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Good Experience with FMODs

199.46.200.230

Posted on November 19, 2015 at 11:56:09
Dimitry
Audiophile

Posts: 457
Location: Massachusetts
Joined: January 26, 2006
So, my system has gotten complicated, with LFT8Bs now driven by two pairs of mono amps - Canary CA-339s for the planar driver and Emotiva XPA-1s for the woofer. The low frequency amps are driven through MFX10V3 buffer/Goldpoint stepped attenuator box/MFX10V3 buffer. 500 Hz LP FMODs are used at the input of the XPA-1s.

Emotivas have a DB more gain than the Canary and the two tube buffers supply maybe 6 db of gain in total. FMODs have about 2-3 db loss. In the end I have a woofer level knob that allows for +3db/-12db control, which is nice to have with some material.

Why all this complexity? Well, the LFTs have a small but well audible flaw - the woofer has out of band junk in the 1 Khz range, which makes for a nasal coloration on some voices. 500Hz LP FMODs make this go away, so the speaker sounds really pristine through the entire frequency range. It sounds like FMODs are working well in this application.

As an aside, I have not listened to the XPA-1s full range, but the make a dynamite woofer amp. They also appear well built and VERY heavy - over 70 lbs each. Run very warm in Class A mode. Mechanically silent and pretty to look at, if you have the space. They are also "on spec" to the FMOD vague guidance that the rated frequency is achieved with 22K input.

 

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RE: Good Experience with FMODs, posted on November 19, 2015 at 12:27:09
Satie
Audiophile

Posts: 5426
Joined: July 6, 2002
I tried the Fmods as well, but a simple 2nd order passive RC network with cheap foil caps from parts express' Dayton brand do far better. Sonicap MKPs also did far better than the Fmods. The fmods did not do better than an old Ashly acctive 3rd order XO but for lower noise.

That said, given the convenience it is not a bad idea so long as the input impedances of the power amps are favorable for it.

So other than the fmod low pass you are passive biamping with the stock XO?

 

RE: Good Experience with FMODs, posted on November 19, 2015 at 13:13:30
Do you have a measurement of that "junk" to look at?

From what I understand, the woofer of the LFT-8b's is designed with high inductance, heavy cone, etc, etc, specifically to reduce radiation above the woofer region. Bruce claims the woofer (even without crossover) "begins to roll off above about 500Hz and has no usable output above 1khz. This allowed the crossover to be 6db per octave."

FMOD's are 12db/octave, yes? Those, in conjunction with the existing inductor, would seem to alter the bass/midrange transition in a substantial manner. In fact, theory says you should now reverse the polarity of the woofer to (sort of) compensate.

Dave.

 

RE: Good Experience with FMODs, posted on November 19, 2015 at 17:08:18
Dimitry
Audiophile

Posts: 457
Location: Massachusetts
Joined: January 26, 2006
Yes, it shows up in the measurements on the ET site and was confirmed to me by Bruce T. It's woofer cone resonance.

I disagree about the balance. The second crossover is 500 hz, with attenuation coming in above that where the woofer has very little real output....except cone resonance.

Recent glowing review of LFTs preferred woofer out of phase. Bruce T says it's a matter of preference.

 

RE: Good Experience with FMODs, posted on November 19, 2015 at 19:33:32
Can you point me to the measurements on the ET site showing this? (I can't find them.)
I see a graph on this page, but it looks like a dip at 1khz, not a peak.
http://www.eminent-tech.com/LFT8specs.html

A 2nd-order (FMOD) filter introduces a group-delay into the woofer path of about 600usec. (Equivalent to about 8 inches.) Any square-wave performance you might have had in that region is degraded when you do that.
Also, the reverse polarity preference noted by REG provides a much better phase match to the midrange driver in the crossover region, but it would degrade the advertised square wave performance too. I'm not sure why that's considered a matter of preference. It's an objective change to the system design.

