Darlington Labs MM-5 Review (Phono Stage)

[JDS]

This is a review and detailed measurements of the Darlington Labs MM-5 Moving Magnet phono pre-amplifier. It was kindly purchased new by a member and drop shipped to me. It has a few options but as purchased, it costs US $199 from the company.

The front panel doesn't look half bad for a budget device:

View attachment 143208

The back side, not so much:

View attachment 143209

While the RCA connectors are high quality, the labeling and input power hole are as Dave Jones would say, "how you doing."

I must confess the name Darlington takes me back decades to my childhood when I was learning electronics. Darlington is a simple pairing of two transistors where their gain multiplies almost like magic. I would always have a smile on my face when I would spot it in a circuit diagram.

Here, I am not sure if any Darlington circuit is used or not. Company says it uses J-FET discrete class A amplifier with no feedback. I suspect we are going to see plenty of distortion.

Darlington Labs MM-5 Measurements
With only moving magnet supported, our job here is pretty simple as far as measurements starting with our dashboard:

View attachment 143210

We have two problems here. One is mains and harmonics which I could not get to change one bit no matter how I grounded things. And quite high second harmonic distortion as we expected. Ranking therefore is quite low:

View attachment 143211

Company makes up for that partially with good implementation of RIAA equalization:

View attachment 143212

As noted, there is no high pass filter here so if your woofers are dancing around without making any sound, you know you should have gotten a phono stage with one.

I was really impressed by the high level of headroom available:

View attachment 143213

Immunity to pops and clicks should be good then.

What distortion there is, is almost constant with frequency which is good:

View attachment 143214

Conclusions
A design choice was made to leave feedback behind which then resulted in quite elevated distortion. Usually we don't see any distortion spikes in decently implemented phono stages with SINAD completely dominated by noise. Not here. There is plenty of distortion but it is second order so maybe folks can feel good about that. Given the high distortion of the LP media itself, I decided to not be too critical of that and assign some weight to nice headroom and decent RIAA implementation.

Of note, company provides pretty accurate measurements that correlate well with mine. Kudos on that.

Personally I can't recommend the Darlington Labs MM-5. You can choose otherwise with the data provided.

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As always, questions, comments, recommendations, etc. are welcome.

Any donations are much appreciated using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/

If there were a way to vary feedback with a knob (and somehow keep gain constant), this device would be a perfect tool for a double blind test for distortion preferences -- let listeners dial in as much 2nd harmonic as they want. Has anyone ever done a test like that?

 

[Helicopter]

If there were a way to vary feedback with a knob (and somehow keep gain constant), this device would be a perfect tool for a double blind test for distortion preferences -- let listeners dial in as much 2nd harmonic as they want. Has anyone ever done a test like that?

I suggest a computer with some software as the best tool for that job.

 

[krichard2496]

I do wish I could discuss with you on more of an equal level to yourself, but I can't, so profuse apologies..

I do have to ask though, what is all this about low or zero feedback in the audiophile community, when other makers seem to use it carefully (local, global and possibly carefully adjusted amounts of both) and get a superb performance and 'great sound' as well. I'm not a Bruno Putzeys disciple as such, but he seems to feel feedback isn't the enemy it's been made out to be, but he's designing power amp circuits and not to my knowledge, phono stages.

After that possibly too-naive question above, I must congratulate you on a nice high overload margin, as that's one thing that I feel many vinyl lovers aren't aware of - the effects of a phono stage momentarily overloading on surface ticks and splats and making it appear subjectively worse than it really is.

Our units do use some "local degeneration" to accurately set the gain parameters. In a tube circuit, this is through use of a cathode resistor. In our high-voltage J-FET units, this is through the use of source resistors. What we do not have is "loop feedback" whereby a portion of the output signal is returned to a summing node and there is a phase inversion. This discussion is wide and broad and has been occuring for more than 80 years.

