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In Reply to: RE: Good Experience with FMODs posted by Dimitry on November 20, 2015 at 10:35:34
Here's a better look at the woofer resonance (blue trace) from the Stereophile measurements of the LFT-VII many years ago.
Dave.
Follow Ups:
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.
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.
True, but it wouldn't be as fun as breaking it!
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.
So you went for them! How do you like them overall?
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.
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.
"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.
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).
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.
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.
Buzzes, rattles and delamination fixes trump everything else and go to the top of the to-do list. :)
Dave.
Fixing them is less fun than comparing amps, though . . . :-|
So what freq LP FMOD did you get?
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