Mofi StudioPhono Stage Review

[restorer-john]

Can I ask a perhaps trivial question?

I'm not a technician, but I always notice that normal amplifiers and DACs have much higher performing circuits with a much better SINAD and lower noise than the average prephono. what I don't understand is whether it is a circuit problem with all the phono preamps or a problem with the source which is analogue?
As you rightly say, an $80 DAC performs much better than even the best phono preamp tested.

There is a huge amount of gain in a phono stage. 40dB difference (RIAA curve) between the frequency extremes and the signal were are amplfiying outputs just a few millivolts (a few hundred microvolts and 60dB of gain for moving coil). 60dB is a thousand times amplification!

DACs have it easy. There is no gain in most cases, just perhaps an IV stage and a buffer.

We've had DAC and CD player converters outputting 1uV residual noise levels for 35 years.

 

[Mikig]

There is a huge amount of gain in a phono stage. 40dB difference (RIAA curve) between the frequency extremes and the signal were are amplfiying outputs just a few millivolts (a few hundred microvolts and 60dB of gain for moving coil). 60dB is a thousand times amplification!

DACs have it easy. There is no gain in most cases, just perhaps an IV stage and a buffer.

We've had DAC and CD player converters outputting 1uV residual noise levels for 35 years.

Thanks for the reply!!

 

[Watire]

How to measure a phono preamp at home without a turntable with a test record ?
Do classic audio files RIAA treated from a software like Audacity or SOX are good enough ?
How to feed the tested preamp a signal with the required impedance and voltage ?

 

[Digital1955]

Well at least you didn't find it internally converting to DSD and then back to analog again .

 

[Michael Fidler]

If you use dull cartridges such as Rega's home made examples (the cheap carbon is a 'poshed up' AT91), mid priced Sumiko or many modern Grado's, this MoFi box will help eq them just fine. Ortofon OM's seem to have an otherwise benign gentle rise over 10kHz and mid level AT's have a rise at around 10 - 12kHz or so. This box isn't severely out by 'analogue' standards, but a slight roll-off at hf I'd suggest would be preferred as so many modern MM types actually rise at hf now rather than fall, as old fashioned MM's used to before the pole piece advances that makers began to introduce, which encouraged a much flatter playback response I remember.

The Mani 2 is pretty darned all you need in the US and we here have the Spartan 15 which I'm trusting maker @Michael Fidler does as good a job on (as far as costs allow) as he does the more advanced models. MoFi do turntables too and over here, they're rather expensive, may I say...

Regrettably, I would say that +-0.8dB is significantly below the average type of circuitry we might find in a typical cheap receiver. By using 5% capacitors (very low cost C0G SMT devices) and readily available 1% resistors (they don't even have to be thin film types to beat the distortion performance that we see here), we could exceed this specification for a few pennies. It's nice to see the mono switch and subsonic filter, though. I really wouldn't be comfortable with the 'studio' moniker applied to this product given the tonality that it's bound to impart on the sound signature. It's quite amusing to see the marketing copy for this one...

StudioPhono replicates the wide bandwidth transparency Mobile Fidelity Sound Lab mastering engineers demand. Tim De Paravicini, the designer responsible for the Mobile Fidelity Gain2™ Ultra Analog cutting system, assisted with the internal circuitry to ensure your analog system at home faithfully reproduces the music buried in the grooves of your LPs.

I think if you look at the designer's track record, it explains a lot! Wide bandwidth transparency!?

 

[Michael Fidler]

There is a huge amount of gain in a phono stage. 40dB difference (RIAA curve) between the frequency extremes and the signal were are amplfiying outputs just a few millivolts (a few hundred microvolts and 60dB of gain for moving coil). 60dB is a thousand times amplification!

DACs have it easy. There is no gain in most cases, just perhaps an IV stage and a buffer.

We've had DAC and CD player converters outputting 1uV residual noise levels for 35 years.

