Combi XLRs and Phantom Power

[jimh76]

Hi,

I have a Tascam DP32SD. It has combi XLR/Jack sockets. The phantom power is only switchable in banks of 4.

My problem is this, I need to use capacitor mics on inputs 1 & 2 and therefore need phantom power (which as I say will be on inputs 1-4) but will be going through external preamps on inputs 3 & 4 (GAP Pre73s).

How would I connect the GAPs to inputs 3 & 4 without frying anything? Can it even be done?

Thanks

Jim.

[James Perrett]

Properly designed gear shouldn't have phantom power on the line inputs. I know that this is something you need to watch with the Golden Age units as they have phantom power on both the jack and XLR inputs but I would expect that the Tascam is properly designed.

[Rich Hanson]

I can confirm as an owner of the baby brother DP24SD that there is no phantom power on the line (jack) inputs.

[Folderol]

FWIW my humble KA6 gets it right too

[Mike Stranks]

Not all Tascam gear inhibits phantom-power on the line inputs of combi inputs (or at least didn't in the past).

I had one of their 6 input portable multitrack-recorders from which I was getting odd results from using the line inputs until I realised what was going on.

Just a word to the wise...

[Wonks]

All you can do is stick a TRS cable in a combi input, turn on phantom power and use a multimeter to check for 48v DC between the T and S and R and S of the other end of the cable. If it's not there, you'll be fine.

If it is, then you can buy phantom power blockers. These are normally XLR based, so you'll also need suitable TRS to XLR cables.

[Hugh Robjohns]

How would I connect the GAPs to inputs 3 & 4 without frying anything? Can it even be done?

Everything is possible... you just have to figure out a solution!

Unfortunately, there are lots of unknowns here, so you'll need to do some research for yourself.

Firstly, 'standard' practice is that the centre TRS jack connection of a Combi-jack shouldn't carry phantom power. Most manufacturers now stick to that policy, but certainly not all and I would never assume it to be the case.

If there is no phantom present you can plug your preamps in to the quarter-inch input sockets using TRS-TRS cables (or XLRf-TRS) and carry on without concern... but you need top check first.

As Wonks described, if you don't have a convenient phantom-power checker, a cheap multimeter will get the job done. Plug a TRS-TRS cable into the combi input, Switch on phantom power, set the meter to a DC volts setting, and measure between the sleeve (black) and ring (red) and/or sleeve (black) and tip (red). If you measure 48V or something close to it, phantom power is present. If you see zero (or something close to it) you're fine.

If there is phantom power present then you may still be fine if The Gap73 has been designed to cope with phantom power present on its outputs. This will depend on how the outputs are connected. Devices with output transformers are often quite happy if phantom power is present, for example. Most devices have DC blocking capacitors feeding the outputs, and these might be able to cope with phantom power on the line... or they might not!

I don't know what the situation is with GAP, so I would suggest contacting them directly and asking whether your specific model of preamp is safe to handle phantom power on its output connection.

If it isn't then you will need to block phantom power from reaching the GAP, and you can do that in a couple of ways. My preference would be to use a line isolation transformer box, like the ART Cleanbox 2 or DTI box (both two -channel)... but if you're running the preamps hard you may find that the transformers tend to saturate and alter the tone somewhat... which could be a desirable effect! More expensive transformers like those from Jensen or Lundahl are less prone to this!

You can also get inline XLR phantom blockers that use capacitors.

A third option, and actually the one I'd favour personally, would be to invest in a mains-powered standalone phantom power unit to power your capacitor microphones. That way you can leave phantom turned off on the Tascam, and no longer have to worry about anything at all!

[jimh76]

Wow cheers Peeps.

All great advice.
I might try the multimeter way first and then contact GAP if the lines are hot.

I chatted with Bo Medin (GAPs founder) via email in the first lock down as there were a few spelling mistakes on their website which I noticed. I offered to check the website and Bo himself was emailing me back and forth as we corrected spelling and words etc (English is not their first language). As a thank you they sent me a specialist rack tray for my 2xPre 73s for free! What a guy and what a company....

Thanks for all your help.

Jim.

[jimh76]

I checked it.

All good. The line-in doesn't receive +48v.

Thanks for everyone's help with this.

Interestingly, when there is +48v on the xlr, I was amazed how slowly it takes after turning off, to return to +0v.

Thanks

Jim.

[Sam Spoons]

That was discussed in another thread recently and it is usually by design* to prevent switch on/off noise spikes, pops, splats etc. The power ramps up and down slowly so it doesn't cause a transient and it's associated noise.

* Or is it discarging capacitors?

[ConcertinaChap]

As a thank you they sent me a specialist rack tray for my 2xPre 73s for free! What a guy and what a company....

You're right, that is a nice gesture.

CC

[Hugh Robjohns]

I checked it.
All good. The line-in doesn't receive +48v.

Excellent. That's as it should be, but I've come across a few examples where phantom appears on TRS connectors so it's worth checking.

Interestingly, when there is +48v on the xlr, I was amazed how slowly it takes after turning off, to return to +0v.

In real life, with mics connected, it would decay much quicker as charge drains from the reservoir capacitor as current flowing into the mic. A digital voltmeter has a very high input impedance, so will draw hardly any current and consequently the supply caps will take a very long time to discharge.

[PhilS]

Hi. I recently bought a Steinberg UR824 on eBay. A great buy it is too!

I was pondering the question about phantom power being present on the jacks of the combi inputs (or not) so this thread was exactly what I was looking for. I checked the block Diagram in the Operation Manual, and although I'm not expert in reading electronics diagrams, I *think* it is saying that the jacks WILL have 48V applied when phantom power is switched on.

So I checked with my multimeter and here are the results:

With phantom power switched on there was a fluctuating reading of < 2 mV present. After switching phantom power off and measuring again, there was initially around 500 mV present(!) which fell rapidly so that after a few seconds it was down to less than 100 mV, and then continued to fall, but more slowly down to less than 10 mV. Repeating the experiment (switch the phantom on and then off again) yielded similar results with the initial reading reaching as high as 1V on a couple of occasions.

Bear in mind that this is on the jack socket, not the XLR, so it is not quite the same behaviour as described above, although it does suggest a discharging capacitance. But can someone explain how this can happen, when there is next to no voltage when the phantom power is actually on? Also, is 1V something I should worry about being applied to the outputs of my line level gear?

Thanks.

[Hugh Robjohns]

1V isn't going to upset anything, and it was only the charging transient, anyway.

The small milliVolts values will be leakage through the blocking capacitors and probably varying slightly as the contact impedance of the meter varied.

So, overall, there are no unwanted phantom worries there.

[PhilS]

Thanks Hugh. The reassurance is appreciated