High Pass Filter (HPF) and Low Pass Filter (LPF) Mega Thread

[beans-on-toast]

In phonograph systems, there is low frequency modulation caused by warpage of the media. It doesn't have to be much either, and the same issue can occur when the record is pressed slightly off center.

No issues with the pressing that I can recall. The vinyl was flat. The bass and drums seemed to drive the excursion, the extent depended on the song. The JBL speakers were somewhat loose, you know those old foam surrounds.

 

[agedhorse]

I managed a JBL authorized service center in the late 1970's and early 1980's, I'm quite familiar, having reconed many with rotted foam.

It doesn't take much warp to cause this, you may not even see it in the record. All drivers become acoustically unloaded at those frequencies, the excursion depends only on the motor strength and the raw driver compliance. The JBL compliance was very low.

 

[beans-on-toast]

...still, one reason for using a HPF, even at low (home) levels.

 

[Stumbo]

@DeepSouth started a thread and asked:

...I can't figure out why I have god awful amounts of loud boomy bass behind the cab.

...I really not sure what to do about it.

He already tried a HPF and it didn't solve his problem.

610 cab sounds great out front but is knocking drummers over with boom

 

[agedhorse]

@DeepSouth started a thread and asked:

...I can't figure out why I have god awful amounts of loud boomy bass behind the cab.

...I really not sure what to do about it.

He already tried a HPF and it didn't solve his problem.

610 cab sounds great out front but is knocking drummers over with boom

This goes back to my comments about wash off the and around the backs of cabinets because the very low frequencies are quite omnidirectional as well as conducted through the materials of the cabinet.

 

[Steve Dallman]

Anyone tried this Gob HPF?

I amplified a few upright basses in the 80’s for myself and a couple bluegrass bands. I’d put a flat piezo disk on the BACK of the bass, right where the sound post was.

The result was a great, full range tone, with none of the “paper-y” string noise.

Problem was that most basses were a bit too boomy and feedback was a problem.

So I made a passive box with a “Gob-like” HPF...just a series small cap and a bypass pot. Sure the roll off was shallow, but it was very effective. The HPF was followed by a simple transistor buffer.

The roll off was shallow, and reached well into the mids, but it gently rolled off the low end, and that was just what we needed.

I had a smaller, 1/2 sized upright...35” scale. I didn’t have much problem with low end. In fact, the guy who bought it from me, used that pickup into a GK800 head, into an Ampeg fridge in a rockabilly band.

 

[Stumbo]

I amplified a few upright basses in the 80’s for myself and a couple bluegrass bands. I’d put a flat piezo disk on the BACK of the bass, right where the sound post was.

The result was a great, full range tone, with none of the “paper-y” string noise.

Problem was that most basses were a bit too boomy and feedback was a problem.

So I made a passive box with a “Gob-like” HPF...just a series small cap and a bypass pot. Sure the roll off was shallow, but it was very effective. The HPF was followed by a simple transistor buffer.

The roll off was shallow, and reached well into the mids, but it gently rolled off the low end, and that was just what we needed.

I had a smaller, 1/2 sized upright...35” scale. I didn’t have much problem with low end. In fact, the guy who bought it from me, used that pickup into a GK800 head, into an Ampeg fridge in a rockabilly band.

Would you be able to post a schematic?

 

[Steve Dallman]

Would you be able to post a schematic?

That was in the 80’s, when I still had a working memory. It was just a small cap...maybe .0047uF, bypassed with a 500k or 1meg pot. The signal went through the cap from input jack to output, and the pot progressively shorted across the cap. I don’t remember the values, but that would get you into the ballpark.

 

[Al Kraft]

I had a Bob Marley Live vinyl record that would move the speaker cones in the stereo’s JBL cabs like nothing else. Even at low volume levels. This is a perfect example of where a HPF would be useful at low listening levels.

With vinyl mastering they would cut the low end to prevent the needle from skipping out of the grooves. I don’t know how they did it but those speaker cones really moved. Interesting to not that the CD didn’t have the same effect on the speakers.

I had a pair of JBL L-19's with the foam surrounds and that suspension seemed like it was only a touch stiffer than hanging in mid air. IIRC all of my receivers in that timeframe (1970's/80's Yamaha and Luxman) had a "rumble filter" that was a version of a HPF to help with unwanted dynamics from turntables and/or vinyl runout.

When CDs came in I think they deliberately filtered out unusable/undesirable low end, but "no mechanical contact playback" from the medium coupled with with laser precision likely played a big part as well.

 

[Mike Arnopol]

one thing I've been thinking about---
Thermal compression is a really big deal. You start out in a set and your sound is great. Halfway through your set it seems like everyone in the group has gotten louder. But--your sound has also gotten thinner.
Thermal compression.
I realized that a big reason for thermal compression is that without a GOOD HPF---your amp and speakers are using a lot of their resources to try to amplify the subsonics present. So--the voice coil heats up as this demand continues. Even at a medium volume---the demand to reproduce these frequencies takes up much more resources than the actual frequencies that need to be reproduced.
So---subsonics will propel you into thermal compression quite easily.
Get a good HPF!
BTW---I stupidly realized something. When working on the HPF for my powered speakers, I set the knee at 30hz. Some of my speakers do have some noticeable content down there. I should have known---with the knee set at 30hz the attenuation starts higher---even with a 24db/octave slope. I had to back down to around 26hz until I didn't notice it. I could still tell that it was doing its job---those subsonics are really low!

