Phantom power drop resistor

[abbey road d enfer]

Ideally I want to build one Psu feeding 4 channels for 48v

At the minute I have 2 Psu feeding 2 channels each

So typically, you would design for 100mA at 48Vdc.
1000uF will produce about 1V ripple, which should result in less than 1mV ripple at the output. Brute force, like increasing the cap tenfold would result in 0.1mV, but it is better to use an output filter, like 47 ohm/1000uF after the regulator. The series resistor will create a slight voltage drop (less than 1V), but noise would be in the uV region.
You need to make sure the circuit delivers 15mA min. This is usually done by using a 82r resistor between output and adjust. The resistor that goes from adjust to ground will dissipate about 0.7W, so make sure to dimension it properly and make sure it is not too close to other components.

 

[PRR]

>> 4 channels
> design for 100mA at 48Vdc.

Seems high. IIRC there is a 10mA max. 48V through 3.4k to a dead short is 14mA. OK, to avoid burn-up if all mikes short, 100mA is not an excessive goal. But over 56mA even 40mA can sag.

 

[Winston OBoogie]

>> 4 channels
> design for 100mA at 48Vdc.

Seems high. IIRC there is a 10mA max. 48V through 3.4k to a dead short is 14mA. OK, to avoid burn-up if all mikes short, 100mA is not an excessive goal. But over 56mA even 40mA can sag.

Maybe Abbey is allowing for a few mA extra per channel for LED 'on' indicators? 

 

[abbey road d enfer]

Maybe Abbey is allowing for a few mA extra per channel for LED 'on' indicators?

Exactly. 4x10mA  + 4x10mA for the LED's + 15mA for proper operation of the TL783 make a total of 95mA, rounded at 100.

 

[Spencerleehorton]

Ok how’s this looking?

 

[abbey road d enfer]

Ok how’s this looking?

You need another elcap right at the ouput; 100uF should do.

 

[moamps]

My next question is what is better in this diagram and why?

If you already have DPDT switch for phantom, why not using +24V for LED powering? And also LE diodes should be low current, high efficient types, 10mA is way too much IMHO.

You should be sure that switching phantom on on an one channel will not disturb phantom voltage on other channels.

 

[Harpo]

With current setting resistor 82R for 15mA min.load, the series string of fixed and variable shunt arm resistor will be about 3K with a min.required rating of 0.73W for 48VDC out. Better use a 2K7 fixed resistor in series with a 500R trimmer to catch tolerances, if you want to use a trimmer with required rating at all.

 

[[silent:arts]]

If you already have DPDT switch for phantom, why not using +24V for LED powering?

I see this in many designs but I prefer to have a real phantom power present LED.
I think it is wrong to have a glowing phantom power LED but the phantom power PSU is broken.

 

[moamps]

I see this in many designs but I prefer to have a real phantom power present LED.
I think it is wrong to have a glowing phantom power LED but the phantom power PSU is broken.

Your choice. In your last design, there is a pass transistor after the switch and LED.  So if it fails, there is no phantom at mic also, but LED will happily lit.  Anyway, I think that the LED on phantom isn't so important at all.  I worked on some consoles without this feature. Others had an usual  CC string for LED powering.

I will just add at the end that the LED shouldn't be connected to +48V, but a resistor should be connected to +48V and then LED to it and GND. Always LEDs on front panels should be connected to GND if it is possible, I had once a catastrophic failure of a big power supply where someone mechanically destroyed a control LED on front panel and remaining contacts touched the chassis.

 

[Spencerleehorton]

That why I asked before about the config 2 arrangement.
The 48v went into the 220R resistor then into the + of the led then of the neg leg of the led to gnd was the 10k?
Shouldn’t it have been the other way round?
I’ve tried this arrangement and it works!!?

 

[moamps]

Just swap places of resistor and LED.

 

[Spencerleehorton]

If I put the 3.9k on the + side of led and just get rid of the 220R is that better?

Also what would I now change on the Psu to deliver enough current for 4 channels of Neve 1290.
I think each channel needs about 100ma but not sure?

 

[abbey road d enfer]

If I put the 3.9k on the + side of led and just get rid of the 220R is that better?

No, you need the additional filtering from teh 47R and associated cap.

Also what would I now change on the Psu to deliver enough current for 4 channels of Neve 1290.
I think each channel needs about 100ma but not sure?

You want this 48V supply to also provide the 24V for the audio? You would need to drop the voltage from 48 to 24. What a waste! and a lot of heat.

 

[Spencerleehorton]

I was thinking i could use a feed off one of the 24v AC winds and build a separate circuit which is also robust and low in noise.

Also with the DPDT switch which i am soldering the 3.9k resistor, 220uf cap and LED, from this do i want a separate wire direct to pin1 or can i use a screen of a wire which is closes at that point in the enclosure which is connected to pin1?

hope you understand what i mean, just finished a 12 hour shift and have been trying to write this and think!!

 

[Spencerleehorton]

so from having another quick look and trying to make this a bit more specific:

I have 4 x Neve 1290 channels, they need 24v at 500mA each, allowance for 4 x 48v and LED's = 100mA

I have a 4 channel unit i've built so i need to get 2.1A total or build it in two halves.

The 783 regulators are only 1.5A, i wanted just to build one PSU to accommodate everything rather than the two i've already built from LM317's.

whats the best approach?

 

[abbey road d enfer]

i wanted just to build one PSU to accommodate everything rather than the two i've already built from LM317's.

Don't pursue in this direction. You want the 48 circuit to be completely independant of the 24V circuit. The specs are so different there is no synergy.

 

[Spencerleehorton]

when you mean separate, you mean from the AC transformer, as in, one AC transformer but different circuits from there?

 

[Winston OBoogie]

I have 4 x Neve 1290 channels, they need 24v at 500mA each

Surely this is a typo yes?  I can't see how a Neve pre amp circuit would be more than around 90 -100mA, most of that being the single-ended class A output transistor.

 

[Spencerleehorton]

Yes originally thought 100ma per channel but then looked in another thread and it said 500ma?
Would be good to know, I might test mine