reasoning for doing it this way can be found there.

Cheers

Ian

Click to expand...

Thanks Ian, I felt a little uneasy about it and your comment was enough to discourage me from trying it. It probably works well for some people but it just feels wrong. Like having a toilet in your kitchen. I'm still keen to keep it on the same transformer though, but electrically isolated. Any recommendations on a simple 48V Greinacher doubler? I only need to power 2 mics max and the schem I posted earlier seems a little overkill.

Click to expand...

You need to keep the phantom voltage as clean and noise free as possible so a regulator is always a good idea to my mind. On the other hand, the spec allows the actual dc voltage to vary 4 Volts either way so regulation is not mandatory. However the output voltage and ripple of the Greinacher depends on the load so it might pay you to add a zener shunt regulator at its output.

Cheers

Ian

ruffrecords said:

You need to keep the phantom voltage as clean and noise free as possible so a regulator is always a good idea to my mind. On the other hand, the spec allows the actual dc voltage to vary 4 Volts either way so regulation is not mandatory. However the output voltage and ripple of the Greinacher depends on the load so it might pay you to add a zener shunt regulator at its output.

Cheers

Ian

Click to expand...

Hi Ian,

Correct me if I am wrong, but I thought zeners are generally noisy.  Of course so are fixed linear regulators, but the LM317 adjustable regulator is said to be good.  It can handle +48 and I have used it in a supply with +24 and +48 with no problem.

Mike

Connect the secondaries in series and you have a 0-24-48 tapped output. Create the +24V supply from the first tap (0-24) and the 48V supply from the second tap (0-48V). A 20-0-20 transformer will probably be better as there will be fewer losses in the regulators (ie less heat).

ruffrecords said:

I am afraid I am going to be the voice of dissent here. I think you should keep the phantom supply completely electrically separate from any others. For best noise/interference performance you should connect the zero volts of the phantom power only to the pins1 of the mic input XLRs. The XLR pins1 should be connected directly to the chassis at the XLR. The audio zero volts should be connected to the chassis at the power supply at the same point where the mains safety earth is connected to the chassis.

Click to expand...

Indeed, mic ground is always quieter when kept separate from other loads. So, for this reason, the stacked secondary is another scheme best avoided. Best that the doubler circuit be implemented from it's own winding, as originally suggested.

Zener shunt ... hmmm, dunno. OK right after the doubler I guess, nice film cap next to it, more filtering downstream?

electric_diaries said:

Like having a toilet in your kitchen.

Click to expand...

Bit harsh!  ;D

mjrippe said:

Hi Ian,

Correct me if I am wrong, but I thought zeners are generally noisy.  Of course so are fixed linear regulators, but the LM317 adjustable regulator is said to be good.  It can handle +48 and I have used it in a supply with +24 and +48 with no problem.

Mike

Click to expand...

Zeners can be noisy. I only suggested it as the OP seemed to be suggesting using just the voltage doubler on its own as a phantom supply because the voltage reg. was overkill for his application. The voltage doubler has poor output regulation andhigh  ripple so the next best thing is a zener - but of course it will need some noise suppression too. By the time you have done that you might as well use a regulator.

Cheers

Ian

take advantage of the fact that current for the phantom supply will be much lower than the other supply, that means if you split the supplies then ripple will filter out a lot easier, there will not be much ripple difference between the half wave doubler and the full wave at those lower current levels (pg 1187 RDH4), so you will not have to use huge caps for the phantom filtering (even though i would anyway since they are cheaper now days), and with the half wave, you will have 2 terminals in and 2 out, unlike the full wave,