Freezing cold here in the Northeast USA has got me holed up in the apartment, so this seems a good time to apprehend the unique circuit found here.
Want more control over the mid-range for guitar. Found a different 4-Band ParaEQ PCB out in Europe but no answer to email, so no money will be sent.
Having trouble apprehending the action around what I have named, "Range Node", specifically, the phasing of the dual pot connections, i.e., the direction of the wipers and their effect. I assume it moves the range around somewhat.
Here is what I have figured out so far: when the wiper of VR2a moves downward, adding series resistance to C1, there will be less low end boost due to the higher XC, thus effectively moving the band higher. In the case of VR2b, it seems to me a conundrum whichever way that wiper goes in relation to the other. I have spent hours trying to understand what would be going on either way, with the exception that C1 and C2 will be in series, regardless of wiper direction.
Another question is there are no input caps on each opamp. A main incoming cap is elementary, but what about the rest of them? Adding them will produce a phase shift at each band stage around 90 deg. from the preceding. I wonder what that will do the to tonal qualities. Might try that after the ranges are adjusted to suit guitar, clean channel and heavily distorted channel. At that point, the dual pots will be replaced with fixed resistors.
Well, I could build it, and if it doesn't work, the pot leads could be reversed until it does (if it does), but that is really "bush league". This is akin to the difference between an automotive technician and a "parts changer".
Any comment appreciated.
Want more control over the mid-range for guitar. Found a different 4-Band ParaEQ PCB out in Europe but no answer to email, so no money will be sent.
Having trouble apprehending the action around what I have named, "Range Node", specifically, the phasing of the dual pot connections, i.e., the direction of the wipers and their effect. I assume it moves the range around somewhat.
Here is what I have figured out so far: when the wiper of VR2a moves downward, adding series resistance to C1, there will be less low end boost due to the higher XC, thus effectively moving the band higher. In the case of VR2b, it seems to me a conundrum whichever way that wiper goes in relation to the other. I have spent hours trying to understand what would be going on either way, with the exception that C1 and C2 will be in series, regardless of wiper direction.
Another question is there are no input caps on each opamp. A main incoming cap is elementary, but what about the rest of them? Adding them will produce a phase shift at each band stage around 90 deg. from the preceding. I wonder what that will do the to tonal qualities. Might try that after the ranges are adjusted to suit guitar, clean channel and heavily distorted channel. At that point, the dual pots will be replaced with fixed resistors.
Well, I could build it, and if it doesn't work, the pot leads could be reversed until it does (if it does), but that is really "bush league". This is akin to the difference between an automotive technician and a "parts changer".
Any comment appreciated.
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This is only about the input capacitors.
An input capacitor will create a high-pass filter isolating the DC levels between the previous output and the input.
For guitar usage the cutoff frequency should be below 80Hz (actually quite a bit below as you have 4 stages in series - maybe 20-40Hz).
A capacitor (or actually the high-pass filter) will create a phase shift - but only at frequencies approaching the cutoff frequency and below.
Expect 45 degrees at the cutoff increasing to 90 degrees at frequencies far below the cutoff.
At frequencies above the cutoff there will little or no phase shift.
So with sufficiently large input capacitors (giving a low cutoff frequency) there will be no phase shift to worry about.
To give a rough estimate of the required input capacitor size, I would equate the input impedance to some 8KOhms and suggest around 10Hz cutoff - giving 2uF.
Likely anything between 1 and 10uF would work.
Cheers, Martin
An input capacitor will create a high-pass filter isolating the DC levels between the previous output and the input.
For guitar usage the cutoff frequency should be below 80Hz (actually quite a bit below as you have 4 stages in series - maybe 20-40Hz).
A capacitor (or actually the high-pass filter) will create a phase shift - but only at frequencies approaching the cutoff frequency and below.
Expect 45 degrees at the cutoff increasing to 90 degrees at frequencies far below the cutoff.
At frequencies above the cutoff there will little or no phase shift.
So with sufficiently large input capacitors (giving a low cutoff frequency) there will be no phase shift to worry about.
To give a rough estimate of the required input capacitor size, I would equate the input impedance to some 8KOhms and suggest around 10Hz cutoff - giving 2uF.
Likely anything between 1 and 10uF would work.
Cheers, Martin
@diyMartin: Agreed, 2.2uF are used almost everywhere in my overall design. A roll-off beginning around 50Hz is okay. However, any phase shift is inconsequential from the amp input all the way to this circuit, and subsequent to this circuit's output, which is the output power section. I have not yet considered inter-stage phase shifts with series opamps in EQs. Some frequencies will be either be boosted or attenuated but with phase shift. I will try this later; it might be interesting maybe not. Thank you for your comment.
@diyMartin: Come to think of it, I never considered the phase shifts at C1-C2 and their effect on the total operation !! At the moment, I'm primarily interested in how the range adjustment works.
If you are a true DIYer, here's a good one to build:
https://sound-au.com/project84.htm
It says "subwoofer", but can be adapted for guitar use.
https://sound-au.com/project84.htm
It says "subwoofer", but can be adapted for guitar use.
@dotneck335: Good idea !! I just wrote him to see if it's possible to split it into two, 4-Band. I need two channels anyway.
Another benefit is that I know it will work. A ready made board far outweighs the hassle of adaptation.
Another benefit is that I know it will work. A ready made board far outweighs the hassle of adaptation.
@Osvaldo de Banfield: No tapers indicated anywhere. It wouldn't matter for the dual gang, since they will be eliminated with resistors after tuning. Fortunately, my application is mono. Some designs use W and/or C tapers, which is undesirable, especially for dual pots.
But thos pots are who set the frequency of the filters, do you need a fixed filter?, then go to a simpler design IMO.
@Osvaldo de Banfield: I was using a Baxendall, which I finally got to work fairly well by using the unused back half of the Treble pot to influence the Mid-range. But the need now is to divide the Mid-range in half for better control. Rod Elliot's Project 84 will work except that it has to be split in two, in addition to component value substitution. I was also considering expanding the traditional Baxendall, also but will require extensive modification to the boards available.
Yeah. I have my own 3 band Bax and works fine. The board has about 15 years and has broken repairing the pots. "Industria Argentina" (made in argentina) a thrash.
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There are many on eBay but you will have to wire the sliders to the board.
Che por qué puteás la Industria Argentina?
Mes siento atacado personalmente.
A parametric eq that I have from an old popular electronics article by john Roberts is two band, it has a “q” or bandwidth control which sets it apart from the design you posted
"@dotneck335: Good idea !! I just wrote him to see if it's possible to split it into two, 4-Band. I need two channels anyway. Another benefit is that I know it will work."
YES! I have built many of Rod's designs and they always work!! Rather than try to split it into two, just buy TWO of his boards (supports one of the VERY BEST resources we have here at DIYaudio), and put in as many filters as you need.
YES! I have built many of Rod's designs and they always work!! Rather than try to split it into two, just buy TWO of his boards (supports one of the VERY BEST resources we have here at DIYaudio), and put in as many filters as you need.