Pro-Ject Tube Box S2 Phono Preamp Review

[McFly]

I ducking hate people who hate things

 

[SIY]

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.

Miller. ECC83 has a mu of 100. Typical input C for ECC83 designs is 150-200 pF.

 

[anmpr1]

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...

Could you please elaborate? Thanks. The most discussed use for the filter, at least from the days of analog and as I remember it, was removing non musical LF crud from the amplification chain which would a) keep your amp from working needlessly and b) keeping your woofer from pumping back and forth when the amp amplifies the 'subsonic' crud. Is this what you are referencing?

 

[AudioSceptic]

i fucking hate tubes

C'mon, stop beating about the bush and say what you really mean.;-)

 

[AudioSceptic]

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.

Is rumble really an issue anyway, except for a really cheap TT?

 

[AudioSceptic]

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:

View attachment 60441​

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:

View attachment 60442

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:

View attachment 60443

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:
View attachment 60445

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

View attachment 60447

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:

View attachment 60448

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

View attachment 60449

Distortion+noise versus frequency shows good behavior:
View attachment 60450

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!

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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/

Now, if only someone would send you a Pro-Ject Phono Box S2 Ultra. I'd love to see how it compares with the Cambridge Duo.

 

[Robin L]

Thanks for sharing, now would anyone else in the group like to share their thoughts ?

A squid eating dough in a polyethylene bag is fast and bulbous.

 

[amirm]

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

THanks. Forgot to mention and show a picture of dip switches underneath that sets the capacitance.

 

[SIY]

THanks. Forgot to mention and show a picture of dip switches underneath that sets the additional capacitance.

FTFY. With a starting point likely to be a minimum of 150pF, that throws those numbers off significantly.

 

[LTig]

FTFY. With a starting point likely to be a minimum of 150pF, that throws those numbers off significantly.

... but only if the tubes are in the first amplification stage. Do we know this is the case?

 

[SIY]

... but only if the tubes are in the first amplification stage. Do we know this is the case?

Nope, which is why it needs to be measured. The baseline input capacitance is an important and audible parameter.

 

[Rifmon]

I love audio but most of what's on this site is WAY over my head. But I read enough of I think, the Audio Critic years ago to know that Tubes do introduce some distortion. Glad to know Amir's basic stance on tubs confirming this.

 

[AudioSceptic]

... but only if the tubes are in the first amplification stage. Do we know this is the case?

Reading elsewhere, it seems the input stage is FET. <https://www.project-audio.com/en/product/tube-box-s2/#features>

 

[AudioSceptic]

I love audio but most of what's on this site is WAY over my head. But I read enough of I think, the Audio Critic years ago to know that Tubes do introduce some distortion. Glad to know Amir's basic stance on tubs confirming this.

Of course. It wasn't just for reliability and durability that transistors replaced valves (tubes).

 

[AudioSceptic]

Reading elsewhere, it seems the input stage is FET. <https://www.project-audio.com/en/product/tube-box-s2/#features>

Also from there, manufacturer's spec

Noise floor 80 dB (A weighted) at 40 dB input gain, 75dB (A weighted) at 50 dB and 63 dB input gain
THD <0.02% at 40 dB input gain <0.05% at 50 dB and 63 dB input gain
RIAA-equalisation curve accuracy +/- 0.4 dB/20 Hz - 20 kHz
Subsonic filter at 20 Hz with 18 dB/octave

 

[Nathan_A]

i fucking hate tubes

But, if you wire them in series, you get the Internet.

 

[Dimitri]

Reading elsewhere, it seems the input stage is FET. <https://www.project-audio.com/en/product/tube-box-s2/#features>

Best case it's then a hybrid preamp, and there is a good chance all the heavy lifting is done with solid state components and the tubes are there either as a final stage amplification and an excuse to call it a "Tube" preamp or just for eye candy .
The "eye candy" part worked , if you are into that sort of thing

( If tubes are covered and you can't see them, do they sound the same ? )

 

[Robin L]

i fucking hate tubes

Aw, come on, get with it:

 

[Robbo99999]

What is the point of tube amplifiers, if you want to add colour why not just EQ to your taste...probably with less 'noise' and more customisable...tubes seem like the love of vinyl, just sounds like pawn!

 

[amirm]

What is the point of tube amplifiers, if you want to add colour why not just EQ to your taste...probably with less 'noise' and more customisable...tubes seem like the love of vinyl, just sounds like pawn!

Well, audio gear does not come with such an EQ. EQ is a linear process whereas second harmonic is a non-linearity.

Importantly, a lot of times the tube distortion is too low to be heard. So what the listener is preferring is imagined, not real. They put the tube gear in the system, remember people saying it has smooth mid-range, more analog like, warmer, etc. and that is what they perceive. There is no way to simulate this with real hardware because there is no change to system sound that matters.