Phantom power drop resistor

[Rob Flinn]

Actual current draw is more like 100mA as Winston says.

 

[Winston OBoogie]

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

Around 70mA would bias the output transistor sufficiently, a couple of mA for the other two transistors driving it.  And a guess would put each of the two pre stages of a 1290 at no more than 7 mA all told so, 80mA for a 1272 and 87mA for a 1290.  Allow 100mA per channel plus a bit more to keep the power transformer happy. 

 

[Winston OBoogie]

Actual current draw is more like 100mA as Winston says.

OK cool, thanks.  So I was a bit off but you get the picture

 

[abbey road d enfer]

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.

I've tried to figure out but I couldn't find any spec regarding current draw and the schemo is not much help in that regard. The OT has a ratio of 1:1.7. In order to deliver +26dBu into 600r, this computes to about 80mA peak at the primary, so the idle current in the 2N3055 must be about 100mA. I would take a margin for the other stages.

 

[Spencerleehorton]

Ah ok, now we seem to be getting somewhere, so 500mA should cover the 4 channels and the 48v supply.

So I take it that i need to build two psu, one for the 400mA supply at 24v and one for the 100mA supply for the 48v.

I have a 2 x 24v toroidal txf, could i then take a feed off one 24vAC winding for the 24vDC supply and use both windings for the 48v supply?
or would I need to have two separate toroidals?

 

[Winston OBoogie]

I've tried to figure out but I couldn't find any spec regarding current draw and the schemo is not much help in that regard.

Couldn't find an original Neve spec sheet either, the nearest I found was the clone built by BAE here: http://www.baeaudio.com/products/1272-module
Which says 24V @ 80mA.  No idea if that is as per the original.  Either way, the 1290 has an additional 3 transistor pre amp gain stage which won't add much.

 

[abbey road d enfer]

I have a 2 x 24v toroidal txf, could i then take a feed off one 24vAC winding for the 24vDC supply and use both windings for the 48v supply?

  It could work somewhat but I wouldn't recommend that. As I said earlier, the two PSU's should be separate as much as can be.

or would I need to have two separate toroidals?

That's what I would do. What power rating is your toroidal? For best performance I would use two identical xfmrs, although one would be very lightly load, I would mount them humbucking, so their magnetizing fields cancel.

 

[Spencerleehorton]

well i'm stumped, on one channel i have put the 48v to DPDT switch and connected the 3.9k then the led, put the 220uf cap across and put a gnd connection, when i put on the 48v yes the led lights up and yes i get 48v on pin 2 and 3, but on the scope the waveform goes to sh*t, loads of noise?

the other 3 channels i've got them configured like i had them which is wrong, the 48v goes to 220R then into LED and cap . 220uf cap, then a 10k on negative leg of led down to gnd but they all work and no noise?

gremlins or what!!!

The only difference is that I’m using a red led rather than blue leds on the other 3 channels without noise!?

i need some sleep!!!

 

[Spencerleehorton]

Tl783 has same pin out as LM317 so will look at repairing the Psu with the 317 which has blown.

 

[mhelin]

Doesn't TL783 require min. 15 mA load? That should be taken care somehow.

Discrete circuit like https://sound-au.com/project96.htm build using SMT on small PCB with 317 pins (pinout) would  be handy.

 

[abbey road d enfer]

Doesn't TL783 require min. 15 mA load? That should be taken care somehow.

This has been mentioned several posts ago. This should be taken care of by changing teh adjustment resistor to 82r instead of 240.
However, my experiments showed that it works with 240r, which results in 5mA, as soon as a load that draws more than 10 mA is connected. That would be the case if the LED draws 8mA or so and the mic 2mA.

 

[Spencerleehorton]

well i've swapped out the blown LM317 and put in the 783 and i'm getting 60v with 220R to output and 8.2k resistor to gnd.
so do i need to change 220R to 82R? and the 8.2k to 2.7k?

 

[abbey road d enfer]

well i've swapped out the blown LM317 and put in the 783 and i'm getting 60v with 220R to output and 8.2k resistor to gnd.
so do i need to change 220R to 82R? and the 8.2k to 2.7k?

yes, or draw about 10mA to a load.

 

[Harpo]

well i've swapped out the blown LM317 and put in the 783 and i'm getting 60v with 220R to output and 8.2k resistor to gnd.
so do i need to change 220R to 82R? and the 8.2k to 2.7k?

With 82R current setting resistor, a voltage setting 2K7 to gnd will give you about 43VDC out. Depending on parts tolerances (FI TL783 Vref 1.2 ... 1.3V with typical Vref 1.27V), you'll need about 3K to gnd for 48VDC out. A 500R trimmer in series to your 2K7 for a 2K7... 3K2 range to gnd will catch these tolerances for the wanted 48VDC output voltage. Just keep an eye on power dissipation for these shunt arm resistors.

 

[Spencerleehorton]

Ah ok great stuff, I’ve got some time this week to weed out and test this.
So for the 24v I’m gonna use a 7824 and for the 48v I’ll use the TL783 just need to test it all works.
Thanks for all your advise everyone.