[LAD] (Modular) Synth and Clipping

Dominique Michel dominique.michel at vtxnet.ch
Sun Jun 16 19:15:51 UTC 2013

Le Sun, 16 Jun 2013 12:13:30 +0100,
John Rigg <ladev6 at jrigg.co.uk> a écrit :

> On Sat, Jun 15, 2013 at 11:33:45PM +0200, Dominique Michel wrote:
> > > An output transformer will saturate if the frequency is low
> > > enough, but the signal level required to saturate it is directly
> > > proportional to frequency. In a properly designed guitar or bass
> > > amp there will be some transformer distortion at the lowest
> > > frequencies but not much above that. If you lowered the frequency
> > > enough to fully saturate the transformer it wouldn't sound very
> > > good, as you say. (I design guitar amps among other things).
> > 
> > Me too, and I repair them too. I was talking here about cheap power
> > transformers used in some brands of commercial guitar amplifiers,
> > not about their output transformers. The main frequency is low
> > enough to easily saturate them when they are not properly
> > dimensioned, and this saturation will go through everything to the
> > speaker.
> Power transformer saturation only occurs if the voltage applied to the
> primary is too high. It is not affected directly by the load on the
> transformer.

This is true for a transformer that is not or normally loaded. But when
it is overloaded, we can get a behaviour similar to a coil: the
saturation is fixed by the current, not by the voltage, due to the
knee of the hysteresis curve. That imply the voltage drop will not only
be a function of the resistive losses, but also of the saturation of
the flux in the iron.

> > A typical example are the old Peavey Mace, good transistor preamp
> > and driver stage, 6x6L6 for the output, but a too small power
> > transformer to drive such a power (160 w RMS), and a bias circuit
> > for the power stage that kill the dynamic when it is in saturation.
> > The power transformer is definitely too small to drive the tubes at
> > full saturated volume. I measured such an amp, the maximum power is
> > the same with a clean sound and at full saturation. The sound is
> > very good when the power stage is not saturated, but very bad when
> > the power stage is saturated, that not only because of the lack of
> > dynamic, but also because of the saturation of the power
> > transformer.
> The effect you are describing is due to the internal resistance of the
> transformer windings and other power supply components, not
> transformer saturation. When more current is drawn the supply voltage
> drops due to resistive losses. If there's a tube rectifier the effect
> will be more pronounced. Some people like that effect but not me. I
> agree that power transformers in many commercial designs are
> undersized.

The Mace have silicon rectifiers. And the supply voltage drop on the
output transformer was at least of 100V, that between the full volume
point (160 w with a clean sinus, I know its hard...), and the fully
saturated point, when at the same time, the output power was almost the
same. At the output, the voltage was dropping with the saturation. 
I don't think resistive losses alone can explain such a huge voltage
drop on the supply voltage.


> John
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