• WANTED: Happy members who like to discuss audio and other topics related to our interest. Desire to learn and share knowledge of science required as is 20 years of participation in forums (not all true). There are daily reviews of audio hardware and expert members to help answer your questions. Click here to have your audio equipment measured for free!

Why preamps or DACs really should have high pass filters

617

Major Contributor
Forum Donor
Joined
Mar 18, 2019
Messages
1,682
Likes
3,697
Location
Somerville, MA
As is ASR dogma, the best way to get good bass in a room is to use multiple subwoofers, which will generally have a shallow, low pass filter of perhaps 80hz 2nd or first order. There's no standard - some people advocate lower or higher or shallower or steeper. Bass management isn't really the issue I want to discuss here, however.

Ideally, your main speakers would have a complimentary high pass filter on them. This is much more uncommon, but that is a shame. Here's why:

1633660220652.png

This is a graph of a Rival woofer in a typical bass reflex enclosure. It has an f3 of 43hz in a small box. This is a high quality 7" woofer similar to what you get from ScanSpeak or Wavecor. This graph shows excursion in a vented box, and it shows that at only 39.5 watts, you are at the limit of excursion (6mm or so) at 60hz. If your content has any low bass, it will run out of excursion with much less power. Speakers tend to produce a lot of distortion as they near their limits and at a certain point it will just stop producing more sound.

In the past I've always advocated for plugging a bass reflex port. This essentially turns your speaker into a sealed alignment. I never really thought about this, but I assumed you would get more excursion and be able to play louder. However, this is what happens (blue line):
1633660557167.png

Yes, the over-excursion no longer happens at 60hz, but you still are dealing with the fact that at below the tuning frequency you run out of excursion fairly fast. Here, you run out of output at 14.4 W at 30hz.

It's clear that these midwoofers, while quite capable of producing pleasant bass at low levels, are not really made for higher output levels. Simply plugging your speaker results in a low, extended response that will run into excursion issues and have limited output at low frequencies.

But what if your preamp/DAC had a high pass filter on it?

1633661201311.png

This is what happens if you add a 2nd order high pass filter to your vented speaker. This ends up being what looks like a 3rd order ish response when combined with the roll-off from your vented speaker. F3 is now 80hz, which is a conventional crossover frequency. However, look at your excursion now:

1633661430914.png

Now you're running into xmax at 121.6 W! This woofer has a fairly typical IEC power handling of 70W, but still, it's nice to have that extra excursion for momentary signals. At this (maximum) power level, we are realizing the full output potential of this woofer, which is around 105db/1m.

The moral of the story - the ubiquitous hifi midwoofer is a limited full-range performer. Even next generation designs like the illuminators/purifi/anarchy woofers are not going to give you crazy output levels at low frequencies. However, they are capable of significant output if you relieve them of bass duties. Plugging a port doesn't really do much for you - a high pass filter is the way to go.

The woofer in this simulation is a full 180mm model with a pretty hefty magnet and xmax. When you consider how many people have bookshelf speakers with far less capable woofers (like a KEF LS50, almost any affordable passive speaker, even the Revels use 17cm models) it becomes clear how advantageous line level high pass filters could be for them. Indeed, the addition of subwoofers wouldn't simply 'fill out' the lower octave, it would allow for much greater dynamics as well, in a system that is visually unobtrusive and affordable.
 

restorer-john

Master Contributor
Joined
Mar 1, 2018
Messages
8,758
Likes
24,340
Location
Gold Coast, Queensland, Australia
Most proper old-skool preamplifiers and integrated amplifiers had two low filters and two high filters. The low filters were infrasonic and another filter around 70-80Hz- primarily for hum issues, but quite usuable with modern 2 way baby speakers that are so popular these days.

But let's stop calling these 4"-6.5" drivers "woofers". They aren't. They are mid-bass drivers at most. They aren't even good at midrange.
 
OP
617

617

Major Contributor
Forum Donor
Joined
Mar 18, 2019
Messages
1,682
Likes
3,697
Location
Somerville, MA
Most proper old-skool preamplifiers and integrated amplifiers had two low filters and two high filters. The low filters were infrasonic and another filter around 70-80Hz- primarily for hum issues, but quite usuable with modern 2 way baby speakers that are so popular these days.

But let's stop calling these 4"-6.5" drivers "woofers". They aren't. They are mid-bass drivers at most. They aren't even good at midrange.
Thats a very interesting factoid about preamp filters. I assume these were all first order.

Regarding the quality of these drivers I think they can be pretty good if you high pass them. Ideally you'd go even higher- a typical 3 way speaker might cross over at 300hz. A 7 inch midwoofer will do 300hz and up very loud with very low distortion.

Now, are they ideal for that application? No, the high Xmax they need to produce bass becomes a liability for midrange, at least in theory. That's why scanspeak, morel and others make dedicated 5-7 inch woofers with limited Xmax specifically for midrange.
 

