The Output Transformer (OPT) of tube amplifiers, regardless of their differences in circuit designs, is considered the heart of the amplifier.  It is the single most influential component to the amplifier sonics.

Understanding the operation requirements, having a good knowledge of the strength & associated limitations of the material used, and coupled with experienced winding techniques is the key in producing good OPT.

For the enthusiast who has tried winding their own OPT, they will realized it is more then just pure science, it is a form of Arts. An OPT with excellent electrical measurement does give us a good indication of performance, however it does not equate how “musical” the OPT can be. The fact that no one will purchase a pair of loudspeaker base on just the technical specification speaks for all.

Looking around at most high-end audio transformer manufacturers from Japan to America, you will discover there is a common trait in their product lines – Permalloy Transformer as their top of the range & usually the most expensive too. 

Permalloy is a special blend of Nickel alloy that offer superior properties especially for audio circuits. A good example is the use of Permalloy Microphone Transformers in recording studios. While it has an excellent magnetic property for audio transformer application, it is extremely costly.  Typically 8-9 times more expensive than the common audio-grade silicon steel used in OPT and 30-40 times more expensive than the common industrial transformer core. This explains why Permalloy OPT is very rarely used in commercial tube amplifiers.

In SilverTone Audio, we recognized that Permalloy OPT produces its own unique musical tone, which is distinctively different from the common Silicon Steel OPT. 

Feedback from all the "golden ears" audiophiles who had their first experience with Permalloy OPT noted it breathes music with natural energy that allow all instrumental coloring and micro-harmonics to be faithfully produced. Without sounding forceful or unnatural, every instrument is encapsulated within it's own circle of air. And upon close listening, the bass is much more extended.  Every bass note ends with a beautiful decay, dissolving slowly into the serenity of the listening room.

In Model 3.2, we have design and voice the amplifier building on the strength of SilverTone Audio Permalloy OPT.  Not only we are making statements in audio engineering to include Permalloy in a commercial amplifier, we did it so with a down to earth pricing without compromising on sound and product quality.

In the DC current path, the DC current flows through the Output Transformer (OPT). To counter the effect of the transformer core flux saturation due to the DC current, the OPT will have to be of substantial size and air-gapped. In simple term, air-gapping means introducing a magnetic resistance in the magnetic path by establishing a break (gap) in the transformer core. This reduces the sensitiveness of the OPT (permeability of the transformer core) to small signal input which make the amplifier loses it's resolving power of micro-dynamic, speed and low-level resolution.

In the AC signal path, the signal passes through the Output Tube (1), the OPT (2), the big power supply filter capacitor(3) and the Output Tube's cathode bias capacitor (4). Now it is easy to see why the power supply of single-ended tube amplifier is in the direct signal path and why the sound quality of the power supply components (especially those with big electrolytic capacitors) degrades the overall sound quality.

air-gapped Permalloy OPT since there is no longer any DC current flowing through it. Many have argued that the Parallel-Feed Capacitor which is needed to block the DC current from flowing through the OPT, is decremental to the quality of the sound. But then again, the Parallel Feed Capacitor is non-polar and is of very low value (typically less than 4% of the power supply filter cap value). The later also allow us to use superior signal coupling capacitor than what is possible for power supply filter capacitors.

In Model 3.2, anode chokes are explicitly used on both the Driver tube and the Output tube. Anode Choke is basically a DC choke that is designed to be able to handle all the DC current and AC signal swing without transformer core saturation. Another important property is that the resonant of Anode Choke is design to be out of the audio bandwidth. This is the main reason why power supply filter choke should not be used as a Anode Choke.

Refering to many textbook, you can read that flux leakage of a transformer is maximum when it is at core saturation. But just how far can we push towards the saturation point before the leakage

become significant enough? Or how far can we push in a audio transformer before we can hear any different in sound? Frustrated with little success on relying purely on electrical measurement that does not make any sense to the sound quality we can achieve, we went through a whole series of painful cut, install, run-in, listen and throw it away, until we can finally conclude what is good enough to go into our amplifier. Although we can probably reduced our cost by using a very much smaller anode choke, we find that it is a neccessity and not a luxury or marketing-hype that we must use Anode Choke of such substantial size.