Transformer vs. transformerless microphones ! ! gas efficient cars 2015

First, many dynamic mics produce so little voltage (Shure SM57/58, Senheiser 412, AKG D12, etc) that any induced noise is significant compared to the signal. A step-up transformer makes the voltage HIGHER in these cases, while not making the impedance too ridiculously high.

Tube condenser microphones on the other hand frequently produce larger signals at the anode or cathode of the tube, so the signal-to-noise voltage ratio (in terms of surviving a long trip down a mic line) is pretty good… however, the impedance might be too high for even moderate length cables (the higher the source impedance, the more high end is lost to the inevitable cable capacitance, which increases proportional to cable length). -In addition, the tube produces a single, unbalanced signal. A transformer also has the wonderful benefits of galvanic isolation AND balancing.

Now, for phantom powered microphones, the simplest circuits are something like the KM84, where a single FET is used in a comparable way to a tube in a simple tube mic. It produces a single unbalanced output signal, and the transformer effectively balances the signal, as well as tweaking the impedance to suit the typical range of mic preamp loads.

In addition, they use fairly low-impedance drive circuitry, which is capable of driving longer lines with little loss of high end (in the case of many transformerless Chinese mics, a little HF rolloff might actually be a NICE thing! -although that’s a personal taste issue, of course).

Some transformerless mics produce an unbalanced signal, but use the ‘silent’ pin (from which they still draw half of their phantom power current) as a matched-impedance connection to ground, which allows most balanced-input mic preamps to still cancel noise fairly effectively.

-So there are a few designs out there, and they can work QUITE differently, but off the top of my head, I’d say that the only transformerless mic designs that spring to mind which WOULD be particularly sensitive to long lines are the transformerless cathode follower tube designs which I’ve seen… and they’re not a common topology, in my experience.

When I was little I was fascinated by the fact that I could put a big cog on one axle, and a small cog on another, turn a crank and get the smaller one to turn MUCH faster than the crank which I turned. -It was like getting ‘speed’ for free.

One afternoon, I built a step-up gearbox with an input-ouptut ratio of about 100:1. -If I turned the input shaft at 1RPM, the last shaft turned at 100RPM. -But I realised how HARD it was to turn the input crank. -I noticed that I could make the crank IMPOSSIBLE to turn by just applying the SLIGHTEST braking force to the output. -My young brain was puzzled by why this ‘free energy’ (as I thought it was) was being overcome.

Of course, with any gearbox -like a car transmission, or a bicycle derailleur gear set- as you INCREASE the speed multiplication and increase the ‘speed ratio’ you DECREASE the ‘torque ratio’. Conversely, as you DECREASE the speed ratio, you INCREASE the torque ratio.

So, a transformer for a microphone might need to be a STEP UP (voltage multiplying) transformer, as in many dynamic mic designs -ribbon mics being an EXTREME example- or a STEP DOWN (voltage dividing)transformer, but it can ONLY produce enough voltage and current at a low enough impedance, if there is enough actual POWER present initially.

The IMPEDANCE is probably the most important factor which affects long line driveability. So long as this is nice and low, the mic should be able to push a decent length of copper. If it’s balanced (either voltage balanced, transformer balanced or impedance balanced) then the receiving preamp should be able to reject most noise picked up along the way.