Mofi StudioPhono Stage Review

[DualTriode]

I use a custom box I made a while ago that contains an AT91, with a switch on the front that provides short and open options. When you go from the short to the cartridge, the noise floor typically increases by about 6dB.

I like to test from 220Hz to 22kHz. Testing from 22Hz with a cartridge load is tricky because it's so difficult to eliminate hum from the measurement, as well as flicker noise kicking up around 100Hz which isn't exactly audible but also skews the measurement. Most use A-weighting.

View attachment 365209

Hello,

I am borrowing your tech for personal use. Just placed a order for a AT91 mm carriage. $25.00 for a "reference" mm cartridge is doable.

I will also add another position to the switch for (analyzer) generator input.

Perhaps add another switch position for a "reference" Noise resistance. 60R to start with.

I do not see the AT91 in your photo.

Thanks DT

 

[Michael Fidler]

Hello,

I am borrowing your tech for personal use. Just placed a order for a AT91 mm carriage. $25.00 for a "reference" mm cartridge is doable.

I will also add another position to the switch for (analyzer) generator input.

Perhaps add another switch position for a "reference" Noise resistance. 60R to start with.

I do not see the AT91 in your photo.

Thanks DT

Thanks for the kind comments! I think you might want a rotary switch if you're adding extra settings like that.

The AT91 is in the bottom left hand side of the photo, just above the ground binding post.

EDIT - I wouldn't bother with a resistance of 60 ohms, as it only gives a noise voltage of 1nV/sqrtHz which would pale in comparison to some of the best MM inputs typically around 5nV/sqrtHz. If you compute these together, it only increases the noise from 5nV/sqrtHz to 5.1nV/sqrtHz which is pretty meaningless for this kind of measurement. Perhaps a 2.2k resistor would be better for a fixed resistance test (6nV/sqrtHz)?

 

[Robciak]

Most important test for a phono stage is the accuracy of RIAA equalization

Hi,
How do you determine the fit to the RIAA curve? Do you use any reference curve to which you compare the frequency response from the phono preamp?

 

[DualTriode]

Hi,
How do you determine the fit to the RIAA curve? Do you use any reference curve to which you compare the frequency response from the phono preamp?

Audio Precision has that reference frequency response data on their web site.

That is the reference curve that I use.

Thanks DT

 

[Michael Fidler]

It's not too tricky. If you know the algebra that creates the function well enough, you can even make a CSV, XML etc. file with 20 or so points per octave in Excel or LibreCalc that will give the generator amplitude values... If you have all the scripts ready-made from the manufacturer of the analyser, then that's a bonus, but if you're creating a script for a non-standard curve (or adding 20Hz IEC amendment etc.), then this is a good approach.

 

[amirm]

Hi,
How do you determine the fit to the RIAA curve? Do you use any reference curve to which you compare the frequency response from the phono preamp?

As noted, Audio Precision software has inverse RIAA EQ which I apply to the test signal. It is one of the few old style measurement features left.

 

[board]

I'm a bit surprised at Amir's results when it comes to the RIAA curve. Although it's not a massive deviation, I'm wondering if this unit could be slightly faulty.
I have this preamp myself, and while I haven't measured it per se, I recorded some records with the Mofi and then also with an NAD PP-4, and then compared the two with the plugin CurvEQ, which shows the difference in EQ.
Stereophile measured the previous incarnation of the NAD, which had around 0.25 dB deviation from the RIAA curve:

NAD PP 3 digital phono preamplifier Measurements

Sidebar 3: Measurements

www.stereophile.com

When I loaded my recordings with the two preamps into CurveEQ (I did the recordings right after each other), this was the result with the NAD having more deep bass, as it has a weaker subsonic filter. The orange line shows the difference between the two preamps with an MC cartridge:

Also, HiFi News measured the Ultraphono, and while this is not the same unit, then as far as I have understood, the only difference between the two was some features and perhaps lower noise in the Ultraphono, but the rest should be identical. HiFi News' measurements look pretty much like my chart above:

MoFi UltraPhono Phono/Headphone Preamp Lab Report

Lab Report At first sight the row of 12 DIP switches governing input loading and gain may be enough to make all but the most ardent of audiophile's hearts sink. However in practice only the two outermost switches are required to cycle through MoFi's selection of +40dB/+46dB (MM) and +60dB/+66dB...

www.hifinews.com

Lastly, a question:
I moved to another house recently and thought that I had damaged my cartridge while moving, because I recorded a few records that I had recorded before, and when I loaded them in the CurveEQ to compare the new recordings to the recordings from before moving, the newer recordings showed a gradual drop-off in high frequencies of around 1 dB from 2 kHz to 12 kHz.
My cartridge is an Audio Technica AT33PTG/II, and I bought another one a while ago, which I only used to record a few songs, just to make sure the cartridge was okay.
So, I mounted this new cartridge today, recorded the same records and compared them with the recent recordings done with the other cartridge in CurveEQ, and they were almost identical.
So, this gradual drop-off doesn't seem to be the cartridge, so I'm wondering if it could be the Mofi phono preamp, possibly my Focusrite 2i2 converter, the cables or something else. I tried moving the dip switches on the Mofi back and forth with no difference.
Does anyone have any idea?
Unfortunately, I currently don't have the ability to switch out any of the equipment.
Everything is set up exactly the same way as in my previous house (on the same rack and everything), and when I've moved before, I've only seen at most a slight difference in rumble or similar, as when I moved the turntable to my current rack instead of having it on top of another piece of furniture.

