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Review and Measurements of Little Labs Monotor Headphone Amp

amirm

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#1
This is a review and detailed measurements of Little Labs Monotor Headphone Amplifier. It is on kind loan from a member. The Monotor costs USD $540 from Amazon including Prime shipping. Target market and company focus seems to be professional audio users.

That pro pedigree shows up in the design language of the Monotor:

Little Labs Monotor Headphone Amplifier Review.jpg

I would consider it a bit quirky and oddly cute at the same time.

Speaking of odd, there is a selector switch to choose various modes of listening from mono to only Left, Right, Left-Right, etc. which should come in handy in audio post production.

There are no less than four headphone connectors, two of each size. I tested the two 1/4 inch ones and they are identical in performance.

Volume control is small and has very few notches so was rather hard to adjust for measurements. For listening that may or may not be an issue for you. The control is analog by the way.

The backside naturally has balanced inputs even though this is a very compact enclosure:

Little Labs Monotor Headphone Amplifier Connectors Review.jpg

In addition, you also have 1/4 and 3.5 mm input jacks so you don't need any adapters to hook up anything to the monitor. Very well done.

As you see to the left, a very chunky external power supply comes with the Monotor which provides +- 16 volt DC. The negative supply makes the amplifier circuit much simpler and usually provides better performance than converting a positive to negative inside the amplifier. The downside is the rather unique external power supply jack.

Measurements
For consistency with my measurements of the Massdrop THX AAA 789 Headphone Amplifier, I set the output to 5.7 volts and this is what it looks like:

Little Labs Monotor Headphone Amplifier Audio Measurements.png


If you look at the FFT spectrum, you can barely see a distortion spike poking up below -130 dB! That is superb. What is not superb is that our SINAD is only 109 dB. Why? Because SINAD is sum of distortion and noise. If we had no noise, our SINAD would have been 130 dB. Since it is much lower, we know that shortfall in SINAD is due to higher noise floor (not visible in the FFT). As a way of reference, THX AAA 789 has a SINAD of 117. So this is still very good.

We can see the elevated noise floor using our signal to noise ratio measurement:
Little Labs Monotor Headphone Amplifier Signal to Noise Ratio Audio Measurements.png


As you see, it is almost identical to SINAD confirming what I explained above. If noise was better managed, the Monotor could have sailed way past the the Massdrop THX AAA 789.

I have started to measure the SNR at just 50 millivolts for sensitive IEMs:
Little Labs Monotor Headphone Amplifier 50 mvolt Signal to Noise Ratio Audio Measurements.png


As you see, you don't nearly have the same dynamic range that you think you have, looking at the previous measurement that is at 5.7 volts output.

Frequency response is fine for listening but as far as measurements, I was not expecting a roll off so early past 20 kHz:

Little Labs Monotor Headphone Amplifier Frequency Response Audio Measurements.png


Output impedance is comfortably low at 1.0 ohm:
Little Labs Monotor Headphone Amplifier Output Impedance Audio Measurements.png


Channel imbalance is not as good as I like to see:

Little Labs Monotor Headphone Amplifier Channel Balance Audio Measurements.png


Most important test for headphone amplifier is power versus distortion. Here is that for 300 ohm load:
Little Labs Monotor Headphone Amplifier Power at 300 Ohm Audio Measurements.png


That is ample amount of power! There is however more noise than our reference THX amplifier as shown earlier. The high power here is due to very high output voltage available (I think the max is around 9 volts). The dual dc voltage at 16 volts is responsible for this.

Switching to 33 ohm load, we test for current delivery:
Little Labs Monotor Headphone Amplifier Power at 33 Ohm Audio Measurements.png


Ouch. That is a huge drop in performance. We don't eave have 200 milliwatts. A desktop amp in this day and age needs to produce more than one watt especially at these prices. On top of that, we have the same higher noise level once again.

Listening Tests
People keep asking me how to relate these measurements to listening experience. Well, in the case of headphone amplifiers, the last two power measurements are excellent predictor of what you can expect. We had tons of power with 300 ohm. No wonder that it could rattle the headband off my Sennheiser HD-650 if I let it! :) It was one of the few times that I got too scared to go to 0 dB level. :) If you have not experienced a high power amplifier driving the Sennheisers, you need to experience them to know what I am talking about. The character of these headphones changes completely. Bass becomes thundering as if there is a subwoofer in there. Clarity and resolution improves. At lower powers I just find the sound dull and uninteresting.

