[iFi audio]

  .... it sure doesn't sound like you're just modifying feedback level. 

 
It is precisely what we are doing. In the iTube 2 (and in the Pro line) we adjust the open loop gain to differing amounts.
  
If you reduce the open loop gain within a feedback circuit, it will also reduce gain slightly. 
 
The ideal feedback circuit has infinite open loop gain. Then overall gain is set absolutely by the amount of feedback. 
 
If by comparison we take a circuit with an open loop gain of 1 (however this may be achieved) and a normal feedback loop applied, the gain is set only by the open loop gain and the feedback loop is in effect defeated.
 
Using above unity but finite open loop gain gives results in-between the two extreme cases set out above. Changing open loop gain will also affect overall gain and yes, SET has the least open loop gain and so the least closed loop gain. The differences in gain are however quite small even so.

 

[iFi audio]

The valve giant just got bigger… - part 2/5

GE 5670 JAN double triode

One of the most important aspects of every valves based audio equipment are tubes themselves. In case of iTube2, we pushed ourselves to find one of the very best double triodes – General Electric 5670 JAN. Our latest headphone amplifier - the Pro iCan - is also loaded with a pair of these. And so are the AMR 777 SE machines too. 
 

 

GE JAN 5670 NOS type is sonically nearly as good as the best and most expensive NOS valves (WE396A and 6922 types in general) and made to very strict specifications. It flew under the audiophile tube radar for many years. Ostensibly an RF device for VHF use, thanks to a usable frequency range to 800MHz it also offered good linearity, low harmonic distortion and low-noise in audio applications. Relatively expensive to make (at least 10 times more in comparison to identical civilian parts) and less attractive for mass production, GE JAN 5670 NOS never really went into audio gear much during the golden age of tubed audio.

GE JAN 5670 NOS is a marvelous piece of glass built to last (100,000 hours’ of lifespan are easily possible if the electron valve is operated conservatively), with marvelous sound quality: 

• Excellent consistency of electron valves between batches
• Glass is thicker than usual, 
• Controlled warm-up
• Tight specifications on grid current, 
• Low noise and microphonics and exceptional longevity 

In conclusion, this electron valve has an incredible price-to-performance ratio, it honestly doesn't get much better than this. We've been there, we've done that – we’ve got the t-shirt and the blinking key ring. It's highly unlikely that you'll find anything better for sane money. We don’t joke around when it comes to sound quality. The GE 5670 JAN application in iTube2 was a no-brainer decision.

 

[EVOLVIST]

So, what happens if I push 6.7vRMS from my DAC into the RCA inputs of the iTube2, and then use the preamp function at +9db from the RCA outs? A mess?

 

[iFi audio]

So, what happens if I push 6.7vRMS from my DAC into the RCA inputs of the iTube2, and then use the preamp function at +9db from the RCA outs? A mess?

 
Unless you set the volume appropriately to keep the output levels in check, you will have severely clipped signal that will overload pretty much any amplifier or active speakers. It's possible to overload and damage speakers. Hence it's for the best to avoid this. 
 
But hey, its your gear...

 

 

[EVOLVIST]

Unless you set the volume appropriately to keep the output levels in check, you will have severely clipped signal that will overload pretty much any amplifier or active speakers. It's possible to overload and damage speakers. Hence it's for the best to avoid this. 

But hey, its your gear...:wink_face:  

Heh. Maybe I should have worded this better. I will be experimenting with driving high efficiency speakers with a preamp, alone. My DAC outputs 6.7vRMS which would go into the iTube2, and from my understanding the max output at +9db from the iTube2 is 7.7vRMS, yes? The speakers in question are 95db - 8ohm - 2Watt, which are great for nearfield listening. So, I'm saying that 1.) Will my input damage the iTube2, and 2.) at 7.7vRMS I should achieve a little more gain to my speakers without clipping, yes?

There's no way any of this would damage the speakers. I'm more concerned about the iTube2.

 

[iFi audio]

The valve giant just got bigger… - part 3/5
 
The Harmonic Envelope control adjusts the amount of feedback in the circuit, trading-off transient performance and harmonics generation, allowing the sound signature to be adjusted between a modern tube preamp, a modern push-pull amplifier or a single-ended amplifier.
 

The difference between ‘classic studio’ and ‘push-pull’ is in the levels of harmonics. These are greater in the latter case (just as they would be in this kind of a tube amp in comparison to a studio preamp) and feature appx. equal amounts of 2nd order harmonics and 3rd order harmonics. There are similar relations between the even/odd harmonics at higher harmonics.         For the ‘single-ended’ signature overall, 2nd order harmonics are much higher than these of 3rd order, whereas the other harmonics fall-off in a similar manner with the even order harmonics in each pair significantly higher. This would be the case with a low/non-feedback SE Amplifier.

 
3D Holographic+ Matrix for Speakers®
 
Based upon a fundamental understanding of how we hear, stereo separation is different at different frequencies. However, very few recordings and no playback system compensates for this issue. With the introduction of 3D Holographic Matrix® for Speakers, the iTube2 provides the means to specifically and correctly reproduce stereo recordings for both floor-standing speakers and desktop speakers regardless of price even US$100,000 speakers.

 

 

Our proprietary XBass+® and 3D Holographic+® enhancements are in fact ASP (Analogue Signal Processing) filters based on Alan Blumlein’s work. There is no DSP, therefore not handled in the digital, but analogue domain.   Microphones are not ears, whereas speakers + rooms and/or headphones are not really just microphones in reverse. As early as the 1930's, the inventor of modern stereo recording/playback systems - Alan Dower Blumlein - realised that microphone based stereo audio recordings distorted the 3D perspective in comparison to the real thing listening.

