"The Natural Philosopher"

You can use a transformer to adjust power output via taps without
upsetting the impedance

** Very dodgy connecting a matching transformer to any old SS amp. Many
output stages cannot cope with the very low primary resistance and
propensity to core saturation at low frequencies.

- but resistive pads are not good for bass units.


** Perfectly OK for the purpose of an extension speaker.


As part of my essays into reproducing 'valve sound' for guitar amplifiers,
artificially raising the output impedance of the power amplifier was
routine.

** By adding a 0.1ohm 5W resistor to the ground in the feedback loop and
returning the speaker to it - right ?


The good old Vox A30 amplifier has an output impedance of some 80 ohms.
Driving an 8ohm pair of loudspeakers, the effects are dramatic.

** The speakers in an AC30 are series connected, giving a nominal 16 ohms.
At bass resonance, that will rise to about 60 ohms - so the amp's output
will rise by 4 times giving a 12dB peak at about 75Hz.


Massive bass resonance, about the bottom A on a guitar, plus overall
treble boost as the rising impedance of the speaker voice coil (a lot of
leakage inductance in a voice coil) is no longer a significant effect in
reducing treble output. Boom an chink we used to call it.

** That is the sound of an AC30 - despite the amp testing almost flat with
a dummy load.

Lotsa valve guitar amps use PP output pentodes with little or no NFB, just
like the AC30.


..... Phil