Step down input transformer for Sennheiser MD211N

The MD211N sounded bass light on the Pleiades V6 because the inductance of the input trasformer used is only 140mH.


The Grampian DP4/L sounded great as it is a 25 ohm mic.


Why does the primary winding of an audio transformer affect so much the bass response?. Impedance is proportional to both inductance and frequency. Just like a door, its impedance to us moving it back and forth depends on its mass and the frequency of the oscillation.


And we have a potential divider. The mic's output impedance in series and the inductive load in parallel or shunt. The mic the mic is loaded the more the voltage drop so the less volatge that appears at its output terminals. Like we trying to move the door back and forth at a great amplitude but we cannot. It is the energy conservation principle chasing us (perhaps a new episode for Canelina the cat cartoons).


But if the MD211N impedance is stepped down by a transformer to 25 ohms! Or Sennheiser designs another world class version of this mic with 25 ohms voice coil...


Anyway reducing the impedance by 10 needs a turns ratio of about 3:1. Since 3x3=9 or nearly 10.


That should conserve the bass response of the mic.


But this is equivalent to saying that for the given secondary the primary can be increased in turns so that the transformer ratio which now is 1:10 can become 1:3.


The 200ohm impedance is converted to 200x3x3 or roughly 2000 ohms.


The EF183 at the Pleiades V6 conditions is so low noise that it can operate in a great way with a driving impedance of just 2Kohms, see next euroelectron post on grampian DP4/L.


More experiments need to be done. The step down idea. This can be easily tried with the 1:2 BBC transformer connected the other way round.


Or making a 1:3 to 1:5 transformer, or adding a winding to the exostibng Pleiades trasformer.


The turn ratio is so small that an excellent studio quality transformer can be wound easily enough on a Magnetec 070 core with relatively few turns.


Another way of looking at the same thing. If the existing 1:10 transformer has a primary inductance of 140mH then by doubling the primary turns the inductance will go 2x2 or four times up. This will increase bass response by 2 octaves! And this becomes a turn ratio of 1:5, since the primary is double turns with secondary fixed.

 
The loading on the MD211 will be very small.


If the turns ratio is 3 and the impedance conversion is 9 (say10) then looking from the grid backwards...
 

Since the impedance of the grid at the Pleiades V6 conditions is 100Kohms, this impedance will be convrted to 100,000 ohms/3/3 or roughly 100,000/10 ie to 10Kohms.


If the turns ratio is 1:5. The impedance becomes 100000/5/5 or 100000/25 or 4Kohms.


These cases should give a very extended high frequency result.


Another interesting experiment could be trying a standard in line transformer of 1:14 turns raio and primary inductance of 400mH. What would the sound be like. Would a possible restriction in HF make for a very natural vocal sound?


Reference:


Pleiades V6 Schematic


Electromagnetism - Grant , Philips (an excellent chapter on trasformers from this great physics textbook)


 The Feynman Lectures in Physics  (No introduction, possibly the best physics book on the planet by the Nobel Price Laureate Feynman on quantum electro-dynamics)


Conceptual Physics - Hewitt (An amazing book by a great student of Feynman)