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- Jan 20, 2019
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Wasted my vacation day today doing some MM measurements on my Mofi StudioPhono phono preamp.
My equipment:
Hardware:
- Soundcard is a lowly ASUS Xonar DSX 7.1, but it actually has decent ADC performance when run at 44.1 KHz. ADC input used for signal analysis. ADC saturates at 1.025 Vrms. For signal generation, SPDIF out used.
- Benchmark DAC1 used as DAC for test signal generation.
- Goldpoint RCA stepped attenuator used to optimize signal levels into soundcard ADC.
- RatShack multimeter used to read voltages as needed.
Software:
- ARTA software on PC used for signal generation and analysis.
- Octave (Matlab clone) used for RIAA curve calculations.
Setup:
Soundcard SPDIF out -> Benchmark DAC1 -> Phono Preamp -> Goldpoint Attenuator -> Soundcard ADC -> ARTA Software
SPDIF connection is optical.
All other connections are single ended RCA.
Test #1: Gain @ 1KHz
Preamp input voltage: 1mVrms
Preamp output @ 1KHz: 20.6 dBFs referenced to 1.025 Vrms (0 dBFs).
This equates to 95.65 mVrms, yielding a gain of 39.6 dB, very close to the spec'd 40 dB.
Test #2: Overload margin referenced to 5 mVrms input
ARTA spectrum analyzer used to incrementally increase input until phono preamp clipped.
Goldpoint attenuator used between preamp output and soundcard input to keep signal below fullscale ADC.
Results:
@ 60 Hz = 6 dB overload margin (clean to 10 mVrms input)
@ 1000 Hz = 22 dB overload margin (clean to 63 mVrms input)
@ 10000 Hz = > 40 dB overload margin (clean to >500 mVrms input)
These overload margins correspond to a maximum output voltage of about 6-7 Vrms before clipping.
Test #3: RIAA Accuracy
ARTA Steps program used to sweep the phono preamp gain with 5 mVrms input.
Octave used to compare gain sweep with RIAA curve.
Results are shown. First plot is with subsonic filter engaged, second is without.
I have noticed a slightly thin presentation on some records, and this confirms it.
Test #4: Distortion @ 1KHz
Signal input at +18 dB overload relative to 5 mVrms.
ARTA spectrum analyzer used to capture.
Results are shown:
Even at this high input level, very clean. Harmonics start at approximately -95 dB.
Note: the power supply noise in the output is not due to the phono preamp. It is in the generator signal and shows up because the phono preamp is boosting that region by approximately 55 dB. When connected with turntable as input, as actually used, the power supply spurs are >95 dB below signal level.
My equipment:
Hardware:
- Soundcard is a lowly ASUS Xonar DSX 7.1, but it actually has decent ADC performance when run at 44.1 KHz. ADC input used for signal analysis. ADC saturates at 1.025 Vrms. For signal generation, SPDIF out used.
- Benchmark DAC1 used as DAC for test signal generation.
- Goldpoint RCA stepped attenuator used to optimize signal levels into soundcard ADC.
- RatShack multimeter used to read voltages as needed.
Software:
- ARTA software on PC used for signal generation and analysis.
- Octave (Matlab clone) used for RIAA curve calculations.
Setup:
Soundcard SPDIF out -> Benchmark DAC1 -> Phono Preamp -> Goldpoint Attenuator -> Soundcard ADC -> ARTA Software
SPDIF connection is optical.
All other connections are single ended RCA.
Test #1: Gain @ 1KHz
Preamp input voltage: 1mVrms
Preamp output @ 1KHz: 20.6 dBFs referenced to 1.025 Vrms (0 dBFs).
This equates to 95.65 mVrms, yielding a gain of 39.6 dB, very close to the spec'd 40 dB.
Test #2: Overload margin referenced to 5 mVrms input
ARTA spectrum analyzer used to incrementally increase input until phono preamp clipped.
Goldpoint attenuator used between preamp output and soundcard input to keep signal below fullscale ADC.
Results:
@ 60 Hz = 6 dB overload margin (clean to 10 mVrms input)
@ 1000 Hz = 22 dB overload margin (clean to 63 mVrms input)
@ 10000 Hz = > 40 dB overload margin (clean to >500 mVrms input)
These overload margins correspond to a maximum output voltage of about 6-7 Vrms before clipping.
Test #3: RIAA Accuracy
ARTA Steps program used to sweep the phono preamp gain with 5 mVrms input.
Octave used to compare gain sweep with RIAA curve.
Results are shown. First plot is with subsonic filter engaged, second is without.
I have noticed a slightly thin presentation on some records, and this confirms it.
Test #4: Distortion @ 1KHz
Signal input at +18 dB overload relative to 5 mVrms.
ARTA spectrum analyzer used to capture.
Results are shown:
Even at this high input level, very clean. Harmonics start at approximately -95 dB.
Note: the power supply noise in the output is not due to the phono preamp. It is in the generator signal and shows up because the phono preamp is boosting that region by approximately 55 dB. When connected with turntable as input, as actually used, the power supply spurs are >95 dB below signal level.
Last edited: