[chadh]

I'm embarking on an experiment using a swarm of subwoofers.  This lets me relieve my otherwise full range drivers from some of the heavy lifting in the bass region.  To do so, I need to employ a high pass filter, and I want this filter to be passive but to operate at the line level.  It has been suggested to me that it would be easy to build such filters into a pair of interconnects.

Specifically, I'd want to use a 6dB/octave slope, kicking in at 100Hz.

The problem is that I have no idea how to do this.  I'd be grateful if somebody could tell me how to do it.  Alternatively, if some competent individual wanted to build a pair in return for the cost of parts and a (negotiable) fee to reflect my appreciation, that would work just as well.

Chad

[Speedskater]

Three or four decades ago, people sometimes did this.  You need to know the input impedance of the full range amplifier.

[mgalusha]

All that is needed for the simplest form of 1st order high pass filter is a cap in series. As speedskater mentioned, you need to know the input impedance of the amp you are driving.

A handy calculator can be found here: http://www.the12volt.com/caraudio/crosscalc.asp

This is a car audio site but it works just fine for line level applications. The math is the same. Just enter the impedance and the frequency and it will calculate the cap needed. Note that you may find the calculated value rolls off more bass than you expect as the frequency you input is the -3bB point.

mike

[stereocilia]

You could also buy FMOD filters, I think they accomplish what you seek.  http://store.hlabs.com/pk4/store.pl?view_product=10

[MaxCast]

not quite what you are looking for but it may be more versatile.
http://www.audiocircle.com/index.php?topic=70312.0

http://www.outlawaudio.com/products/icbm.html

[jrebman]

Chad,

Are you still using the em7 amp?  I ask because mine came with a second set of highpass inputs set at 100 Hz.

Keep in mind you're going to need a tightly matched pair of fairly small value caps (at least for the em7 amp), and with higher input impedances, the number of frequency intervals covered by standard caps is going to be pretty restricted.

Also, if you want to use different amps with this cable, they will have to have the same input impedance or the cutoff frequency will be different.

HTH,

Jim

[chadh]

Whoa, 

Somehow I missed all of these extremely helpful posts.  Thanks guys.

Chad,

Are you still using the em7 amp?  I ask because mine came with a second set of highpass inputs set at 100 Hz.

Keep in mind you're going to need a tightly matched pair of fairly small value caps (at least for the em7 amp), and with higher input impedances, the number of frequency intervals covered by standard caps is going to be pretty restricted.

Also, if you want to use different amps with this cable, they will have to have the same input impedance or the cutoff frequency will be different.

HTH,

Jim

Jim,

Yes, I'm using the EM7 amp.  But Roger modified it for me so that the second set of inputs offers a little bass boost below 400Hz instead of the highpass filter.

Essentially, I'm looking to replicate the passive filter built into the input of your EM7, but to have it built into the cable.

This is really just a temporary solution for me, until I save up enough to have Roger build me a new pre-amp.  The pre-amp will include the passive highpass filter as well as an active low pass filter.

As a temporary solution, I was thinking that those FMOD things might work.  But, how can they work as described if the crossover point for the passive filter depends not only on the passive components in the filter, but also on the input impedance of the amp?

Chad

[jrebman]

Chad,

In the em7 amp, the high pass inputs are nothing more than a second set of RCAs with the hot pin connected to the first pair by a .01 uF cap -- in accordance to the formula Mike G. gave earlier and assuming 100k input impedance.

As for the other, commercial filters, they are making an assumption somewhere, but I don't know what it is, and you're correct to be confused.  I always was and thus avoided such things.

-- Jim

P.S. -- YGPM on another subject.

[zybar]

I know you want to build these hi-pass filters, but what are describing is exactly what Vandersteen sells (and requires) for some of his speakers.

To handle the different impedances, these filters have dip switches that allow one to match the hi-pass filter to the amp being used.

Also, the hi-pass filters use a battery bias.

They come up from time to time on Audiogon in both xlr and rca versions.

Here is a picture:



George

[stereocilia]

I'm in complete speculation mode, but it looks like the FMOD things may be more oriented to car audio use.  It's possible that the impedance characteristics in that world are more predictable.  Again, total speculation.

[Brown]

The Fmods have a hard sound in the treble region IMO. Tried them more than once and again a bit Hi fi sounding. The Vandersteens work very well if you are willing to spend. Best bet make them.