 

[MusicNBeer]

@board I measured my Studiophono a few years back and got the same result as Amir. I have a link to my measurements on page 2 of this thread.

Not sure about your question, but to start, you can rule out the ADC by recording a tone sweep and verifying flat response.

 

[formdissolve]

@board I measured my Studiophono a few years back and got the same result as Amir. I have a link to my measurements on page 2 of this thread.

Not sure about your question, but to start, you can rule out the ADC by recording a tone sweep and verifying flat response.

Two different units, same results. I think that rules out "defective unit" hypothesis.

 

[board]

Two different units, same results. I think that rules out "defective unit" hypothesis.

I feel a bit silly now. I looked at MusicNBeer's link, and back then I proposed exactly the same thing, but I completely forgot .
Anyway, I suppose it's possible, although perhaps not super likely, that my unit has a different frequency response. Besides the close results to the NAD + HiFi News' results, when I've recorded records that seem to be flat transfers from the CD master, I get a curve that matches my cartridge.
Anyway ...

 

[board]

@board I measured my Studiophono a few years back and got the same result as Amir. I have a link to my measurements on page 2 of this thread.

Not sure about your question, but to start, you can rule out the ADC by recording a tone sweep and verifying flat response.

I will try that. I didn't even think about that. Thanks!

 

[cgallery]

I feel a bit silly now. I looked at MusicNBeer's link, and back then I proposed exactly the same thing, but I completely forgot .
Anyway, I suppose it's possible, although perhaps not super likely, that my unit has a different frequency response. Besides the close results to the NAD + HiFi News' results, when I've recorded records that seem to be flat transfers from the CD master, I get a curve that matches my cartridge.
Anyway ...

I wonder if there were any revisions to the design after production started.

 

[board]

I wonder if there were any revisions to the design after production started.

I was wondering the same thing. Mine is serial number 0531416, bought in November 2019, if that's of any help.

Another suggestion: Do any of you think it's likely that the RIAA filter is different for the MM and the MC sections in the Mofi?
Amir and MusicNBeer only tested the MM section, whereas I've mainly used the MC section.
When I've used my Audio Technica VM750SH (which is an MM), it does tip up slightly from around 300 Hz and up:

When I compared the MM section of the Mofi to the NAD PP4, the Mofi also had slightly more energy from around 300 Hz and up (but less energy around 10-12 kHz). I assumed that this was due to a difference in capacitance between the two preamps, but I'm wondering now if simply the RIAA filter could be different between the MM and the MC section in the Mofi ...?
Here's the difference between NAD and Mofi for MM:

 

[LTig]

Another suggestion: Do any of you think it's likely that the RIAA filter is different for the MM and the MC sections in the Mofi?

Yes, this is possible, and not unlikely, if one wants to make no compromises in the stage design.

 

[LTig]

Another suggestion: Do any of you think it's likely that the RIAA filter is different for the MM and the MC sections in the Mofi?

Yes, this is possible, and not unlikely, if one wants to make no compromises in the stage design.

 

[formdissolve]

Technically they should be the same RIAA with just a lower impedance and a much higher amout of gain needed at 60+ dB for MC.

I sent this one in and it's also from around 2019/20. I also exclusively used this with MC carts.

 

[board]

Technically they should be the same RIAA with just a lower impedance and a much higher amout of gain needed at 60+ dB for MC.

I sent this one in and it's also from around 2019/20. I also exclusively used this with MC carts.

Do you mean that the one Amir measured was your unit?
You didn't by any chance happen to measure the MC section or make a recording and use CurveEQ to determine the frequency response like I did?

 

[formdissolve]

Do you mean that the one Amir measured was your unit?
You didn't by any chance happen to measure the MC section or make a recording and use CurveEQ to determine the frequency response like I did?

It was, yeah. I didn't measure it no. The MC is the same RIAA network, just lower ohms and much higher gain

 

[board]

It was, yeah. I didn't measure it no. The MC is the same RIAA network, just lower ohms and much higher gain

I would also imagine that it would be the same RIAA network, but LTig suggested that it might not be. I seem to recall having seen other phono preamps where the MM and MC RIAA curves were different. So, I suppose with this Mofi one we won't know for certain unless we measure it (which I'm not able to do, besides using the CurveEQ method, and if mine is also broken, then ...).
However, since there's only one set of inputs that would rule out two separate RIAA networks, wouldn't it?

Anyway, all this might not be the most important thing in the world .

 

[LTig]

I would also imagine that it would be the same RIAA network, but LTig suggested that it might not be.

I looked hard at the pics of the preamp and its PCB and think that there is only on RIAA stage (the opamps with those 2 red caps, 1 per channel) and probably also only one amplifying stage (U5 for one channel) in front of it, with the Dip switches changing the gain. Those preamps which are optimized for both MM and MC usually have separate inputs, separate amplifying stages and very often separate RIAA stages. I might be wrong though.