Woofers getting workout, but no bass

it has a subsonic filter (18hz with 24db slope) and that worked

there is the answer. there are a few highend stages that have built in filters. Tom Evans comes to mind

Dougdeacon said, "Nick doesn't include a rumble filter per se, but his circuit is designed to roll off around 16 Hz or so IIRC, which helps."

Is that not a subsonic filter?

Bob

"Is that not a subsonic filter?"

Technically, perhaps not, but essentially the same effect most likely from a practical perspective, and perhaps not switchable in/out. Probably a good thing still.

So, this wasn't an issue until the woofers were "seen" doing the dance. I don't think there was much mentioned about "hearing" any issues. Certainly finite detail at low freq. might be improved by defeating the woofer dance, but it usually comes at the cost (price and performance) of additional components in the case of using sub-sonic filtering.

The issue that most miss can arise IF there is not enough headroom in the amp to prevent clipping when those cones are moving an inch in both directions.

Acoustat,
I don't know the circuit details (probably wouldn't understand them if I did) but Mapman's comment seems about right from what I remember Nick saying. It's certainly not switchable. I got the impression there's no active filter *added* to the circuit, but rather its a characteristic of the circuit itself. Sorry to sound so mystical about it.

Agree with Dan_Ed, with some elaboration. Certainly if the amp stage or its power supplies clip due to the extra LF energy you'll have sonic problems.

More subtly but more commonly, if the amp or its power supplies are easily modulated by the extra LF energy then it will congest on complex or dynamic passages. Dan's amp and mine (more Doshi stuff, sigh!) are about the best we've heard in their power class for the ability to resist such congestion. The power supplies are radically overbuilt and very carefully grounded, which reduces signal modulation of the PS to very low levels. Similar class amps played side by side are often less transparent, and I'd expect those amps would suffer more from woofer pumping.

If the amp has sufficient isolation and headroom then I agree the sonic effects would be lessened.