• WANTED: Happy members who like to discuss audio and other topics related to our interest. Desire to learn and share knowledge of science required as is 20 years of participation in forums (not all true). Come here to have fun, be ready to be teased and not take online life too seriously. We now measure and review equipment for free! Click here for details.

Pro-Ject Tube Box S2 Phono Preamp Review

amirm

Founder/Admin
Staff Member
CFO (Chief Fun Officer)
Joined
Feb 13, 2016
Messages
25,949
Likes
57,781
Location
Seattle Area
#1
This is a review and detailed measurements of the Pro-Ject Tube Box S2 phono preamplifier. It is on kind loan from a member. The S2 costs US $399 on Amazon including Prime shipping.

I must this say this is the cutest tube audio product I have seen:

Pro-ject Tube Box S2 Phono Preamplifier Audio Review.jpg

A nice set of controls let you pick one of six (6) different gains plus a useful subsonic filter.

Power is provided through an external switcher, a bit larger than a typical USB charger:

Pro-ject Tube Box S2 Phono Preamplifier Back Panel Connectors Audio Review.jpg


Sorry, don't know what tube is in there. Perhaps the owner can chime in.

Phono Preamplifier Audio Measurements
My standard dashboard is a 1 kHz tone at 5 millivolts simulating a moving magnet cartridge. Here is the output with 40 dB gain setting:

Pro-ject Tube Box S2 Phono Preamplifier Audio Measurements.png


SINAD is mainly determined by the "tube 2nd harmonic distortion" to the tune of -72 dB. Add a bit of noise to it and you arrive at our SINAD of 69 dB. This naturally ranks near the bottom of our rankings:
best phono tube preamplifier review tested 2020.png


One of the most important tests of a phono preamplifier is the proper implementation of RIAA equalization and the S2 does well here:

Pro-ject Tube Box S2 Phono Preamplifier Frequency Response Audio Measurements.png


Other than tiny bit of emphasis at low frequencies the rest is pretty flat. Subsonic filter (dashed purple) does what it is supposed to do to get run of LP rumble.

Signal to noise ratio is not great by our standards but likely better than what the format (LP) can do:

Pro-ject Tube Box S2 Phono Preamplifier SNR Audio Measurements.png


Distortion versus input level is again "tube like" with proportional rise and without hard clipping until the end:

Pro-ject Tube Box S2 Phono Preamplifier distortion vs level Audio Measurements.png


Distortion+noise versus frequency shows good behavior:
Pro-ject Tube Box S2 Phono Preamplifier distortion versus frequency Audio Measurements.png


It gets a bit upset at 20 Hz and rises a bit at highest frequencies. For the most sensitive part of our hearing (2 to 5 kHz), it is the lowest at -70 dB.

Conclusions
Most of the time when a tube is put in some audio product the result is a disaster. It is as if they think stuffing the tube in there is all they need to do, performance to be damned. Not here. The Pro-Ject Tube Box S2 seems to have been designed well to do what people expect a tube to do: produce a gradual rise in distortion with level.

Personally I have no use for a tube audio product. When I have listened to them in the past, they either do nothing (i.e. distortion is below audibility) or obscure detail due to higher distortion. To the extent you do, the Pro-ject Box S2 scratches that itch well and looks adorable to boot!

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

You all know how addicted I am to money and shopping. I thought I see if I can shake off the habit and stop buying things cold turkey. Well, it didn't last but a few hours before I went on Amazon and bought something!!! Can't help it. Until I figure out the method to kick the habit, I need to rely on your money. So please donate what you can using : https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
 

tomelex

Addicted to Fun and Learning
Forum Donor
Joined
Feb 29, 2016
Messages
787
Likes
315
Location
So called Midwest, USA
#2
Nice looking little unit as you said. I agree that tubes do not shine in all circuits but this one does pretty well given the price point and I am pretty impressed that the rumble filter is pretty tight at the low end and does not bother the overall FR very much, that's good to see. A little boost on the low end is not too bad in normal either, and few records go that low anyway. Tubes shine when they play with transformer loads and are making power, other than that the "tube" sound is not really audible in a circuit designed for fidelity. I appreciate your work as always. Cheers!
 

wwenze

Active Member
Joined
May 22, 2018
Messages
249
Likes
275
#6
This looks like a good tube preamp to recommend if I need to recommend a tube. Clean spectrum with a heavy emphasis on second-harmonic that is just outside of audibility. I mean, if you're going to do something on purpose, at least do it without nasty side effects like most other tube equipment.

Tho I never get into situations where I need to recommend a tube.
 

LTig

Major Contributor
Forum Donor
Joined
Feb 27, 2019
Messages
1,722
Likes
2,423
Location
Europe
#7
Could you measure S/N and THD at maximum gain, to find out if it is suited to MC pickups, please?
 

SIY

Technical Expert
Technical Expert
Joined
Apr 6, 2018
Messages
4,244
Likes
8,174
Location
Phoenix, AZ
#9
Having polystyrene caps near a heat source is not a good idea for long term reliability.

@amirm What are the tube types? That may help us in guessing the input impedance, critical for MM operation.
 

SIY

Technical Expert
Technical Expert
Joined
Apr 6, 2018
Messages
4,244
Likes
8,174
Location
Phoenix, AZ
#11
Thanks. So the input capacitance will be rather high, knocking out a good match with quite a few MM cartridges.

It's still a mystery to me why this measurement isn't done- it is actually a very important one for MM preamps, since it will strongly affect frequency response.
 

Michael Kelly

Member
Manufacturer
Forum Donor
Joined
Aug 22, 2018
Messages
58
Likes
62
#14
Having polystyrene caps near a heat source is not a good idea for long term reliability.

@amirm What are the tube types? That may help us in guessing the input impedance, critical for MM operation.
There is little to no heat at the socket. There is some radiated through the pins, but it is very minimal. All of the heat literally rises to the top of the tube!
 

Frank Dernie

Major Contributor
Forum Donor
Joined
Mar 24, 2016
Messages
3,122
Likes
5,888
Location
Oxfordshire
#18
Subsonic filter (dashed purple) does what it is supposed to do to get run of LP rumble.
The most important feature of the high pass filter, though it is not often mentioned, is to remove the output from the cartridge below 2x its natural frequency which, in a seismic transducer, is wrong in both amplitude and phase. Removing any rumble generated by the turntable is a less important side benefit, though it is the thing it is mostly mentioned for.
 

Michael Kelly

Member
Manufacturer
Forum Donor
Joined
Aug 22, 2018
Messages
58
Likes
62
#19
Thanks. So the input capacitance will be rather high, knocking out a good match with quite a few MM cartridges.

It's still a mystery to me why this measurement isn't done- it is actually a very important one for MM preamps, since it will strongly affect frequency response.
Grid to plate and grid to cathode on a Telefunken ECC83 is under 2pf. Not sure if this would actually cause problems? Input resistance is dominated by the cartridge load, not the tube.
 
Joined
Dec 19, 2017
Messages
46
Likes
41
#20
Some useful adjustability...

Input impedance10 ohms, 100 ohms, 1 kohm, 2 kohms

Input capacitance/impedance 47pF, 147pF, 267pF, 367 pF, 487 pF and 587 pF* / 47k ohms

Input gain 40 dB, 43 dB / 50 dB, 60 dB und 63 dB
 

Similar threads

Top Bottom