u-turn Original Pluto MM RIAA Pre-Amp - Look Inside

[Pete B]

u-turn is a Boston based company with well reviewed products that are made in the USA.

On Analog Planet people listened to samples of the Pluto and Schiit Mani that Fremer made
and voted. The Pluto won 54% to 23%:
U-Turn Pluto Versus Schiit Mani: The Votes Are In! | Analog Planet

This was from 2015 and both were the original versions but both now have version 2. I just
bought a used Pluto on ebay for $45 shipped which I believe is a great bargain.

The first thing I did was to take a look inside, it has a very solid all metal feel, simple design, and is made with
decent quality parts. Found this picture on the web, same as mine except this circuit board is V1.4 and
mine is V1.5:
I really like this design and build. It's a single stage OP amp (TI NE5532 in mine and if you zoom in you'll
see the same in this one from advertising), red film caps are Wima all electrolytics are Nichicon Fine Gold
except for one ordinary Nichicon in the power supply. There are gold plated test points, very nice. Also note
the clean layout, this looks to be done by hand and very well done:




One might think that the NE5532 is an outdated OP amp however some say that this is the best, lowest noise
OP amp for moving magnet applications which this is - it is a good choice on this basis. Here's the thread,
see posts #88 and #99, etc:
https://www.diyaudio.com/community/threads/lme49720-vs-lm4562.107344/page-5

It is well known that there are better choices for much lower impedance MC applications, but I believe that
the above is true. Douglas Self has been an advocate of the NE5532 for decades.

I'd like to see the schematic and perhaps some rainy day I'll trace it out - not a high priority right now.

I was thinking about mods for this but overall I like it and feel it is too nice and well done to hack. I do want
to measure the RIAA compliance, it is rated tight and so is probably fine. There are a lot of electrolytic caps
in the signal path and that's something to consider for mods.

 

[Pete B]

Variable BOM:
Imagine that you read a very positive review of a product, say a solid state drive (SSD), and then you find
out that the one you buy has a different controller than the one that was reviewed. In fact, it might also have
different flash memory chips. This is happening with very low cost SSDs from major brands, SanDisk and
others. This is called a product with a variable BOM:
SanDisk SSD Plus 120GB, 240GB & 480GB Review (tomshardware.com)
11-Drive DRAMless SSD Round Up Conclusion (tomshardware.com)
IMO, it doesn't matter as long as the parts are
functionally identical especially with a digital design but there are reports that these dirt cheap SSD's do not
all perform to the same speed spec. What companies do, who're trying to maximize their profits, is to go out
for bids for each batch of components for the best price for each production run.

It seems that u-turn is using a fixed BOM, the type and brand of components remains fixed as much as
possible and it is a good thing.

 

[Pete B]

Detailed measurements at Audio Science Review:
Review and Measurements of u-turn Pluto and ART DJPRE II Phono Preamps | Audio Science Review (ASR) Forum

It is interesting that he states that the Pluto has an always on LF filter, and his frequency response plot
shows the Pluto to be about -6dB at 20 Hz and -3dB at 40 Hz which is really going to be audible. This is
something worth correcting if it is actually that bad.

I'd expect an RIAA pre to be a minimum phase filter design which means that if the frequency response is
flatter then the phase response should also be flatter and based on this Amir's measurements do not make
sense. I actually wonder if he got them mixed up since he shows the Pluto as having a very flat phase
response, when it should be the ART DJ in flat mode that has the flatter phase response.
The measurements just don't pass a sanity check.

 

[Hajidub]

Variable BOM.

Just ready to comment on the super clean layout, until I read it farther down your description. Great deal for $45 bones! Keep the thread going and give us your thought after listening a while.

 

[tabarddn51]

If you want to improve it noticeably for just a few bucks, then change the op=amp to something like a OPA2604 & run the unit from a 12 VDC battery.

Cheers

 

[Pete B]

I'd have to see that confirmed in blind testing.
I'm going to check the supply voltage at the OP amp but there's only 9 Vac coming in.
Higher voltage at the OP amp will provide more headroom which can't hurt.

 

[stanwebber]

i have a growing collection of budget preamps that i'm always looking to add to. do you by chance have any in this list to which you could compare the u-turn pluto?

technolink tcc tc-750lc
art dj pre ii
art usb phono plus v2
xpsound xp203 (new zealand)
audiophile ak-750s (china)
microphono pp500 (china)
dynasty proaudio ua2d
moukey mpamp1 (china)

 

[WizardWG4]

I'd have to see that confirmed in blind testing.
I'm going to check the supply voltage at the OP amp but there's only 9 Vac coming in.
Higher voltage at the OP amp will provide more headroom which can't hurt.

Based on the pictures it's using a L7809 regulator after the rectifier bridge then splits the 9vdc to +/- 4.5vdc with a resistor divider virtual ground. It's interesting they would use less than the NE5532 datasheet minimum of +/- 5vdc. It could certainly benefit from more voltage with a different regulator as long as the capacitors are rated for it.

 

[Pete B]

Yes, good point about the L7809, I really don't understand why they didn't go for much more voltage.

