A good friend of mine brought the measurement review of the Little Labs Monotor to my attention.
I'm the designer of the Monotor.
I very much appreciate the effort Amirm put into it. It’s interesting and useful. I just hope it doesn’t discourage people from seriously listening and comparing, using their own ears as the final judge when making a headphone amp purchasing decision.
I do not dispute the accuracy of the measurements. I do however disagree strongly with the conclusions drawn by Amirm on some of the measurements. In my 40 years of working professionally designing, maintaining and manufacturing audio electronics for recording and mastering facilities, I can assure you a layman audio fanatics biggest mistake is judging a unit to purchase on specs alone.
As a designer working with professionals with serious listening chops, you over time learn what makes a circuit sound better and what specs matter, and what specs are a wank.
One can make two identical circuits with different chosen components that measure identically but can sound very different.
One can also add to a circuit to make a noise floor even quieter when the noise is already audibly imperceptible.
One can also add to a circuit to make more current available when in reality it won't be used.
Each active addition to a circuit is one step further away from the purity of the source.
My design philosophy is to use minimum active circuitry in the signal path to bring the headphone to a respectable volume and command that headphone to be as transparent to the source as is possible. There is a reason a power amplifier makes a poor headphone amp. You don't put a dragster engine in a Porsche.
Some notes:
Headphone Imbalance vs. Volume Position.
I challenge anybody out there that says they can perceive an l/R imbalance of less than 1dB. Yes, it's nice when you can find a simple analog carbon pot that tracks closer than 1dB top to bottom in the whole logarithmic scale, but if you find one that has 30 steps within 1dB you're doing great. The monotor pot is not a discrete stepped attenuator, but it tracks pretty damn well for what it is. Of course, you can use an IC based potentiometer that can track perfectly, but then you just added another active step further from transparency. Oh and regarding steps, how many more than 30 is necessary?
How much power is necessary for driving a headphone?
I am using daily both the HD600 Sennheiser 300 ohms and an Audeze LCD-X 20 ohms and both work wonderfully with the monotor, and certainly without distortion at very loud volume.
Those two phones are the two most popular used by professionals paired with the monotor.
I listen to all genres of music and not once did I notice distortion even at dangerously high volumes. Now I'm not familiar with the Hifiman HE-400i, but I'll take Amirm at his word that the monotor distorted before the HE-400i did.
But, and this is very important, the casual reader of this review would most likely overlook this. This HE-400i is a rare case, a new breed of headphone that is very low sensitivity, and also low impedance (FYI low impedance phones are typically very sensitive). I think another headphone with that spec is the Mr. Speaker Aeron (closed back), a headphone I like a lot. I have never pushed it so loud the monotor distorted, but I don't dispute that you can.
In my experience with headphone amps, voltage gain, which is necessary to drive phones to a respectable level, is far more important a spec than power output. Rarely is over 100mw of power necessary to happily drive a well-designed headphone. The Monotor has 13.8 dB of gain. I chose that gain for a perfect pairing with my most popular headphone the HD600. That gain on the HD600 gives you a great range from soft to ridiculously loud, and oh so clean... Now where that gain becomes a problem is with super sensitive phones, mostly IEMS. Some IEMS are crazy sensitive, those IEMs I do not recommend with the monotor.
The monotor is not a one size fits all, you don't use a Porsche for off-roading now, do you?
That being said I have some drummer friends that love it super loud and use the monotor on stage to power their IEMS, they couldn't be happier. I worry about their ear health.
Frequency response:
The monotor is .3 dB down at 20kHz, at 50kHz it's 1.7dB down this is on purpose. I can assure you, you cannot only not hear less than 1dB imbalance left to right you sure as hell can't hear .3 dB down at 20kHz. Amplifying stuff that's not music does not add to a sonic experience. Overlooked in this review, the monotor has an excellent low-frequency response, flat to 3Hz (where you can actually feel it).
Mono functions and other pro features and price:
The monotor found its way into audiophile circles, but it is truly a pro device.
The mono functions do add greatly to the cost of the monotor. The phase function makes checking azimuth on tape machines and phono cartridges a breeze.
The monotor remains balanced, completely differential internally through to the output driver. We don't use any balanced to unbalanced buffers.
Only a single active stage is used surrounded by top-notch passive components in a hybrid thru-hole/smt component selection. This includes Dale Vishay thru hole resistors, Nichicon Muse bipolar capacitors, polystyrene capacitors, and massive power supply reserve caps using some of the quietest voltage regulators available. None of these components add to what can be measured, but definitely bring you closer to the source sonically, and makes the unit more costly. I laugh when I hear comments of the monotor being overpriced. They wouldn't say that if they saw the BOM (bill of materials).
In closing I didn't come here to bitch, I came here to enlighten. I appreciate what Amirm has done but I want to encourage the consumer to look past the spec. Any EE can make textbook audio gear that measures well, but it takes ears and years to learn what really sounds well.
Cheers
Jonathan Little
ps
Amirm I measured the monotor output impedance to be 0.5 ohms!