Hi-Pass Filters w/ Steep Roll-Off?

[Count Bassie]

I've been looking into a hi-pass filter for use with an SWR SM400 head. There's a thread about it here.

I'm now understanding that such a (shelving-type) filter with a 24dB per 8va roll-off would be desirable. Anyone know who might make such a device?

Alternatively, could a capacitor-loaded box be easily made to cut off a given frequency, and inserted into the signal-chain at the speaker cab's input?

Hmmmm??

[bongomania]

A passive one (capacitors and resistors) will not have a steep enough slope. I'm still looking for a suitable solution. I just today received a Carl Martin parametric EQ which has a low corner of 20 Hz, but apparently it has a fairly shallow slope, so it's no help in this application.

[Count Bassie]

Maybe a laptop in the chain with a software Hi-Pass filter. like in a Waves plug-in. Lot of extra bs though, not a real simple solution you want to carry in your gig-bag.

[niftydog]

Cascaded passive filters will achieve 24dB/octave (fourth order filter), but you start hitting issues with phase and insertion loss. What you're asking for is essentially one half of a crossover, try searching for crossovers instead and perhaps settle for 18dB/octave and you might have more luck.

I've glanced at the other thread and my gut feeling is that there must be a better solution to this problem. Surely modifying the amplifier itself is a better engineering solution? You need a competent EE to do some circuit analyses for you. There is a question of where the excess low frequency is coming from. If it is inherent in the preamp then simply filtering it at the FX loop would seem to be the most practical solution. Building a high power, high quality 24dB HPF is not trivial.

[Silent Fly]

Originally Posted by Count Bassie

I've been looking into a hi-pass filter for use with an SWR SM400 head. There's a thread about it here.

I'm now understanding that such a (shelving-type) filter with a 24dB per 8va roll-off would be desirable. Anyone know who might make such a device?

Alternatively, could a capacitor-loaded box be easily made to cut off a given frequency, and inserted into the signal-chain at the speaker cab's input?

Hmmmm??

Are you looking for a commercial product, the schematics or someone that can build it for you?

[synaesthesia]

Just run an electronic crossover in your loop that will meet your specs. Most off the shelf 1U E Cs will shelf off 24dB and if you want to cascade it for sheer OTT shelving you could as most are stereo units.

[OldogNewTrick]

Originally Posted by bongomania

I just today received a Carl Martin parametric EQ which has a low corner of 20 Hz, but apparently it has a fairly shallow slope, so it's no help in this application.

The Q of the slope is quite wide on low settings, but at -15dB the slope is pretty steep.

I use it to filter out the very low frequencies when I play a 5.string through my Dual Showman setup, as that one tends to get a bit too boomy down low.
One nice side effect is to be able to dial in the right amount of preamp tube compression and grit, without actual driving the whole amp into clipping...

[zac2944]

Originally Posted by synaesthesia

Just run an electronic crossover in your loop that will meet your specs. Most off the shelf 1U E Cs will shelf off 24dB and if you want to cascade it for sheer OTT shelving you could as most are stereo units.

Sounds like a good idea. It would be cool if there was an x-over out there smaller than a 1U unit for those who don't rack.

[Count Bassie]

The x-over sounds like a good idea, if the roll-off is enough of a killer.

I had a passing thought of a mod to the amp- best idea actually, I would think. I'm going to ask a tech I know about that.

Thanks for the replies.

[Hoover]

Originally Posted by Count Bassie

I've been looking into a hi-pass filter ...(snip)... a (shelving-type) filter with a 24dB per 8va roll-off would be desirable.

