Max. obtainable signal output voltage with P48 phantom power

[MicUlli]

Hi All,
driven by some discussions around the topic "output voltage" of a condenser mic i tried to find some useful math for calculating the "impossible".
Here is the first simple approach.
The max. DC power the P48 input can deliver to a microphone is 170mW.
If we use a class A driver stage for the AC-signal it has a max. power efficiency of 0,25 -> 42,4mW AC output power.
Using a class B driver stage with a power efficiency of 0,785 delivers 133,5mW AC output power.

Most audio interfaces rely on a differential input impedance of 3kOhm.

This leads to 11,3V rms for class A and 20V rms for class B.

BTW: I have never seen a mic that is capable to deliver such a high output level..

Your thoughts are welcome

BR MicUlli

 

[kingkorg]

Hi All,
driven by some discussions around the topic "output voltage" of a condenser mic i tried to find some useful math for calculating the "impossible".
Here is the first simple approach.
The max. DC power the P48 input can deliver to a microphone is 170mW.
If we use a class A driver stage for the AC-signal it has a max. power efficiency of 0,25 -> 42,4mW AC output power.
Using a class B driver stage with a power efficiency of 0,785 delivers 133,5mW AC output power.

Most audio interfaces rely on a differential input impedance of 3kOhm.

This leads to 11,3V rms for class A and 20V rms for class B.

BTW: I have never seen a mic that is capable to deliver such a high output level..

Your thoughts are welcome

BR MicUlli

22:30 time stamp. Tube mics tho.
Røde NTK, K2.

 

[MicUlli]

Granted, but not the use case i asked for.

In simple directly driven mic designs (i.e. Schoeps, Octava, MicUlli..), that means no transformer and no DCDC converter,
the maximum obtainable output voltage (rms) is

Urms = Idc * Zin * 0,3.

Idc is the total current driven from the phantom supply by the output stage,
Zin is the differential input impedance of the power supply,
0,3 accounts for a safety factor multiplied with 1/(2*sqrt(2)).

With Idc = 4mA, Zin = 3kOhm: Urms = 3,6V (11,1 dBV and 13,3 dBu).

For extreme conditions P48-spec. allows to source up to 10mA -> 9V (19,1 dBV and 21,3 dBu).

Some audio interfaces like Neumann have a very high Zin (10kOhm). Other interfaces have a high Zin when PAD is switched.
For my taste a higher Zin is always preferable (max. theoretical value is 13,6kOhm).

So long MicUlli

 

[dimitr1]

higher Zin is always preferable (max. theoretical value is 13,6kOhm)

You can bootstrap 6.8k resistors for higher Rin

 

[Khron]

In simple directly driven mic designs (i.e. Schoeps, Octava, MicUlli..), that means no transformer and no DCDC converter,
the maximum obtainable output voltage (rms) is

Urms = Idc * Zin * 0,3.

That's probably "permanent" or "long-term" though, right?

 

[MicUlli]

That's probably "permanent" or "long-term" though, right?

Yes, and it is a worst case value. In a circuit with a Reservoir cap and class ab output stage the rms output voltage is up to 3 times higher..