I got myself a Behringer X32 Compact mixing desk and Behringer S16 Stageboxes. Because sometimes I record classical concerts, or have ambient microphones that get very little signal, I wanted to verify the preamp performance to avoid nasty surprises.
To establish the voltage scale when recording, a sine-generator generating 1kHz was fed into a passive DI box, and adjusted in amplitude, so that a 1Vrms signal was present on the DI box XLR output. This 1Vrms signal could be easily measured with a TRMS multimeter. Then both the 20dB pad on the DI-box and a 40dB pad on the function generator were engaged, so that a 1mVrms signal is output on the DI box. This sinewave was fed into all devices under test at maximum gain. (See Fig. 2)
Sucessively the XLR connection to the DI box was removed and the input was shorted. A 2nd recording was then made, with identical gain settings only capturing the noise. (See Fig. 1)
Preamps tested were:
- EchoAudio EchoFire 4, which uses a now obsolete SSM2017 from which noise performance is known and documented
- Preamps within the X32 Compact mixing console, using the local XLR input
- Preamps within the two S16 stage boxes
In python (using numpy), the rms of the sinewave-recording was calculated relative to digital full-scale and the noise recording scaled with the resulting factor (e.g. -21.0 dBrms/full-scale for the EchoFire4 recording). From this data the noise density in nV/√Hz was calculated using Scipy's scipy.signal.welch results.
The spikes present are most likely caused by switching voltage regulators within, or in vincinity of, the devices. Generally the Behringer preamps have slightly higher noise than the very respectable SSM2017 within the EchoAudio firewire interface, but all in all the difference is pretty much irrelevant (approx. ~1.5dB more noise than the SSM2017, adds approx. 3 dB to the thermal noise of the 200Ω resistor) for practical use.
Fig1: Noise Analysis (with inputs shunted by 200Ω)
Fig2: Reference Recording of 1mVrms, 1kHz Sine Wave