Anyways, your solution only works for those that are bi-amping.
Fashioning a simple trap into the stock woofer filter could easily address a cone resonance issue. Why has Bruce not implemented that?

Always there are trade-offs with speakers. :)

Dave.

 

RE: Good Experience with FMODs, posted on November 20, 2015 at 05:39:21
Dimitry
Audiophile

Posts: 457
Location: Massachusetts
Joined: January 26, 2006
The peak is at ~1.4 Khz.

The real woofer output is well down at >500Hz, between the 180Hz first order crossover filter and the natural rolloff of the cone. What is being attenuated is the out of band resonance, which obviously contributes nothing to square wave performance.

The sketch of the crossover in the manual shows woofer connected out of phase already.

I don't know why Bruce did not choose to add to the crossover in order to reduce or eliminate this coloration. He does not seem too worried about it and it is a small thing. I do hear it quite clearly on lower key female voices, like Cassandra Wilson's.

 

RE: Good Experience with FMODs, posted on November 20, 2015 at 06:38:45
"What is being attenuated is the out of band resonance, which obviously contributes nothing to square wave performance."

I didn't say it did. I said implementing a filter and/or reversing the polarity would.
I still can't find a graph on the ET website showing the woofer response alone. Can you point me to it? Is it buried in one of the PDF's in the support pages somewhere?

Dave.

 

RE: Good Experience with FMODs, posted on November 20, 2015 at 07:32:04
Dimitry
Audiophile

Posts: 457
Location: Massachusetts
Joined: January 26, 2006
My understanding is that the phase effects of the filter will not be manifested significantly prior to the corner frequency, but perhaps I am not correct.

The manual shows square wave at 500 Hz, but I am pretty certain it is not the woofer. I don't think this cone woofer can do a square wave. I think only the LFT panel can do that due to its magnetic field linearity.

The main response graph shows a clear peak at 1.4 Khz. Bruce agrees that there is woofer cone resonance there. Maybe its not the entire peak. In any case attenuating the woofer above 500 Hz makes this region much less prominent and the nasal coloration on low female voices is gone.

 

RE: Good Experience with FMODs, posted on November 20, 2015 at 08:07:32



Well, if you're interested I can compute some values for a notch filter. You'd need three components.....which almost doubles the amount of components in the existing crossover. :)
Something with a response like this....as an example.

Cheers,

Dave.

 

RE: Good Experience with FMODs, posted on November 20, 2015 at 10:35:34
Dimitry
Audiophile

Posts: 457
Location: Massachusetts
Joined: January 26, 2006
Thank you, but my electronic peak was reached when I opened the woofer cabinet and changed the 4.7uf film bypass capacitor to my preferred Multicap type (RTX).

I still need to listen to LFTs driven by the big Emotivas full range to see if having higher power with these speakers is really beneficial.

 

RE: Good Experience with FMODs, posted on November 23, 2015 at 08:48:29



Here's a better look at the woofer resonance (blue trace) from the Stereophile measurements of the LFT-VII many years ago.

Dave.

 

RE: Good Experience with FMODs, posted on November 23, 2015 at 14:06:51
Dimitry
Audiophile

Posts: 457
Location: Massachusetts
Joined: January 26, 2006
Thank you for finding it.

That's a decent sized peak. I am glad I have a simple way to reduce it.

There is very little information on the FMOD internal construction and sound quality. Their own website has a blurb from AS touting devices' transparency, but I have not used them on the panel itself yet, so very difficult for me to tell.

The same blurb and a fuzzy graph on their web says "fraction of a db loss in band," but my own experience with the woofer section suggests about 2 db loss or so.

I may buy one and break it to see what is inside.

 

RE: Good Experience with FMODs, posted on November 23, 2015 at 15:40:25
The low-pass FMOD's are just two RC combinations cascaded.

You can somewhat deduce the component values by measuring externally.
A simple resistance measurement from input/output will yield the net of the two series resistors. That net resistance and some arithmetic with your amplifier input resistance will yield the insertion loss.

