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Harrison Labs FMOD inline HP filters - opinions?

Zombie

Zombie

pfm Member
Eyeing these HP filters to offload my recently refurbed Quad 988s below 100 Hz with a sub.
I read several different opinions about them, like not actually being 12 dB/octave, but rather 6 dB, etc, while others swear by them.
I thought I'd try those first before I get entangled in active filter or miniDSP type filters.
I'll begin with a standard 10" without a HP, that's why I ask. :)
So, do you have any opinions about the FMOD HP filters? Are they available in Europe?
 
Gosh there's a blast from the past! I used FMOD low-pass filters back in the mid-to-late noughties when I 'made' DIY subwoofers by using KEF B139 bass drivers in free KEFKIT3 enclosures I picked up. IIRC the FMODs cost me more than the subs (the pair of B139s cost me £35!). This was long before I had measuring equipment so I couldn't tell you how accurate the filter slopes are but I ended up stacking two same value FMODs together to double the steepness of the slope, otherwise there was too much bleed through of unwanted frequencies coming from the subs. I never tried the high-pass versions so cannot comment on their transparency etc.
 
If the high pass is just a stacked RC, it is ultimately 12 dB/octave, but with a very gentle roll off. Putting two in series is going to not work well as the second filter will be driven by the high impedance of the first.
 
Basically a 6 dB slope is to put a suitable cap in front of the power amp input...maybe that's better. The preamp has bass cut shelf with -4,5 dB at 60 Hz (Quad Artera pre).
 
Btw, can someone confirm that the cap size for f = 100 Hz, R = 20k is 8 x 10-8 = 80 nF
A bit rusty with equations nowadays...if not correct, which is the correct formula?
 
Yep, that' s right for a single stage (-6dB/Octave) RC filter.

At c. 80nF, look for polypropylene caps nearest standard value (E24-series of values) is 82nF, but you might need a couple in parallel to add-up to this depending on what you can find.

That will set the HP filter negligibly -higher, single-figures, above your target. No bad thing.
 
What would be the best setup? A small metal box with the caps inside? But should I connect all phono connector grounds to the box or just one or one per channel?
 
martin clark said:
Yep, that' s right for a single stage (-6dB/Octave) RC filter.

At c. 80nF, look for polypropylene caps nearest standard value (E24-series of values) is 82nF, but you might need a couple in parallel to add-up to this depending on what you can find.

That will set the HP filter negligibly -higher, single-figures, above your target. No bad thing.
Click to expand...
Why 20k? Remember that your load is usually anywhere between 10k and 100k, often 47k.
This load is in parallel with your 20k.
I would use a higher capacitance eg 220n or even 470n and work backwards.
 
The 909 is a little less. R2 22k in parallel with R5 62k and T1 input resistance, with C2 330n between them. R1 560R in series with the lot.
So 22.5k at dc, rather less at audio, maybe effectively 15k?
 
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