Measurements predicted we would not do well with low impedance headphone and that was absolutely correct with Hifiman HE-400i. I could easily get the amplifier to distort, well before the limits of the driver. Can't recommend usage of the Monotor for such headphones.

Conclusions
The Little Labs Monotor exceeded my initial expectation as far as performance. It has incredible ability to drive high impedance headphones. If you have a headphone at 300 to 600 ohm, you will have a very good match here. As I noted above, I don't recommend the Monotor to be used with low impedance headphones. It is a complete mismatch in that situation.

On the value side, the $540 cost is hard to justify. If you don't need balanced inputs, the JDS Labs Atom at USD $100 performs much better and of course leaves you a ton of money to spend on other things. If you need balanced, Massdrop THX AAA 789 is the king but availability is poor right now.

I like my headphone amplifiers to be flexible so I don't have to change them if I change headphones. The low current delivery of the Little Labs Monotor is problematic there so I can't quite recommend it, even though it seems to be well designed.

EDIT: manufacturer was kind enough to join the forum and respond to the review. See: https://www.audiosciencereview.com/...e-labs-monotor-headphone-amp.7079/post-162094

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#2
I have been waiting for this review ever since I found out about the Monotor about a year ago. These are the first real measurements released for the device. I know because I have spent many days searching for measurements of this in the past. Thank you. I am surprised this is so specialized to high impedance headphones.
 
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#4
In a professional studio setting, high impedance headphones would be the standard for the person mixing and/or mastering. From my research, this is specifically designed for studio use and found it's way to the audiophile market by accident.
 

solderdude

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#5
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.
 
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MZKM

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#6
Measurements predicted we would not do well with low impedance headphone and that was absolutely correct with Hifiman HE-400i. I could easily get the amplifier to distort, well before the limits of the driver. Can't recommend usage of the Monotor for such headphones.
I honestly don’t see how. InnerFidelity measured the regular HE-400’s and found they need 1.62Vrms for 110dB (matching spec), assuming the HE-400i is not much different (specs are similar), and assuming the amp is 200mW into 45ohm (3Vrms), the HE-400i should get to >115dB. @amirm, I worry for your hearing if this is true.
 
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solderdude

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#7
116dB would be the max SPL that can be achieved.
It is a bit misleading though to use these numbers.
The 116dB would be present when all sinewaves happen to add up.
With most music the emphasis would be on the bass.
Like shown below:



As can be seen most of the 'power' is around 100Hz and about 12dB down from 0dBFS
So lets assume peaks in the bass would be 105dB SPL.
When we grab the equal loudness curves we see that 105dB SPL is about 90 Phon.
Mids peak at around -30dBFS so about 85Phon.

This shows that while the SPL may calculate as 115dB and one would say ... wow that's deafening bass notes and voices etc. would be perceived as 85 to 90 Phon (or around the same value when measured in dBA).
Loud... for sure but not dangerously loud and the main reason why amps need to have more headroom than one might think and the headphones need to have low distortion even at higher SPL.
 

MZKM

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#8
116dB would be the max SPL that can be achieved.
It is a bit misleading though to use these numbers.
The 116dB would be present when all sinewaves happen to add up.
With most music the emphasis would be on the bass.
Like shown below:



As can be seen most of the 'power' is around 100Hz and about 12dB down from 0dBFS
So lets assume peaks in the bass would be 105dB SPL.
When we grab the equal loudness curves we see that 105dB SPL is about 90 Phon.
Mids peak at around -30dBFS so about 85Phon.

This shows that while the SPL may calculate as 115dB and one would say ... wow that's deafening bass notes and voices etc. would be perceived as 85 to 90 Phon (or around the same value when measured in dBA).
Loud... for sure but not dangerously loud and the main reason why amps need to have more headroom than one might think and the headphones need to have low distortion even at higher SPL.
I get what you are saying, but I’m lost a but in your calculation choices. When setting 100Hz to 105dB, is that because it’s -12dB down from full scale which we agreed is >115dB? Shouldn’t you instead bump up 105dB to treat it as full scale? Don’t know if that’s an RMS or peak graph, because peaks would for sure be much closer to full scale. If that’s an RMS graph, then I guess I could see your calculation choices.

However, the sensitivity rating is B-weighted; which if my understanding is correct, will help even out the disparity between frequency amplitude as shown in your graph (100Hz is about -20dB compared to C-weight).
 

solderdude

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#9
In music it's always an addition of all 'signals' combined.
The song used has it's peaks very close to 0dB.
When I play back a 0dB tone and analyse I get 0dB on the plot so I assume its peak.
So the bass seems to peak around -12dBFS (as there is not only bass but also the other signals added)
So 115 - 12 = 103dB SPL is around 90 Phon perceived.