In short, some commentators believe that the ear is less sensitive to low-order harmonic distortion, and much more sensitive to high-order harmonic distortion. E.g., D.E.L. Shorter of the BBC in a famous October 1949 paper on BBC transmission practices, and Harry Olson of RCA in the 1940s. Radiotron 4 (1953) has useful references; and Norman Crowhurst with his late 1950s papers (some reprinted in TAA around 1985) and Jean Hiraga (of L'Audiophile in the late 1970s, and some republished in TAA circa 1985) are two prominent commentators.

In short, we want minimal distortion. Too much 2nd harmonic can lead to an overly 'warm' or 'colored' sound, and contributes to Intermodulation Distortion, which in excessive amounts leads to a 'grundgey" sound and distorted bass and mids.

We feel that 2nd harmonic in the mid -60dBs combined with 3rd Harmonic around -100dB or lower, creates a result that is nearly transparent to the ear. Our units have the openness of tubes but don't tend to have the prominent second harmonic sound which is produced with 2nd harmonic above 0.5% or more.

 

[abdo123]

Thank you Amir for the review.

The single-ended Class-A feedback-free gain circuitry is a conscious design choice.
We could have chosen differential-operation of the gain stages and still not used feedback; the result would be around -70dB to -80dB second harmonic distortion.

Could you please explain your reasoning behind that choice?

You would find that the majority of people here thinks that the quality of sound in the context of amplifiers is only effected by 4 factors, noise, distortion, frequency response and time-based errors (in other words, straight wire with gain).

And from everyone's persepctive it seems like you delibrately picked the design with higher distortion, it would be nice if you could explain the reasoning behind a feedback-free design.

 

[krichard2496]

Could you please explain your reasoning behind that choice?

You would find that the majority of people here thinks that the quality of sound in the context of amplifiers is only effected by 4 factors, noise, distortion, frequency response and time-based errors (in other words, straight wire with gain).

And from everyone's persepctive it seems like you delibrately picked the design with higher distortion, it would be nice if you could explain the reasoning behind a feedback-free design.

We accurately report 4500 hours of Research and Development for this current line (since 2015) and I've calculated 20,000 hours of broader study.
I wish it was amenable to a simple answer. The shortest answer is probably personal audition.

 

[DanTheMan]

This looks like a great phono pre with one flaw that the strict objectivist my disapprove of. Otherwise it shouldn’t be a problem and many, if not most will likely enjoy especially if they don’t know it’s a “flaw”. I’m personally getting more relaxed in my old age when it comes to things like this. When we were in the audio dark ages and published information was scarce or misleading, I was far more concerned With things like this. Thanks to increased understanding and availability of information due to sites like this, I’d give this one a high grade on my own psychoacoustic weighting scale. Especially when it comes to vinyl. I think we enjoy it warts and all. I enjoy more than digital perfection, but Can’t fit it in my pocket.

 

[Helicopter]

Someone should make a 110dB SINAD balanced phono pre with MM/MC inputs for <$400

 

[CausticStorm]

Thanks Amir! Can't wait to put it to use next I do have an external linear / regulated power supply I've built for another phono stage I've used before with C-R-C filtering to reduce the AC/hum noise. Maybe that will be of use hmm.

Hi Keith- any thoughts on reducing the hum noise, would I be going in the right direction if I replace the supplied wall wart with a regulated low ripple psu?

Cheers!

 

[Francis Vaughan]

I do have an external linear / regulated power supply I've built for another phono stage I've used before with C-R-C filtering to reduce the AC/hum noise. Maybe that will be of use hmm.

Hi Keith- any thoughts on reducing the hum noise, would I be going in the right direction if I replace the supplied wall wart with a regulated low ripple psu?

