what to do with 48v winding - hairball's toroid

[toneboner]

a couple months ago i would have said a toroid is something that requires medication.  after reading much of these forum pages i think i'm ready to start my first project - a pair of greenpres and a pair of access 312 in 2u space to allow for another pair each later.  i snatched a couple hairball kean ocean 16/16/48 toroids and am wondering what to do with the 48v winding.  Given that i'm planning to use green psu v6 i guess i would simply not use it?  seems like a waste.  would there be much benefit in altering the psu a bit to get phantom exclusively from that winding?  what would that look like?  i know its got to be rectified ---> smoothed ----> regulated?  is this toroid more geared for a 500 style setup?

go on, unleash your toroidal wizardry upon my starving noobish mind

thanks!

 

[3nity]

I have one too...havent been used yet!
However i remember once i got problems with one 15-0-15 toroidal to get 48V.

What you can do is get the +/- from the 16-0-16.
and use the 48V for phantom wich should be more stable and clean!

Thanks

 

[tomas1808]

Whoaa my 16/16/48 from Hairball has just arrived and came here to ask exactly this!

Its much bigger than I expected.

BTW, does anyone know what the VA is?

 

[chrispsound]

http://www.groupdiy.com/index.php?topic=42745.msg579450#msg579450
or PPA and Whistlerock audio have a good power supplies that will work with that toroid as well.  ChrisP

 

[toneboner]

interesting thread, chrisp... totally missed that one!  that board looks pretty much what i imagine is needed but it wants 50v? where did that come from?  it would probably be just as happy with 48v, right?  because its from a single winding, will there need to be considerations made for 1/2 vs full wave rectification as mentioned in the thread:

You need to be careful deriving phantom from the 50V tap on the Hammond transformer. As it is a single tap you can only use a half wave rectifier so you cannot use the bridge on the phantom power supply PCB...

I'm guessing most people that are using this toroid are just leaving the 48v out of it.  but I can't help thinking theres some benefit to using it as dedicated phantom power.  nitpicking here? yes!  trying to learn something too.  (i also checked out the whistlerock psu which looks really nice, tho looks like it's power input is ac ~ ac ~  // ac ct ct ac  --->  would that be 16 - 0 16 - 0  ???  still no use for the 48v!)


tomas, this is info from mike at hairball:

LOADING: DC65VA

S1:16Vdc 1.8A dc
S2:16Vdc 1.8A dc
S3:48Vdc 0.15A dc

 

[toneboner]

ok so its really 15/15/44 vac looking at the data sheet

44 is not a center tapped winding so can full wave/ bridge rectify

 

[toneboner]

After rectifying, smoothing per schem provided on the data sheet.

looks like the phantom pcb without regulator, protection diode for regulator (?), and 120/ 4k7 in series in place of the "rheostat"/ pot.  and some extra caps for flavor/ some genius reason i'm sure.  what would the value R be for my purpose?  would i be better off sticking a trimmer there?  reached the depth of my understanding here pretty quick! someone put me out of my misery!!! 

 

[toneboner]

phantom psu schem for reference

 

[toneboner]

i like it better when you guys just give the answers.  *gasp*, reads tl783 data sheet, nyaaa!

Vout = Vref * [1 + R1\R2] + Iadj * R2

is this what i'm after?

 

[toneboner]

http://www.datasheetcatalog.org/datasheet/texasinstruments/tl783.pdf

TL783:

1.2v minimum Vref
15mA minimum load
83uA typical adj input

R1 = 1.2v/15ma = .08 = 80ohm

Vout = 1.2v(1 + R2/80) + 83uA * R2

R2 = 46.8v/15.083ma = 3.1028 = 3k1ohm

check:
1.2v(1 + 3.1028/80) + .083 * 3.1028 =
1.2v(1 + 38.785) + 0.2575324 =
47.742 + 0.2575324 = 47.9995324

If someone can confirm that i'm even on the right track, I'd be eternally grateful!
Thanks, Sasha

 

[Harpo]

seems right, but from TL783 datasheet Vref can be in range of 1.2 ... 1.3V with 1.27V typical. Keeping your -for my taste too low- 80R and ignoring resistors come with tolerances, this 15mA will rise to 16.25mA with Vref 1.3V and value of your R2 has to decrease to 2K874. With R1 80R this R2 will generate 0.7W up to 0.76W of heat. If you'd connect a LED with current limiting resistor of about 8mA for a constant 48V status control, you could double up R1 and R2 for half of R2s needed power rating. R2 further might be split up into a 5K6 fixed and 1K rheostat in series for a target voltage range in between 42V .. 53.6V. Just my 2ct and YMMV.

