High-pass line filter to relieve main speakers from low frequency


Hi everybody
I recently purchased a subwoofer, KEF Kf92. It has a built in low-pass filter, 24 dB, that can be set from 40-140 Hz. They go with ATC SCM20sl. Their specs are: 80-20 kHz +/- 2 dB, 55-25 kHz : - 6 dB (free standing, which they are, actually: free hanging).
I wonder whether it would be beneficial to insert an high-pass filter before the power amp, consisting of a capacitor with a set point of 65 Hz. This filter might not have a huge impact because the frequency of the ATC SMC20sl rolls off quickly by itself below 80 Hz, apparently by about 12 dB/octave.
My thinking however is: the loudspeakers still receive the complete energy of the low bass from the amp. They might not be able to convert it into sound, but instead convert it into heat and distortion. So a filter will make the life of the amp and speaker easier, since at 33 Hz, only a quarter of the energy is produced.
Do you agree with me?
b_mueller
Thank you, yes it does indeed - and I tested it. The thing is it is way much less transparent compared to the direct route. I suppose the signal is converted A->D, then back to A. Anyway it sounded kind of dull, and it was hard to judge whether there is a benefit from less bass.
Years ago when I had a subwoofer I used this high pass filter to take the load off my mains!
https://www.hsuresearch.com/products/high-pass-filter.html
Did you like the effects?
I was planning to solder a Duelund silver cap 47 nF after the RCA input of the power amp.
I am doing just what you describe in my strength to roll of the bottom end below about 65Hz. I am using a Pair of Miflex KPCU caps for each channel (balanced connections).

I think the improvement in sound quality is pretty subtle, but it does seem to have a bigger effect the louder I play the system. 
I think the improvement in sound quality is pretty subtle, but it does seem to have a bigger effect the louder I play the system.
@jaytor Thank you for your feedback. Your observations would be consistent with the theory: the louder, the more energy and distortions, scaling up exponentially with perceived loudness.
https://www.parts-express.com/FMOD-Crossover-Pair-100-Hz-High-Pass-266-274?gclid=CjwKCAjwmqKJBhAWEiw...

They make inline filters (low and high) from 20hz up to at least 100 hz and down, THEN 15khz and up. AT LEAST.. It's a pro application, but works very well..

No need to add anything but the filters you like PRE power amp..

It won't affect the passive OX in the speaker.. Just the signal it receives from the amp. No SUB involved with  steeper slopes like 24 or 48. The BOOM goes away and the timing (which is the problem) will become coherent. Reduced distortion, muddiness, bloated bass ALL goes away.

Room treatment, main and sub decoupling are the real eye openers too.

BUT limiting bass to the main is the BEST place to start, decoupling is right there though.. Room treatment is just the polish on a good, from ground up design..

Regards
Any new sub will have a crossover filter in it for that purpose, which is to relieve your main speakers of bass duties, but if it does not, sure that is a great idea. It used to be that the satellites ran full range to augment he sub, but times have changed.
I was curious about the same thing.  I decided to go this route. 
https://sound-au.com/project81.htm

I haven't built it yet, this fall or winter, but I have all the parts.  I figured it would be a nice cold weather project.
It lools like you guys are looking for an active crossover. Per ocd hifi guy - check out Marchand. - https://www.marchandelec.com/electronic-crossovers.html

Many folk discourage. I have limited knowledge but I believe crossovers cause phase shift. If you have expensive time aligned drivers you may be doing more harm than good.
Vandersteen also makes a high pass amp but I doubt you'll like the price.

https://www.vandersteen.com/products/m5-hpa
Built one of these for a friend to use with his Spendor  BC-1’s for the exact reason you are wanting to implement.

Here is a link to capacitor values as the apply to the input impedance of the power amp. 
I suggest buying a bunch of cheap capacitors around the values you think you might want  to drop in for “tuning”, and then buy proper ones once you sort it out.

http://www.troelsgravesen.dk/High-Pass-Filter.htm
Would this be of any help?
https://6moons.com/audioreview_articles/sublime-k231/

There's a link to the maker at the end of the article.

All the best,
Nonoise
It lools like you guys are looking for an active crossover. Per ocd hifi guy - check out Marchand. - https://www.marchandelec.com/electronic-crossovers.html

Many folk discourage. I have limited knowledge but I believe crossovers cause phase shift. If you have expensive time aligned drivers you may be doing more harm than good.

Marchand has both active and passive crossovers.

On their web site is a paper that discusses why there is no phase shift with a 24dB/octave slope.
I use two of their XM9 active crossovers. The newer one came with a selection of inserts that allow you to choose your crossover point.
Hello,
I have been using the JL Audio CR1 crossover. This is an Audiophile piece of gear. It has XLR and RCA which can be utilized at the same time. Also it has home theater bypass for the subs. It is not cheap. But it is awesome and works with all speakers and subs. It also has a few other features like damping and your choice of roll off. 
That's right Tom, analogue crossovers introduce phase shift. But, as Sd40 points out, 24 dB crossovers don't; then again, 24dB crossovers do retard the signal by 1 complete cycle at the crossover frequency. Actually, that's an oversimplification and it's more complicated than that, but you get the idea: analogue crossover design is hard.

Then there's the problem with components. You just can't expect $10 parts to sound like $500 parts, and even the most expensive parts are not neutral and will introduce their own sound.

This is the one arena where digital is far superior. Since your speakers fall off sharply, I suspect that your best bet is to not bother with a highpass crossover. But no-one knows until you try it.

IMO
Thank you all, I suppose almost all aspects have been covered by your comments.
Anyway, I bought a set of high-quality capacitors (Duelund JDM Silver Foil, 47 nF) and will insert them at the amp (input impedance 51 KOhm). I will report findings.
I‘ve done it - added a 47 nF Duelund silver cap in line at the power amp‘s input. I soldered between Eichmann bullets plugs and sockets, allowing quick removal. I dislike any modification that adds sockets, wires, soldering points, but there was no space inside the amp. When I used pink noise and measured frequency response in the room, the drop below 60 Hz is steeper, as was predicted for a high pass. Furthermore, the woofer extends much less, indicating less low freq power reaches the chassis.
On sound quality (with KEF Kf92) the effect ist not huge. There is a bit less stress and increased clarity. I will do more comparisons.
https://ibb.co/gFP5Vxw