Re: Critique this internet sourced Bax design...?
« Reply #20 on: May 20, 2020, 04:11:41 PM »
I noticed that the 535 schematic from xqp separated the high and lows in order to avoid interaction there...perhaps this would be necessary for an accurate mix bus EQ, and I'm wondering aside from component count if there is any other downside to having all 3 bands on their own op amp...odd number not fixing the phase inversion perhaps?



JohnRoberts

Re: Critique this internet sourced Bax design...?
« Reply #21 on: May 20, 2020, 05:34:01 PM »
I noticed that the 535 schematic from xqp separated the high and lows in order to avoid interaction there...perhaps this would be necessary for an accurate mix bus EQ, and I'm wondering aside from component count if there is any other downside to having all 3 bands on their own op amp...odd number not fixing the phase inversion perhaps?
Using 3 op amps, one for each section,  is perhaps better on the test bench but not really necessary in my judgement. The HF and LF Baxandall sections are spaced far enough apart that they don't interact noticeably.  Shoehorning a third in between them is noticeable.  The fact that two inverting stages in series restores polarity is a bonus, because inside consoles you don't want a polarity flip when you hit bypass. .

JR 
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Newmarket

Re: Critique this internet sourced Bax design...?
« Reply #22 on: May 20, 2020, 07:23:59 PM »
Looking at the original circuit - it does seem to me that the resistor values are unusually high (= noisy) for an opamp based circuit.
The input impedance is 100K but implemented by means of an inverting configuration. Hiss Alert !
I'd suggest investigating a non-inverting configuration at the input stage.
Yes- you will need an extra non-inverting opamp stage to retain polarity but that might still be less noisy if implemented with low value resistors. You'd need to run the numbers but worth a look (and I do realise that this has basically been addressed by Douglas Self before me).
On the other hand - do you need 100K input impedance in your application ?

Newmarket

Re: Critique this internet sourced Bax design...?
« Reply #23 on: May 20, 2020, 07:31:42 PM »
[quote autgiven that hor=JohnRoberts link=topic=75135.msg952656#msg952656 date=1589996260]
 a resistor is not a "current source", not even close. My current source summing invention replaced summing resistors with audio modulated current sources.
[/quote]

I've often thought about what I understand this approach to be - true current sources rather than a series resistor.
I understand why JR states that digital summing means that it's not really viable now,
But I still think it's a valid approach  given that it's not always convenient / desired to convert analogue signals to digital and back to analogue ?

Re: Critique this internet sourced Bax design...?
« Reply #24 on: May 20, 2020, 08:13:04 PM »
...perhaps it is worth me in that case following the example in the Small Signal design...the reason why I was attracted to this one was because of the setup which described the way I wanted to use the bax design which is where it has a long gentle slope and the shelving is out of the audible range...and I like the plugin version (which actually used the passive version on the site)...

I've been confused previously about the 'bax' thing because it seemed like there were different versions...apparently the Studer 169 diy project is a bax design but it looks like a typical shelving EQ...and the image of the Self bax design also looks like a typical shelving eq (left)...whereas the dangerous bax has a long slope but no shelf...same with the 535 XQP Audio one (right)...is this difference solely down to setup of the frequencies (and gains)?


Brian Roth

Re: Critique this internet sourced Bax design...?
« Reply #25 on: May 20, 2020, 11:40:13 PM »
Swan: the curves on the right side of your post are combinations of a Bax along with low and high cut filters.  See PDF page 8 of the Dangerous Bax manual for an example:

https://dangerousmusic.com/wp-content/uploads/2019/08/BAX-Manual-v6-LR.pdf

Bri

edit:  If you look through other curves in the Dangerous manual you will see the curves without filters match with classic Bax curves.

« Last Edit: May 20, 2020, 11:50:10 PM by Brian Roth »
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JohnRoberts

Re: Critique this internet sourced Bax design...?
« Reply #26 on: May 20, 2020, 11:45:49 PM »
Peter Baxandall RIP was (is) a well respected engineer. 

Bax is easier to spell.

JR 
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Re: Critique this internet sourced Bax design...?
« Reply #27 on: May 21, 2020, 12:46:10 AM »
Self has a very nice Bax implementation in this book, lower noise and with clear notes on component scaling and band interaction. 

I see no reason to favor the above design over Self's take.

I love Self's books, I just order SSAD 3rd edition!, however, there is only one problem with Self's implementation, and in fact with his book in general, there is absolutely zero math on how to calculate the circuit parameters, you either take it as it is or you are basically screwed, you can fool around with a simulator trying to modify it to fit your needs, or you can try and figure out how the circuit works by doing the analysis and math by hand, but you are on your own with this. Thats the only downside I see on Self's books, if he at least included  simple formulas on how to calculate basic parameters such as cut-off frequency the book would be truly legendary.

JohnRoberts

Re: Critique this internet sourced Bax design...?
« Reply #28 on: May 21, 2020, 10:18:54 AM »
I love Self's books, I just order SSAD 3rd edition!, however, there is only one problem with Self's implementation, and in fact with his book in general, there is absolutely zero math on how to calculate the circuit parameters, you either take it as it is or you are basically screwed, you can fool around with a simulator trying to modify it to fit your needs, or you can try and figure out how the circuit works by doing the analysis and math by hand, but you are on your own with this. Thats the only downside I see on Self's books, if he at least included  simple formulas on how to calculate basic parameters such as cut-off frequency the book would be truly legendary.

