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:
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:
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:
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:
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:
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:
Output impedance is comfortably low at 1.0 ohm:
Channel imbalance is not as good as I like to see:
Most important test for headphone amplifier is power versus distortion. Here is that for 300 ohm load:
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:
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
------------
As always, questions, comments, recommendations, etc. are welcome.
Someone has told me about a great Neapolitan Pizza Restaurant. I called then and they said the food is not free. So please donate some money using:
Patreon: https://www.patreon.com/audiosciencereview), or
upgrading your membership here though Paypal (https://audiosciencereview.com/foru...eview-and-measurements.2164/page-3#post-59054).
That pro pedigree shows up in the design language of the Monotor:
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:
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:
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:
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:
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:
Output impedance is comfortably low at 1.0 ohm:
Channel imbalance is not as good as I like to see:
Most important test for headphone amplifier is power versus distortion. Here is that for 300 ohm load:
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:
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
------------
As always, questions, comments, recommendations, etc. are welcome.
Someone has told me about a great Neapolitan Pizza Restaurant. I called then and they said the food is not free. So please donate some money using:
Patreon: https://www.patreon.com/audiosciencereview), or
upgrading your membership here though Paypal (https://audiosciencereview.com/foru...eview-and-measurements.2164/page-3#post-59054).
Last edited: