Schematics for Barcus Berry model 1720?

[J M Fahey]

Thanks Teemu for the search effort and links provided.

I quickly browsed those Patents (each one requires re-reading a lot of times to cut through the Legalese  ) and found that he basically patented current drive in 1979

means and method are provided which correct for variations in an amplifier load such as a speaker by sensing the load current, developing a feedback signal related to the load current, comparing this feedback signal with the amplifier program, and instantaneously adjusting the gain of the amplifier to compensate for load current deviations from the program applied to the amplifier.

and later ones were tweaks on the basic first one, to which he constantly refers.

He seems to be trying to get full, flat frequency response from full range drivers, up to 20 kHz  and recognizes that adding crossovers and tweeters complicates the issue.

Then he talks about fast rise time amps as being better, which is true.
At least, can't hurt, at all.

And then mentions an issue which has been intriguing me, since I saw it in an "Exciter" pedal: he claims that by advancing phase up to 180 degrees "high frequencies separate and become easier to hear" relative to"muddy" mid frequencies.

I'm not so sure I buy such a simplistic explanation (human ear is quite insensitive to absolute phase), but all said pedal (I can post the schematic if you wish) does is first passing highs through a single phase shift cell (think any classic guitar Phase Shifter) and then mixes it with original signal.
There's a switch to mix in/out of phase.

Net effect is a strong treble boost ... so maybe that's what Mr Crooks is hearing.

Mixing out of phase gives the classic Phase Shifter response: values are chosen to provide a notch at 5kHz (might be good to tame some annoying speaker peak) while the peak at 20 kHz is perceived as strongly rising treble up to the limits of audibility.

Mixing in phase provides just a shelf type boost above 5 kHz .

In both cases, a single speaker will be perceived as "more transparent sounding".

So maybe the "Accent" control is some of that.

The idea is interesting and has great potential, but maybe didn't go further because doing it in "hardware" (adding anything from a cheap Piezo to an expensive driver or bullet)  provides more and truer highs than EQing a "guitar speaker" which typically drops at 18 to 24dB/oct above some 3500 Hz

[teemuk]

I'm somewhat familiar with early BBE "Enhacers" and later BBE "Sonic Maximizers".

The phase shift idea... I think it's nothing but common "time alignment" thing: Introduce electrical phase shift to compensate the acoustic one created by the loudspeaker system.

I'm not too familiar how all this "phase" stuff behaves in practice so I can't comment on whether BBE's "enhancer" idea works like it's claimed to do, but I do know that time alignment with phase shift or phase lag filters is extremely common design practice and in practice one can make it work effectively.

...But I would think you need to match it to specific drivers though, not offer it in some mysterious "black box" that approximates some generic phase lead/lag behaviour.

Edit: Refreshed my memory and took a glance too:
- Amplifier load correction system, US 4260954, is a generic mixed-mode feedback scheme. Note's are made about the scheme correcting phase errors at certain frequncies. Never investigated that.
- Method of correcting for variations in a load driven by a power amplifier, US 4482866, is a scheme somewhat "external" to power amp. Instead of sampling loudspeaker load current the circuit senses an approximated load current from a reactive circuitry that emulates characteristics of a real loudspeaker. Now current feedback can be applied in earlier, low current stages, indenpendent to the load. At least that seems to be the idea by quick glance...
- Reference load amplifier correction system, US 4638258, continues based on earlier patent's ideas.
- Notch filter system, US 4496859, is a resonant filter (with gyrator) that notches deep at 60 Hz.