 

[beans-on-toast]

Less amplifier energy wasted if sub-sonics are reduced.

I once read a paper that outlined how much PA energy is required to support sub-sonics. It’s a crazy percentage of the total power.

 

[agedhorse]

A bigger component of perceived compression is not thermal but as a result of ear fatigue.

 

[Mike Arnopol]

In my case---very much not true. I've experimented. I have a preamp into a 5 band Symetrix or Klark Technik parametric. Both have variable HPF's. Into a 1200 watt Ice module. So---I have plenty of power to try to reproduce subsonics.I'd play a set without HPF . Medium volume jazz with a piezo pickup. Piezos have much more pronounced subsonics (but magnetics have them, too). By mid set I could hear my volume start to drop and get thinner. Noticeably. Next set I'd engage the HPF set at 30hz. (fine for upright, but as I said--with a 5 string electric--I'd go a tad lower). No change in volume by the end of the set. No thinner low end. This was at a jazz quartet volume level. Even at that level--with all of the available power-- the subsonics from the piezo were definitely causing thermal compression. I think that I'm smart enough to know the difference between ear fatigue and thermal compression.

 

[agedhorse]

By any chance did you actually verify the temperature rise of the voice coil when coming to this conclusion? This is one of the steps in identify the actual cause versus the perceived or postulated cause.

I have a test jig at my shop to allow me to extract thermal information from a test run of a speaker, as well as the acoustic transfer function. What I have found fairly conclusively and consistently across a large number of speakers is that at low frequencies the temperature rise is really quite small even when showing measurable power compression on the transfer function.

The reason for this is that there are 2 distinct components to power compression, there is a thermal component and an electro-dynamic component. At low frequencies, the electro-dynamic component typically dominates (by a wide margin), and is composed of magnetic circuit losses, electro-mechanical losses, and at very high power and excursion there will be electro-magnetic losses as well as non-linear losses from the voice coil not being uniformly immersed within the magnetic field.

At higher frequencies, where displacement is small compared with the gap/VC dimensions, it's quiet possible for thermal compression to become the dominant factor, this is confirmed by the rise in VC temperature relative to the transfer function losses.

The electro-dynamic compression effect typically occurs at between 75% and 100% of the driver's Xmax, whereas the thermal compression effect has little to do with Xmax but instead is almost purely a function of the VC temperature which is more a function of mid and higher frequencies as the cooling effect of low frequency motion decreases.

Perception and reality are often confused, this is why the engineering and math behind these effects are important to understand and recognize the differences.

A HPF can greatly decrease the excursion due to subsonics, and can improve articulation of the signal due to the removal of the non-musical components that get in the way of the music portion of signal itself, as well as the harmonics that are generated by the cabinet in trying to reproduce signals well below the cabinet's linear capabilities.

 

[beans-on-toast]

Not easy to accurately measure voice coil temperature. In one approach they wind a thermocouple into the coil. Data logging changes in coil DCR is also quite involved. An IR thermometer would make the task easier if it can be used as an indirect temperature measurement tool to infer coil temperature.


http://cyrille.pinton.free.fr/elect...rs/AES_paper_1998_4667-thermal simulation.pdf

 

[Mike Arnopol]

Ok--- but why did this occur after 40 minutes of playing and then not occur at all when engaging the hpf? I've seen this occur consistently when playing a 5 nighter. Same rig, same setup, same gig. It seems that the other compression that you're talking about would occur immediately, where this compression that I was experiencing occurred only after playing a while and didn't occur when using a hp

 

[agedhorse]

Ok--- but why did this occur after 40 minutes of playing and then not occur at all when engaging the hpf? I've seen this occur consistently when playing a 5 nighter. Same rig, same setup, same gig. It seems that the other compression that you're talking about would occur immediately, where this compression that I was experiencing occurred only after playing a while and didn't occur when using a hp

This is consistent with hearing fatigue, which could be compounded by (or augmented by the effects of) the additional non-musical information present without the HPF. By using the HPF, you reduce the non-musical information that's generated, which improves articulation that might very well reduce the perception of hearing fatigue.

It may also be the combination of factors, including some thermal compression along with hearing fatigue.

 

[Mike Arnopol]

On a medium level jazz gig? And consistent set-to-set? And every day? And the speaker is essentially unable to reproduce this non-musical information?

 

[agedhorse]

On a jazz gig, I can't see how you would get even close to the thermal limits.

 

[Mike Arnopol]

Perception and reality are often confused, this is why the engineering and math behind these effects are important to understand and recognize the differences.

Apparently, it seems that more often than not, those with the understanding of "the engineering and math behind these effects" seem to be unable to produce a product that I am comfortable playing through.