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Review and Measurements of Little Labs Monotor Headphone Amp

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#41
Speaking of the RNHP, since that is the Monotor's main competition in the pro market, it would be nice if someone could send one to Amir in order to get a direct comparison of the two amps. Hint hint.
 
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#43
As a (happy) owner of a Monotor myself, very excited to see this review and kudos again to Amir for a fine review. My personal headphone is a KRK KNS8400 with an impedance of 36 ohms, so I suppose I'm now justified in looking for a new pair of headphones with higher impedance ; )

Amir, one thing that surprises me a little bit is that your measurements found the Monotor to be "noisy", as in the many reviews out there note how quiet it is, and that's been my experience as well. Is there something I may be missing? For example, is the noise in your measurements more "measurable" than "audible"? Or, are there particular situations where the noise would be more audible? Just wondering how to interpret this in real life.
 

amirm

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#44
Amir, one thing that surprises me a little bit is that your measurements found the Monotor to be "noisy", as in the many reviews out there note how quiet it is, and that's been my experience as well. Is there something I may be missing? For example, is the noise in your measurements more "measurable" than "audible"?
Correct. Over time I hope to develop more realistic tests that predict audibility of the noise. You can see the start of that with the "50 millivolt" test.
 

solderdude

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#45
The KNS8400 is 97dB/mW so can reach 120dB peak on the Monotor.
Just 3 dB less than the HD650.

When you haven't driven it to clipping levels yet and like the sound of the KRK I see no need to buy a 300 Ohm headphone unless you are looking for better or different sound quality.

With a 100 Ohm headphone the Monotor can even reach 550mW
 

Jimmy

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#46
Fair enough, that's what I understand, too, but to put it in perspective, the Topping DX7S packs much more functionality and the HP amp section is also TPA6120 based (2x since it has balanced output).

This review says more or less what I do :

https://m.bonedo.de/artikel/einzelansicht/rupert-neve-designs-rnd-rnhp-test/3.html

Good but pricey, if it used an in house developed amplification topology instead of a very standard design things would change, R&D is costly specially for small production runs.

It would be intersting to see what's inside the Monotor to judge its value proposition, too.

It is a small run product, made in the US with quality components all round. Personally, if I was in the market for a HP amp (I'm not) it'd be on my short list.

And yes, you need to pay for his name too. Just like Apple.
 
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#47
The KNS8400 is 97dB/mW so can reach 120dB peak on the Monotor.
Just 3 dB less than the HD650.

When you haven't driven it to clipping levels yet and like the sound of the KRK I see no need to buy a 300 Ohm headphone unless you are looking for better or different sound quality.

With a 100 Ohm headphone the Monotor can even reach 550mW
Ah, a voice of reason, just when I thought I had enough justification to pull the trigger :facepalm:

All kidding aside, thanks for your post, very valuable. I've never lacked amount of volume nor experienced any clipping with my KNS's, so your analysis does seem to match my experience.
 

Jimster480

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#48
300 Ohm = 220mW = tons of power
40 Ohm = 200mW = falling short.

Both impedances receive the same 'power' but the culprit here is efficiency differences in dB/mW
HD650 = 100dB/mW, HE400i= 93dB/mW so the HD650 simply plays 7dB louder at max volume.
When a HP50 would be used for instance (32 Ohm) it would play 1dB louder than the HD650 and the Focal spirit one even 7dB louder than the HD650.

Edit: corrrected for 32 Ohm and assuming max. output current.

In this case the amp inside can provide a high output voltage but is rather limited in output current 100mA peak (70mAeff) for this device..
So the amp is not really suited for low impedance inefficient headphones.
For high sensitivity headphones it may well be sufficient.

But I agree that 200mW is a bit too little for driving all lower impedance headphones and the amp section is much better suited for high impedance headphones.
Yes it surely wouldn't be able to drive my Aeon
 

solderdude

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#49
That would reach max 113dB (0.9V) and play 4dB softer than the HD650 but somewhat louder than the HE400i.
Would play about as loud as you could drive it straight out of a mobile phone.

