Power is everything (Part 3)
But in the end the output transformer is what makes or breaks a tube amplifier. And the one we use is as good as any of the Golden Age and of the modern times. If used in straight line in configuration (tone control in bypass) the only further element in the signal circuit is a motorised 16mm Alps Black Volume control. That is it!
If the tone control is engaged, the passive, James type network optimised by us, is put into the circuit before the Volume control and a preamplifier circuit derived from the Class A TubeState circuitry found in the iCAN micro. Again, MELF resistors and C0G capacitors are used for all signal critical parts.
Yet there is an old saying - any amplifier is only as good as its power supply. Here is where we break with tradition big time. We use what is called a resonant power converter, it is somewhat similar to the much (and often justly) maligned switched mode power supply (SMPS) in operation, but it avoids the hard, high current switching of classic SMPS, thus avoiding many of the SMPS's sonic problems.
This power converter operates at very high frequencies, making it much faster than the common AC supplies and it is regulated. This means no matter what the mains voltage from below 85V to over 265V the voltages remain rock steady on nominal. Equally, common tube power supplies tend to "sag" as more power is drawn, this is often claimed as the reason for the "woolly bass" of some tube amplifiers. With a regulated supply the power remains steady from no signal to maximum output and bass is solid and taut.
This has an added benefit. As we have a reliably regulated voltages for heater and high tension there is no need to leave any headroom or over-voltage margin to accommodate varying mains voltages. Instead we can use the design maximum specifications producing more power than common. As we have no drop in power supply voltage with increased power output again, more power is produced by the Amplifier than a classic design using the same output tubes.
And of course regulated power supplies have very low noise. The Retro Stereo 50 manages > 95dB(A) Signal/Noise ratio at 2.83V output.
Source: AMR/iFi measured on AP2
This compares to the around 70dB (A) of many classic designs and even today many traditional design tube amps struggle to offer better than 80dB (A) SNR.
Note: these two graphs have the exact same x and y values, so you can make a proper side-by-side comparison.
More crucially, all the noise in the Retro Stereo 50 is tube noise (often called tube rush) there are no significant hum components that can be ascribed to the mains frequency. What this means in practice is that if a 90dB/W/m Speaker (e.g. the matching LS3.5) is connected to the Retro Stereo 50, any noise at 1m distance is around 5dB lower than the hearing threshold.
So for the Retro Stereo 50 beauty is more skin deep, combining the latest modern technology with classic tube design gives the best of all worlds, the classic tube, but without the noise, limited bandwidth, limited power and woolly bass that many have come to negatively associate with tubes.
In fact, like a good sports car, the more you rev it, the better it gets. Wanna play Nirvana? the Stereo 50 just says "stop tickling me at such a low volume and let's crank it up".