 

[Pete B]

i have a growing collection of budget preamps that i'm always looking to add to. do you by chance have any in this list to which you could compare the u-turn pluto?

technolink tcc tc-750lc
art dj pre ii
art usb phono plus v2
xpsound xp203 (new zealand)
audiophile ak-750s (china)
microphono pp500 (china)
dynasty proaudio ua2d
moukey mpamp1 (china)

Hi, sorry I don't have any of those.
I have a Holographic Audio RIAA 2, Emotiva XPS-1, and a few complete preamps with phono.

 

[gadget73]

Based on the pictures it's using a L7809 regulator after the rectifier bridge then splits the 9vdc to +/- 4.5vdc with a resistor divider virtual ground. It's interesting they would use less than the NE5532 datasheet minimum of +/- 5vdc. It could certainly benefit from more voltage with a different regulator as long as the capacitors are rated for it.

kind of surprised its not using a dual bridge off the AC input to get +/- supplies and feed each of those to an appropriate regulator. Could easily do +/- 8vdc with a 9vac input.

only down side to doing it that way, it forces an AC adapter, battery wouldn't do it.

 

[DavidTT]

I owned the Pluto 1 and also had one internal to an Orbit I owned. I was happy with it and remember that it earned more votes against the Mani, which at the time was getting MUCHO positive press. U-Turn is a company worthy of the support of hobbyists, as they design and make their products in the US and sell high quality at very reasonable prices. I'm glad to see a poster looking inside one of their products.

 

[E-Stat]

If you want to improve it noticeably for just a few bucks, then change the op=amp to something like a OPA2604 & run the unit from a 12 VDC battery.

There are many approaches to improving the design, but the goal seems to be producing a good sounding $50 device.

Suggesting an obsolete product doesn't help either.

https://www.digikey.com/en/products/detail/texas-instruments/OPA2604APG4/1572288

Understand that the NE5532 costs barely more than a quarter in quantity. My take: good performer in today's world with incredible value, but ultimately opaque sounding as compared with better. As for your suggestion, I applied that concept specifically upwards with the garage DAC. But would be a complete budget buster for the phono board. And power the RPi based endpoint with a battery using a star quad DC cord.

https://audiokarma.org/forums/index.php?threads/new-op-amps-for-a-music-hall-dac-25-3.812527/

 

[Pete B]

The 9 Vac rated wall wart measures 12.2 unloaded and 12.16 loaded. With all the complexity
in the power supply the OP amp is run on +9V single ended. The pair of caps that look to be
for a split supply probably filter the 9V, and the 1/2 supply to bias the OP amp input.
The large diameter caps are 100 uF 16V.
The small diameter are 22uF 25V
The PSU cap is 220uf 50V

I wonder if the amount of 60Hz leakage is proportional to the AC input voltage. They should have
just brought +24V or even 36V DC into it to get the most out of the OP amp.

 

[Pete B]

I was getting an old HP printer ready to take to recycling and remembered that these
often have higher voltage power adapters. It has 32V at 700ma, now that would
provide some nice headroom! It is a switching supply and might be noisy but in the
u-turn the regulator could be removed and replaced with just a filter. It would get
messy changing all the caps to higher voltage rated caps so no plans to try it at this point.

The 32V supply on the Pyle PP999 with all new caps, and tight RIAA, might make it a giant killer.

 

[stereofisher]

Interesting post. Bought one of these from a Barnes and Noble. Had my Marantz recapped and it developed motor boating noise when I hooked my cassette deck to to it. Bob S moved away so I couldn’t fix it. Have used the Pluto three years with no issue. Nice post. Enjoyed reading it,

Eric

 

[Pete B]

kind of surprised its not using a dual bridge off the AC input to get +/- supplies and feed each of those to an appropriate regulator. Could easily do +/- 8vdc with a 9vac input.

only down side to doing it that way, it forces an AC adapter, battery wouldn't do it.

The only straight forward way of getting a split supply from 2 wire AC is to ground one side
of the AC and use half wave rectification for each side (two diodes). This is what's
done in the Holographic RIAA 2, and the AC in is 16V. Pos and neg regulators are also used.

 

[Boltman92124]

Hi, sorry I don't have any of those.
I have a Holographic Audio RIAA 2, Emotiva XPS-1, and a few complete preamps with phono.

How does the UTurn compare to the Emotiva? The Emotiva was a more expensive unit when they were selling for $199. They are on close-out at Emotiva for $99 right now.

 

[Pete B]

I'm at our other home that doesn't have an LP setup, I suppose I should set something up here also. I've not yet listened to the u-turn.

 

[Boltman92124]

I'm at our other home that doesn't have an LP setup, I suppose I should set something up here also. I've not yet listened to the u-turn.

A great phono preamp to check out under $200 is the Darlington Labs MM-5. It is a bargain. I replaced the Emotiva with the MM-5 and sent it back for an upgrade to the MM-6. The MM-5 is a class A JFET single ended design. No Op Amps. The DL preamps compete with much more expensive stuff.
https://www.darlingtonlabs.com/