It's also well worth adding that your typical moving magnet cartridge has a high output impedance, due to inductance, that exacerbates the problem of noise by converting the current noise of the loading resistor and the input amplifier very effectively into considerable amounts of voltage noise. The input amplifier has to be optimised for low current noise and voltage noise at about a 1pa/10nV ratio. It's very hard indeed to exceed 79dB SNR with a 5mV cartridge connected.

With DACs, the operating impedance is generally so low that the IV converter amplifier and buffer are optimised just for low voltage noise, as current noise effects are negligible. We can therefore just select for low voltage noise, which can give us another 6dB of noise advantage (or more with some of the new op-amps that we're seeing that do something like 1pa/1nV).

 

[restorer-john]

It's very hard indeed to exceed 79dB SNR with a 5mV cartridge connected.

Michael, out of interest, what is your preferred setup for testing an MM front end? An attached M44/55 type cart and the analyzer generator? We are getting a better than typical implementation by driving the stages with a low source impedance (gen) that has no reactive component otherwise.

What about residual noise? Shorted input A-WTD or unweighted for you? 20-20k BW or greater?

 

[Michael Fidler]

Michael, out of interest, what is your preferred setup for testing an MM front end? An attached M44/55 type cart and the analyzer generator? We are getting a better than typical implementation by driving the stages with a low source impedance (gen) that has no reactive component.

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.

 

[restorer-john]

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.

View attachment 365209

Oh that's beautiful! I want one! I've got a pile of AT91 bodies without plastic mounts/unscrewed too, mainly pulled from cheap moulded headshell TTs. Very consistent coil to coil values in those older Japanese ATs.

Is it something you'd consider making a few of for sale/kit?

 

[Michael Fidler]

Oh that's beautiful! I want one! I've got a pile of AT91 bodies without plastic mounts/unscrewed too, mainly pulled from cheap moulded headshell TTs. Very consistent coil to coil values in those older Japanese ATs.

Is it something you'd consider making a few of for sale/kit?

Shipping to AU might be a bit pricey, but if you're happy to source a centre-off switch (or just a normal DPDT if you wish to forgo the open option and just have 'cartridge' and 'short' modes) then I can send you a few bits and pieces such as the board, RCA connector, and some 'B metal' enclosure pieces. The original enclosure is just cosmetically rejected bits of a product I already make.

 

[restorer-john]

Shipping to AU might be a bit pricey, but if you're happy to source a centre-off switch (or just a normal DPDT if you wish to forgo the open option and just have 'cartridge' and 'short' modes) then I can send you a few bits and pieces such as the board, RCA connector, and some 'B metal' enclosure pieces. The original enclosure is just cosmetically rejected bits of a product I already make.View attachment 365227

Shoot me a PM. I've got centre-off miniature DPDT toggles and several AT91 bodies.

The case, PCB, RCAs, earth term. and the MC R and L components are all I'd need to keep it the same as yours.

 

[Mulder]

Mofi was heavily criticized a few years ago when it emerged that their audiophile vinyl reissues were digitally mastered. They even got sued. Now they apparently also has a phono-stage that does not live up to TOTL. How much faith should one really place in Mofi as an audiophile prestige label? https://www.rollingstone.com/music/...-made-with-digital-mastering-tech-1234580198/

 

[formdissolve]

Can confirm that this sounds a bit "bright" with a Hana SL moving coil cart, as well as a cheap Ortofon Red MM cart. If you can tame it with EQ, it actually can sound pretty good subjectively (although I can't hear above 17khz anyways), and doesn't sound distorted. Definitely more expensive than it should be though. MoFi also makes a $6000 phono stage now

Specs from the site:

Gain:
Adjustable from 40dB - 66dB

THD:
< .01% (MM) / < .012% (MC)

Load Impedance (MM):
47,000 Ohms

Load Impedance (MC):
75, 100, 500, 1k, 10k, 47k Ohms

Input Capacitance:
100pF

FEATURES
• Selectable gain for MM or MC 40dB, 46dB, 60dB, 66dB

• Adjustable loading 75Ω - 47kΩ

• Switchable Mono mode

• Subsonic filter

• External power supply

 

[Toni Mas]

The RIAA is not so bad but the response above 20khz is very strange as if there is a resonant peak above 40khz. Might be interesting to get the full response and see how is the cut off...

