Bottom octave compensator

[Dave Jones]

[Mr. Moderator: You may want to move this thread to the Solid State BB, but I would appreciate it if you left it here for a little while, because I know a couple of our loudspeaker guys are experts on this stuff.]

The May 2004 AudioXpress magazine has an article titled "A Bottom Octave Compensator." It describes a general purpose second order active filter that can be used to shape the bass response of a woofer or subwoofer.

I have a number of questions. It is not clear to me how to interpret the instructions for choosing R and C component values, but I will defer those questions for now.

I find it strange that the author makes no mention of the Linkwitz transform. One would think he would be aware of it. He makes a point of the Bottom Octave Compensator's relative antiquity (ca. 1971), which I believe may predate the Linkwitz transform by a little. How do the two compare? On a cursory examination, it looks to me as though the BOC has two integrators, whereas the LT has only one.

Any commentary on the two circuits would be appreciated.

(I am thinking of doing YAS [yet another subwoofer], and the idea of stuffing an EQ circuit between the pre-amp and amp is interesting.)

[sreten]

They are two very different beasts, as the LT has no low
frequency roll-off, and the 2nd order high pass by definition
adds 2 orders to the low frequency alignment.

The dip at main resonance with an LT is impossible with a
2nd order high pass filter, which generally is used with a
peak of ~ 6dB (Q=2) well below the box resonant frequency.

They are not remotely interchangeable.

The LT suits underdamped sealed alignments.

The 2nd order high pass suits overdamped sealed and reflex
alignments, converting them to 4th and 6th order respectively.

sretenncy.

[Dave Jones]

Originally posted by sreten
They are two very different beasts, as the LT has no low
frequency roll-off, and the 2nd order high pass by definition
adds 2 orders to the low frequency alignment.

The dip at main resonance with an LT is impossible with a
2nd order high pass filter, which generally is used with a
peak of ~ 6dB (Q=2) well below the box resonant frequency.

They are not remotely interchangeable.

The LT suits underdamped sealed alignments.

The 2nd order high pass suits overdamped sealed and reflex
alignments, converting them to 4th and 6th order respectively.

sretenncy.

When you say "2nd order high pass" are you referring to the BOC? I don't think that's what it is. Well, it could be configured as a highpass filter, but the way the article says to choose the RC values, it is a lowpass filter.

I am aware that the so-called "boost" on typical plate amps is actually just a high Q highpass filter. Correct me if I'm wrong, but I think the "BOC" described in the AudioXpress article is a different kind of thing. I think it may "contain" the Linkwitz transform as a special case. But I'm just starting up the learning curve on filters, so I could be wrong.

[BillFitzmaurice]

The filter is a 12dB/octave that kicks in at the Fb to compensate for the rolloff of the sealed box at 12dB/octave below Fb. There are two problems with the design. One is that at 12dB/boost per octave it is very power hungry. The other is that it's already patented and licensed out to Bag End Speakers, who call it the ELF.

[Dave Jones]

Originally posted by BillFitzmaurice
The filter is a 12dB/octave that kicks in at the Fb to compensate for the rolloff of the sealed box at 12dB/octave below Fb. There are two problems with the design. One is that at 12dB/boost per octave it is very power hungry. The other is that it's already patented and licensed out to Bag End Speakers, who call it the ELF.

Say wha? The Bag End patent is for using EQ on a sub that is operated entirely below Fb. If the unit described in AudioXpress is in violation of that patent, well, it's not -- because the BOC was invented, described publicly, and built for sale in 1971. Or at least that's what the article says. Prior art. Way prior, I would guess.

Besides, the Linkwitz transform (1980?) will also equalize below Fb.

What I am asking is what the essential difference is between the two? Does the extra integration stage in the BOC buy anything? What does the extra integrator do?

[BillFitzmaurice]

Thew main difference is that while the ELF is designed to operate wholly below Fb the other circuit is designed to do nothing down to Fb and then kick in, so it is not intended for pure sub usage as the ELF is. The patent thing is not of much consequence as long as you don't intend to sell any of your creations, but the power hungry aspect remains. The whole point of the system is that it trades a large cabinet to get low bass for a large amplifier to squeeze bass from a small box. Another variation on the same theme: Bose 901/w controller (a fancy word for EQ).

[Dave Jones]

Originally posted by BillFitzmaurice
Thew main difference is that while the ELF is designed to operate wholly below Fb the other circuit is designed to do nothing down to Fb and then kick in, so it is not intended for pure sub usage as the ELF is. The patent thing is not of much consequence as long as you don't intend to sell any of your creations, but the power hungry aspect remains. The whole point of the system is that it trades a large cabinet to get low bass for a large amplifier to squeeze bass from a small box. Another variation on the same theme: Bose 901/w controller (a fancy word for EQ).

I'm asking what the difference is between the circuit in the AudioXpress article, (have you read it?), and the Linkwitz transform.

I've been doing a little research. I believe both circuits are "bi-quad". Both implement a filter whose transfer function is the ratio of two quadratic polynomials. The one in the AX article is more generalized. I think it is what's called a "state variable filter". The Linkwitz transform is a special case, and uses fewer parts. Someone please correct me if I'm wrong.

[BillFitzmaurice]

There I can't help. My thing is speakers, and while I can muddle through a schematic and cobble up anything I may need I'm not sufficiently versed in the theoretical aspects to the extent that you require for a definitive answer.

[Konrad]

Reading the artickle:
The amount of 1.order kompensation can bee made variable by making R6 and R9 to a potmeter. Thus the dominator b in the 2.order eq
The amount of 2.order kompensation can bee made variable by making pot of R10 and R11 Thus the amount of a in the 2nd order eq
and the resistors R2 and R5 preforming a pot will determine the ratio c in the 2. order eq ( as^2 +bs+c = A(s) ) dekribing the rollof from a speaker in a sealed box.
The Q is stated = sqrt(ac) / b and then adjustable
The wo is stated = sqrt(c/a) thus also adjustable
Thus the circuit is fleksible and easily bekoms generaly adjustable for any speaker.

abt the first question (now edited away) i think it was abt u were getting a polynom instead of the ekspected 1/s in the integrating stages:

as the resistor R4 and C2 is equal to 1/(1+s)
and wil bee multiplyed by the opamp A2 response (1+ 1/s) and s is obtained from R3 and C3
1/(1+s) * (1+ 1/s) = 1/s. but only when R3=R4 and C2=C3

[phase_accurate]

The proposal for tuning a box half an octave lower and adding a highpass with a 6 dB hump to generate a sixth order alignment comes from D.B. Keele (with Electrovoice back then).

His formulae were:

fb = f3 = faux = 0.3 * fs / Qts

Vb = 4.1 * Qts^2 * Vas

faux = pole frequency of the auxiliary highpass, having a Q of 2

Regards

Charles