Help on analyzing/designing Eq section

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kurotenshi
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Post by kurotenshi »

Hi everyone. I'm working on something that is not a stompbox but is based on one so I hope you don't mind the question.

I working on designing a circuit to make a pre-amp similar to Tech21's RBI unit. Since the RBI is based on the Bass driver DI, I'm using the Triangular Ear Bass21(Behringer BDI21) schematic to make mine.

I'll leave the details for when I post the design here, for now I want to focus on the EQ section. Both the BDDI and the BDI21 have only bass and treble controls but the RBI have a mid control that I want. The thing is, I want to keep as much as possible the SansAmp sound characteristics and so I went searching for info on that. From what I've found, both the BDDI and the GT2 use exactly the same EQ and judging for the frequencies on the manuals, so does the RBI (on the bass and treble), searching for Tech21 pedals with 3band EQ i've only found schematics for the old Tri-A.C. wich has a different (but similar) bass and treble design from the rest so i can't just copy the mid section of the circuit and i don't want to copy the hole EQ section without knowing the frequencies of each band.

So basically what I need is either a schematic with 3band EQ where the bass and treble are the same as the BDDI or (what would be more productive) some help on analyzing the EQ sections I have and designing the mid section for my project. This last option would be even more cool because then I'll have the knollege to and a Mid Frequency selector to the design.

Thanks
Attachments
SansAmp_TRI-AC.pdf
Tri-A.C. schematic
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teb21.pdf
Triangular Ear Schematic
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Brian M
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Post by Brian M »

In refrence to the trangular ear-
Signal from u3b only has two paths to u3c's inverting input.... both go through c25, and from there diverge in one of two directions.
On the top side c18, treble pot's wiper (for theories sake I am ignoring r31 to simplify) to the inverting input of u3c creates a HPF. When the wiper is close to c18 the impedance to the inverting input is very low. The opamps output will do all it can to keep the inverting input at the same voltage as the non-inverting input through the feedback resistor. This is sometimes referred to as a virtual ground. It can be treated as such when calculating filters (so long as the opamp is not clipping)
your low impedance signal HPF signal is amplified a lot.... the 1M resistor actually looks quite large to me, but not completely out of the ordinary.

The more interesting part is when you turn the wiper towards c19. You have the same basic filter, but instead of the original signal you have the the HPF as negative feedback. This is where you get your treble cut.

The bass control is similar, but is a LPF instead.

This is not how I would design a 3 band active tone control if I wanted to keep it simple using a single opamp stage. There is going to be some interactivity between each side of those tone pots at the in between settings, unless they are in the middle, in which case they do basically nothing. Essentially the frequency corners of the filters are not locked... they will move a bit depending on how much cut or boost set the controls for. This sound, while different than you would get from a more “mathematically accurate” circuit, could add something that would be missing with something easier to work with. If you like it, and that's what you want then so be it.

Adding a mid control that would only boost would be pretty easy. You could add a bridged T filter in the feedback loop, and a pot to vary how much negative feedback makes it back to the inverting input... but this circuit would not cut. It would only boost. All of the other ways I can think to do it would require changes to the rest of u3c's surrounding parts if you want a mid cut and boost, or would require another opamp stage.

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kurotenshi
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Post by kurotenshi »

Thanks for that fantastic response. But it turs out you have raisen another doubt, if the HPF and LPF are designed to be mutualy influenced, why is the mid control designed around U3B?? (i'm refering to the tri-AC) Will that create a band pass filter that is not influenced by the other filters? Would that require messing with the rest of the EQ section?

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Post by JiM »

These are Baxandall-style tone controls.
The BDI21 uses the classic 2-band circuit, whereas the Tri-A.C. adds one single-band before that. :hmmm:

But you can do a 3-band Baxandall !
Look at figures 2 &3 here : http://www.headwize.com/projects/showfi ... al_prj.htm
It's intended for HiFi, but you can easily adapt it for bass.

The easiest way to tweak is to run some simulations. Unfortunately Duncan's Tone Stack Calculator doesn't have the active Baxandall topology, just the passive James. I recommand then to use SPICE, for example LTSpice : you can feed a .wav file to the simulation, and then hear the result ! (not in realtime though :( )
I only give negative feedback.

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kurotenshi
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Post by kurotenshi »

Sweet.
This Baxandall EQ is the same used on the Alber Kreuzer Bass pre-amp design. I was looking at that but (in my ignorance) thought they were too diferent...
Thanks for the help people. Keep the info coming :D

As for simulations, If some one here knows how to use Matlab for circuit analysis and can give me some lights I would appreciate. Send me a PM or something.

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Brian M
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Post by Brian M »

I thought of something last night that should work, and should not require changes to the existing parts... I'll see if i have time to draw it up later.

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kurotenshi
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Post by kurotenshi »

So brian, any news?

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