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In Reply to: RE: "extending the lower bass of the panels" ... posted by josh358 on March 18, 2017 at 19:52:00
The caption says the bass was measured separately below 300 hz in "quasi anechoic" conditions which is long form for near field.
The speaker level XO should reduce the damping available from the amp. At least vs. not having it in the way.
The transient dynamics are a matter of power supply and the IV curves of the output devices. Pass points out somewhere in his white papers that besides power supply sag the output devices are just not linear outside the narrow range at their center where the IV curve is straight. so he chooses where to operate the output devices to get the least compression and then to make that compression come off as naturally as the drop off of SPL in air. That way he is thinking that the effect would be similar to sitting a bit further away from the sound source rather than being perceived as drooping output.
Pass also discusses the effects of transformer saturation. The effect is compression of dynamics and he gauged its audible effect by using two giant spools of magnet wire on pallets .stacked on top of each other as an air core transformer for the same amp design. There he noted how it differed in there being a lack of compression, among other things. He then lamented the impracticality of air core transformers since they would be as big as speakers beyond a few watts and be entirely immobile..
Follow Ups:
I'd say, just off hand, that maggies don't store enough energy for amp damping to be a Huge Factor.
Too much is never enough
Remember this room?
http://i331.photobucket.com/albums/l454/cmbfoto/_P0F5690.jpg
It is now slightly different, Tympani IIIA Bass Speaker added.
http://tinyurl.com/lwxdcwh
Any idea how those are crossed over?
I see he is using the new concentric U profile diffusers. They really took off recently.
Originally his room housed some conventional box speakers. In order to tame them the room was filled with a lot of Svanå Diffusors. After switching to Magnepan 3.7 and later on to 20.7, the room has been changed a couple of times. The side walls are now without most of the diffusors, the wall behind the speakers have them partially coverd with absorbers from Sounds of Silence (http://www.sofsci.com/). The transparent diffusors are from Svanå (http://www.diffusor.com/). The Tympani Bass Speakers are driven by a LAB poweramp with built-in crossover (maybe even DSP).
Yikes! I'm having a somewhat difficult time believing a 20.7 is that deficient in delivering "lower bass" that it needs to be supplemented by four Tympani Bass panels. Perhaps their individual total bass output has been adjusted (by use of a X-O of some type?) to meet the task. Nevertheless, just too much of a good thing?
The panels couple together. So having them as an add on increases lower freq output and extension beyond that of either set.. Besides which the T III bass panels are likely crossed over in the vicinity of 50hz while the 20.7 likely play full range.
Thanks Satie, points taken.
They spec'd the measurement at 100", though. That isn't near field. And if they tried to gate it at that distance to make it quasi-anechoic, they screwed up *unless* they were outside or in a huge space in which the reflections were outside the window.
Some other wrinkles -- if the gating is too short, you won't get a contribution from the full height of the line source or the floor and ceiling reflections that you do want. And if you do it outside, you won't get a contribution from the ceiling reflections.
Just very hard for these reasons to do meaningful quasi-anechoic or even anechoic of a planar. What counts is the in-room response -- and then you have to accept that bass levels vary widely between rooms and speaker/listening position.
Sounds like an interesting white paper. I remember Pass saying that he tries to mimic the harmonic distortion of the air but I'm afraid I'm missing the effect of device linearity -- that's what feedback is for. Of course you'll hear effects of feedback such as the adverse effect on the harmonic spectrum, but I'd expect the feedback to take care of obvious effects of the magnitude I was hearing. That I think is too high for the harmonic distortion spectrum of a modern amp, usually what I hear of harmonics is subtle except for crossover notch distortion and this didn't sound like that to me.
They say it is nearfield right on the caption. That is how they get "quasi anechoic" in the bass. Atkinson details these measurements somewhere on the Stereophile website.
Part of Pass' philosophy is to avoid using too much feedback, so he can't rely on it to entirely flatten out non linearities in the output devices. He is mainly avoiding high order odd harmonics so far as I understand it and that relates to the HD spectrum in air - which does not contain those. And his DIY projects going into the First Watt concept also try to avoid time domain issues due to overly aggressive feedback, which is something he also worked into his "regular" AB and Class A amps.
He's wrong, though. For the typical planar, 100" isn't near field, it's medium field -- on the border of the near and far field, which is to say it's near field at some frequencies, far field at others. This is very apparent when you look at a plot of dispersion as a function of both distance and frequency.
"Quasi anechoic" just means the measurement has been gated to suppress room reflections and at that distance in the typical room you *can't* gate out the reflections. Unless he measured outdoors, but then it would likely be described as an outdoor measurement and would include the reflections from the ground -- which are actually crucial to the LF response of a dipole since they effectively double the baffle size.
I seem to remember having read Pass on the harmonic distortion of the air some years back. The man is an artist. On the other hand, high order harmonics sound more dissonant and have to be weighted in a harmonic distortion measurement because they're a good deal more audible. One of the main arguments against high levels of negative feedback is that the distortion spectrum changes to emphasize higher-order harmonics. They really should dispense with THD and use the formulas for audibility to create a meaningful weighted measurement. It would take into account both the relative audibility of the harmonics and the Fletcher-Munson curves.
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