Amp Size for Bi-Amping

[marcl]

Mar 12, 2024 14:14:37 GMT -5 KeithL said:

I took a quick look... and their explanation seems to be pretty thorough... and pretty straightforward.

From their description it seems like their driver has a second "sense coil" that is somewhat specialized.
They made a point of the fact that "they aren't just using the second voice coil on a dual-voice-coil woofer" and mentioned "thinner wire".
I would assume that it's thinner, lighter, probably has more windings, and may be spaced away from the main VC for thermal isolation.
(However, from their description, it seems obvious that it's a sense coil, so saying that "it acts like a microphone" is a bit misleading.)
Their descriptions of the benefits they were able to achieve also seem quite reasonable... and not at all unexpected.
And, yes, it's sort of obvious that both the driver and the amp are custom designed, and their entire system is optimized to work together.

The other thing you need to keep in mind is that, with systems of this sort, there is going to inevitably be a lot of "manual fine tuning" and "trial and error".
For example, back in the old days, I recall seeing articles about people who designed and built DIY subwoofers using accelerometers for feedback sensing.
(And, as they mentioned, a few that used the second voice coil in a dual-voice-coil driver for feedback.)
However, designing and building something like that, and then getting all the bugs out, so that it works well, is a MAJOR undertaking...
(There are both a lot of things that must be taken into account - and a lot of things that can go wrong.)

I have to admit that I haven't heard one of their subs in person (at least not when I was paying attention)...
But I've "always heard good things about them"...

Mar 12, 2024 12:42:18 GMT -5 marcl said:

Wow ... as usual you dove in with a ton of context and broad-based pros and cons.  And as usual ... way down in there you kind of answered the question with respect to subs, I think:  a servo solution could be a benefit for subs due to their size and mass.

So I think the broad answer for subs is, there are still lots of variables ... you can't make a generic servo amp and expect it to work with a lot of drivers.  Rhythmik subs tend to be favored by Magnepan owners because it seems in their specific implementation with their drivers and amps designed to work together ... they have overcome the pitfalls.

Extensive discussion of their technology here ....

www.rythmikaudio.com/technology.html

Of course I'll test them with the servo engaged at the three damping positions ... and disengaged.  The measured results should show some effect in the impulse response plot.

[mgbpuff]

Mar 12, 2024 14:46:58 GMT -5 KeithL said:

That's a nice idea... but not at all practical... for several reasons.
And, no, it will NOT eliminate even most of the variables.
[NOTE that it would eliminate one of the variables quite nicely.]

Let's assume that you can measure the location of a single point on the cone instantly and with perfect accuracy.
That really still only addresses some of those variables... and not all of those completely.
- You STILL have the fact that, since the cone is not perfectly stiff, it will not act like a perfect piston.
   (So you cannot assume that the entire area of the cone is represented by your single measurement.)
- And, even worse, the surround, which makes up a significant part of the area, doesn't act like a piston at all.
- And, even with a "closed loop", your ability to correct errors is far from absolute.
   (There is a limit to the power of your "motor" and to the amount of force that the voice coil and cone can survive.)
   (The inductance of the voice coil is also a limiting factor because it limits the speed at which you can feed energy into that motor.)
   (It's also worth mentioning that, since your cone is not perfectly stiff, corrections applied at the voice coil will also "not be applied perfectly to the entire cone".)

It's not impossible that the 8-9 dB improvement that Rhythmic claims to be able to achieve with their feedback coil system may actually be
approaching the limit of what can be achieved without somehow eliminating those other variables.

Incidentally... and just for fun... here's your LASER measurement device... sort-of...
www.klippel.de/fileadmin/klippel/Bilder/Know-How/Literature/Papers/Measurement_and_Visualization_Cone_vibration_06.pdf

Mar 12, 2024 11:33:12 GMT -5 mgbpuff said:

Maybe the feedback device could be a laser distance measuring instrument looking directly at the driver (with high enough response, of course - if that is possible). Closing the loop this way would eliminate all of the variables Keith is mentioning.

Well, o.k. but all those variables still occur in standard open loop speaker designs. I don't agree with you that most of these variables would not be eliminated with proper motion or position feedback. That's what feedback systems do. Different materials and different drivers may be needed, but it is possible. That's quite a reference paper on laser analysis of speakers, not really using the laser as a feedback device for movement control. But this is all future science supposition. I have a very small laser measuring device like this which is probably not fast enough, but it shows the sensor can be pretty small.

[leonski]

Mar 12, 2024 14:53:07 GMT -5 marcl said:

Mar 12, 2024 14:14:37 GMT -5 KeithL said:

I took a quick look... and their explanation seems to be pretty thorough... and pretty straightforward.

From their description it seems like their driver has a second "sense coil" that is somewhat specialized.
They made a point of the fact that "they aren't just using the second voice coil on a dual-voice-coil woofer" and mentioned "thinner wire".
I would assume that it's thinner, lighter, probably has more windings, and may be spaced away from the main VC for thermal isolation.
(However, from their description, it seems obvious that it's a sense coil, so saying that "it acts like a microphone" is a bit misleading.)
Their descriptions of the benefits they were able to achieve also seem quite reasonable... and not at all unexpected.
And, yes, it's sort of obvious that both the driver and the amp are custom designed, and their entire system is optimized to work together.

