Why 48v for phantom power? - explained

[Wonks]

Short video from the Audio University YouTube channel with Neumann's microphone director, Martin Schneider, on why 48v DC was finally chosen as the voltage for phantom powering mics.

https://youtu.be/3oLfx36shI0?si=AmJ0wvZg9DRUl75z

[Hugh Robjohns]

...And the famous KM84 was Neumann's first phantom powered mic!

Lots of European broadcasters had 48-50V DC supplies to hand as backup (redundant) supplies — not just Norway — not least because much of the technology and installation practices came from the telecoms industry which had maintained 48-50V supplies since the turn of the 20th Century. Self-dial phone systems across Europe and wider had also used a 50V phantom power system to energise subscriber phones since before WW1. So Neumann's 'invention' was more a novel re-application, really.

Interestingly, there was an earlier form of phantom introduced by Schoeps in the 60s which used -12V instead of the +48V we use now. I've seen it on some Nagra tape machines, but it didn't really catch on.

The most popular mic powering system prior to phantom was AB or Tonader (T-) powering which was 12V applied between the hot and cold signal wires. It was used widely on portable recording equipment right up until the late 90s.

A major disadvantage of T-power is that any power line noise is added directly to the audio signal. Phantom power doesn't suffer from that problem (provided the 6k8 feed resistors are very closely matched to maintain common-mode rejection).

[Martin Walker]

A major disadvantage of T-power is that any power line noise is added directly to the audio signal. Phantom power doesn't suffer from that problem (provided the 6k8 feed resistors are very closely matched to maintain common-mode rejection).

Ooh, that's a good point Hugh - 1% resistors are the order of the day!

[ef37a]

A major disadvantage of T-power is that any power line noise is added directly to the audio signal. Phantom power doesn't suffer from that problem (provided the 6k8 feed resistors are very closely matched to maintain common-mode rejection).

Ooh, that's a good point Hugh - 1% resistors are the order of the day!

Or better! If you are building/refurbing a rake of 1 percent MFs are cheap enough and you can match them to better than 0.1% with even a cheap DMM.

They don't of course have to be bang on 6k8 just very close to each other.

(I do hope my abbreviations are not confusing peeps!)

Dave.

[Martin Walker]

A major disadvantage of T-power is that any power line noise is added directly to the audio signal. Phantom power doesn't suffer from that problem (provided the 6k8 feed resistors are very closely matched to maintain common-mode rejection).

Ooh, that's a good point Hugh - 1% resistors are the order of the day!

Or better! If you are building/refurbing a rake of 1 percent MFs are cheap enough and you can match them to better than 0.1% with even a cheap DMM.

They don't of course have to be bang on 6k8 just very close to each other.

(I do hope my abbreviations are not confusing peeps!)

Dave.

Exactly!

When I wrote up DIY details for others to add multiple filter 'shape' options to their Mutable Instruments Shruthi synths, I stressed that in various cases the exact values didn't matter, but that:

"N.B. The unusual resistor vales are the exact values needed for the ‘perfect’ filter response, but in practice you can probably get away with nearby stock values. However, if you have a selection of resistors on hand to choose from, the important thing is the RATIO of the resistors used in each network."

And for example:

"For the Notch filter, you could use two 4k7 instead of 4k99, as long as they are both the same value, but then the one marked ‘10k’ should be exactly double this (or two resistors in series such as a 9k1 and 330 ohm). The quality of the match here determines how deep your notch is."

[Folderol]

Indeed, very good points both the above.
In filters, capacitors are the bigger headache as it's more difficult to get close tolerance ones - or indeed to measure and match. Where possible, my policy is the try to work to the most common capacitor values that you'll be likely to want elsewhere and buy bulk, then adjust the resistor values to suit the capacitor values you actually have.

[ef37a]

Indeed, very good points both the above.
In filters, capacitors are the bigger headache as it's more difficult to get close tolerance ones - or indeed to measure and match. Where possible, my policy is the try to work to the most common capacitor values that you'll be likely to want elsewhere and buy bulk, then adjust the resistor values to suit the capacitor values you actually have.

For the hobbyist where time and to some extent, money is not an issue you can improve capacitor tolerance by using multiples in parallel. I would have to check but I think tolerance improves by the root of the number of caps of the same value? Then of course, most decent DMMs will have a capacitance range but building a cap' bridge is pretty easy if you have a stable signal source. OP amps are very accurate these days.

Dave.

