Neve 1290 questions

[Spencerleehorton]

Yep, trimmer problem bang on!!

I’ve changed the 47R 2w to a 1k 5w and I’m getting 62v at first 1000uf cap now!
Don’t know if this is the way it should be done but it all works now!

 

[PRR]

> 63v cap is getting 75v!!

If you don't want to replace that cap for decades, it should be 100V. Even if it comes down on load. You never want to be close to the rating for long-term life.

 

[Spencerleehorton]

Yes, I should change that one for 100v, I just wondered why in these schematic designs that it states 50v or 63v for that cap?
Also with the voltage divider arrangement of the 2 x 22k and 2 x 1000uf at this point as per diagram does this bring voltage down?

 

[Spencerleehorton]

Ok I think I understand what Rob has put now.
Using 2 x 1000uf in series along with 2 x 100k or (22k on the JLM circuit) resistors in parallel with the caps this then voltage divides so each cap sees 37.5v rather than 75v.
But I take it that the regulator still gets a potential 75v?
If this is now correct I will change circuit to accommodate the extra cap and resistors.

 

[Rob Flinn]

Ok I think I understand what Rob has put now.
Using 2 x 1000uf in series along with 2 x 100k or (22k on the JLM circuit) resistors in parallel with the caps this then voltage divides so each cap sees 37.5v rather than 75v.
But I take it that the regulator still gets a potential 75v?
If this is now correct I will change circuit to accommodate the extra cap and resistors.

Yes, it distributes the voltage better across 2x 1000/63 caps.  As you say 37.5 across each.  However 2 similar caps in series halve the capacitance, so the effective capacitance would be 500uF.  Therefore if you upped the value to 2200/63 you would wind up with 1100uF which would probably be better.  Or you could just get a 1000/100 ........ As they say "the choice is yours"

 

[Spencerleehorton]

Much appreciated with that, I understand it now, have done a prototype and works nicely.
Will stick with 2 x 1000uf for the moment, at a later stage I will get some 2200uf / 63v or 50v.

You suggest 2 x 100k but I ended up putting in 2 x 22k as per JLM, is this ok?

Also when i put 48v on there is a drop of about 3-5v, should i put a cap (220uf) in there and would this stop the drop?

 

[Rob Flinn]

Much appreciated with that, I understand it now, have done a prototype and works nicely.
Will stick with 2 x 1000uf for the moment, at a later stage I will get some 2200uf / 63v or 50v.

You suggest 2 x 100k but I ended up putting in 2 x 22k as per JLM, is this ok?

Also when i put 48v on there is a drop of about 3-5v, should i put a cap (220uf) in there and would this stop the drop?

The 2 x 100k or 22k are there to ensure that the voltage across each cap is half of your 75v.  If they are not there & one cap starts to fail then the other cap could have more than 63v across it.  These resistors can serve another purpose to discharge the caps slowly when the unit is powered off, which can be useful if, for example, you have 400v across some psu caps in a valve unit & I normally add them so it's safer to work on the unit more quickly.

As for the value (100k instead of 22k) it won't make a massive difference at 37.5v I just plucked the value out of thin air.    I normally use 100k in valve gear just to stop them getting too hot.  The higher voltage means that if they were 22k several times the current would be flowing in them compared to 100k.  However it's worth noting that they are actually discharging when they are operating normally, albeit slowly, so they do very marginally affect the amount of ripple rejection that the caps supply, but not enough to effect things really. 

With regards to the slight voltage drop when connecting condenser mics, changing that cap value I don't think will affect the static voltage. The caps are there to reduce ripple, not increase or decrease static voltage.  That is the job the regulator can do.  Even if your 48v is 45v it's not a big deal.  most condenser mics can run on quite a wide range of phantom power voltages, take a look at some spec sheets.

For phantom power unless you are supplying loads of mics the current draw is next to nothing so you probably don't need to worry about using larger value caps to make the combination 1100uF.  Unless you hear hum or have a problem I would just keep it as is.  If it aint broke ..............

 

[Spencerleehorton]

Fair play mate, all noted.
I think I’ve seen my mistake in the wiring of the led and the 48v and that’s why I’m getting a drop.
Will rewire once I’ve walked the dog!!

I’ve tried the api way of phantom power but I only get 1.2v?
The led lights up but tested pins 2 and 3 on xlr and only getting 1.2v?

