Hmmm, where do I start...
Mr. Erath has passed on a couple of years ago and he hasn't made one of these in over a decade.
I knew him personally and he directly told in me the 2000's that he did that circuit in the 80's and wasn't going to redo it but I was welcome to. Unfortunately I don't have anywhere near his skill ( he was the closest thing to a genius I ever met, or maybe mad scientist
). He was focused on his "newer" full range servo used in his active speakers the Trout.
Anyway the whole reason I was trying to reverse engineer it was so I could split it into active electronics at the speakers. In the end he made me a pair just for that and they are now in my active speakers.
Mr. Erath has passed on a couple of years ago and he hasn't made one of these in over a decade.
I knew him personally and he directly told in me the 2000's that he did that circuit in the 80's and wasn't going to redo it but I was welcome to. Unfortunately I don't have anywhere near his skill ( he was the closest thing to a genius I ever met, or maybe mad scientist
). He was focused on his "newer" full range servo used in his active speakers the Trout.Anyway the whole reason I was trying to reverse engineer it was so I could split it into active electronics at the speakers. In the end he made me a pair just for that and they are now in my active speakers.
Trying to reverse engineer a Commercial product, which is in current production and patented is a big NO NO.
No matter what reasons you claim for it.
Doing it in a Public Forum, for all the World to see , and even worse, a DIY Forum, even less.
My guess, the circuit extracts a back-EMF component. The two large resistors (upper left and upper right), I'd guess to be about .2 Ohms (please look and then tell us what they are - or easily measured since one end is untethered). These are in series between the speaker and ground. That should be easy to confirm (or disconfirm) by looking at the heavy wires connecting speaker input and speaker output.
The resistor produces a current feedback, as per the large literature on motional feedback. Or you can view it as a poor-person's bridge. Or possibly it is part of a bridge.
I can't see the toroid, but it may be part of a bridge in which it counterbalances the driver inductance since the woofer driver likely plays well above the range where MF usually ceases to work right and a suitably tuned bridge is better than just a single series resistor.
Some of the op-amps could be in the feedback circuit as well as the crossover. Velocity MF inherently corrects for a speaker resonance (even at 70 Hz) but it always needs EQ below, as we all know. And the feedback loop needs to be very thoroughly pruned top and bottom.
In short, it looks to my amateur judgment like a nice MF implementation, likely (and wisely) using a mild feedback factor. A good way.
But for DIYaudio, most of us want to take good drivers and make great speakers.... not trying to get pretty good sound from bookshelf speakers.
My 2-cents.
Ben
I may vomit if I one more time hear "Mr." Erath... hagiography and sounds like Saint Erath.
The resistor produces a current feedback, as per the large literature on motional feedback. Or you can view it as a poor-person's bridge. Or possibly it is part of a bridge.
I can't see the toroid, but it may be part of a bridge in which it counterbalances the driver inductance since the woofer driver likely plays well above the range where MF usually ceases to work right and a suitably tuned bridge is better than just a single series resistor.
Some of the op-amps could be in the feedback circuit as well as the crossover. Velocity MF inherently corrects for a speaker resonance (even at 70 Hz) but it always needs EQ below, as we all know. And the feedback loop needs to be very thoroughly pruned top and bottom.
In short, it looks to my amateur judgment like a nice MF implementation, likely (and wisely) using a mild feedback factor. A good way.
But for DIYaudio, most of us want to take good drivers and make great speakers.... not trying to get pretty good sound from bookshelf speakers.
My 2-cents.
Ben
I may vomit if I one more time hear "Mr." Erath... hagiography and sounds like Saint Erath.
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