The difference between microphones with transformer vs. transformerless output can in short be outlined as:

	Transformer	Transformerless
Sensitivity	Lower	Higher
Noise immunity	Higher	Lower
Cable drive capacity	Longer	Shorter
Low frequency handling	Less Precise	Good

Microphone examples in this article features the well known Type 4006 with output transformer versus the acoustical identical but transformerless Type 4003 with the PCC4000, 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 the same ratio as the signal is transformed down, as the sensitivity specification concerns the microphone's output signal in relation to the pressure on the diaphragm (mV/Pa).

Diaphragm Size	Small	Medium	Large
	4004	4003	4041-S
	(Ø12mm)	(Ø16mm)	(Ø24mm)
Self Noise	Higher	Lower	Lowest
	24 dB (A)	15 dB (A)	7 dB (A)

Sensitivity

The sensitivity of the large and more compliant microphone diaphragm is generally higher than the small and stiff diaphragm. The large diaphragm is easier to move, even with low sound pressure levels, and will therefore provide a larger output.

Microphone	4006	4003 + PCC4000
Transformer	4:1	-
Sensitivity	10 mV/Pa	40 mV/Pa

Noise immunity

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 electric components introduce a slightly unbalanced load on the audio lines. The unbalanced load will allow a percentage of the induced noise to appear in different mode, which therefore not will 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.

Microphone	4006	4003 + PCC4000
Transformer	Yes	No
Cable drive capacity	300 mt.	100 mt.

Low frequency handling

Transformers will introduce some low frequency distortion, as transformers are in danger of becoming saturated by the high energy in the low frequency signals. This fact will also influence the lower limiting frequency (LLF) of the microphone as illustrated below.

Microphone	4006	4003 + PCC4000
Transformer	Yes	No
Lower Limiting Frequency	20 Hz	10 Hz

Conclusion

Both transformerless as well as transformer output design have different advantages and disadvantages. This is illustrated in the table below. For this reason DPA offers both types of preamplifier design. It should, be noted however, that the potential drawbacks of the transformerless design illustrated here, may be eliminated through the use of a Hi-voltage Microphone Amplifier (e.g. Type HMA4000), when placed near the microphone.

	Transformer	Transformerless
	4006	4003 + PCC4000
Sensitivity	Lower (10 mV/Pa)	Higher (40 mV/Pa)
Noise immunity	Higher	Lower
Cable drive capacity	Longer (300 mt)	Shorter (100 mt)
Low frequency handling	Less precise at high SPL's (LLF 20Hz)	Good (LLF 10Hz)