Does this control the damping factor of the speaker?
I’ve never played around with this before, is it something you can hear does it change the feel of the guitar (feel is very non-scientific, but just trying to get a handle one what difference it makes).
What does the impedance look like from the perspective of the speaker for your design?
Absolutely right, yes, it does.
That in turn changes the bass response, usually making it woofier and beefier.
But high amplifier output impedance does more than that. Guitar speakers tend to have lots of voice coil inductance, meaning the speaker impedance tends to rise significantly through the treble frequency range. When fed from an amplifier with a high source impedance, more power is delivered to the speaker at these higher frequencies. The result is often quite dramatic treble boost, built into the speaker/amp combination.
If the speaker was designed to be used with a tube amp (with high Zout), this treble boost will be intentional, and part of the intended sound of the amp/speaker combo.
If you now take away the high-Zout amp, and replace it with a solid-state one with very low Zout, all this treble boost goes away. The sound will be dull and flat by comparison, with all that shimmering treble boost removed.
-Gnobuddy
You're very welcome. And please feel free to ask your questions - if I know the answer I'll chime in, if not, someone else who does will. 🙂
-Gnobuddy
Hi @boyfarrell , Gnobuddy describes this well. I've measured the effective output impedances of a few tube guitar amps, in the range 10 to 20 to 40 or more ohms out of an 8 Ohm tap. So when I design my attenuators, I target around a 20 Ohm output impedance and keep this consistent. It gives a rise of about 5db at 5kHz with typical 12" guitar speakers.
Apart from this difference tube to SS amps, the effect also comes into play in most simple passive attenuators using an L-Pad, which have very low output impedance at high attenuation.
Apart from this difference tube to SS amps, the effect also comes into play in most simple passive attenuators using an L-Pad, which have very low output impedance at high attenuation.
It took me years before I finally figured out why my L-pad speaker attenuators suffered from "tone suck". 🙂
I was too chicken to measure the output impedance of any of my guitar amps (worried they would self-destruct in the process), but I did a simple calculation by taking the datasheet plate resistance of an output tube, and stepping that down through the known output transformer impedance transformation ratio.
If the amp had negative feedback, I accounted for that as well (feeding back the output voltage lowers output impedance).
My quick-n-dirty calculations predicted output resistances in the same ballpark as your measurements. Tens to hundreds of ohms.
Having cut my teeth on solid-state electronics, where all modern audio amplifiers have very low output impedance in fractions of an ohm, I was shocked. Tube guitar amp ouput impedances were way, way, way higher than I had anticipated!
And thanks for the kind comments!
-Gnobuddy