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Harrison FMOD filters

B

Bass Unique

Nov 3, 2011
298
8
Wiltshire
www.lowfrequencysolutions.co.uk
#1
I have been doing some work with these filters recently and have found fantastic results. The findings show that the filters reduce the unwanted subsonic cone movement so that more of your available power goes to producing the actual notes you want.

The filters are inline with the signal lead and use gold connections. They could be fitted on the main feed to the amplifier inputs or in the effects loop.

I have tried the 30 Hz rumble filter and the 50 Hz filter. The 30 Hz appears to make no difference at all to the sound but does reduce the large cone excursions associated with slapping or hard picking, particularly on amplifiers with less agressive subsonic filters built in. These have proven reliable wiht no failures so far.

The 50 Hz filters do reduce the lowest fundamentals and as the manufacturer claims 12 dB per octave, you would be 12 dB down at 25 Hz and 6 dB down around 37 Hz. I found this filter to be a great help on the smallest combo or practice cabs where it is pretty inaudible but increases the available power out. With a large low tuned cab, there is less LF boom but this may be a good thing on a PA supported stage.

The results is less farting out of speakers when driven hard and more amplifier headroom for useful notes.

One word of warning, my first 50 Hz one died. They are supposed to take 10V without damage but if you use an active 5 string with the low end turned up and expect it to handle strong fundamentals below 50 Hz, there is a lot of energy to be lost somewhere - I think mine overheated but I am unsure of why it failed. If I break another, I will let you all know. It may be safer to fit this in the effects loop instead.

You need an RCA male connection on the input and a female on the output to link it. I fitted the appropriate connections to an older lead I had with a duff jack one end and I can now choose to fit the FMOD or not with the same lead. Very simple to fit between tracks if you like.

Anyone else with a similar experience?
 
#2
I used them on a few car stereo projects from my youth. I had good experience in that application with 100-150hz filters for door speakers. At the time 4v was a hot signal in the car stereo amplification world, so no failures to note.

I'm nto the most help here, except that I can verify along with you that they do waht the say they do, relatively how they say they do it. Comparing them to my 12db/octave active filter I had for the same application yielded nearly identical rfesults.
 
#3
Yes, except I generally do the filtering with active circuits. I think the idea of passive filters is fine; the only potential fly in the ointment is the impedance the filter is working into. I've seen line level inputs that run from below 10 KΩ all the way up to slightly over 100KΩ. I've seen effects return impedances run all over the place, too. This will have an effect on the filter's performance. My gut tells me the 30Hz (spec) model would be the wiser choice because it'd be less apt to knock off wanted low end when the impedance it sees is on the low side.

Running the filter into an instrument input would probably (again) change the filter's nature: Instrument inputs typically run from just under 1MΩ to several MΩ -- a far cry from your typical line-level input impedance -- possibly making the filter less effective. However, the proof is in actual field use, and if you're using an FMOD "off label" but you're still seeing benefits in terms of reduced cone hop, then [DEL]when[/DEL] what you're doing has merit.
 
#5
Bass Unique said:
Anyone else with a similar experience?
Click to expand...

Not with the FMOD, but in extensive testing of various active filters I have found that for my purposes it's often most desirable to have the filter as late as possible in the signal chain, with right in front of the power amp being by far the sweetest spot. I really like to let the subsonics and especially low fundamentals throw harmonic content into the upper frequency bands, and filtering early negates that possibility IME. On the other hand, some effects can benefit greatly from pre-filtering.

I'm also not too keen on unnecessary insertion loss or unbalanced solutions, and RCA connectors are an absolute deal breaker for me. In general, I would prefer this to be a built-in feature of the power amp, via DSP that I can program myself. As it so happens, I have that covered. But if it sounds good to you, and the other parts work to your liking, it is good in my book.;)
 
#6
It is interesting about where to put it in the signal chain. I tend to run my bass clean so I would not really see any significant difference putting it early or later in the chain but if you like to create some distortion by over-driving the pre-amp tube etc, maybe it is better in the effects loop than on the input. Good comments about the matching impedance too.

thanks so far. Has any one else seen any problems with these though? I have an active bass which is usually at high output, I have not yet measured it but I assumed it would not be at 10V.....
 
#7
Bass Unique said:
It is interesting about where to put it in the signal chain. I tend to run my bass clean so I would not really see any significant difference putting it early or later in the chain but if you like to create some distortion by over-driving the pre-amp tube etc, maybe it is better in the effects loop than on the input.
Click to expand...

It's not about distortion at all for me as primarily an old school jazz EUB kind of player, but rather pitch definition and psychoacoustic processing of low frequency information. If you're running a Fender tone stack for instance, there's a lot going on between 8 and 25 Hz that early high pass filtering neuters. That may or may not matter to you, or you may easily prefer doing things some other way, of course...:cool:
 
#8
If you have even a little soldering skills you can google "filter calculator" to make your own filter.
Put frequency you want and the component values will spit out.

Many pro amps have rumble filters built in. There is a few out there that don't. There is no reason to amplify rumble.
 
#9
I have used an FMOD-Sub in the effects loop of my solid-state Bmax for many years. It done what I wanted it to do, and very well. However, for some reason it wasn't enough when I tested a fEARful 1515/66/1 with my rig... Actually got that cab into overexcursion with maybe half-power (DCM600)..
 
#10
Could have been a narrow window between filter range and where the cab unloads its driver, especially if you were using the 20 Hz variant rather than the 30 Hz. I'm thinking the window probably spanned 30 to 50 (?) Hz. But even a 30 Hz FMOD-Sub may have been the wrong choice in that application. The FMOD "High Pass RCA Level Crossover" -- the 50 Hz variant -- might have done a better job. As you know, the BBE can supply massive (sometimes dangerous) amounts of bass boost in a very low region, and so would probably have been able to overcome any unwanted low-bass (40-50 Hz) attenuation caused by the 50 Hz filter, and thus make up (benignly) for any leanness caused by that high of a -3dB frequency.

Another unknown is the input impedance of the BMAX's fx return. A higher-than-typical input impedance would have lowered the FMOD's -3dB point and made it less effective than it would have been at a lower input impedance.
 

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