I'll save Keith the trouble. Read the FAQ. https://www.darlingtonlabs.com/faq

Short answer: No.
Longer answer: Absolutely do not do this. (It won't work.)
From the FAQ:

Should I upgrade the external power cube?
No…there is NO NEED to do so. Darlington Labs technology differs substantially from other units with which you might be familiar.
Our power supply cube is NOT a switching power supply.
It is also NOT an external DC power supply.
It’s simply an external, AC to AC, 5:1 ratio step-down transformer, 120VAC (or 230VAC, 10:1, for non-US) to 24VAC. This transformer mounts on the wall and keeps the radiated hum field away from the electronics. It may look like other power cubes but this type of true, simple AC-to-AC transformer is now quite rare in consumer electronics.

I'm sort of impressed with the approach displayed by Darlington. Whilst I might have some issues with some of the claims about the ills of negative feedback, you would have to say that this particular pre-amp is a bit of a stand out in the crowd, and for the base price very good value. They do of course provide mechanisms for buyers to spend ever more money, just optioning the front LED and the top end connectors nearly doubles the price. I'll bet that is where they make their money, not on the base product.

 

[CausticStorm]

Thanks for your input, but I'll wait for Keith to respond. I am not asking about upgrading a simple power cube, I am talking about specifically replacing it with something that has LOWER ripple. This has worked for me with another amp, and if you've built amps before (especially high current class A amps. lower ripply thanks for regulated PSU can help dramatically decrease hum noise that's coming from the PSU.

This is of course assuming the noise Amir measured is from the PSU section.. Does it exist WITHOUT anything connect? I'll have to see if that's the case, or is it a case of ground loop.

It's not going to blow up, as I'll match the voltage obviously. Cheers.

 

[Francis Vaughan]

Someone should make a 110dB SINAD balanced phono pre with MM/MC inputs for <$400

It would be a good trick, however the thermal noise from the source slams into this especially when you add any needed gain. A perfect MM cartridge pre is going to be limited to about 90dB*. The lower source resistance of a MC will help it, but the higher gain needed will wipe out those gains.

* depending on reference signal voltage.

 

[Francis Vaughan]

Thanks for your input, but I'll wait for Keith to respond. I am not asking about upgrading a simple power cube, I am talking about specifically replacing it with something that has LOWER ripple.

The link is to the FAQ Keith no doubt is the author of. Read it first, he goes on at length as to why this is a bad idea.

The supplied PS supplies AC, there is no such thing as low ripple AC. The unit expects AC.

 

[DVDdoug]

If there were a way to vary feedback with a knob (and somehow keep gain constant), this device would be a perfect tool for a double blind test for distortion preferences -- let listeners dial in as much 2nd harmonic as they want. Has anyone ever done a test like that?

Not practical... When you increase the (negative) feedback you reduce the gain.

Open-loop, op-amps have "nearly infinite" gain (which makes open-loop operation unusable for audio or linear applications). You can make a unity-gain buffer (no gain, no attenuation) by using an op-amp with 100% feedback.

And in a phono preamp the RIAA equalization is usually accomplished in the feedback circuit.

 

[dfuller]

I am too lazy too google it but why would one need a phono stage ?

A lot of turntables don't have one built-in, so they need a standalone one.

I do have to ask though, what is all this about low or zero feedback in the audiophile community, when other makers seem to use it carefully (local, global and possibly carefully adjusted amounts of both) and get a superb performance and 'great sound' as well. I'm not a Bruno Putzeys disciple as such, but he seems to feel feedback isn't the enemy it's been made out to be, but he's designing power amp circuits and not to my knowledge, phono stages.

So this is a complicated topic, but long story short too much global NFB can be a bad thing if you implement it wrong - it can introduce artifacts that actually increase distortion and worsen overload recovery. That's what happened, more or less, and why there's a whole thing about "feedback bad" - some audiophilia beliefs that look like hogwash at first do have some basis in truth, but not for the reasons they tend to think. The issue doesn't crop up at lower levels of NFB but does as you use more and more to further linearize the device in question.

 

[CausticStorm]

The link is to the FAQ Keith no doubt is the author of. Read it first, he goes on at length as to why this is a bad idea.

The supplied PS supplies AC, there is no such thing as low ripple AC. The unit expects AC.