[Wonks]

The theoretical bell curve distribution for tolerance will improve with multiple caps or resistors, so you are more likely to get a value closer to the ideal, but if you happen to fit two items both at +10%, then the total value will be at +10% as well.

You’d be hoping that one would be +X% and the other at -Y% to get closer to the nominal value, and you may well get that. Even one at +10% and one at +6% is better than just one at +9%.

But I generally find checking values that a given supplier’s products are normally almost all under, or almost all over, the nominal value, with a few outliers. E.g. I have boxes of 500k and 250k audio taper pots for guitars use that I’ve measured the resistances of and marked on the pots. Invariably almost all the CTS pots will be under the nominal value, with maybe a couple just sneaking over, whilst Alpha pots will tend to be closer to the nominal value but on the positive side.

[James Perrett]

Over the years manufacturers have improved their processes and it is now fairly easy to produce close tolerance components. Nowadays, if you test a bunch of capacitors you will often find that they are under the specified value but still within their tolerance. The reason is that the manufacturer can use less material to produce the capacitor if they aim for a value, say, 5% under the nominal value yet all the capacitors will still conform to a +/- 10% tolerance.

[Wonks]

I bought a mixed box of ceramic caps from Amazon for occasional use. The smaller values were pretty much spot on but the bigger values were pretty much all at the very bottom end of the tolerance limit.

[James Perrett]

I bought a mixed box of ceramic caps from Amazon for occasional use. The smaller values were pretty much spot on but the bigger values were pretty much all at the very bottom end of the tolerance limit.

You were lucky! I bought a similar mixed box from Amazon and the first one I tested was around a third of the value that it should have been. Others were similarly out. Fortunately Amazon took it back with no problems and I now stick to reputable suppliers for components.

[Martin Walker]

This is the digital multimeter that I bought a year or two back (at the time it was only £19.99, but is still a reasonable £26.99).

Its capacitance function works well, since although by default it typically measures around 1nF with nothing connected to its test leads, once you press the REL button this value is 'zeroed', whereupon the measurement seems accurate and repeatable within a few picofarads. It's certainly helped me match capacitors quite a few times!

https://www.amazon.co.uk/Proster-Multim ... r=8-5&th=1

[The Red Bladder]

I occasionally get asked "But why 48 volts? Why not 55 or 60 or whatever?"

The answer is The Law. Under post-war German law governing domestic and working environments, any voltage above 50V had to be treated as a mains power supply and therefore must comply with electrical installation laws. That would have meant connecting your mics and telephones with a Schuko plug and all installed cabling would have had to be behind plaster, 10cm below ceiling height for horizontal runs and be perfectly vertical for vertical runs and pinned securely throughout. (Similar laws applied in most European countries.)

The exception is animal barns where the highest 'open' permitted voltage was 25V. Fortunately cows and horses seldom use microphones.

[ef37a]

I occasionally get asked "But why 48 volts? Why not 55 or 60 or whatever?"

The answer is The Law. Under post-war German law governing domestic and working environments, any voltage above 50V had to be treated as a mains power supply and therefore must comply with electrical installation laws. That would have meant connecting your mics and telephones with a Schuko plug and all installed cabling would have had to be behind plaster, 10cm below ceiling height for horizontal runs and be perfectly vertical for vertical runs and pinned securely throughout. (Similar laws applied in most European countries.)

The exception is animal barns where the highest 'open' permitted voltage was 25V. Fortunately cows and horses seldom use microphones.

Hmm, how do they treat 100V line speaker systems then? Our colonial friends use 70.7V centre tap to ground IIRC.

Dave.

[James Perrett]

I occasionally get asked "But why 48 volts? Why not 55 or 60 or whatever?"

The answer is The Law. Under post-war German law governing domestic and working environments, any voltage above 50V had to be treated as a mains power supply and therefore must comply with electrical installation laws.

I seem to remember that there is a 60V limit in the UK (and possibly in the EU too nowadays). It was a consideration when we were designing the batteries and charging system for our underwater vehicles.

[The Red Bladder]

Hmm, how do they treat 100V line speaker systems then?

Excellent question! And I honestly do not know. As part of my training, I had to learn about installation laws and the rather boring history of same. I never even thought to ask back then about 100V speaker lines!

I am guessing that 100V lines have to be installed just like a mains line and I do know that certain plugs (possibly Speakons?) are permitted for limited applications. I looked online at German 100V speaker suppliers and they show the cables being captive (i.e. no plug.)

On the back of the amp, they have cable clamps that are covered with a plastic cover. So it looks as if they are treated as mains supplies.