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FE-804 / FE-210 Noise Comparison

We performed a direct noise comparison between our FE-804, FE-210, and Dayton Audio's EMM-6 measurement microphone. The EMM-6 has a stated self-noise rating of 33dBA. In the following spectral graphs, the blue trace is the microphone under test, and the red trace is a second preamp channel of the same type (ProFire 2626) with the input terminals shorted. This shows the noise floor of the preamp itself. The dark lines represent peak values over a period of 45 seconds.

The dB figures on the Y-axis are not absolute values – the only purpose of this test is to compare the relative noise output of the three microphones sampled.

First the EMM-6, with a noise rating of 33dBA:

Next the FE-804, with a noise rating of 19dBA:

Finally, the FE-210, with a noise rating of 13dBA:

As you can see, the noise at greater than 10kHz is essentially the same between the FE-804 and the FE-210. However, the noise at 1-3kHz (the most sensitive range of our hearing) is about 6dB lower in the FE-210. By applying an A-weighting to these measurements, you can see that noise in this frequency region has more of an audible effect than noise at high or low frequencies. Low frequency noise is also lower in the FE-210, but due to environmental noise during measurement this is not apparent in these graphs.

The FE-804 and FE-210 in this test used OPA-627's; a single op-amp with a noise spec of 4.5nV/sqrt(Hz). All op-amps on our list have similar noise specifications to the 627. The output sensitivities of the microphones tested here were 9mV/Pa (EMM-6), 9mV/Pa (FE-804), 11mV/Pa (FE-210.) This means that the FE-210 actually has 2dB of additional signal gain for the same noise level. The EMM-6 and FE-804 are directly comparable here.

 

Opamps:

We offer the unique ability to customize your microphone by allowing you to choose the op-amp that is used in the preamplifier circuit. With the information provided here, you'll be able to pick an op amp that best fits your needs. Find the link at the bottom of this section to read subjective reviews of some popular op amps.

For use as a microphone output amplifier, these are the parameters that are important:

     -Low input-referred voltage noise
     -Must operate at low voltage (10V)
     -Low current draw, phantom power spec allows for a max draw of 10mA
     -Low harmonic distortion, especially at high frequencies
     -High slew rate (tends to indicate lower distortion)
     -Stability at low gain and driving capacitive loads (the mic cable)

"Input referred noise" means noise generated inside the op amp, which is represented as a separate noise source at the input of the op amp. In our microphones, input referred voltage noise dominates, because the output impedance of the FET stage (driving the input of the op amp) is relatively low. Op amps with either input type (FET or bipolar) can be used because the previous stage is able to supply enough current to even a bipolar transistor input.

Low voltage: Because of the 6.8k feed resistors which are always present in phantom power circuits, the voltage will drop lower than the nominal 48 Volts when current is drawn by a microphone. A single op amp will draw anywhere from three to eight milliamps, and a dual op amp will usually draw between seven and 10 milliamps. All of our microphones have an internal circuit that regulates the phantom power voltage down to a consistent level of 10 Volts. Therefore, all circuitry within the microphone must operate off this 10 Volt supply. This practice is used in almost every condensor microphone that does not have a dedicated power supply.

Single vs Dual: After careful consideration, we believe that the single op amp is the better choice in our microphones.

A single op amp will usually draw much less current, and so will be more likely to work with less-than-perfect phantom power, or battery powered designs. Impedance balancing the output and using a single op amp has no negative effect on the sound, or on the microphone/preamp signal to noise ratio. Specialty op-amps usually only come in single packages. The performance benefit of these special devices (low noise and distortion, high speed) comes at a cost of up to 8 milliamps of current draw. Using a single op amp with a low current draw, like an OPA134, you can make a microphone suitable for use in a battery powered system.

Harmonic and intermodulation distortion are rarely (if ever) measured in enough detail for audio specifications. For a thorough harmonic distortion measurement, the level of each harmonic above the fundamental would be measured, at every frequency within the audio band. The same applies for intermodulation distortion testing. The results of such a test would be difficult at best to translate into how the op amp sounds, because the threshold of audibility changes at every frequency, and for each type of distortion. In addition, some amount of distortion may be preferred depending on artistic intentions (we are recording music, not measuring it.) Listening impressions are the best method to fully compare op amps in this regard.

OPA627: We can make the FE-804 and FE-210 using the highly regarded OPA627 op amp. Due to the unusually high cost of the 627, we charge an additional $30 per microphone. This op amp circuit draws about 8mA from the phantom power supply.

Sources:
Here are some subjective reviews of several popular op amps.
Read here about internal op amp noise. (Analog Devices application note 940)
Read here for a different description of internal op amp noise. (Linear Technology design note 355f)
Read here for information on driving long cables.
Read here for distortion measurements of a large group of op amps.

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