ZTX851 and ZTX1053A Noise Measurements and Comparisons

[mediatechnology]

ZTX851 and ZTX1053A Noise Measurements and Comparisons

I've had a chance to run some additional noise tests on the ZTX851 and ZTX1053A that include 1/f measurements. My interest in the ZTX1053A is for mic preamps. Although the ZTX851 is the lowest noise NPN I've measured and is superb for moving coil preamps, the ZTX1053A has higher current gain and may be better-suited for microphone source impedances.

The test circuit I used is from the MC preamp with 5.5 mA Ic and 499Ω/leg bias resistors. If this circuit were to be optimized for the ZTX1053A and mic preamp applications I would conside an Ic of 3 to 3.5 mA and bias resistors ranging from 1KΩ to 10KΩ/leg. To permit direct comparison I used the MC preamp's 5.5 mA Ic and 1KΩ (499Ω/leg) bias network. For these test the servo was disabled extending the preamp response to DC. The gain is 61.5 dB. The test circuit is found here: https://proaudiodesignforum.com/images/ ... ematic.png

I used the Focusrite 2i2 (Gen 3) for 1/f noise measurements. The 2i2 input and A/D has an approximate 1 Hz -3 dB point. The FFTs are 1M point 50 sample averages.

I first checked the 2i2's 1/f noise with its input shorted. There is a slight 5 dB rise from 10Hz to 1Hz but as we'll see it's usable for measurement.


Focusrite 2i2 Gen3 1/f Noise Input Shorted

Comparing the ZTX1053A to the ZTX851 with a 10Ω source impedance - for a moving coil comparison - shows the ZTX851 to be about 2 dB quieter in the 20-20 kHz bandwidth. The ZTX851 has a significantly lower 1/f corner and rise. Within the audio band the ZTX1053A performs admirably.


ZTX1053A (blue) vs ZTX851 (red) 1/f Noise Rsource 10Ω

Comparing the ZTX1053A to the ZTX851 with a 150Ω source impedance - for a mic preamp comparison - shows the ZTX851 to be nearly identical to the ZTX1053A in the 20-20 kHz bandwidth. Once again the ZTX851 has a significantly lower 1/f corner and rise. Within the audio band the ZTX1053A performs as good as the ZTX851 and due to its higher current gain and lower input noise current.


ZTX1053A (blue) vs ZTX851 (red) 1/f Noise Rsource 150Ω

20-20 kHz Noise Bandwidth Comparisons

The next test is with a 1KΩ input termination which shows the reduced input noise current provided by the ZTX1053A. The ZTX1053A overall is about 0.8 dB quieter.


ZTX1053A (blue) vs ZTX851 (red) Noise Rsource 1KΩ

To get an idea of the voltage noise I compared them with a shorted input. The ZTX851 is about 3 dB quieter. The test circuit includes op amp noise and common mode rejection stage noise making indirect measurement of rbb error-prone. With 1.75Ω being the somewhat-accepted value for ZTX851 rbb this would suggest that the ZTX1053A rbb is about 1.4 times higher or around 2.5Ω.


ZTX1053A (blue) vs ZTX851 (red) Noise Rsource 0Ω

Comparing the ZTX851 to to ZTX1053A with a 10Ω source impedance over the 20-20 kHZ bandwidth shows the ZTX1053 to only be slightly noisier.


ZTX1053A (blue) vs ZTX851 (red) Noise Rsource 10Ω

Comparing the ZTX851 to to ZTX1053A with a 150Ω source impedance over the 20-20 kHZ bandwidth shows the ZTX1053 to be identical to the ZTX851.


ZTX1053A (blue) vs ZTX851 (red) Noise Rsource 1500Ω

[emrr]

Fascinating. Thanks for digging in to this.

[olafmatt]

Any idea how to plot the output impedance over frequency for a dynamic mic? I'd guess that doing it the normal way (injecting test signal into the XLR through a voltage divider) will not give proper results because the mic would then be a "loudspeaker" (i.e. nothing that holds the membrane in the "zero" position).

Reason for asking is that I'd bet that a dynamic mic has an impedance that will rise quite a lot at higher frequencies. That might make that requirement for lowest noise at 150 ohms less of a critical thing. In condenser mics the output is usually hotter (and has some noise from the circuitry in the mic), so less gain is needed and the "relative" noise contribution by the preamp is less than with dynamic mics.