I know that, but guess what, you can supply regulated DC into AC bridge rectifier and nothing blows up...it simply does what its supposed to do. AC side of the bridge rectifier can take DC in, without any harm. The key thing is make sure you supply the correct voltage.

As a side not - many circuits also include simple diode, to prevent someone from connecting DC with wrong polarity (dc going through diodes in the correct polarity, and doesn't otherwise).

The point is, that I think you missed is my question is to understand WHERE the source of the hum is... is it from the PSU section or is it from the source/ground loop.

From my experience, you absolutely CAN lower the noise IF it's from power supply section with additional filtering. Hope this clarifies it for you.

Anyway, I paid for this unit and is why I am curious on the source for hum. It also doesn't cost me anything to try and improve it. Just for giggles you could even try to power a phono section from DC batteries (as I am sure you've seen this done, to completely eliminate ANY PSU noise period).

Have a good day.

 

[krichard2496]

I know that, but guess what, you can supply regulated DC into AC bridge rectifier and nothing blows up...it simply does what its supposed to do. AC side of the bridge rectifier can take DC in, without any harm. The key thing is make sure you supply the correct voltage.

As a side not - many circuits also include simple diode, to prevent someone from connecting DC with wrong polarity (dc going through diodes in the correct polarity, and doesn't otherwise).

The point is, that I think you missed is my question is to understand WHERE the source of the hum is... is it from the PSU section or is it from the source/ground loop.

From my experience, you absolutely CAN lower the noise IF it's from power supply section with additional filtering. Hope this clarifies it for you.

Anyway, I paid for this unit and is why I am curious on the source for hum. It also doesn't cost me anything to try and improve it. Just for giggles you could even try to power a phono section from DC batteries (as I am sure you've seen this done, to completely eliminate ANY PSU noise period).

Have a good day.

This is possible but it will essentially be a reengineering of the product. We do not support external power supplies nor provide technical details, and any damage that may result will not be covered by warranty.

A user would need a regulated external DC supply furnishing approximately 70 to 78VDC at 80mA or greater and would connect it after the internal voltage doubler but before the pre-regulator (which precedes the high performance discrete regulator).

Amir's measurement of the residual mains noise is higher than we expect to see, so there may be a grounding or interface issue with his setup.
In any case, there is a slight bit of measurable mains noise in our products and this is due to the high current rectification and regulation in reasonably close proximity to high-impedance nodes.

Most competitive products which use "better external power supplies" have notably improved sound - better bass, better soundstaging, etc.
We predict that any such changes with our products will be very slight to inaudible, due to our advanced internal regulation (see our website FAQ).
Therefore, a user who wishes to use an external power supply should be doing so only as an academic exercise; we have had no complaints of 60/120/180Hz (or 50/100/150Hz in foreign countries) in actual real-world use. Most users report our very low background noise to be one of our strengths.

 

[CausticStorm]

Thank you, I was mainly asking this to understand if the noise reported above is perhaps from the PSU filtering, exactly as you covered in your reply.

If I don't hear any AC noise with headphones on when nothing is playing I'll leave it alone

Cheers and thanks for your reply!

 

[cgallery]

I'm still intrigued by this unit. Some of the phono stages I've enjoyed most, have measured waaay worse than this.

I think some of the design considerations mentioned by Keith would be less easily identified in other areas of hi-fi, but in this case we're looking for approx. 40db of gain. 40db is a helluva magnifying glass.

Some day I'm going to try one.

 

[thefsb]

Must admit nice name for an amp though, considering the most basic and early amplification stages were https://en.wikipedia.org/wiki/Darlington_transistor

To me it's a railway station. The London-Edinburgh trains that I used in my youth would always stop there,

 

[welsh]

As noted, there is no high pass filter here so if your woofers are dancing around without making any sound, you know you should have gotten a phono stage with one.

Isn't this like, very, very bad especially with smaller speakers?

It certainly looks alarming! I use a KAB subsonic filter after my Schiit Mani. Seems to work without changing the sound in any way.