varible LPF/ HPF/Baxandall EQ

[simonsez]

all gone, druu got the last one.

Thank you

 

[simonsez]

all has been shipped, BAXX amp pcb, and 4 Q8 DOA pcb's.

Doc attached (rename the extention into rar), ask any question here .


Regards,

Simon

 

[simonsez]

This can be easy to integrate into BAXX

 

[simonsez]

We can choose how many freq step, just omit  the corresponding RC that we did not want to have.
for stereo ganged control, we need 4 pole rotary switch for bass freq selector, and 2 pole for treble.

 

[abbey road d enfer]

for stereo ganged control, we need 4 pole rotary switch for bass freq selector, and 2 pole for treble.

That's because you've chosen the dual-cap variation of Baxendall's implementation. the single cap implementation requires only two poles and offers better curves IMO.

 

[abbey road d enfer]

Single cap implementation has more predictible LMF response.

 

[simonsez]

yes indeed, single cap looks better and simpler to me,  Douglas self has some word about that. I found on most hifi tone control use 2 cap version.

 

[simonsez]

Still learn about this baxandall behavior, it said here that :

"The main Disadvantage of the basic circuit is that it has its greatest effect at the extremes of the audio range, as shown in Fig . 1.For example , if a 6dB boost is required at 4kHz, one must simultaneously tolerate a Much  greater boost of perhaps 18dB at 16kHz. Furthermore, the turnover frequency of the bass control depends on its setting, but this does not apply to the treble control!”

can you explain this?

 

[abbey road d enfer]

Still learn about this baxandall behavior, it said here that :

"The main Disadvantage of the basic circuit is that it has its greatest effect at the extremes of the audio range, as shown in Fig . 1.For example , if a 6dB boost is required at 4kHz, one must simultaneously tolerate a Much  greater boost of perhaps 18dB at 16kHz.

This is just a description of the shelving response of the Bax circuit, in opposition to a bell curve. In order to achieve a certain amount of boost at a frequency situated between the mid range point and one of the extreme, one has to apply a larger boost; with a bell curve, one just applies the right amount at the right frequency.

Furthermore, the turnover frequency of the bass control depends on its setting, but this does not apply to the treble control!”

That's the behavior of the dual-cap LF implementation; it tends to act as a variable-frequency low-pass or high-pass filter, when the single cap implementation acts as a fixed-frequency, variable-slope type.

 

[simonsez]

Is  fHB and fLB in the pic below is what called turnaround freq ? so when we  boost at say 3db at 3khz, we actually boost much more in the higher freq? (or lower with bass). Is this true for all shelving eq?

 

[abbey road d enfer]

Is  fHB and fLB in the pic below is what called turnaround freq ? 

Correct.

so when we  boost at say 3db at 3khz, we actually boost much more in the higher freq? (or lower with bass). Is this true for all shelving eq?

Yes. That's because shelving filters (or EQ's) are characterized by a sloping response.

 

[druu]

From the Dangerous manual:

The frequencies noted on the faceplate reside in the middle of this slope, thus corresponding with the most audible band. The highest frequency shoulder is approximately twice the listed frequency (where the cut/boost is exactly 5dB). So for example, when modifying 1.6kHz, a 5dBu boost occurs at 3.2kHz.

 

[abbey road d enfer]

From the Dangerous manual:

The frequencies noted on the faceplate reside in the middle of this slope, thus corresponding with the most audible band. The highest frequency shoulder is approximately twice the listed frequency (where the cut/boost is exactly 5dB). So for example, when modifying 1.6kHz, a 5dBu boost occurs at 3.2kHz.

That's weird! Traditionally, Bax EQ's are specified by their cut-off frequency at max boost/cut, which makes sense technically;  specifying the "tilt" frequency usefully complements the information.
But this doesn't make any sense to me. Fortunately, a shelving EQ can be operated without any markings on the knobs.

 

[druu]

Definitely, markings should only be for reference, if required.

 

[simonsez]

it's looks that here's few different perspective about this,  douglas self  have used the term ‘break frequency’ to indicate where the
tone control begins to take action. This is defined as the frequency where the response is +/- 1 dB
away from flat with maximum cut or boost applied.

same wrote in http://sound.westhost.com/articles/eq.htm, but he use term "turnaround freq" at +/-3dB

in the end we (some) always use our ear to play with all of this gear 
we love analog, but sometime we digitized our mind with numbers?

 

[dirty1_1garry]

Hello!
I was impressed on eq that  Simonsez build. Great result!
It’s interesting for me to enjoy and may be build sometime similar for me a little bit later.
As I was inspired, I spend last night for understanding bax type eq and trying to simulate it with LTSpice.
I’ve used one cap schematic that Simonsez marked in his docs. I’ve experimented with resistors and capacitors values.  As a reference I’ve taken dangero*s bax eq curves. It has 74, 84, 98, 116, 131, 166, 230, 361Hz low frequencies on +/-5dB.


Here is what I’ve got:

 

[dirty1_1garry]

Interesting moment - dengero*us bax eq HF marked as 1.6kHz, 1.8kHz, 2.1kHz , 2.5kHz, 3.4kHz, 4.8kHz, 7.1kHz , 18kHz.  But if you will see on it's HF curves , HF points match +/-3dB boost/cut instead of +/-5dB boost/cut on LF



and here is what I've got:

 

[abbey road d enfer]

Interesting moment - dengero*us bax eq HF marked as 1.6kHz, 1.8kHz, 2.1kHz , 2.5kHz, 3.4kHz, 4.8kHz, 7.1kHz , 18kHz.  But if you will see on it's HF curves , HF points match +/-3dB boost/cut instead of +/-5dB boost/cut on LF

Yes some authors choose to specify the frequency corresponding to half the max boost or cut. It makes sense  in metrologic terms since it is in the highest slope portion of the curve, but in terms of adequation with the physiological perception it is inadequate.

 

[dirty1_1garry]

Yes some authors choose to specify the frequency corresponding to half the max boost or cut. It makes sense  in metrologic terms since it is in the highest slope portion of the curve, but in terms of adequation with the physiological perception it is inadequate.
[/quote]

May be main theory that reference frequency is where the curve get +/-3dB of gain does not work with baxandalls because they have such a wide shelf. And that's  why it's more useful to use irrespective method of calculation frequency.

 

[alexc]

Hope there is a rev2 of your pcb with your bax implementation. 

Ticks some useful boxes with me AND a few nice-to-haves as well as some quite impressive features.

Well done