 

[DSJR]

Can confirm that this sounds a bit "bright" with a Hana SL moving coil cart, as well as a cheap Ortofon Red MM cart. If you can tame it with EQ, it actually can sound pretty good subjectively, and doesn't sound distorted. Definitely more expensive than it should be though. MoFi also makes a $6000 phono stage now

Specs from the site:

Gain:
Adjustable from 40dB - 66dB

THD:
< .01% (MM) / < .012% (MC)

Load Impedance (MM):
47,000 Ohms

Load Impedance (MC):
75, 100, 500, 1k, 10k, 47k Ohms

Input Capacitance:
100pF

FEATURES
• Selectable gain for MM or MC 40dB, 46dB, 60dB, 66dB

• Adjustable loading 75Ω - 47kΩ

• Switchable Mono mode

• Subsonic filter

• External power supply

The 2M Red is as far as I can tell, a re-dressed OM10 and this latter, is a bit 'lively' and 'raw' with it to say the least. Still a fair choice at the price, but I'd love to do a comparison with the AT VM520 which I suspect is a touch more restrained possibly as it's AT110E ancestor was all those years ago. Apologies for the digression...

 

[aschen]

I have had this since it came out. Maybe getting close to a decade? Anyways wont argue with the measurments. However, I think the packaging is great. It is nice and slim so it can sit on top of things in a low shelf, I actually had it hidden under my turntable for a while.

Built in the USA to a pretty high standard and a ton of adjustabilty. The dips are a bit of a pain, I actually taped the owners manual under mine since that sticker was not on the early ones. However I never really need to mess with them. I think the metal chassis with the bespoke plastic lid looks nice. Its much nicer than the common generic extrusion boxes that are so common at this price.

Anyways no emotional attachment to it but it has worked well for many years now.

 

[sq225917]

How to measure a phono preamp at home without a turntable with a test record ?
Do classic audio files RIAA treated from a software like Audacity or SOX are good enough ?
How to feed the tested preamp a signal with the required impedance and voltage ?

Just plug it in, turn the volume to your normal listening level, if you can hear hum and transistor roar it's a POS.

 

[LTig]

The RIAA is not so bad but the response above 20khz is very strange as if there is a resonant peak above 40khz. Might be interesting to get the full response and see how is the cut off...

Maybe the implemented the mythical (non existing) "Neumann pole".

 

[USER]

The 2M Red is as far as I can tell, a re-dressed OM10 and this latter, is a bit 'lively' and 'raw' with it to say the least. Still a fair choice at the price, but I'd love to do a comparison with the AT VM520 which I suspect is a touch more restrained possibly as it's AT110E ancestor was all those years ago. Apologies for the digression...

There are measurements of two of those on this site (though the AT VM540 is available):

Introducing the Phono Cartridge Measurement Library

After years of development we are finally launching our phono cartridge measurement library. These measurements were created using the brilliant cartridge response python script written by @scott wurcer and @JP. Information about its origins can be found in the following threads...

audiosciencereview.com

I find that A-T cartridges generally fall into 3 different but similar FR curves so you can likely make a good guess as to where the AT VM520 lies.

I would personally never let this phono preamp anywhere near those three cartridges. That FR is abysmal, especially when you consider that most modern cartridges are plus minus 2 or 3 dB. What a piece of junk.

 

[Michael Fidler]

Maybe the implemented the mythical (non existing) "Neumann pole".

Or more likely not enough HF stability to bring full feedback as frequency increases, so the gain doesn't drop down low enough as it approaches maximum possible feedback...