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Puritan Audio PSM156 Review (AC Filter)

amirm

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This is a review and detailed measurements of the PSM156 "Studio Master Mains Purifier" from the British company, Puritan Audio. It is on kind loan from a member and costs £1,449. I can't find a US price for it.

The PSM156 is a much more stout and serious looking AC conditioner than we find from typical audio companies:

Puritan Audio PSM156 Review AC Mains Purifier Filter.jpg


I was surprised to see these massive outlets with auto closing lids:

Puritan Audio PSM156 Review AC Mains back panel Purifier Filter.jpg


The little green thing on top is the grounding lug which was floating in the box when I received it. The screw was sheered off even though the unit was well protected and packed.

A super thick 20 amp cord is provided which conveniently terminates in a 15 amp connector. Despite being thick, the cord is super flexible which I really appreciated. It was a delight to hold and use. A big lesson for audio companies who build these stiff cables that risk breaking off the mains IEC plug.

Puritan PSM156 Measurements
As always, we attempt to tease out the transfer function of the conditioner using normal audio level signals. The unit doesn't care about the voltage and this way, we can fully quantify its performance. So let's start with a frequency sweep to see what kind of filtering it provides:

Puritan Audio PSM156 Measureemnts Frequency Response.png


I was pleased to see substantial filtering of input, down some 57 dB at 20 kHz. Compare this to Audioquest Niagara 1200 which only filtered less than 6 dB. Filtering is not flat though. It starts to make a difference around 500 Hz and has a resonance around 4.7 kHz which causes its attenuation to drop fair bit.

The application is AC mains filtering so let's start with measuring my AC mains first:

Puritan Audio PSM156 AC Mains Measureemnts Mains Purifier Filter.png


This is using a differential 100:1 probe so "1.17" volt is rally 117 volts RMS. The 60 Hz component is visible in FFT and so is its harmonics which make the bulk of "junk" that is in there.

Now let's route that AC through PSM156 and see what it filters:

Puritan Audio PSM156 AC Mains Through Measureemnts Mains Purifier Filter.png


As we expected, there is good amount of filtering in higher frequencies. Alas, as noted just now, our "problem" was below 500 Hz which this device doesn't filter. As a result, THD+N which is sum of distortion + noise for our mains input remains identical to not using this filter.

We don't listen to AC mains directly so let's test it with some audio devices. First up is the Topping Pre90 which is a state of the art analog pre-amplifier. Let's test it again with generic AC mains input:


Topping Pre90 Measurements Preamplifier.png


Superb performance as always with distortions at -140 dB and lower. Now let's route its input through the PSM156 and see if it makes a difference:

Puritan Audio PSM156 Measurements Topping Pre90.png


It doesn't. Nothing remotely is changed here. So much for "very resolving gear" showing the difference.

But others say it may help devices that have performance problems. I just tested such a device: the Leaf Audio PA-03 tube preamplifier. Let's test that by feeding it 4 volts and setting the volume to output something reasonable:

Leaf audio PM03 MKII Generic AC Mains.png


Above is with generic AC mains. Let's route the plug through PSM156 filter and see if there is a difference:

Leaf audio PM03 MKII Puritan PSM156 Measurements.png


There is a tiny difference in output voltage but that is due to vagaries of tube amp and it power cycling. Otherwise there is no impact on level of noise and distortion products.

Above is at one frequency and measurement is limited to 22.4 kHz bandwidth. Let's sweep all frequencies and increase bandwidth to 90 kHz which should let the PSM156 show what it can do:

Leaf audio PM03 MKII Puritan PSM156 THD+N vs frequency Measurements.png


Ignoring that blip which is a random fault in this preamp, performance as you see remains identical. The variation in the peak around 20 Hz is also random so don't assuming something improved. It is just run to run variation and faulty design of this equipment in bass region.

Conclusions
As you see, I have run a number of tests to give the PSM156 ample opportunity to show it can do something to improve audio but it can't even move the dial one hair. There is no indication or logic that would tell us that it can make an audible improvement. Yet the company says this in the feature list of this product:

1630123343700.png


None of these things is observed in the measurements.

The device however seems to have real filtering in there as opposed to toy implementation we see in other audio devices. So if you have audible mains related interference that is above a few hundred Hertz, then the PSM156 may have an effect.

As an audio fidelity improvement device, I can't recommend the Puritan Audio PSM156. As a mains filter in general, it seems to be well built and does a far better job than typical consumer gear sold for this purpose.