 
 
The Blumlein Shuffler
 

 
 
Source: British Patent: 394,325
 

Alan Blumlein devised a compensator known as shuffler. for the speaker side of things and also did some work at the headphones end. Said compensation technology was implemented later by EMI as ‘Stereosonic’ process and applied to early stereo recordings (including some by The Beatles). The headphone compensator never was completed. At later time, inventors such as Bauer presented their takes.

 

In the end, both Blumlein/Stereosonic shuffling and the Bauer/Linkwitz/Meier cross-feed systems remain worthwhile. But attempts to correct a fundamental dichotomy or flaw in modern stereo recordings were unsuccessful.     What the iFi 3D-Matrix has to offer is an improvement over the previous corrective systems and we feel we have achieved this. And while there are of course parallels, which overlap with classic Blumlein shuffling and Bauer crossfeed, the iFi 3D-Matrix goes well beyond.   The iDSD micro was actually iFi's first product to contain 3D-Marix ready for both speakers and headphones. Only our Retro and iCAN Pro models share this technology, whereas 3D-Matrix headphones side was introduced in iCAN and its speakers side was to be found in iTube and now in iTube2 as well.

 
The whole line-out processing implements a Head Related Transfer Function (HRTF from here-on) that is designed to correct standard microphone recordings played back on speakers. 
 
XBass+ Matrix®
 
Many speakers show limitations and deficiencies in low frequency response. With the introduction of XBass Correction® for speakers, the iTube2 provides the means to specifically bypass these limitations and correctly reproduce low bass frequencies through a wide range of speakers.
 

 
More to come!

 

[iFi audio]

The valve giant just got bigger… - part 4/5
 
 
Part 4: Analogue Volume Control
 
Many computer audio systems use a digital volume control which typically truncates the signal. The iTube2 precision analogue volume control system stays true to the source at all volume levels and it works in completely different manner than typical preamplifiers.
 

 
This circuitry has no active components at all. Although more expensive, this solution holds many advantages over more common applications. Linear presentation, less noise and cleaner sound are the most prominent ones. Usually the main issue is with relatively low input impedance, which translates to less full sound with some sources than expected. We’ve countered this problem by gain switch addition. If one misses body in music, flipping the iTube2 upside down and adjusting the ‘mice keys’ accordingly will make it appear again.
 
 
Audiophile Component quality
 
Inside the micro iTUBE 2,we’ve used Japan TDK C0G type capacitors, complimented by Japan Panasonic ECPU film capacitors explicitly designed for audio use with extremely low distortion (<0.00001% @ 1V/10kHz). Along with ELNA Silmic capacitors, Panasonic ECPU and MELF resistors, these parts are only found in components with an extra ‘0’ to the price. The proof is in the listening. Because of this no-compromise approach, the best tube buffer/pre-amp out there just pulled even further ahead. These ‘boutique’ components deliver the very best sonics.
 

 
 
The Circuitry
 
In comparison to its predecessor, iFi Audio iTube2 has drastically changed circuitry. This product is in fact a step-down from our Pro iCAN top-of-the-line amplifier and is directly coupled. No output coupling capacitor for the purest, most direct signal path. The actual circuit used is single-ended Class A.
 

 
 
Part5: The power supply
 
And finally, every iTube2 comes with super low-noise iPOWER (15v) which is a remarkably quiet, advanced DC power supply that features our proprietary Active Noise Cancellation+® drawn from military radar technology. This power supply’s topology is based on the 12-Element Output Array/6-Element Input Array. The outcome is stunning. Measured on the Audio Precision 2, the iPower has an astonishingly low audio band noise floor of just 1uV, while it has the ability to power the iTube2 to put out a maximum 7.75v which is quite remarkable.
 

 

 

[smartin]

Just wanted to say I've been using one of these with a Peachtree Nova300 (it has a built in loop circuit) and really enjoy it.  Pretty significant changes with each setting.

 

[EVOLVIST]

You know, I might just have to try the iTube2 out. iFi has never let me down before.

 

[Topspin70]

I have the previous iTube and that already improves sound quite significantly. I recently swopped it into my speaker system with the 3D effect on and it's stunning. I might say the positive effect it adds to the music is even more pronounced than in a head-fi set up.

 

[slex]

Anyone has it connected to jotunheim?

 

[gr8soundz]

Here's a nice review of the iTube2 by Head-Fier @ostewart 
http://www.head-fi.org/products/ifi-micro-itube2-vacuum-tube-buffer-and-preamplifier/reviews/18110
 

 

[slex]

Here's a nice review of the iTube2 by Head-Fier @ostewart
 
http://www.head-fi.org/products/ifi-micro-itube2-vacuum-tube-buffer-and-preamplifier/reviews/18110

 

Great hope it can match up with ipro using jotunheim since itube2 is of 90% of ipro's DNA. Will hop by for a demo.

 

[gr8soundz]

Great hope it can match up with ipro using jotunheim since itube2 is of 90% of ipro's DNA. Will hop by for a demo.

 
I was thinking the same thing. Like a pseudo iCan Pro for half the price.
 

 

[ostewart]

the iTube2 did make a huge difference to my desktop listening with the 3D settings and even the xBass (normally I am not a fan of bass boost).
 
It would work really well with some headphone systems too, but I have a valve amp already so the iTube2 slotted into my speaker system better, it's well worth a listen, I have never been let down by iFi products either, every one I have tried has been awesome.