How much does cab impedance influence sound?

mark_jwedge

Probably a very stupid question but please forgive my ignorance. I have a 50watt Plexi style head and I use it with a 8ohm 2x12 cab. I have recently also run it into a 4x12 and 2x12 both of which were 16ohms. There is an impedance selector switch on the back of the amp.

Now although the volume levels were pretty comparable and I'm aware there is a huge influence of speaker type and cabinet construction, but generally through the two 16ohm cabs the sound was brighter, slightly more harsh and 'stiff' feeling to play and warmer and more compressed and forgiving through the 8 ohm cab.

As the bright, harsh, stiffness was common to both the 2x12 and 4x12 16 ohms cabs it made me wonder if the cab impedance has a significant influence on the sound?

ICBM

Not a stupid question. Yes, it does have an effect because the amount of winding in the output transformer, and hence its inductance, is different.

You can eliminate the effect of different speakers and cabinets if you use a cab which has a series/parallel option and can run at either 16 or 4 ohms - either a 2x12" or a 4x12" like the modern Marshall 1960s - although there is a still a small difference due to the damping factor of series and parallel wiring being different, it's reasonably close to a proper comparison.

What you describe about 16 ohms sounding brighter and tighter, and 4 ohms sounding warmer and more 'squashy' is also my experience.

ecc83

Yes, most certainly going to be a factor.  A 'perfect' amplifier would have a zero output impedance and hi fi solid state amps come very close*. This means that the variations of load with frequency due to the speaker have no effect on the driving signal.  This is absolutely NOT the case for guitar amps, especially valve types.

A 30W   'AC30' style amp might have an output impedance of 50 Ohms or more. A big, 100 wattter with 4 paralleled 25W pentodes and a whiff of feedback will be lower but still 10 Ohms or so.

The result of all this is that the frequency response and distortion characteristics are all over the shop and at the mercy of the gyrations of speaker load impedance.  No wonder guitar amps are such dynamic 'Marmite' devices!

*It is perfectly possible to make an amplifier with an ACTUAL zero output Z or even a negative one. The old amp company Pamphonic made such an amp many years ago.

Dave.