Goodman

Active Member
Joined
Jun 1, 2021
Messages
170
Likes
61
This would suggest that a good 30 year old bookshelf you can pick up for a couple hundred euros has better bass performance than a 15,000 € two way modern column. Shocking!!
 

thewas

Major Contributor
Forum Donor
Joined
Jan 15, 2020
Messages
4,180
Likes
9,175
Everything is a compromise, if you close the port you almost loose 6dB around level (and thus also max SPL) at the tuning region and a loudspeaker like in your above example that has dropped already 10dB at 50 Hz is sounds very limited with many music, unless you manage to "fill" a big portion of it with near wall placement and room gain or even better subwoofer(s). Also the higher the steepness of the slope and the lower the centre frequency of the high pass function is the higher the group delay becomes which can be audible, unless you use FIR filters.
 

wwenze

Addicted to Fun and Learning
Joined
May 22, 2018
Messages
844
Likes
1,141
Hmm... I noticed many active speakers have HPF. Figured it is for a similar reason, but did not realize the significance.

*Ignores it for my +13dB boosted passives anyway coz I'm lazy*
 

restorer-john

Master Contributor
Joined
Mar 1, 2018
Messages
8,758
Likes
24,340
Location
Gold Coast, Queensland, Australia
Thats a very interesting factoid about preamp filters. I assume these were all first order.

Mostly they were 2nd or 3rd. 1st were common on amplifier input HPFs (DC blocking) and preamp outputs (cap coupled). Here's an example on the classic 1970s Pioneer Spec1 preamp:

1633681202749.png


Here's the response. Look at the 30Hz filter (12dB per octave), it'd be great for a small modern two way:
1633681272520.png

Regarding the quality of these drivers I think they can be pretty good if you high pass them. Ideally you'd go even higher- a typical 3 way speaker might cross over at 300hz. A 7 inch midwoofer will do 300hz and up very loud with very low distortion.

I agree. But 300Hz Bass/Mid XO for a three way? Seems pretty high to me.

My Jamo 507a floorstanders XO the 2x 6.5" internal woofers at (electrical) 150Hz and the twin 4" mids come in from 110Hz with the measured woofers' response falling at 30dB per octave above 100Hz.

But yeah, IIRC my Sony SSG-333es (12" woofer) XO at ~500Hz.
 

Ultrasonic

Addicted to Fun and Learning
Joined
Dec 30, 2018
Messages
693
Likes
527
Location
UK
I use miniDSPs in part because they offer the feature of distinct high and low pass filtered outputs. It would be good to see this sort of feature being more widely available but it's no minor tweak to an existing stereo DAC design.
 

Willem

Major Contributor
Joined
Jan 8, 2019
Messages
2,673
Likes
4,059
My solution has been to use a passive High Pass Filter in the cable from the balanced output of the RME ADI-2 DAC to the power amplifier, in the same way as the Harrison Labs Fmods. The subwoofer with its own Low Pass Filter uses the RCA output of the ADI-2.
 
OP
617

617

Major Contributor
Forum Donor
Joined
Mar 18, 2019
Messages
1,682
Likes
3,697
Location
Somerville, MA
My solution has been to use a passive High Pass Filter in the cable from the balanced output of the RME ADI-2 DAC to the power amplifier, in the same way as the Harrison Labs Fmods. The subwoofer with its own Low Pass Filter uses the RCA output of the ADI-2.
How did you calculate the passive line level filter?
 

ad_fletch

Active Member
Forum Donor
Joined
Jul 22, 2020
Messages
111
Likes
54
Location
Australia
I am on a quest to get a reasonably priced DAC/Pre solution with both LPF and HPF, as per this thread.

Ended up ordering Allo Piano 2.1 DAC which allows you to create filters using built in micro DSP and TI Purepath software. Similar to MiniDSP but without the user-friendliness and cheaper

Unfortunately a month later it seems to have gotten lost in the mail, but I still like the idea....
 

pjug

Major Contributor
Forum Donor
Joined
Feb 2, 2019
Messages
1,566
Likes
1,333
As is ASR dogma, the best way to get good bass in a room is to use multiple subwoofers, which will generally have a shallow, low pass filter of perhaps 80hz 2nd or first order. There's no standard - some people advocate lower or higher or shallower or steeper. Bass management isn't really the issue I want to discuss here, however.

Ideally, your main speakers would have a complimentary high pass filter on them. This is much more uncommon, but that is a shame. Here's why:

View attachment 157793
This is a graph of a Rival woofer in a typical bass reflex enclosure. It has an f3 of 43hz in a small box. This is a high quality 7" woofer similar to what you get from ScanSpeak or Wavecor. This graph shows excursion in a vented box, and it shows that at only 39.5 watts, you are at the limit of excursion (6mm or so) at 60hz. If your content has any low bass, it will run out of excursion with much less power. Speakers tend to produce a lot of distortion as they near their limits and at a certain point it will just stop producing more sound.

In the past I've always advocated for plugging a bass reflex port. This essentially turns your speaker into a sealed alignment. I never really thought about this, but I assumed you would get more excursion and be able to play louder. However, this is what happens (blue line):
View attachment 157794
Yes, the over-excursion no longer happens at 60hz, but you still are dealing with the fact that at below the tuning frequency you run out of excursion fairly fast. Here, you run out of output at 14.4 W at 30hz.