Sensitivity is usually rated at 500Hz or 1kHz and sometimes with noise and weighting does nothing when rated at 500Hz or 1kHz but indeed is somewhat different from noise which would/could be weighted.

This calculation is for this song only and may (will differ) with other songs and DR ratings.
It was merely to show that while 115dB would be measured at a certain frequency with a specific power/voltage that doesn't mean it will sound as loud. Of course peaks in the signal reaching that level would be at 115dB but be very short peaks (unless DR is very poor)

The amp section may well not be powerfull enough for some low impedance low sensitivity headphones. This is what Amir concluded.
I agree sortof with it but one has to factor in the efficiency.
 

MZKM

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#10
Sensitivity is usually rated at 500Hz or 1kHz and sometimes with noise and weighting does nothing when rated
Oh, ok. I’m more of a speaker guy, and speaker sensitivity is usually averaged from a frequency range (300Hz-3000Hz is a common parameter).

So, for the HE-400i to get to reference levels for mids, that would be about +20dB, so around 20W, not even the Emotiva A-100 can do that (8.5W into 47ohm). Most people listen around 10dB from reference, so that’s 2W. Again, this is going solely off the graph you linked. Of course though, you almost never have vocals close to clipping, usually around -20dBFS to -30dBFS, so most of my calculation is meaningless for real-world usage.
 
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JohnYang1997

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#11
But the problem is that you only need one opa1612 running at +-18v to perform this well at high impedance. Maybe two will be optimal. Over 60ohm is the region that not much current is used and over 150ohm is the region that it just draws little current. A slightly modded o2 will perform just as good, and outperform noise wise. It's glad to see it's not bad like neve at least.
 

JohnYang1997

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#13
Maybe you can use final e5000 and mr speakers aeon for low impedance listening test in the future, amirm.
 

SIY

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#14
I know you've answered this before, but I'm old, senile, and forget- what's the bandwidth you use for the noise measurements?
 

JohnYang1997

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#16
I asked him about it a little while ago and if I recall correctly it's 90 kHz.
I wasn't replying to SIY because i believe he wanted amirm's own reply.
From what I know, 22khz for dashboard and 90khz for thd+n vs frequency graph. So the answer to SIY should be 22khz since there isn't thd+n graph in this yet.
 

JohnYang1997

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#17
Considering the output is 5.7v, the noise is very high. That's like more than 10k ohm resistance of noise.
 
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#18
I wasn't replying to SIY because i believe he wanted amirm's own reply.
From what I know, 22khz for dashboard and 90khz for thd+n vs frequency graph. So the answer to SIY should be 22khz since there isn't thd+n graph in this yet.
I'm pretty sure the SNR measurement is also measured with 90 kHz bandwidth just like the THD+N vs frequency but I may be wrong.
 

JohnYang1997

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#19
I'm pretty sure the SNR measurement is also measured with 90 kHz bandwidth just like the THD+N vs frequency but I may be wrong.
Oh right. I overlooked that. I never read DNR/SNR so i thought he meant the dashboard. My bad. In this case, I have no idea.
Just checked the numbers, it has to be 22khz. The number is too close to thd+n and since the distortion is minimal. It will be much lower dnr/snr if the bandwidth is 90khz, like 100db or less.
At 5V+ output, the dnr should be at 120db+ range for high performance low noise devices.
 

solderdude

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#20
Oh, ok. I’m more of a speaker guy, and speaker sensitivity is usually averaged from a frequency range (300Hz-3000Hz is a common parameter).

So, for the HE-400i to get to reference levels for mids, that would be about +20dB, so around 20W, not even the Emotiva A-100 can do that (8.5W into 47ohm). Most people listen around 10dB from reference, so that’s 2W. Again, this is going solely off the graph you linked. Of course though, you almost never have vocals close to clipping, usually around -20dBFS to -30dBFS, so most of my calculation is meaningless for real-world usage.
The HD650 seemed to be satisfactory for Amir which only played 7dB louder. For the HE400i to reach the same SPL as he got with the HD650 the HE400i thus needs 5x more power in that case = 1W. (slightly more than double the current is needed, the voltage wasn't a problem in the first place).
The output stage is most likely just an opamp.
 
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