This should be so easy!!!!

I want to wire this with dpdt switch and led, will check I’ve put in 6.8k rather than 68k or 680k!!
But think I’ve got this wired correctly but obviously haven’t!!
Could anyone point me to a clear schematic of this please?

 

[Spencerleehorton]

Double checked wiring and it’s still not right!!
6.8k resistors, within 1%, used 3.9k for led, used 510R for resistor which connects to 6.8ks.
Getting 48v at Psu but only 1.2v at pin 2 and nothing at pin 3?

Aaahhhhhh!!!!!

 

[Spencerleehorton]

Ok I’ve done some maths and this is what I’ve found.

48v , need to drop 46v to give 2v at 15ma for the blue led to be within it ideal operation.

46 / 0.015 = 3000 ohm (3k)

So I’ll lower the drop resistor and see what gives.

 

[Spencerleehorton]

Did a quick test and without a xlr lead plugged in I get 43v to pin 2 and 43v to pin 3, as soon as I plug in an xlr cable the voltage goes down to 1.2v on pin 2 and nothing on pin 3?

Very strange?

 

[Rob Flinn]

WHat happens if you use  resistors to draw 10mA from pin 2 & pin 3.  That is a a typical(ish) current draw for a condenser mic ? Measure it with the load. 

Also I'm not sure I would use a feed resistor to the pair of 6k8 personally.

Since you are getting a massive drop out when the xlr is plugged in are you absolutley sure you have the xlr wired up correctly ?    You would be shorting the phantom if for example you had pin 2 & 1 swapped.

 

[Spencerleehorton]

I will test xlr but I’m not even plugging in the mic it’s just the cable im plugin in!!!
What a nightmare, seems all I get is problems at the moment!!!
I’ll take the 510R feed resistor out and should I reduce the 3.9k to 3k?

 

[Rob Flinn]

I will test xlr but I’m not even plugging in the mic it’s just the cable im plugin in!!!
What a nightmare, seems all I get is problems at the moment!!!
I’ll take the 510R feed resistor out and should I reduce the 3.9k to 3k?

If you are just plugging the cable in there shpuld be no drop at all.  It does seem rather odd that plugging a cable in with no mic is pretty much giving you a dead short. Can you easily disconnect the 10468 & check the phantom voltages without them beng connected to the mic amp, if you get my drift ?


For the led I normally run them at around 10mA or less with modern ones.    So for me I would be looking at 4k7. 

 

[Spencerleehorton]

Ok, I’ll try disconnecting input txf and also try another one and change resistor for 4.7k

 

[Rob Flinn]

Only make one change at a time before retesting or you will lose track of what's going on.

 

[Spencerleehorton]

Ok I got it going now!! Finally!
The xlr cable is shot, also the switch was playing up so I replace both and now it seems fine!?
I’ve set at Psu to 51v and when phantom is on it drops to 48v at Psu.
Then at pin 2 and 3 it’s at 35v.
On the 1290 I’ve raised the voltage at Psu to 31v so I get about 22v at 3055 transistor

 

[Rob Flinn]

I’ve set at Psu to 51v and when phantom is on it drops to 48v at Psu.
Then at pin 2 and 3 it’s at 35v.
On the 1290 I’ve raised the voltage at Psu to 31v so I get about 22v at 3055 transistor

Do you get the phantom dropping after the 6k8's when you only have a meter loading the PSU ?  If so that doesn't seem right to me.  The meter will prob have a Z of 10M & therefore you wouldn't be loading the supply enough to see any drop.  You could try disconnecting the led & checking because the led will be the only thing drawing any current if you have no mic connected.  If no significant current os being drwan then you should be measuring the same V on the 48v side & the 10468 side of the 6k8's

 

[Spencerleehorton]

Well I’m getting hum when dynamic mic is plugged in without 48v and not when using 48v the hum goes?

Actually there is a slight hum which comes and goes and I think I know why.

The cap on the 48v I’ve attached the neg leg to pin 1, so I’m going to put this after the input txf and attach to the 48v switch.

If there is still hum I’ve probably not done the gnd correctly.

From what I can see the 0v doesn’t attach to gnd?
A 10R is used between 0v and gnd, just checking I have I got this right!

My input txf has been put in mu-metal and I’ve connected the can to 0v.

 

[Spencerleehorton]

I’ll try a different input txf that I know is good just to check it’s ok