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Mofi StudioPhono phono preamp homebrew measurements

MusicNBeer

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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.
RIAAaccuracy.jpg

RIAAaccuracy_noSubsonic.png

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:

Spectrum@_1KHz_+18dB_small.png


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.
 
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Old Grey Punk

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So what do all those measurements add up to in terms of how the Studiophono sounds?
 
OP
M

MusicNBeer

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So what do all those measurements add up to in terms of how the Studiophono sounds?

They mean the StudioPhono will sound good with cartridges that are bass heavy, and sound overly bright with any cartridge that's remotely bright. For example, my VM740ML sounds really really bright with the StudioPhono. My Concorde ProS with Stylus 40 sounds good with it.

Distortion is low and noise is pretty low so it's a good phono stage with the right cartridge. It has good overload margin so that's another good thing.
 

Old Grey Punk

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They mean the StudioPhono will sound good with cartridges that are bass heavy, and sound overly bright with any cartridge that's remotely bright. For example, my VM740ML sounds really really bright with the StudioPhono. My Concorde ProS with Stylus 40 sounds good with it.

Distortion is low and noise is pretty low so it's a good phono stage with the right cartridge. It has good overload margin so that's another good thing.

Smashing, thank you for explaining in laymans terms, much appreciated, on behalf of all us laymen!
 

board

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I know this is an old topic, but I'm wondering if you had a defective unit, because a deviation of almost 2 dB in frequency response (from highest to lowest) seems odd. I'm currently using a StudioPhono, and although I haven't measured it precisely, the testing I've been able to do has shown me that my unit is quite accurate, and it seems to match the UltraPhono that Hi-Fi News measured, with a deviation of around 0.3 dB, which would pretty much be inaudible:

520mofiphono.lab1.jpg


My unit seems a bit noisy though. I used to use a NAD PP-4, which had lower gain, but even when adjusted for the level of gain, my MOFI seems a tad noisier.
 
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MusicNBeer

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@board, it's possible Mofi changed one or more components along the way. My unit is 3 years old or so. Not sure about defective because both channels measure the same and distortion is low. Oddly, my unit is quiet too, not quite as quiet as my Cambridge Duo though.
 

formdissolve

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I have been using this with a Denon DL-103 with gain set for low output high gain and yes.. it sounds super thin and bright with that cart. I listened to the same record using the MM with a cheap-ish Ortofon OM 10 and it wasn't nearly as bright. I then tried the Denon 103 on a different preamp and it wasn't nearly as thin and bright as the MoFi.

I'll probably end up selling it at some point (but maybe send to Amir first).
 
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B00tySalads

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They mean the StudioPhono will sound good with cartridges that are bass heavy, and sound overly bright with any cartridge that's remotely bright. For example, my VM740ML sounds really really bright with the StudioPhono. My Concorde ProS with Stylus 40 sounds good with it.

Distortion is low and noise is pretty low so it's a good phono stage with the right cartridge. It has good overload margin so that's another good thing.
How do you think it would pair with an Ortofon Blue? Would the Pro-Ject S2 be a better fit in theory based on measurements?
 
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