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Transformer vs. Transformerless Microphones !??!

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Bubba#$%Kron:
When did they start making transformerless mics and why?  

Do you have to worry about using long cable runs with transformerless mics?

What are some of your favorite tranny-less mics??

Why did Bambi's mom have to die?

Thanks, Bubba

ssltech:
Well, there's a lot of different mic types, but here's a few explanations:

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.

Now... on to transformerless mics:

There ARE transformerless tube designs. You don't encounter them much, but they do exist. -A friend of mine builds all of his this way. With these, you do have to be mindful of cable length.

Many designs however (like the much-imitated Schoeps), produce a differential output signal which is itself essentially balanced. Therefore part of the 'need' for an (expensive when done right) transformer is suddenly less pressing.

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.

It's a good thing to ask however. -While I'd say that most mics manufactured for general sale have been engineered to be capable of driving fairly long lines without great problems, there may well always be one which catches you out!

But as to whether being 'transformerless' has any automatic bearing on ability to drive long lines... I'd say basically that the answer is no.

Hope that helps more than it confuses the issue!

Keith

ssltech:
Oh, and another thing about transformers: -think of them like gearboxes.

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.

Of course, (using 'Meccano') I used to then connect another large cog onto the second shaft, then turn a third shaft EVEN FASTER... then a fourth and so on.

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.

This is because power equals speed multiplied by torque. Power cannot be 'created' out of nowhere (as I'd hoped as a 5-year old!) but CAN be transformed to a different ratio of speed versus torque.

In an AC transformer, Voltage is like 'speed' and current is like 'torque'. Double the transformation of one and you halve the availability of the other.

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.

Keith

Peter Weihe:
Great posts Keith,

thanks for your time and energy.

Peter

Bubba#$%Kron:
+1   Thats very kind of you sir!!!  Fantastic info!!
Peter Weihe wrote on Thu, 30 December 2010 11:34
Great posts Keith,

thanks for your time and energy.

Peter

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