I noticed back in the 1970s that college students buying my audio kits were tasked with modifying them as a course requirement for their engineering studies. To sell more kits I included some basic design equations where appropriate in my kit instructions.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Re: Critique this internet sourced Bax design...?
« Reply #29 on: May 22, 2020, 06:14:01 AM »
I am about to pull the trigger on the BOM for this design to test on the breadboard but thought it might be sensible to check in advance it is worth pursuing from the more knowledgable types around here, seeing as it's an internet source...




I've built that 3 band EQ many years ago (2004 ish)  and the interaction between the mid and the treble controls was awful.
I always thought i made a mistake somewhere until I came across a piece of equipment with 3 band EQ that had the same interaction. I took the cover off and looked inside and it was the same circuit.

I first saw it in an IC data book , page 586 and I used to drool at the pretty looking curves


JohnRoberts

Re: Critique this internet sourced Bax design...?
« Reply #30 on: May 22, 2020, 10:08:19 AM »
I've built that 3 band EQ many years ago (2004 ish)  and the interaction between the mid and the treble controls was awful.
I always thought i made a mistake somewhere until I came across a piece of equipment with 3 band EQ that had the same interaction. I took the cover off and looked inside and it was the same circuit.

I first saw it in an IC data book , page 586 and I used to drool at the pretty looking curves
Never ASSume that all typical application circuits are well engineered. They are typically developed by junior engineers tasked with selling more ICs.

I learned a lot from the old National Semi applications handbooks in the 70s but I was starting from a blank sheet... Of course some senior application guys were head and shoulders above the rest (Bob Pease, Jim Williams, etc....). Dennis Bohn of RANE fame started out as a National Semi app engineer.

JR

PS: I repeated this anecdote here recently but back in the 80s when dealing with CBS I learned that one of their CX licensees copied their mistake verbatim from a typical application circuit, manufacturing tens of thousands of playback devices with an incorrect side chain time constant.   :o
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

john12ax7

Re: Critique this internet sourced Bax design...?
« Reply #31 on: May 22, 2020, 03:21:33 PM »
Never ASSume that all typical application circuits are well engineered. They are typically developed by junior engineers tasked with selling more ICs.

+1

The same is also true of eval boards. I've never quite understood why companies feel sub-par app circuits and boards is the way to help sell product.  Perhaps they feel the cost / benefit is not justified.

Re: Critique this internet sourced Bax design...?
« Reply #32 on: May 22, 2020, 05:53:44 PM »
...talking of ICs...how specific to op amps are these type of circuits? I notice in Selfs design it just says 'IC' with no specific selected...this one recommends an OPA132....can I just use an Ne5532 (and can I power that with a 9v battery on my breadboard...)?

Re: Critique this internet sourced Bax design...?
« Reply #33 on: May 28, 2020, 06:19:51 PM »
I need to find a pot with centre detent to fit the values of this design (100k and 500k) and/or the Douglas Self one which is 10k...which is a 6mm spline shaft and 15mm shaft length...(the Tayda unfortunately are 17mm)

question: if I used a 20k pot in the place of a 10k could this be accounted for by other resistance?

other question: in these types of EQ circuit which component value stipulates the boost amounts?

Re: Critique this internet sourced Bax design...?
« Reply #34 on: May 31, 2020, 07:30:35 PM »
Swan,

You may find this 1952 article from Mr. Baxandall interesting and helpful.  I did.

https://learnabout-electronics.org/Downloads/NegativeFeedbackTone.pdf

You'll notice it's called a Negative-Feedback Tone Control, bass and treble controls in the feedback loop.  The way I integrated it in a passive fashion with makeup gain which wasn't how it was designed, so I'm going to revisit it and try it like Peter designed it, then compare for the experience.  What I did wasn't really the same thing, technically not a Baxandall.  I didn't know this at the time.

In my testing of the basic circuit idea it's a broad, gentle and effective tone control.  I put a high pass filter circuit/control in front of the bass and treble controls, then used a FET input opamp as makeup gain to recover the insertion loss.  The opamp I used was 1/2 of a TL072.  The other half I used for something else.  Kind of convenient in that regard.

The Baxandall design should be fed from a low impedance source and drive a high impedance load.  That's the reason for the FET input opamp as far as I know.

You will likely get a cleaner result with lower potentiometer resistance as was suggested.  I still think it may be worthwhile to try it out the old way (with high resistance) as well, as if you were using tubes and actually needed the high pot resistance, and compare it to the lower pot resistance that would be more common, and likely considered more appropriate under most circumstances, to use for today's extremely convenient opamps.  Don't be afraid to try a discrete opamp design as well if it's a one off and you like that sort of thing.

And from the reading I've done recently you'd likely be wise to follow the suggestion/advice regarding the mid-range control.  I didn't incorporate one so I can't comment but others have and seems like it's not a very effective way of incorporating it.

As for the gain, I'll leave that up to you to work that out.  If you do lower potentiometer values you will need to tune the circuit to match the original curves by changing the other components in the network.  I don't have Doug Self's book so I can't say if he already did that or not, or whether he explained the equations.  Others may be able to advise you on that.

I didn't use center detent pot but I'd consider that useful for the circuit as it's a flat, nearly unaffected, zone in the center.  I incorporated a bypass switch with LED indicator, but you don't necessarily need one because the center position of the tone controls are so flat.

I was surprised by the tone controls I made.  They were passive, with inductors and didn't have a feedback loop so as I said it's not really the same thing.  Very curious now about following Peter's design more closely.

Adam


 

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