The second capacitor can (sort of) be measured by probing the output with a capacitance meter.
You can't directly measure the other capacitor and you can't determine exactly the ratio of the two resistors.

Dave.

 

RE: Good Experience with FMODs, posted on November 23, 2015 at 20:12:22
Dimitry
Audiophile

Posts: 457
Location: Massachusetts
Joined: January 26, 2006
True, but it wouldn't be as fun as breaking it!

 

RE: Good Experience with FMODs, posted on December 10, 2015 at 14:14:25
I experienced this firsthand now. :) The resonant peak on the woofer driver is quite horrible. On this set I measured it's at 2230Hz with a secondary peak a bit higher. Clearly audible if you play the woofers on their own with the stock inductor in path.

Dave.

 

RE: Good Experience with FMODs, posted on December 11, 2015 at 05:45:45
josh358
Industry Professional

Posts: 12327
Joined: February 9, 2010
So you went for them! How do you like them overall?

 

RE: Good Experience with FMODs, posted on December 13, 2015 at 10:49:21
Interesting speakers. They have a couple of issues, but for the most part do everything well.
The woofer is a (modified) Peerless 830667 but, as noted by Dimitry, has a fairly strong cone resonance that's still audible with the first-order electrical filter.

Dave.

 

RE: Good Experience with FMODs, posted on December 13, 2015 at 11:26:28
Satie
Audiophile

Posts: 5426
Joined: July 6, 2002
So what freq LP FMOD did you get?

 

RE: Good Experience with FMODs, posted on December 13, 2015 at 11:53:59
josh358
Industry Professional

Posts: 12327
Joined: February 9, 2010
Are you planning to add a notch filter?

I've been bothered by a resonance in the IVA's, I think in the midrange. It isn't that loud but it's fairly tizzy and loud enough to be annoying. Not sure yet whether it's a design issue or a consequence of delamination. Either way, I'm looking forward to the arrival of the rest of my Neo 8's -- they guy who shipped them made a mistake so had to send six more. Should bring the quality of the midrange closer to that of the tweeter.

 

RE: Good Experience with FMODs, posted on December 14, 2015 at 07:28:11
"Are you planning to add a notch filter?"

No.....at least not yet. I think a simpler solution would be to just add an appropriate shunt C.

Dave.

 

RE: Good Experience with FMODs, posted on December 14, 2015 at 09:08:47
josh358
Industry Professional

Posts: 12327
Joined: February 9, 2010
Definitely simpler, though you might lose some driver blending (not sure what the intended woofer level is at that frequency, if it's down at least -20 dB I imagine it wouldn't matter).

 

RE: Good Experience with FMODs, posted on December 14, 2015 at 09:37:01
If keeping the existing inductor the "appropriate" capacitor value becomes a trade-off. That's why I was hedging. :)
As with all speakers, these need to be bi-amped. (Many issues can be alleviated.)
But I'm going to investigate the conventional path for a while longer.

That said, they're enjoyable to listen to in stock form.

Dave.

 

RE: Good Experience with FMODs, posted on December 14, 2015 at 18:59:31
josh358
Industry Professional

Posts: 12327
Joined: February 9, 2010
I'll be bi amping the IVA's soon, Mini DSP is here and the Yamaha P7000S proved much more, well, normal sounding than the Crown. Which being said, I was definitely hearing some buzzes and rattles from the delamination this afternoon so I'll probably repair that first.

 

RE: Good Experience with FMODs, posted on December 15, 2015 at 08:24:16
Buzzes, rattles and delamination fixes trump everything else and go to the top of the to-do list. :)

Dave.

 

RE: Good Experience with FMODs, posted on December 15, 2015 at 08:52:02
josh358
Industry Professional

Posts: 12327
Joined: February 9, 2010
Fixing them is less fun than comparing amps, though . . . :-|

 

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