It's clear that these midwoofers, while quite capable of producing pleasant bass at low levels, are not really made for higher output levels. Simply plugging your speaker results in a low, extended response that will run into excursion issues and have limited output at low frequencies.

But what if your preamp/DAC had a high pass filter on it?

View attachment 157801
This is what happens if you add a 2nd order high pass filter to your vented speaker. This ends up being what looks like a 3rd order ish response when combined with the roll-off from your vented speaker. F3 is now 80hz, which is a conventional crossover frequency. However, look at your excursion now:

View attachment 157804
Now you're running into xmax at 121.6 W! This woofer has a fairly typical IEC power handling of 70W, but still, it's nice to have that extra excursion for momentary signals. At this (maximum) power level, we are realizing the full output potential of this woofer, which is around 105db/1m.

The moral of the story - the ubiquitous hifi midwoofer is a limited full-range performer. Even next generation designs like the illuminators/purifi/anarchy woofers are not going to give you crazy output levels at low frequencies. However, they are capable of significant output if you relieve them of bass duties. Plugging a port doesn't really do much for you - a high pass filter is the way to go.

The woofer in this simulation is a full 180mm model with a pretty hefty magnet and xmax. When you consider how many people have bookshelf speakers with far less capable woofers (like a KEF LS50, almost any affordable passive speaker, even the Revels use 17cm models) it becomes clear how advantageous line level high pass filters could be for them. Indeed, the addition of subwoofers wouldn't simply 'fill out' the lower octave, it would allow for much greater dynamics as well, in a system that is visually unobtrusive and affordable.
This is an interesting analysis. I wonder if the 2nd order filter is rolling off steeper than you even need to solve the problem. Would you mind doing this with a first order HP filter? First order makes it easy to do a DIY solution with an adjustable filter.

edit: here is what I get with Philharmonic BMR and port sealed with and without a first order HP filter. The filter is just a 0.22uF cap in-line with the interconnect to my amp, in combination with the amp input impedance. Just this is pretty effective for knocking down distortion at very low freqs. The with a sub really doing the work there I don't think the remaining distortion would be an issue at all. SPL for this is about 95-100dB 100Hz at 1m, but I measured close to the woofer.

SB 6%22 ceramic.jpg

SB ceramic with 6dB filter.jpg
 
Last edited:

Willem

Major Contributor
Joined
Jan 8, 2019
Messages
2,673
Likes
4,059
In the old days with vinyl as your main source many amplifiers had a steep low cut filter. A good example would be my Quad 33 preamplifier that started to filter a bit around 30 Hz and went down steeply around 20 Hz. Peter Walker was a smart designer, since at the time there was no useful information at those frequencies, and only rumble and other noise that could harm your speakers.

1634308716827.png
 

EJ3

Addicted to Fun and Learning
Forum Donor
Joined
Dec 10, 2019
Messages
985
Likes
756
Location
James Island, SC
My solution has been to use a passive High Pass Filter in the cable from the balanced output of the RME ADI-2 DAC to the power amplifier, in the same way as the Harrison Labs Fmods. The subwoofer with its own Low Pass Filter uses the RCA output of the ADI-2.
As I have not been able to yet afford the RME ADI-2 DAC (other projects have gotten in the way, but it is on my list to obtain), I use the Harrison Labs Fmods. It's good to know that I can emulate them with the RME ADI-2 DAC, as the FMODS work quite well for me in my application.
 

Willem

Major Contributor
Joined
Jan 8, 2019
Messages
2,673
Likes
4,059
I am afraid you are misunderstanding what I wrote. I use the unfiltered full range output from the ADI-2 into a cable with FMOD equivalent High Pass filter. The ADI-2 does not do any High Pass filtering, which is not to say that it is not a great DAC.
 

Mnyb

Major Contributor
Forum Donor
Joined
Aug 14, 2019
Messages
1,292
Likes
1,679
Location
Sweden, Västerås
Good post .

In this day and age more manufacturers should reinvent the preamp in the digital domain ADC/DAC DRC and sub output volume control tone control/EQ balance control mono mode etc , there are a few . But you usually have to use some av product
 

EJ3

Addicted to Fun and Learning
Forum Donor
Joined
Dec 10, 2019
Messages
985
Likes
756
Location
James Island, SC
I am afraid you are misunderstanding what I wrote. I use the unfiltered full range output from the ADI-2 into a cable with FMOD equivalent High Pass filter. The ADI-2 does not do any High Pass filtering, which is not to say that it is not a great DAC.
Got it now, thanks. Still, since I know exactly how that filter sounds... I am actually using the PFMOD 2W-1 for my sub-woofer (pair) crossovers.
 

skyfly

Active Member
Joined
May 1, 2020
Messages
188
Likes
49
Some Spendor speakers have quite low port tuning frequency for the price. Low port tuning frequency means less over excursion problem.
 
Top Bottom