Hello
Taking advantage of a new simplified mounting of the cable between the RKV headphone out and the Wee HP input, I had the possibility to compare again the listening of the Stax SR-009 on the tube amplifier (OTL) Audiovalve RKV mK2, with using the Woo-Audio Wee, in direct connection (voltage transformers (1:50) of Wee bypassed) and in classical connection (using voltage transformers (1:50) of the Wee connected).
For the record, with a typical use of the Wee (using voltage transformers (1:50) connected), the RKV amp + Wee + 009 headphone had given very honourable results, superior to those of the SRM - 007t2, at least equivalent to those of the SRM 727, and closer to those of the Electra amp (link : http://www.head-fi.org/t/681814/amplifiers-shootout-for-stax-009-eddie-current-electra-audiovalve-rkv-wooaudio-wee-stax-srm727-srm007t2).
(Nb : I have personally owned a same period SRM 727 amp and the combo RKV-Wee with the Stax SR-009, and prefer the latter ; I separated of the 727 ; I found the RKV-Wee more dynamic than the 727 (at loud listening levels, what is less true at low listening levels) with a sound more full, more matte, though somewhat less defined than the 727).
A few months ago I had "bypassed" the voltage transformers of the Wee to see what this gave and if it brought more transparency.
My first impression was actually a net gain of transparency and details with strengthening of the reverb and echoes, as well as a better presence of voices, as if a veil had been removed.
These impressions being old, I was able to redo the comparison, last night during new connections, listening to the RKV-Wee + 009 with and without the voltage transformers (1:50) of Wee, and see which of these two listens I preferred.
With voltage transformers (1:50) of the Wee connected, in addition to have more gain (Nb : volume of the RKV potentiometer is used on the first centimetres), the sound is much thicker, which initially seems quite pleasant, especially with the 009 giving him more humanity, but this thickness is actually a sound coloration, very evident on listening to the voice who lose a little diversity and seem more monochrome in the sense or this coloration affects all voices that seem less nuanced and varied. Another note, bass are also changed, they are also thicker, but more neglected, less defined and less dynamic.
This thickness of sound gives the appearance of having a little more level in bass, but it loses a lot in readability and definition, as well as impact: bass are also softer.On the treble, it is apparently quite pleasant (with the transformers of the Wee connected) because they are probably mitigated in level, and changed in their tone with also a colouring of the treble and also a loss of definition.
Without the voltage transformers of the Wee, the treble has much more finesse, and appear more extended, and more diversified (less monochrome).
In the end, I wanted to retry this comparison to be sure on of my choice, and not remain on a hasty first impression.
There is no shadow of a doubt that the sound is much better (although obviously with a more low level of amplification) with RKV + 009 without voltage transformers (1:50) of Wee, that with : gain of transparency, details, presence, dynamics, and especially the lack of (falsely pleasant) staining, as well in the bass (which would appear otherwise draft and mashed) than in the mid (which appear otherwise, certainly thickened, but also colorful and monochrome) than in the treble (which would appear otherwise shortened, colorful and dedifferentiated) are obvious. Difficult to back off.
How to explain such differences of listening (with and without transformers) otherwise the well-known fact that the Transformers 'eat' information and shorten the bandwidth ; and well I think explain it by a matter of adaptation of impedance and distortion:
With voltage transformers (1:50) of Wee, the load seen by the RKV amplifier with a 145,000 Ohms impedance headphones is 58 Ohms (145000/50*50), and probably even less for low and high frequency.
However the RKV (without impedancer), in direct OTL mode, works even better that the impedance of the headphones is high ; 58 ohms (and probably even less at extreme frequencies) is a value too low impedance that must "pull" on the tubes and change their response, distortion and harmonics with for results this coloration of sound (in appearance pleasing).
Conversely, without voltage transformers (1:50) of the Wee, the load seen by the RKV amplifier, with the same Stax headphone (009) impedance 145,000 Ohms, is always 145,000 Ohms, so insignificant to the amplifier that works then for tubes (in mode OTL) only in tension, there where the amplifier excels.)
Apparent results: a much better dynamics, better bandwidth, cleanliness of all registers (bass, mid, treble) without artificial colouring. In short, a much better fidelity.
In conclusion: listening headphones Stax 009 with the RKV amp (in live) without use the voltage transformers (1:50) of Wee, is much better than if we connected the transformers.
The resulting gain with the RKV with voltage transformers shunted is obvious and this amplifier should be now considered one of the best amplifiers to be associated with the 009
(for a listening level not exceeding 100 dB, which in practice is more than enough) in any case much better than with the SRM 007t2 amp (even with the Wee transformers plugged), better than the SRM 727 amp (without the wee transformers) and probably also better than the Electra amp, if we considered the gain of the Electra on the RKV (with the Wee transformers connected) as enough moderate (see non-significant in blind listening), while the gain of the RKV (without the Wee transformers) seems to me (yesterday evening) really so obvious compared to the RKV amp with the voltage transformers of Wee connected.
A cherry on the cake : RKV +Wee is not very expensive; we should be able to find them for less than $ 1,500 as second-hand material.
Eric
Audiovalve RKV mk2 - OTL tube headphone amplifier
+ Siemens PCL 805 tubes http://www.audiovalve.de/rkv/audiovalve%20rkv%20mark2.html
WooAudio Wee (Voltage transformers (1:50) bypassed) ; SR-009 Headphone in direct on OTL RKV amp.
(NB : the transformer of the Bias voltage 580 V DC of the Wee box is preserved; It remains connected on the fifth slot Stax female connector from the Wee)
