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Review and Measurements of Little Labs Monotor Headphone Amp

dkinric

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#21
Listening Tests
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.
I have the HD-6XX and I know what you are talking about. Even when paired with the DX3PRO in high gain, although it was clean output, it was not as engaging as I had hoped. I recently plugged them into my Cambridge Audio CXA60 and wow, what a revelation! I have not been able to find any rated specs for this output, I know on many home amps/receivers the headphone out is an afterthought. Not so on the Cambridge. Bass was well defined, overall tone immersive and detailed - improved clarity and resolution as Amir notes.
Obviously CA put some effort into this. If your look at their literature, they claim to put a lot of resources into the electrical design, using all balanced architecture, a very large transformer and designs to minimize interference. In my experience with this amp, it seems to be more than just marketing speak.

So, anyone with an Sennheiser HD-650 / HD-6XX - it transforms into something really special if you can feed it very high power (at least a good bit more than 120 mw). Also, The Cambridge CXA-60 has a very good, very powerful headphone amp (also an outstanding Integrated amp all around).
 

JohnYang1997

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#22
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.
Idk about why amirm would plays at such volume. I wouldn't even stand that. I normal play at less than 100mv region for all my headphones. 10mw is good enough for me.
 

solderdude

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#23
I like to crank it up when I have some very well recorded music (high DR) and really want to listen out for things in the background for just a few seconds. I want the amps headphones to perform admirably at such a moment even though I never listen to music in this manner.
I don't think he listens at this level but cranked it up and noticed the bass clipping ?
Normally I listen to very low levels (70dB on average or lower) I suppose Amir does too.
 

JohnYang1997

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#24
I like to crank it up when I have some very well recorded music (high DR) and really want to listen out for things in the background for just a few seconds. I want the amps headphones to perform admirably at such a moment even though I never listen to music in this manner.
I don't think he listens at this level but cranked it up and noticed the bass clipping ?
Normally I listen to very low levels (70dB on average or lower) I suppose Amir does too.
Probably it's low frequency distortion. 1khz won't tell the full story.
 

solderdude

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#25
In case of the HE400i (is a planar) it has very low distortion in the lows even up to loud levels.



dist. is measured at 90dB SPL and 0.2% is about the measurement limit of this rig.
Most dynamics indeed show much higher distortion levels.
 

andymok

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#26
YES! Finally here thank you so. much. !!

What makes this piece truly special is the functionalties after all. Can't find a similar one that could be so useful for tracking (I.e. recording) like phase checking. Plug it into mixing console's monitor output in control room, setting up mics and check phasing in the studio, all by yourself.

With new firmware, Grace Design's M905 can, but it's an interface after all (DAC+HP) so it can't be used standalone.

Now I wonder what will Neve's HP look like
 
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#27
Thank you for this reviewing this Amir! Little Labs has a pdf (here) talking about the amp and on the last page the owner talks about designing it for a professional setting and using Sennheiser HD600's with it. So it's definitely designed for higher impedances.

It's nice to see it perform as well as it did, although I had issues with the channel imbalance. As a little side note, this unit was hand selected by the owner of Little Labs to have the best possible channel balance. This came about because I bought the Monotor second hand and was off put by the channel imbalance. I reached out to Little Labs and sent it in. They had claimed that while the unit did have some channel imbalance that it was within spec (.5 db). They gave me the option of trading it for a brand new hand selected unit so I took em up on the offer (for a fee). I however could still notice some channel imbalance so I decided to send it to Amir and here we are :)

Also probably late, but fun fact, there are 2 holes on either side of the volume attenuator, inside are depress-able buttons of some sort that when depressed will bypass the corresponding side of the attenuator. So you can actually entirely bypass the volume control altogether and in my experience when you do that the channel imbalance disappears entirely.
 
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amirm

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Thread Starter #28
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.
Short answer:
The best use of sensitivity numbers is to convince yourself you need less power than you do! -- Amir, 2019
:)

Long answer:

1. These are tests, not how I listen to music normally. So don't fear my hearing. The reviewer who takes a car to 100 miles an hour the fastest for testing, doesn't drive that way in city streets.

2. My testing should be representative of countless headphones people throw at these amps even though I only test with two. So I stress test them to see if the amplifier distorts first, or the headphone. I once asked a boat salesman how big of a seas the boat I was test driving would handle. His answer: "you will get scared before the boat does." 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.

3. The most reliable way to use your ears to see if you have enough power is to keep turning up the volume on a bass heavy track until a) you can't stand it any higher or b) the bass either gets thin, muddy or distorted. This works excellently for both power and headphone amplifiers.

4. Our hearing is very insensitive to bass. 70 dB is around threshold of hearing whereas for mid-frequencies it can be -10 dB. If you set the levels for mid-range, then you better have tons of power for when the bass comes.

5. I have never, ever found sensitivity numbers useful for a listening test. Only use for them online seems to be what I said in italics above: making an argument that you need less bass than you do. After measuring countless amplifiers and listening to them, I have developed a feel for how power is necessary for ample bass performance. This correlation is quite good as I showed in this article.

6. I do the listening tests for headphone amplifiers because what the ear hears with music matters with respect to how much power you need. I have carefully selected tracks that are very revealing for this type of testing and results have been excellent. Witness how many people are happy when I recommend an amplifier.
 
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amirm

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Thread Starter #29
Idk about why amirm would plays at such volume. I wouldn't even stand that. I normal play at less than 100mv region for all my headphones. 10mw is good enough for me.
Listener preferences for level varies a ton. I know an audiophile with half a million dollar system. He listens at levels that are way too low for me. Clearly if he were a reviewer, he would not be representing my neads.