You won't be able to play it loud and distortion free.
 

JohnYang1997

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#50
A simple njm2068+tpa6120 composite amplifier will give 0.0001% into 16ohm load and 700ma peak current. Too much energy wasted on barely decent stuff.
 

solderdude

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#51
But what would be the fun or could make a manufacturer 'stand out' if everyone used the same chips and voltage rail voltages and output resistance ?
 

JohnYang1997

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#52
But what would be the fun or could make a manufacturer 'stand out' if everyone used the same chips and voltage rail voltages and output resistance ?
But only when they can make it better? It's probably more hilarious to stand out to be bad like ahhhh schiit? or Wooooaudio?
 

solderdude

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#53
They seem to have a solid fanbase so in their eyes they must be doing something well .:)
 

agjell

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#54
[...] So I stress test them to see if the amplifier distorts first, or the headphone. [...] Nothing is more audible as far as distortion than an amplifier that has run out of power. So I play until I can't stand the level for a second or two to see if the bass gets distorted.
What are the characteristics of amp distortion compared to driver distortion?
 

solderdude

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#55
What are the characteristics of amp distortion compared to driver distortion?
hard clipping vs. compression of upper amplitude so a different harmonic spread.
All tube amps have less hard clipping than SS.
 

agjell

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#56
hard clipping vs. compression of upper amplitude so a different harmonic spread.
All tube amps have less hard clipping than SS.
I wasn't precise enough in my question, sorry. What I'm really wondering about is what they sound like. How can I learn to identify which is which? Can I provoke one without the other, so that I can memorize the different characteristics? Maybe a subject for a new thread.
 

solderdude

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#57
Very short peaks just clipping are not easy to detect.
Next step is bass becoming seemingly 'light' and not nice sounding any more.
When the sound becomes 'loud' and 'nasty'/'gritty'/'shrill' it is most likely already clipping severely already.

An amplifier that has plenty of headroom will sound loud yet still nice and 'fuller' sounding than when playing at more acceptable levels.

Having plenty of headroom is preferred over 'just enough'.
Having waaaayyy too much power (say 2 to 6W) while driving sensitive headphones/IEMs that are lying on a desk and (by accident) receive way too much power will certainly fry those drivers.

The Monotor is not likely to destroy drivers so a quite safe to use device.
 

amirm

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#58
What are the characteristics of amp distortion compared to driver distortion?
Usually, when there is insufficient volume and/or bass and distortion sets in, it is the amplifier. Inversely if you are getting excellent and dynamic sound with strong bass and then hear as if the driver is falling apart, it is the driver. OK, that last bit is recursive. :) I am having trouble using words to describe an audible effect which needs to be heard.
 

littlelabs

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#59
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!
 

MZKM

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#60
@littlelabs

I mostly agree (such as being 0.3dB at 20kHz), but one statement I would like to address:

One can make two identical circuits with different chosen components that measure identically but can sound very different.
Maybe for one measurement (e.g. frequency response), but not true that a real sonic change (i.e. not one you imagine) cannot be measured. How can the flow of electricity be altered but not measurable?

There is a reason a power amplifier makes a poor headphone amp.
I generally agree; I would like to see the Emotiva A-100 measured though, it already has pretty good power natively:
8 Ohms: 60 mW / channel
33 Ohms: 200 mW / channel
47 Ohms: 250 mW / channel
150 Ohms: 430 mW / channel
300 Ohms: 440 mW / channel
600 Ohms: 350 mW / channel


But, once you put it in "direct drive" mode, it becomes insane, they even state no liability for damaged cans:
8 Ohms: 50 watts / channel
33 Ohms: 12 watts / channel
47 Ohms: 8.5 watts / channel
150 Ohms: 2.6 watts / channel
300 Ohms: 1.3 watts / channel
600 Ohms: 0.6 watts / channel
 
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