Current feedback, but not for bass frequencies?

[Kaz Kylheku]

I once had a Trace Elliott solid-state half-stack, and sold it well over a decade ago. I liked some things about it. If I had it today, I'd probably be into modding it.

If you look at the power section schematic, you see that there is current feedback: 0.33 Ω current sense resistor, coupled to the differential stage via a 1 μF cap and 1 kΩ resistor.

Notice how low that capacitor is? That negative feedback faces about 500 ohms of impedance, so the current feedback signal must be rolled off below around 300 Hz. If that is correct, it means that (1) there is louder bass and (2) the low-damping effect around speaker resonance should be nearly absent.

I'd like to explore that idea; any thoughts about it?

It seems these designers at Trace Elliott (known more for bass than guitar!) were onto something.

There are some undesirable effects about extending the high output impedance into bass. Namely, there is a bass roll-off below the resonant frequency. We get a "one note bass" effect.

I always notice there is a more consistent bass across the low octave in voltage drive mode.

If that could be combined with more bass and the good parts of current drive along the midrange and high end, hmmm.

[Kaz Kylheku]

OK, so I tried this: I substituted a 1 uF capacitor in the current feedback line of my power amp, instead of the 47 uF.

There is a lot of bass, as can be expected, and it's pretty even. The midrange and top end has the current feedback characteristics. But the entire tone all over the fretboard is colored by the bass.

I had to cut some bass from the preamp tone, but it's still kind of dark and "creamy". The preamp tone has to be manipulated to get some "bite" back.

I think this would be suitable for the detuned 7 string guys, or the so called "modern metal" or what have you.

I'm going to revert it because it's not my thing. I'd like to hear it cranked, before I do, though!

It possibly could work better with a higher capacitor value, like 2 uF, to drop the frequency by a half.

When I revert it, I will probably go with 10 uF, which is a commonly used value. 47 was needlessly high.

[J M Fahey]

. If that is correct, it means that (1) there is louder bass and (2) the low-damping effect around speaker resonance should be nearly absent.

There is some of that happening but not drastic at all.

I wouldn´t worry about it.