Neve 1290 questions

[Spencerleehorton]

Have got a prototype circuit built but I’m designing a single layer version which will be much smaller.
Going to test, fingers crossed!!!

 

[Spencerleehorton]

Getting smoke!!!
Have ordered some more LM317 as run out and all of them blew!!!
Shame they don’t do a LM7848 as would simplify the circuit using 7824 and 7848!!!
Pretty sure the circuit is correct now but need to use a new LM317 just to make sure.

Psu’s I’m always having a hard time with so this will be very rewarding once I have this down!!

 

[Rob Flinn]

Getting smoke!!!
Have ordered some more LM317 as run out and all of them blew!!!
Shame they don’t do a LM7848 as would simplify the circuit using 7824 and 7848!!!
Pretty sure the circuit is correct now but need to use a new LM317 just to make sure.

Psu’s I’m always having a hard time with so this will be very rewarding once I have this down!!

I have quite often made 2 x 24v regulated psu's from a 2x24va.c transfomer using a seperate half of the trafo and a seperate 7824 for each psu.  I then just stack one on top of the other for 48v.  It means the 48v is way over specced but it is a simple way to get it going.

 

[Spencerleehorton]

Yeah I can’t do that with this config Rob as I have 2x12vac already in series giving me 29vac.
I’m going to print a pcb which I’ve taken from the PoS48 schematic and see how I get on.
I think I need to ditch the ssl9k version as it’s way off.

 

[Potato Cakes]

Spence,

How are you going to generate phantom power? Granted if you are getting 29VAC on your secondaries, that rectified will give you about 40VDC (AC x 1.4), which is plenty for a large number of condensers and active DIs, but it will be short of the 48V that most people are expecting when doing tests.

Thanks!

Paul

 

[Rob Flinn]

Spence,

How are you going to generate phantom power? Granted if you are getting 29VAC on your secondaries, that rectified will give you about 40VDC (AC x 1.4), which is plenty for a large number of condensers and active DIs, but it will be short of the 48V that most people are expecting when doing tests.

Thanks!

Paul

Easy.  Feed a second bridge rectifier from the 29v a.c via some 100uF caps, build a second supply of 24v & stack it on top of the first one.  This is possible because it's isolated by the caps.  It's done a bit like in the attached pdf but the main supply is 24v not bipolar 16v

 

[Spencerleehorton]

I’ve taken this design and built a pcb, this seems to be a nice design?

 

[Rob Flinn]

It doesn't look quite right to me Spencer.

Assuming that you are making 2 x 24v supplies and stacking them since you won't get 48v from one psu with your transformer.  The 0v path of the top PSU including the bottom of C13 need to connect to the output of reg IC1.  Think of them as 2 completely seperate PSU's but you happen to be connecting the negative side of the top PSU to the postive side of the bottom PSU.  A bit like stacking 2 x 9v batteries to give you 0v, 9v & 18v

 

[Spencerleehorton]

It’s a bonafied design by silent arts!!!?

I thought this was how you voltage doubled?

I need a design using just two AC feeds rather than the centre tapped way.

I do have another 2 x 24v but it’s not a toroidal which I could try, as this would have the centre tap for 0v.

Would be good to clear this up as this I think is where my confusion is!

 

[Rob Flinn]

Actually looking at the diagram again it would work because c13 is connected to the lower reg feed it would stack the volts on top.  I guess it's just not quite how I would have done it.  But if the phantom gets shorted the LM317 in the top PSU would die because they are only specced for max 37v across them.  You would do better using a TL783 in this position because it can handle more volts across it.  It is a drop in replacement for a 317.

 

[Spencerleehorton]

Perfect, I’ll order some 783 then!

Thanks again Rob.

 

[Spencerleehorton]

right, ill be honest, i'm getting very annoyed with this, this should be basic stuff and i'm still not getting correct results.Have built new pcb and put in components and hey presto smoke and then several blown 317's!!!!

so ive looked at the JLM schematic again and can see that from the positive bridge rectifier of the first section (i'm calling this the 24v section) this positive feed goes to the negative feed of the second bridge rectifier, this is in tripler mode, so essentially i would get 3 x 24 = 72v which is what i get at the positive of the second bridge rectifier (48v section)

well this is too much for my caps as they are only 63v!!!

so Ive then changed this to doubler now which would be to take the 0v of the 48v section and connect to the negative of the 48v bridge rectifier.
Now yes i get lower voltage but i don't get any adjustment on either trimmer? both voltages sit at 37v?

this is driving me insain!!!

are all these designs just built on the fact that the power transformer needs two AC feeds and a centre tap?

 

[Spencerleehorton]

how embarrassing, had the LM317's the wrong way round!! oops.

Getting 24v adjustable and fine, 48v is just at 72v as i've put it back to tripler now.

Its must be as you've said, too sensitive with the amount of volts going across, i will wait until i get a TL783 to go in the 48v section then retest.

Thought i would be ok with the LM317's!!!

 

[Rob Flinn]

Its must be as you've said, too sensitive with the amount of volts going across, i will wait until i get a TL783 to go in the 48v section then retest.

Thought i would be ok with the LM317's!!!

It should be ok with 317's.  The issue arises if the 48v gets shorted accidently like when people have mic inouts on a patchbay & forget to switch the 48v off before they start patching around.  The 317 will only take 37v across it. i.e input to output.    You should only have 24v (72-48).

 

[Spencerleehorton]

maybe in the confusion i've put in a duff 317 again!!

I'll order some more and see if i cant get to the bottom of this!!!!

 

[Spencerleehorton]

My One bone of contention is that on the 48v part after the diode bridge it will be 72v so 72v will hit the first 1000uf which is only 63v!!!
On some JLM schematics with the tripler it has that first cap as only 50v??
Should I put a high wattage resistor to take the voltage down?
Or is that what the 47R does?

I see on the JLM version that it uses 2 x 22k resistors and 2 x 1000uf, could anyone explain this config please?

 

[Spencerleehorton]

By my calculations the resistor drop should be 10v which with a current for the led and the mic would be approx 0.02A which would mean using a 500R 1/4 watt to drop 10v rather than the 47R 2W?

Can anyone explain this please as when I work out using 47R it’s barely 1v drop?

 

[Spencerleehorton]

Right, I’ve now put in the TL783 and it’s nearly working!!! But not!!

24v line is still fine, on 48v I’m getting 75v at diode bridge and at input to TL783 I’m getting 75v so that means the first 1000uf / 63v cap is getting 75v!!
I know it’s only just over but didn’t leave on for long and no smoke.
The more concerning thing is that I can’t adjust to 48v, I’ve put in 5k trimmers and 240R resistors and no matter whatever I do I either get 12v or jumps up to 75v!!?

So close I think now!!

Help please

 

[Spencerleehorton]

Is it possible that Ive damaged the 1000uf cap on 48v and that’s why I’m getting these strange readings and can’t adjust to 48v?
I will change that cap and see

 

[Rob Flinn]

Is it possible that Ive damaged the 1000uf cap on 48v and that’s why I’m getting these strange readings and can’t adjust to 48v?
I will change that cap and see

Probably not.  If you have 2 1000uf caps put them in series with a 100k resistor across each one (this makes sure that they have 37.5v across each one).      I think your problem is more likely to be something to do with the trimmer.