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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/
 

David_M

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Could it be that your mains are rather clean and have no effect on the AC filter as measured? Is there a way to distort the mains by adding voltage sag, over-voltage, ... etc and other forms of distortion?
 
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amirm

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My mains are not clean. There is a ton of harmonic distortion in it as I showed in the review. The reason these devices don't do anything is because your audio gear consumes DC, not AC. The process of conversion from AC to DC already does this filtering and then some.

This device doesn't correct for voltage sag but even if it did, again, your audio device doesn't care. It is designed to work with wide variation in voltage.
 

jae

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Could it be that your mains are rather clean and have no effect on the AC filter as measured? Is there a way to distort the mains by adding voltage sag, over-voltage, ... etc and other forms of distortion?

He used deliberately distorted AC in a previous review. Didn't change anything.
 

ShinMolina

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If the intention of these companies is good, they should start marketing their products as tools that help removing unwanted noise from electric motors and such, not as audio performance enhancing devices.

In my case, the controller of the elevator from the building I live in adds some 8 kHz noise when in operation. This noise leaks as EMI when I put interconnect cables near AC power ones.

Using a device like the one in this review should easily solve this problem because it has an attenuation of around 35 dB at 8 kHz which should mask the spike completely. For the price, I prefer to keep my cables tidy, but some people may like the idea.
 

fordiebianco

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Excellent review, as ever. Has definitely cured me from my ideas on power conditioners.
 

abdo123

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Did you just use a distortion limited device to test a noise filtering device?

Are you taking us for fools or what?

I mean like i was waiting for that S/N test on any of them but we only got THD+N and it was saturated with distortion anyway.
 

DanTheMan

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Wow! Look how great that performs with the filter! I’m going to get one for both of my systems.
 
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amirm

amirm

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Did you just use a distortion limited device to test a noise filtering device?

Are you taking us for fools or what?
Come again? You are seeing spectrum of both noise and distortion in the FFT:

1630137201317.png


On top of that, this device cleans up both noise and harmonics of AC. It doesn't care which is which. It filters them so it is not correct to say it is a noise filter. The company doesn't call it that either.

I mean like i was waiting for that S/N test on any of them but we only got THD+N and it was saturated with distortion anyway.
I didn't just show you the THD+N. I showed you a full dashboard.
 
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amirm

amirm

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As French during the revolution:
I want to see a headless panther please !!
:) I gave it one notch higher score because it actually does something as far as filtering. For pure audio use, it would get a headless panther.
 

VBSurrey

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Thanks for another excellent review. Two questions please
  1. Would effective AC filtering below 500 Hz be beneficial?
  2. If such a device existed would a component such as the Topping 90 Pre perform better?
 
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amirm

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  1. Would effective AC filtering below 500 Hz be beneficial?
  2. If such a device existed would a component such as the Topping 90 Pre perform better?
It is impractical to build filters that go much lower. They would become large and expensive. But no, it would not do any good. The incredible performance you see from Topping 90 is with my 'dirty' power. The limits there are not how clean the AC power is. We are down to just the noise a resistor produces and such.
 

abdo123

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You are seeing spectrum of both noise and distortion in the FFT:

you can't translate FFT windows directly into actual noise, I don't know which FFT length you're using or how many averages are you taking.

Either way your readers shouldn't go into this much effort to know whether the device is doing anything or not.
On top of that, this device cleans up both noise and harmonics of AC. It doesn't care which is which. It filters them so it is not correct to say it is a noise filter. The company doesn't call it that either.

Fair, but these distortions (of the tube amp) are obviously not AC related, they're harmonics of the fundamental.

I didn't just show you the THD+N. I showed you a full dashboard.

.
You mean the frequency and RMS level? I'm sorry but that doesn't tell me anything about the AC filter.

This review is very shaky, I understand the convenience of using something on your bench already but i really don't think you did this device justice.

That Tube amplifier is just not the best choice, specially since it has a downward sloping noise floor already.

we would need S/N test + A-weighting to discern any measurable differences imo.
 

pma

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I want to see a headless panther please !!

This thing works quite well and is exactly doing what we expect from the mains filter. The meaningful test would be with several audio components connected to its plugs and mutually interconnected to make an audio chain. Especially if some of them use SMPS power supplies. This would show if there is an effect to reduce switching frequency intermodulations. The test with single component like DAC or preamp is meaningless.
 
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