Been doing some rudimentary sims of current feeedback + speaker.

[Kaz Kylheku]

Hi guys!

Having found this electronic model of speaker impedance, I set about doing some simulation.

Here is a screenshot comparing two current feedback situations. Both achieve almost exactly the same voltage transfer curve against identical speaker models. But one is some 7 decibels louder, due to using a smaller current sensing resistor. Using a smaller resistor in the CF line and larger DC blocking cap makes the curves nearly the same.

Indisputably, less is not more: more is more! Except maybe in current sensing resistors.

LTSpice .asc file is incluced; strip the .txt suffix. (Maybe the BBS should allow .asc files).

The speaker impedance page I linked to is guitar oriented: the first set of parameters given are for a "typical guitar speaker", so I used exactly those values.

Keeping things bone simple, I didn't model amp details like the output inductor, and Zobel network.

The CF topology and choices of feedback resistors, as well as the way input is is patterned after a power amp I use; adjust to taste.

[Kaz Kylheku]

BTW looks like I registered here on April 20, 2011. Just passed my 15th birSSday.　🎂🎉🥂

When I joined, Posh Spice and Sporty Spice were still 30 something, right?

The one for me that never grows old: LTSpice!

[Kaz Kylheku]

So I actually tried this exact thing illustrated in my above example: reduce the current sensing resistor to 0.1 Ω and make the other compensating adjustments. I was not looking to explore this exact aspect, but simulation has a way of taking you places.

I have to say, wow! While it has about the same amount of "scoop" as predicted by the bode plot, the sound quality is noticeably better. The top end is smoother and can take more presence out of the preamp. But not in a way that presence is lacking, such that you have to add it. If you know what I mean.
Probably the excessive negative feedback from the larger resistor was causing a problem for the amp, leading to distortion that trashed the high end? Wild-assed guess. Plus there is the fact that the input had to be 7 dB louder to achieve the same level as now.

There is a lesson here: for current feedback, use a small value current sensing resistor; you don't need more than around 0.1 Ω.

That also keeps the wattage down. For current sensing resistors, the dissipated heat is proportional to R. If you cut R by four, you cut the wattage by four.