INSIDE LONDON POWER AMPLIFIERS:
ADVANCED MAGNETICS
TX976 transformer
Toroidal transformer used in our
PSU-L20 kit
150 Henry choke - click to enlarge
London Power's instrument amplifiers stand apart from the mainstream for many reasons, one of which is our approach to the "magnetic elements" in the circuit.
Conventional plate-output tube amps require output transformers (OTs), which can be designed in two ways. The first approach is to design the OT for full power at all frequencies. The frequency response combined with power output is called the "power bandwidth". Having full power bandwidth reduces the sonic contribution of the transformer, keeps its distortion low, assures that the sound will be consistent from "quiet" to "loud", and gaurantees long life for this expensive component. As you might already have guessed, the full-bandwidth approach is the one used by London Power. An example of full-bandwidth design is the use of our TX976 OT in the older line of 50W amplifiers. The "976" can handle much more power than 50W.
Other manufacturers, looking at the guitar as a "mid-range instrument", employ the second design approach of using OTs that only provide full power at middle frequencies. Bass and treble are then rolled off by the transformer, which means the sound is inconsistent from quiet to loud. Also, the OT is in peril if the amplifier is clipped continuously for extensive periods of time.
There are less expensive ways to "voice" the amplifier than by using an under-sized OT.
For greatest bandwidth and highest transparency of sound, London Power uses custom designed toroidal output transformers. For remarkably similar reasons, we use toroidal power transformers (PTs), as well. Toroidal PTs function more ideally than EI types, passing energy from the AC supply to the amplifier with greater fidelity.
As you may know, choke filtering of DC supply voltages fell from favour in amp designs as capacitor prices came down and transformer prices went up. However, it's still a very effective method. Very few guitar amps use a single choke, let alone three as in our STUDIO-10. But then, the London Power STUDIO-10 is quieter than every other brand of guitar amp.
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