Chumps Big Brother
- Written by steve
- Category: Audio DSP Blog
So far so good with the simple valve amplifiers.
All the models so far are single ended designs, in which one single valve powers the output transformer, but most guitar amplifiers are push-pull designs, where a pair of valves ( or more ) drive the transformer.
This type of design is more complex and involves splitting the signal into positive and negative halves of the wave form ( phase-splitter ), amplifying both halves separately and then recombining by using them to drive an output transformer with 2 equal windings. This comes with a new set of problems and possible distortions.
Firstly phase-splitters are rarely perfectly symmetrical, second perfectly matched output valves are again unlikely and third the coils of the transformer never perfectly match. In addition the process of combining the output valves through the transformer effectively cancels EVEN HARMONICS, thus the distortion added at the output stage is predominately ODD HARMONIC. Bang goes the myth that guitar amp distortion is liked because to is EVEN HARMONICS!
Phase-splitting fairly trivial in digital domain and option to subtly alter the signal path of the positive/negative halves means we can intentionally introduce some distortion here.
Same with power valves simple to alter parameters to ensure they are not perfectly symmetrical.
Transform again a very simple model and trivial to slightly alter one so that the "coils" are not perfectly matched.
So put it all together and what happens?
It does work and sounds quite different from The Chump, maybe needs some more detailed experiments with the imbalance of the channels as in theory could introduce a switch or pot that varied the imbalance from none to extreme.
For this amp decided to use 2x12 as the bigger push-pull amps tend to have larger speakers, could also possibly sound good with a 1x12.
The Big Chump