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Microphone Pre-Amplifier Transformer Usage Overview

Primary benefits of using a transformer are to realize high CMRR and to improve noise performance.  Employing transformers enables CMRR that is unmatched by non-transformer designs.  Transformers also can improve noise performance compared to transformless designs.

 

Pre-amp input referred noise voltage and noise current may realize an equivalent noise that is higher than the microphone output noise. If this is the case, the pre-amp noise will dominate, and the signal to noise of the mic and therefore the entire system will be degraded. A transformer can "transform" the mic signal and equivalent output noise to a level where the mic noise matches or is higher than the best case pre-amp input referred noise, and then maximum signal to noise ratio of the system can be realized.

 

Other technical benefits result from using a transformer, but, perhaps as important is the ability of transformers to add a certain color to the audio program.  Transformers have a frequency and signal level dependency to their distortions curves.  In addition to this, transformer’s complex impedances interact with the devices to which they are connected, and can therefore have a positive effect on the audio quality.  These features make the use of transformers desirable, especially with the decreasing use of analog processing equipment in the recording chain.

 

Transformers and Noise Performance

When considering pre-amplifier noise and its affect on the signal to noise of the source signal, the amplifier input referred voltage noise and current noise will add to the electrical and/or thermal noise of the source. Amplifier voltage noise adds directly and current noise is impressed upon the source resistance, transformer primary and secondary winding resistances and transformer load resistance.

 

In order to determine if a transformer will improve mic pre-amp noise performance, the pre-amp noise voltage and current must be known. They can then be used to determine if a transformer will improve noise performance and they can also guide what transformer characteristics must be chosen.

 

Simplified, "rule of thumb" formulas exist for determining a transformer turns ratio that may improve signal to noise ratio. However, the simplified formulas supply the most accurate answer only if it is assumed the transformer is noiseless and the pre-amp equivalent input noise sources and RMS values are fixed. In practice, this is not the case. Equivalent resistances "seen" by the entire circuit and pre-amp bias currents and noise sources are interrelated. Also, transformers are not noiseless devices, and actually contribute thermal noise due to the real resistances in the windings. Because the pre-amp bias currents also affect pre-amp slew rate performance, the use of a transformer for optimum noise performance can also affect circuit dynamic performance if the transformer/pre-amp pairing is considered when the pre-amp active stage is designed.

 

After all of these details are considered, and a real world transformer is to be designed for the system, the loading requirements of the transformer need to be considered on the system performance. Just as a mic needs to see certain impedance in order to realize best noise or transient performance, the pre-amp transformer must be properly loaded in order to have optimum performance. The required transformer load must be considered in the over-all preamp design with respect to noise, dynamic performance and mic loading effects.

 

A series of equations has been developed by Dr. William M. Leach of the Georgia Institute of Technology that enable transformer use to be analyzed with a higher degree of accuracy than previously available for BJT and FET amplifiers.1 Ingram Engineering expanded this previous research to include additional implementation and usage effects, developed modified calculations and used these in the design of the Ingram Engineering pre-amps.

 

To illustrate the concept, mic pre-amp output signal to noise ratio is calculated for an example case. In the example, a mic with the following characteristics is considered:

Mic O.C. Voltage = 1.8mV = -54.5 dBV (similar to Shure SM58)
Mic Noise = Mic source resistance thermal noise

Total input referred noise for a sample mic pre-amp design is calculated, and the resulting signal to noise and noise figure for the system is calculated, both with and without a sample transformer. As can be seen from the summary in the table below, noise figure and system signal to noise is improved by 6.3dB with the use of the transformer. 

Table 1:  Noise Calculations With and Without a Transformer

 

System signal to noise ratio can be further improved by utilizing multi-tap transformers, as do the MPA681, MPA683 and MPA685 (option). Rather than use a transformer with one primary to secondary turns ratio that optimizes performance under one set of mic criteria, a transformer with multiple taps on the primary can be used, and the transformer turns ratio can be changed to match the particular mic characteristics.

 

To illustrate the concept, mic pre-amp output noise figure ratio is plotted for an example case, using the same sample microphone characteristics as in the previous example.

 

Total input referred noise for a sample mic pre-amp design is calculated, and the resulting noise figures for the system are calculated.  In the graph below, these are compared to the case where no transformer is used.  There is a large Noise Figure advantage with using a transformer, and additional advantage by using a transformer with multiple turns ratio options (“taps”). For the lowest noise mics, this transformer coupled design provides more than 10dB better noise figure than if the same design is implemented, but without a transformer.  The graph also shows that the total system noise figure for any single transformer tap is optimized for only a small range of mic output impedance. However, noise figure can be improved if a multi-tap transformer is used and different transformer turns ratios are selected.

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Figure 2: Noise Figure Calculation vs. Mic Output Impedance Across Transformer Taps

 

A graph of the best-case result is shown below. In this case, instead of showing the Noise Figure of each transformer tap separately, it is assumed that the appropriate tap is selected, and the best case Noise Figure is realized versus total mic noise.

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Figure 3: Best Case Noise Figure vs. Mic Output Impedance Across Transformer Tap Position

 

This graph represents the best noise figure that results from the proper choice of turns ratio for noise matching and best-case implementation of a transformer, considering the transformer noise contributions and all the interactions of the transformer and active circuits.
 

References:
1 Noise Analysis of Transformer-Coupled Preamplifiers, W. Marshall Leach Jr. J. Audio Eng. Soc., Vol 40, No. 1/2, 1992 Jan/Feb