 

[CJ]

you could use a half wave circuit, just one diode in series with the 48 volt winding, the duty cycle will knock down the 48 so that you do not have resistors frying away under the chassis, it can be noisy, so you need a good pi filter with plenty of C.

put a 48 volt pilot lamp across the pwr supply to draw constant current and improve regulation.

 

[toneboner]

thanks for chiming in harpo and cj. thats just the kind of gnarly smart sh*t i'm talkin' bout!
you've both given me lots to think about for my phantom psu. 
and, more math!...

Smoothing capacitor for 10% ripple,

C =  5 × Io   
      -------
      Vs × f

 

[toneboner]

finally...  1st power up, nothing blown up. looks alright - 48.1v is close enough, right?!?  The board is a little big tho. will condense it as much as I can at some point. Thanks to all who replied and Ian for the phantom pcb layout which I changed slightly.

 

[toneboner]

close up

 

[ruffrecords]

finally...  1st power up, nothing blown up. looks alright - 48.1v is close enough, right?!?  The board is a little big tho. will condense it as much as I can at some point. Thanks to all who replied and Ian for the phantom pcb layout which I changed slightly.

I think the IEC spec for phantom power is 48V plus or minus 4 volts! unloaded. With the circuit values I gave it always comes well within spec. 48.1 is certainly close enough!

Cheers

Ian

 

[toneboner]

***annoying grounding question alert***

Well, since the man himself has chimed in! ... ::: From the phantom pcb thread Ian, you suggest wiring 0v to input pin 1.  I was planning all pin 1's to chassis as per the rane document.  I know I can't do both. Can I?!  There's plenty of talk about the pin 1 problem but haven't found much considering a separate supply for 48v. These are 4 green pre v1's with green psu v6.

As always, help and advice is greatly appreciated!

 

[ruffrecords]

***annoying grounding question alert***

Well, since the man himself has chimed in! ... ::: From the phantom pcb thread Ian, you suggest wiring 0v to input pin 1.  I was planning all pin 1's to chassis as per the rane document.  I know I can't do both. Can I?!  There's plenty of talk about the pin 1 problem but haven't found much considering a separate supply for 48v. These are 4 green pre v1's with green psu v6.

As always, help and advice is greatly appreciated!

Oh yes you can!  (apologies , not long been to the Panto).

Mic input XLRs are the one and only place where you need to do both but ONLY on mic input XLRs. In my mixers I put all the mic input XLRs on one panel if I can. I connect pin 1 of each direct to the chassis at the XLR. I also run a fat copper bus bar wire strapping all the pin 1s together. From one end of this bus bar I run a wire to the 0V of the phantom supply PCB.

The thing is, phantom power is a bit of a kludge and breaks the cardinal rule that you must not use a screen for signal or power. Screens are for carrying interference currents AWAY from signals. In phantom power the +ve supply goes via the two 6k8 resistors down the mic cable and into the mic. The internal 0V of the electronics in the phantom powered mic is connected to the mic cable screen!!! This has to get back the phantom power supply 0V somehow. Now there are two ways you can do this.

1. Connect phantom PSU 0V to chassis at the power supply just like the regular signal 0V. The problem with this is that all the phantom supply return current flows through the chassis along with all the other crud current the chassis is picking up and safely shunting to safety earth. So what you have just done is put a noise generator in series with the 0V line of the phantom supply. Fortunately the the PSRR of the phantom mic electronics should ignore most of this and any that gets onto the signal lines should be common mode so very little should get through to the mic pre. But why take the risk?

2. If you wire the phantom 0V direct to the mic input pin 1s you avoid this potential problem because you are not taking the phantom 0V via the chassis any more. Phantom is still not ideal because any crud picked up by the mic cable itself also appears in series with the phantom 0V connection but there is nothing you can do about that. In the good old days, condenser mics had their own built in power supplies and there was not need for tha phantom compromise. Screens were screens and never carried signal or power.

It is interesting that Rane does not mention this and none of the other references I have seen mention it either. I know it used to be an issue in some broadcast situations where a mixer was in close proximity to a transmitter. You could get several volts of rf from one end of a chassis to the other and if that got into some phantom powered mics they just packed up altogether.

Cheers

Ian

 

[zebra50]

Thank you Ian for that nicely summarised explanation of the problem and how to avoid it!

 

[toneboner]

You're too much Ian!  Actually, considering the giant void in my understanding of these matters, you're probably just enough...