My normal dial on my DX3 Pro is around -20 dB or so for background listening test. However, on my difficult tests, I can easily stand turning it up to 0 dB and sometimes I wish there was more. That is the difference between testing and listening. Two different tasks.
 

MZKM

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#30
Short answer:
The best use of sensitivity numbers is to convince yourself you need less power than you do! -- Amir, 2019
:)

Long answer:

1. These are tests, not how I listen to music normally. So don't fear my hearing. The reviewer who takes a car to 100 miles an hour the fastest for testing, doesn't drive that way in city streets.

2. My testing should be representative of countless headphones people throw at these amps even though I only test with two. So I stress test them to see if the amplifier distorts first, or the headphone. I once asked a boat salesman how big of a seas the boat I was test driving would handle. His answer: "you will get scared before the boat does." 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.

3. The most reliable way to use your ears to see if you have enough power is to keep turning up the volume on a bass heavy track until a) you can't stand it any higher or b) the bass either gets thin, muddy or distorted. This works excellently for both power and headphone amplifiers.

4. Our hearing is very insensitive to bass. 70 dB is around threshold of hearing whereas for mid-frequencies it can be -10 dB. If you set the levels for mid-range, then you better have tons of power for when the bass comes.

5. I have never, ever found sensitivity numbers useful for a listening test. Only use for them online seems to be what I said in italics above: making an argument that you need less bass than you do. After measuring countless amplifiers and listening to them, I have developed a feel for how power is necessary for ample bass performance. This correlation is quite good as I showed in this article.

6. I do the listening tests for headphone amplifiers because what the ear hears with music matters with respect to how much power you need. I have carefully selected tracks that are very revealing for this type of testing and results have been excellent. Witness how many people are happy when I recommend an amplifier.
#4 is likely the answer as discussed above (as well as more content being nearer to full scale in the bass). Plus the sensitivty number being only at 1KHz, whereas speaker sensitivity is usually an average of a chosen frequency range, which also means B-weighting does nothing for headphone sensitivty, but does get you a more useful number for speakers.
 

chubby11

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#31
YES! Finally here thank you so. much. !!

What makes this piece truly special is the functionalties after all. Can't find a similar one that could be so useful for tracking (I.e. recording) like phase checking. Plug it into mixing console's monitor output in control room, setting up mics and check phasing in the studio, all by yourself.

With new firmware, Grace Design's M905 can, but it's an interface after all (DAC+HP) so it can't be used standalone.

Now I wonder what will Neve's HP look like

Yeah I was going to mention the RNHP. I had one for a short time and it sounded very good with HD700s and HD800.
 

PuX

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#32
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.
and the only downside is going deaf :)

I have the same headphones and an Atom + Arcam rHead, maybe I should try it once... but generally it's best to listen at comfortable level. just above that level the sound is perceived as sounding better, but I'm not sure if it's good for the ears.
 

dkinric

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#33
The JDS Labs Atom puts out 250mw of power with no distortion or clipping at 300ohm according to Amir's measurements, about the same power as the Monotor Amp.
That's double what the DX3PRO has, so that's probably enough for most.
 
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Jimmy

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#34
The RNHP is a very standard and cheap design, it uses NE5532 opamps likely for gain and a single TPA6120 as a buffer, as can be seen here:

http://rockgrotto.proboards.com/thread/11308/rupert-neve-headphone-amp

Not saying that it doesn't sound or measure good, but for the price I would expect something more elaborate or innovative (and a higher BOM).

Curious to know what's inside the Monotor given the asking price, almost the same as the RNHP.

Yeah I was going to mention the RNHP. I had one for a short time and it sounded very good with HD700s and HD800.
 

chubby11

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#36
The RNHP is a very standard and cheap design, it uses NE5532 opamps likely for gain and a single TPA6120 as a buffer, as can be seen here:

http://rockgrotto.proboards.com/thread/11308/rupert-neve-headphone-amp

Not saying that it doesn't sound or measure good, but for the price I would expect something more elaborate or innovative (and a higher BOM).

Curious to know what's inside the Monotor given the asking price, almost the same as the RNHP.

So if you don't how it sounds or measures, what are you saying? Just asking...
 

Jimmy

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#37
That IMHO it's expensive/overpriced given what's inside, probably should be priced between 150-200€, the Atom f.e. is a better design, it just lacks connectivity (compared to the RNHP).

So if you don't how it sounds or measures, what are you saying? Just asking...
 
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restorer-john

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#38
I once asked a boat salesman how big of a seas the boat I was test driving would handle. His answer: "you will get scared before the boat does."
I remember a similar thing with a high powered V8 car I once owned. I ran out of straight road and guts to keep accelerating before the car did. Way off the clock and it was still going. Never needed to 'test' it again. It won.
 

chubby11

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#39
That IMHO it's expensive/overpriced given what's inside, probably should be priced between 150-200€, the Atom f.e. is a better design, it just lacks connectivity (compared to the RNHP).
Thanks for the response.

I'm new to this stuff. I can't imagine that the sum or price of components is the only thing that defines good sound. Certainly you should consider listening or measuring the products you comment upon before commenting upon them.

You base your criticism on another product's design...no absolute evidence or measurements.

Here's where I might be unobjective. Just google rubert neve and look at the sound boards he has designed and installed across the world. It may be cheap to put together a headphone amp based on cost of components, but I think it may have taken him a lifetime of work to get there.
 

restorer-john

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#40
...That it's expensive given what's inside, probably should be priced between 150-200€...
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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