The other thing you need to keep in mind is that, with systems of this sort, there is going to inevitably be a lot of "manual fine tuning" and "trial and error".
For example, back in the old days, I recall seeing articles about people who designed and built DIY subwoofers using accelerometers for feedback sensing.
(And, as they mentioned, a few that used the second voice coil in a dual-voice-coil driver for feedback.)
However, designing and building something like that, and then getting all the bugs out, so that it works well, is a MAJOR undertaking...
(There are both a lot of things that must be taken into account - and a lot of things that can go wrong.)

I have to admit that I haven't heard one of their subs in person (at least not when I was paying attention)...
But I've "always heard good things about them"...

Of course I'll test them with the servo engaged at the three damping positions ... and disengaged.  The measured results should show some effect in the impulse response plot.

What do you do if 'position #1' Measures best BUT 'position #3 Sounds best'

[marcl]

Mar 12, 2024 15:10:52 GMT -5 leonski said:

Mar 12, 2024 14:53:07 GMT -5 marcl said:

Of course I'll test them with the servo engaged at the three damping positions ... and disengaged.  The measured results should show some effect in the impulse response plot.

What do you do if 'position #1' Measures best BUT 'position #3 Sounds best'

Generally. I work toward scenarios that measure the best.  And that's by my standard which is ... flat response, low distortion, time alignment and level matched.   Now, if for some reason I feel like it doesn't sound right ... I first try to characterize what doesn't sound right, why it might not sound right, what affects it (and what doesn't) and how does my perception correlate to the measurements.   I try to make sense of it in engineering terms.  I look for a solution.  But if it measures really good ... it should never sound really bad.  So if the sound is not really bad, I'll live with it as it is for a while and listen to lots of things and then if I still don't like it I'll try a compromise.  But generally I don't abandon good measurements for bad because I don't like the sound ... it generally does not happen.  6 years doing Dirac and 12 years actively studying this stuff ... I always manage to correlate listening with measurements.

But ... it certainly happens for lots of other folks.  LOTS of people do a Dirac calibration, Dirac does what it was designed to do (room correction) and they hate it because "Dirac killed the bass".  They are free to have it their way

[leonski]

I noted a similar set of reactions when talking to peope about MONITOR calibration for photographic purposes......

[PaulBe]

Mar 12, 2024 15:30:07 GMT -5 marcl said:

Mar 12, 2024 15:10:52 GMT -5 leonski said:

What do you do if 'position #1' Measures best BUT 'position #3 Sounds best'

Generally. I work toward scenarios that measure the best.  And that's by my standard which is ... flat response, low distortion, time alignment and level matched.   Now, if for some reason I feel like it doesn't sound right ... I first try to characterize what doesn't sound right, why it might not sound right, what affects it (and what doesn't) and how does my perception correlate to the measurements.   I try to make sense of it in engineering terms.  I look for a solution.  But if it measures really good ... it should never sound really bad.  So if the sound is not really bad, I'll live with it as it is for a while and listen to lots of things and then if I still don't like it I'll try a compromise.  But generally I don't abandon good measurements for bad because I don't like the sound ... it generally does not happen.  6 years doing Dirac and 12 years actively studying this stuff ... I always manage to correlate listening with measurements.

But ... it certainly happens for lots of other folks.  LOTS of people do a Dirac calibration, Dirac does what it was designed to do (room correction) and they hate it because "Dirac killed the bass".  They are free to have it their way

I gave you a 'thumbs up'. I agree with Everything you said up to your comments about Dirac, and why people hate it.

It is true that we live with aberrant bass in real home environments.  It's physics.  Techniques for neutralizing bass/room problems are still in flux. Dirac is one solution - ONE good solution for bass correction. I defer to everyone else for discussion about Dirac and room correction in upper frequencies - It does not interest me.

I hope you understand that other people use the same rational and procedures in your first paragraph, and come to different  and reasonable conclusions - People like me. If you don't understand this, meditate on that.  I love to discuss this topic, but the discussion Often degenerates into political and emotional chaos.

Now, back to passive biamping for no useful purpose... Or, shall we continue with the side show?

[marcl]

Mar 12, 2024 19:34:44 GMT -5 leonski said:

I noted a similar set of reactions when talking to peope about MONITOR calibration for photographic purposes......

And even more than that ... the business of calibrating TV's, and then people don't like the calibration and ask the guy to pump the saturation and contrast.  I'm glad I calibrated my TV myself with Calman .... probably cost the same in the end but at least I had proof that it was true to spec.

[leonski]

docs.minidsp.com/product-manuals/flex-eight/dsp-reference/fir-filtering.html

I was thinking of trying to figure out FIR filters.......


As for bass in a real room?
I can imagine 2 opposite cases
First? You must adapt to the room. Dimensions and construction are fixed constraints.
You can modify a lot around those limits.
or?
Second? Clean sheet starting with a bare patch of ground......make choices starting with basic dimensions and than
from a large menu of options.....