The previous blog post showed some noise measurements using a USB sound card. I just received a QuantAsylum QA400 audio analyzer (link), and got to do some real measurements.
The full Muffsy Phono Preamp specifications are as follows (using the Muffsy PSU, OPA2134 and the components that ship with the kit):
THD: 0.00190% THD+N: 0.01013% SNR: 112.83 dB (Shorted inputs, relative to 1 Vrms/0 dBV) IMD: 0.0195% (ITU-R, 19+20 kHz) Crosstalk: -102.24 dB Dynamic Range: >110 dB (THD+N: 1%) RIAA Compliance: +/- 0.025 dB RIAA Equalization: Passive Gain: 36-46 dB in four steps Input Impedance: 17k-150k ohm in fifteen steps Input Capacitance: Optional (space for capacitors on the PCB)
The measurements were done at 1 kHz, and an output of 1 Vrms. This is an achievable output from the Muffsy Phono Preamp, others get tens of dB "for free" by measuring at 3 Vrms when the actual output from their device is 0.3 volts.
As the RIAA filter removes higher frequencies, I thought it fair to set the bandwidth at 10 to 20.000 Hz.
The Signal-to-Noise ratio is measured with shorted inputs, relative to the 1 Vrms signal. The actual dynamic range (where the THD+N is 1%) couldn't be measured, as my audio analyzer starts clipping at the level where the THD+N has increased to 0.02 dB.
Also note, these measurements are made at 1 kHz. To reliably measure across the whole bandwidth, I would need a high quality inverse RIAA equalizer. The linearity of quality audio opamps is well established in all but the most poorly laid out circuits though.
Finally, my test setup is not optimal. There are two LED light bulbs nearby, the boards are lying bare on my desk and the power cable runs across the low level input. These numbers are most likely quite conservative...