How do I make a DIY FMOD?

[DJNUBZ]

Anyone know how to make a DIY FMOD? I am looking to make one that is a 70hz low pass.

[pinkmouse]

What's an FMOD?

[pinkmouse]

Nope, Google doesn't help either, all I get is references to C++ coding and a Windows sound engine

[theChris]

an FMOD is the name harrison labs gives to their line of inline (RCA signal), passive crossovers. an example can be found at www.thezeb.com i beleive. they are supposed to have 12dB/oct slopes but apparently only acheive 6dB/oct. no word on Q, or zeros. they are popular for rumble (subsonic) filters in car audio, and are used primarily with the jbl 1200 watt class D monoblock which is has no subsonic filter.

[theChris]

info from site:
http://www.thezeb.com/caraudio/harrision_labs.html

"FMOD's are passive crossovers that go in-line with your RCA Interconnect cables before your amplifier. Filter out the frequencies your speakers don't like, and let 'em play the ones they do well. Choose the Low-Pass ones for your subs, and the Hi-Pass ones for your Full-Range Speakers & Components. Comes in pairs: 1 for the left channel, and one for the right.

No power needed to operate (won't cause turn-on thumps)
Can be combined with other crossovers to change the slope (dB/Octave) & crossover frequency point
Virtually NO noise or distortion added to signal
Gold Plated Brass RCA's work with any RCA cable
Handle up to 10 Volts of signal power
12 dB/ Octave "

cost $28.99 USD

[DJNUBZ]

Thats them, now how do I make my own?

[fr0st]

Could it just be a passive crossover like speakers but for a higher impedence?

[JOE DIRT®]

they are an antenuated lcr filter...Harrison Labs is a respected company


DIRT®

[thylantyr]

I found this archived data.. posted
by Jon Risch

"The FMOD parts are unlikely to be of the highest quality, and if you are
seeking the best, then a DIY filter is the way to go.
Teflon caps are the best, followed by the various froms of polypropylene,
and styrene (when you can find it) is almost as good as the teflon.
In terms of brands, everyone has his favorite, but their is no doubt that
Hovland MusiCaps, MIT MultiCap,TRT InfiniCap,Reliable's RelCap are all in
the upper tier as far as sonic quality is concerned.
You simply put the cap in series with the hot lead, make sure the two RCA's
have a good ground connection, and try to shield the whole shebang. A large
piece of copper braid might do the job, slipped over the cap (hopefully with
either insulated leads, or pieces of tefon tubing over the leads).
In terms of assembling a sonically good filter, use a quality insualted wire
for the ground wire, and make it no longer than necessary to bridge the gap
between the two RCA plugs. Nestle the cap in between the RCA jacks, and
using the tubing to insulate if necessary, use the caps own leads to connect
to the hot pins. Then use teflon plumbers tape or a good grade of heatshrink
(polyolefin or better), insulate the assembly in the middle, and use the
copper braid to shield, grounding it at one end only. You may or may not be
able to use the RCA plug barrels, if not, wrap lots of tape tightly around
the threaded end of the plug, and use heatshrink as a strain relief. The
smaller values of caps might fit inside a large barrel, in which case, a
small piece of quality coax could be sued to make the connection from the
other plug.
As far as values go, here are some starting points for a 100 Hz - 3 dB
point:
For a typical SS amp with an input impedance of 10k ohms, use a 0.18 uF cap.
Input impedance of 50k ohms, 0.027 uF.
Traditional tube amps can run as high as 1 Meg ohm, then use 0.0015
Now for the easy adjustments:
if you want 200 Hz instead, halve the cap value. If the imput impedance is
twice as high, halve the cap value. If you want 50 Hz instead, double the
cap, input Z half of the examples, double the cap value.
Caps can be paralleled (placed side by side, and the leads joined at the
ends, keeping the same end together) to double the value, and placed in
series to halve the value. So if you buy (in pairs for stereo) about three
different caps, this will allow you a very wide range of potential crossover
points.
See:
http://www.capacitors.com/pickcap/pickcap.htm
for more information on WHY various types of caps perform differently.

Jon Risch

[planet10]

Passive Line Level XOs

dave