This is a review, comparison and detailed measurements of Review and Measurements of u-turn Pluto and ART DJPRE II Phono Preamplifiers. Both fall in the category of budget products with the Pluto retailing for USD $99 and the DJPRE II for just $49 (both include Prime shipping). Of the two, the DJPRE II has more bells and whistles:
View attachment 13107
As you see, the DJPRE II has "low cut" filter switch together with input capacitance selector (100 or 200 pf). It also has a clipping indicator and variable gain switch.
The u-turn pluto on the other hand, is fixed function. No switches, indicators, or anything else other than the power LED.
Between the two, I like the look of the Pluto better. The DJPRE II screams hobby project box. And that bright blue LED is deadly. Needs some black tape to cover it for sure. It is a sturdy box though so it is just a visual preference.
Both units have external transformers. Pluto has its own branded unit that has 9 volt
AC at 670 milliwatts. The DJPRE II's transformer also outputs AC at 9 volts and 1000 milliwatts. Interestingly enough both are identical in size so not sure about the ratings being correct. The fact that these are AC adapters means that rectification to DC is inside each unit.
There is not much else to them. So let's get into measurements and see how they do.
Measurements
We are breaking new ground here with our first test of phono pre-amps so expect future measurements to vary based on feedback to this thread. In addition, I am using my new
Audio Precision APx555 so bear with me as I refine these graphs and settings over time.
To make the testing easier, I only tested one channel of each unit. That allowed me to test them simultaneously. For input voltage, I picked 5 millivolt. Both of these phono preamps are for moving coil and the research I did showed range of cartridge outputs of 3 to 6 millivolts. I don't see an industry standard for measurement. Let me know if you know of one.
Here is a "dashboard" view of how the two perform using 1 kHz tone at 5 millivolts, with A-weighting filtering of the output (making it more perceptually relevant):
View attachment 13108
I know, lots of data
. Let's take a bit at a time.
For starters, I adjusted the variable output of the ART DJPRE II to match that of u-turn Pluto. This setting was just past the "zero" dial (around +1). This is good as it means the DJPRE II has good range of input sensitivity.
As you see, the output is 0.306 millivolts which corresponds to a gain in db of 35.6 or so. This matches the specification of Pluto and is within the range that DJPRE has.
Distortion with this a-weighting is 0.01% on Pluto. Manufacturer rates it at < 0.005%. Maybe at a different input level it gets closer to that.
DJPRE II's distortion spec is <0.01% and we are getting less than that at 0.007%. So good on them for being conservative.
The DRJPRE II has a clipping indicator. I find out that it lights up at just 0.01% so pretty conservative.
A-weighted signal to noise+distortion ratio ("SINAD") is 83 dB for DJPREE II beating the 80 db for Pluto. Both companies spec 90 dB.
See? It wasn't so bad.
Next, let's look at frequency response:
View attachment 13109
Here we have three graphs: one for Pluto and two for DJPRE II with its filter switch to Flat and Low Cut. When set to flat, the DJPRE indeed has a flat response that extends past 50 kHz. The Pluto has a low-cut built-in per specification.
When selecting the low-cut filter on DJPRE II, we indeed get a low cut (not as steep as Pluto though) but also a high-cut above 19 kHz or so. Not sure that is a bad thing but it is not what the button says it is doing.
Somewhat related to this, I ran a sweep to see how the phase varies based on frequency and got this:
View attachment 13110
Here, it is Pluto's turn to nail the phase response with essentially zero deviation at all frequencies. The DJPRe II though starts to have a negative phase shift below 1 kHz and it progressively gets worse down to 20 Hz at some 66 degrees. Selecting the low-cut filter makes things better with a positive phase shift that starts at 50 hz and goes up 20 degrees or so at 20 Hz. I am not sure what is going with the DJPRE II here. I can see a phase compensation at higher frequencies but not lower.
Next let's sweep the input level from 1 millivolt to 50 millivolt to see when overload occurs:
View attachment 13111
Here, Pluto does better with onset of clipping at 31 millivolt input level. The DJPRE II on the other hand, starts clipping at 22 millivolts. So those pops and clicks will be more obnoxious on the DJPRE II assuming they rise up to such levels.
Finally, let's look at distortion and noise of a 1 kHz tone with 5 millivolt input but without a-weighting:
View attachment 13112
Both devices are remarkably the same with one significant difference: the pluto bleeds noise from its power supply rectifier into its output. You can see this in harmonics of mains 60 Hz in blue. The DJPRE II actually has higher mains hum at 60 Hz, but it is otherwise much cleaner relative to power supply noise.
I think that is it. It is 1:00am so I am going to go to sleep.
Leave me feedback on what you think of these results and I will read them when I wake up.
Conclusions
The ART DJPRE II is a great bargain here. Other than some strange phase shift at low frequencies and lower input dynamic range, it actually beats the more expensive u-turn Pluto in the rest of the measurements.
The main fault I see with the Pluto is the power supply noise. A bit more attention there could have easily landed it on top.
Both are great bargains from what I can see. Personally I find the DJPRE II a more flexible unit so if I had to buy one, it would be that if I could hide it some place.
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As always, questions, comments, corrections, etc. are all welcome.
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