Phantom Power on line level inputs.

[hugo_zair]

I'm stripping and old 12-2 mixer. It's branded "Audiotech" but can't find anything about it on tinterent. I thought about cutting the preamp section off the channel strips (it uses NE5532P opamps) and so set about tracing the circuit. I was surprised to see that the Line input jacks are connected via 22k resistors in parallel with to the XLR mic inputs - no back contacts or capacitors involved.
I haven't actually ever powered this up but it's obvious that there will be volts on the line in jack via about 29K.
I'm just wondering if there are other mixers that do this and whether there would ever be any issue with gear plugged in there with a mono jack.
In this particular mixer the phantom power line on each channel strip is decoupled with a 15v working cap so phantom voltage in this case may have been only 12v.

 

[JMFahey]

I haven't actually ever powered this up but it's obvious that there will be volts on the line in jack via about 29K.

Don't guess, measure.
And post the preamp circuit you've traced.
I don't think you will easily "cut the preamp section off the channel strips"; I would be very surprised if you could actually cut the end of a pcb with a Dremel wheel or hacksaw and ended up with a functional block.
If you need the balanced input stage of a Mic preamp, why just not build that stage on perfboard or something?

 

[hugo_zair]

Hi..This old mixer came to me without it's external psu and it was too much trouble to bodge up the power. I did measure the resistance between the input jack and the XLR and it was the sum of 6.8K and 22K.
Just for fun I've posted pics to show the severed preamp and the circuit diagram..I don't NEED to get a preamp as I've got several but after all this is DIY Audio and I just like tinkering !

 

[JMFahey]

Yes, clearly you'll have phantom DC voltage at Aux in pads, what were they thinking?
Only justification would be having an individual Phantom switch per channel; in that case you would not turn it on unless plugging a Mic which needed it.
But if they have a Master Phantom switch, forget it.
Just a small detail, you should have 1 22K resistor going to each XLR pin (2 & 3), not both to 2.
And the design is poor, they attenuate the already weak Mic signal by 6 dB (470/470 ohms) straight at the input, then they try to gain too much all at once (47K/470r).
They should provide for , say, 5X to 10X at the first Op Amp (470r/4K7 or so) and then , say, 2K2 or 4K7/47K .
Gain range will be much smoother.
Not to mention less noise.
Oh well.

 

[Frank Berry]

You could move the line level input resistors to the other side of the input coupling capacitors. That will keep phantom voltage away from the line input.

 

[hugo_zair]

Quick reply..There is a silly mistake in my drawing. The line input connections should be to each input line, not the same one. I'll reply again asap..

 

[hugo_zair]

JM :

Just a small detail, you should have 1 22K resistor going to each XLR pin (2 & 3), not both to 2.

Yes that was a mistake in my drawing.
JM:

Yes, clearly you'll have phantom DC voltage at Aux in pads, what were they thinking?
Only justification would be having an individual Phantom switch per channel; in that case you would not turn it on unless plugging a Mic which needed it.

This mixer has a common phantom switch!

And the design is poor, they attenuate the already weak Mic signal by 6 dB (470/470 ohms) straight at the input, then they try to gain too much all at once (47K/470r).

I have a feeling that this practice occurs in a lot of consumer/prosumer gear.
As far as the gain is concerned (for the mic input) am I right in assuming the 1st stage gain is unity and the max gain of the second stage is 100. Not really enough for a mic preamp. Perhaps there was more gain in the later strip output circuits ?
Frank:

You could move the line level input resistors to the other side of the input coupling capacitors. That will keep phantom voltage away from the line input.

Wouldn't that mean that the input would be vulnerable to dc ofset in the input signal ? In practice, if I ever actually build something with this (these actually as I have 11 of them !!) I would leave out the line input and make it XLR/mic only.
This mixer is solidly built in a steel chassis and when it was built I would think it wasn't a particularly cheap one. Maybe intended for a gigging band etc.
But the the real purpose of posting was to mention the phantom voltage appearing on the line input jack which I've read many times does not happen. But in this case it does - admittedly at high impedance..With 35v phantom voltage the max current would be limited to just over 1 milliamp.