Not to be a nit-picker, but a hi-pass filter with a 24dB/octave rolloff -- or any hi-pass (or lo-pass) filter -- is not a "shelving-type" filter. Because there's no shelf! By definition, shelving filters and n-pass filters are different beasts:

A Shelving filter has a specific amount of boost or cut (usually variable and user-determined) beyond which the response levels off. So technically there are two corner frequencies in a shelving filter: There's the cutoff frequency at which filtration begins, then the response either rises or is attenuated at a rate determined by the slope of the filter (usually fixed, occasionally variable and user-determined) and then there is a second corner frequency where the response returns to "flat" (sic). Eg, if your low shelf has a cutoff frequency of 100Hz and a slope of 24dB/octave, at 50Hz the frequency response will be 24dB down (edit: or up), but will not be further attenuated (edit: or boosted) below 50Hz. That's the "shelf", that flat portion after the filter's action (represented graphically).

In contrast, a hi-pass filter has infinite cut below the cutoff frequency; the response never levels out below fc (or above fc, in the case of a lo-pass filter). So if your hi-pass filter has a cutoff frequency of 100Hz and a slope of 24dB/octave, at 50Hz the frequency response will be 24dB down, at 25Hz the response will be 48dB down, at 12.5Hz the response will be 72dB down, etc etc. The response never levels off after the filter's action because the filter's action continues essentially to infinity.

[Count Bassie]

So then, Mr. Nit-Picker (j/k!), what would you recommend as a solution to the SM400 problem? You are involved on the other thread somewhere- what's your own opinion on it?

I am considering just having the amp modded to a new center freq... really all I want is for a speaker to not receive a frequency it can't actually handle- and to transfer energy (i.e., power) to a freq that represents a better volume level... in other words, I want to minimize "wasted" wattage.

I do now "get" the fact that a filter placed before the amp's input will do the job. This was confusing at first, but I understand that if there is no 31Hz represented in the incoming signal, then there's no 31Hz for the amp's eq knob to work with. That took a minute, but I now get it.

I appreciate your time.

[synaesthesia]

Originally Posted by zac2944

Sounds like a good idea. It would be cool if there was an x-over out there smaller than a 1U unit for those who don't rack.

Amp heads with a controllable low-cut filter near 20-30 Hz?

[Jim C]

Speaking for myself (and I think Bongo as well) I want a small box that fits in a gig bag to do this job and not a 19" rack unit. Maybe an unrealistic expectation?

Thoughts were 25 - 50 Hz and approx 24dB / octave, or, some sort of tuneable brick wall filter with an appropriate curve

[Hoover]

(I haven't really read that other thread, so I don't know if this has already been suggested, but...)

If it didn't have to be a variable fc but just a fixed turnover, you could build a passive hi-pass filter for about $4 in parts: one cap, one resistor, two 1/4" jacks and some solder. Okay, call it $12 so you can put it in a nice Budd Box.

Not sure if there'd be too much loss with an instrument-level signal going through a passive filter though.

[bongomania]

No, that's already been dismissed as not effective.

[gnasher1993]

EDIT: I'm an idiot...

[Count Bassie]

You mean this? I don't see hi-pass filter...

[gnasher1993]

Originally Posted by Count Bassie

You mean this? I don't see hi-pass filter...

I'm so sorry man, I must be going insane. The funny thing is that I actually own that pedal...

[stingray69]

For years I've always told myself that I'd give one of these a shot, but haven't yet. "Might" be the ticket: http://images.google.com/imgres?imgurl=http://www.performanceaudio.com/images/products/180/4121.jpg&imgrefurl=http://www.performanceaudio.com/cgi/product_view.cgi%3Fproducts_id%3D4121&usg=__ycz8M3 GpqRfDh_u6OFNBW7xEwA8=&h=238&w=270&sz=79&hl=en&sta rt=5&tbnid=XJpAmbMoPoyQpM:&tbnh=100&tbnw=113&prev=/images%3Fq%3DRolls%2BSX21%2BTiny%2B2-Way%2BCrossover%26gbv%3D2%26hl%3Den%26client%3Dfir efox-a%26rls%3Dorg.mozilla:en-USfficial%26sa%3DG

[KPJ]

I have one of those Rolls Tiny 2 way crossovers in my gear box. Thought I could use it at the time. If it takes an instrument level input, it could do the trick.