Le Sun, 16 Jun 2013 12:13:30 +0100, John Rigg <[email protected]> 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. Dominique > > John > _______________________________________________ > Linux-audio-dev mailing list > [email protected] > http://lists.linuxaudio.org/listinfo/linux-audio-dev -- "We have the heroes we deserve." _______________________________________________ Linux-audio-dev mailing list [email protected] http://lists.linuxaudio.org/listinfo/linux-audio-dev
