The difference between microphones with transformer vs. transformerless output can in short be outlined as:
Microphone examples in this article features the well known DPA 4006 with output transformer versus the acoustically identical but transformerless 4006-TL, which ensures an impedance balanced output.
Sensitivity
In most microphone transformer designs the signal is transformed downwards. A transformer will reduce the sensitivity of the microphone with approxmately the same ratio as the signal is transformed down to, because the sensitivity specification concerns the microphone’s output signal in relation to the pressure on the diaphragm (mV/Pa).
Noise immunity (CMRR, Common Mode Rejection Ratio)
Noise immunity on balanced audio lines is totally dependant on how well the in- and output impedances are balanced and the common mode rejection ratio of the audio input channel. The balanced audio transformer introduces a higher common mode rejection ratio than any other electric circuit and is perfect for balancing loads. Using in- and output transformers for balancing audio signal lines will provide the highest possible immunity against common mode induced noise on the audio lines. Transformerless microphone preamplifiers will from the mere tolerance of the electronic components introduce a slightly unbalanced load on the audio lines.
The unbalanced load will allow a percentage of the induced noise to appear in a different mode, which therefore will not be rejected by the input channel.
Cable drive capability
When a signal is transformed down, the signal voltage becomes lower according to the conversion ratio of the transformer, while the signal current becomes accordingly higher. The increased signal current will increase the microphone’s ability to drive long cables before noticeable signal deterioration occurs.
Low frequency handling
Transformers are audible and will introduce some low frequency distortion, as transformers are susceptible to saturation by the high energy in the low frequency signals. Not only will the frequency range grow for the transformerless variant, but more interesting is the tighter bass response in the very important region around 80 Hz.
Conclusion Both transformerless as well as transformer output design have different advantages and disadvantages. This is illustrated in the following table. For this reason DPA offers both types of preamplifier design. However, the cleanest approach to authentic sound reproduction is choosing the transformerless type. Today, most microphone lines do not run more than 100 m.
Mic amps and AD converters are often placed pretty close to the mic which is why the primary advantage of the transformer microphone becomes weaker.
Study the details in the table below to compare the differences between DPA 4006 (transformer), DPA 4006-TL (transformerless) and DPA 4003 (transformerless) wich can be driven by the HMA5000 High-Voltage Mic Amp.
