Does anybody know of a pcb for the following circuit?
(http://img54.photobucket.com/albums/v165/rickydon/simple_parametric_eq.gif)
(Based on http://geofex.com/Article_Folders/EQs/paramet.htm)
Any Diptrace/Eagle wizzard interested in doing a layout for oshpark?
Preferably with a quad opamp like the TL074. The fourth opamp could be used to buffer Vb.
Or if it saves space then one could replace the first opamp with a JFET or bipolar transistor and go with a dual opamp.
I don't use Diptrace or Eagle, but I could probably put this one together in KiCAD in an afternoon. It's pretty simply and I've been playing with the very similar graphical EQ circuits lately.
You probably don't need to buffer the Vref on this one and the op-amp in the simulated inductor circuit could be replaced with a transistor. I'd need to test how much that would alter things, it has a bit of a change on how sharp the peak can be. I'd think though that you could get this to a single dual op-amp and one transistor easily.
Quote from: RobA on September 24, 2014, 04:03:40 PM
I don't use Diptrace or Eagle, but I could probably put this one together in KiCAD in an afternoon. It's pretty simply and I've been playing with the very similar graphical EQ circuits lately.
You probably don't need to buffer the Vref on this one and the op-amp in the simulated inductor circuit could be replaced with a transistor. I'd need to test how much that would alter things, it has a bit of a change on how sharp the peak can be. I'd think though that you could get this to a single dual op-amp and one transistor easily.
Thanks for your answer!
Hm, I thought if you use a transistor for the gyrator then you can't have the Q-pot?
Edit: OK. Maybe that impression was wrong. That pot is only damping the peak anyway, so you can apply it (or leave it off) in both situations, I guess.
I think it's possible to have the Q control with a transistor there. When I was playing with these circuits a bit ago, the transistor versions just had a bit less Q to start with compared to and op-amp version. I think it has to do with the op-amp buffering and ability to be a good unity gain follower. Another possible consideration is noise. At very high Q, a really good op-amp could have less noise and these circuits can be a bit noise prone.
Another problem with these as parametric EQ's is that the Q varies with frequency. So, how good this particular type of parametric is depends on what you want to use it for. There are other designs that take a couple more op-amps and some dual-pots, but they would be better as a generally useful one band EQ.
Where you looking for an Eagle or Diptrace layout in particular, or is getting the PCb up on OSH Park the important part?
Typically I'd use a SVF based design (like http://experimentalistsanonymous.com/diy/Schematics/OOP%20Japanese%20Electronics%20Book/parametric-eq.gif) which requires one of these infamous dual reverse log pots.
But I'd also like a smaller design to put in any dirt box at the input (for example turn a Big Muff Pi into more of a Deluxe BMP) or output, with pots or fixed values.
Sorry, forgot to mention KiCAD wizzards... Any way to a PCB would be great. :)
Even better would be to have the option of an additional treble shelf (like in a tube screamer) and this would be much more flexible than what I often use now: a resonant lowpass where I often miss the possibility of decoupling the resonance from the treble roll off.
Quote from: kothoma on September 25, 2014, 12:40:08 PM
Typically I'd use a SVF based design (like http://experimentalistsanonymous.com/diy/Schematics/OOP%20Japanese%20Electronics%20Book/parametric-eq.gif) which requires one of these infamous dual reverse log pots.
But I'd also like a smaller design to put in any dirt box at the input (for example turn a Big Muff Pi into more of a Deluxe BMP) or output, with pots or fixed values.
Sorry, forgot to mention KiCAD wizzards... Any way to a PCB would be great. :)
Even better would be to have the option of an additional treble shelf (like in a tube screamer) and this would be much more flexible than what I often use now: a resonant lowpass where I often miss the possibility of decoupling the resonance from the treble roll off.
OK, I've got the picture of what you are looking for then.
A couple of questions: Board mounted pots? If so, size and configuration? Do you have a limit on the size of PCB to be useful?
On the high shelf, it's really easy to add into the circuit. You could put a pot in for the slope of the high shelf. It works but isn't ideal. I had an idea when playing with this that kinda makes the really simple high shelf a bit more useful. If you put the cap in a capacitive multiplier circuit, you can tune the frequency of this too. It does add another op-amp to the circuit. I breadboarded the idea and remember it working pretty well. I could test that idea again if you want to put it in the circuit.
Quote from: RobA on September 25, 2014, 01:07:07 PM
A couple of questions: Board mounted pots? If so, size and configuration? Do you have a limit on the size of PCB to be useful?
Aiming for a utility board that could fit many situations off-board wired pots would give the best flexibility.
I imagine that the Q often is wired to a fixed value instead of a pot.
As for size: the smaller the better ;)
Let's say if the regular dirt pedal is of 1590B size then adding the eq should not exceed a 1590BB box.
Could the eq fit in a box of size 1590A?
Quote from: RobA on September 25, 2014, 01:07:07 PM
On the high shelf, it's really easy to add into the circuit. You could put a pot in for the slope of the high shelf. It works but isn't ideal. I had an idea when playing with this that kinda makes the really simple high shelf a bit more useful. If you put the cap in a capacitive multiplier circuit, you can tune the frequency of this too. It does add another op-amp to the circuit. I breadboarded the idea and remember it working pretty well. I could test that idea again if you want to put it in the circuit.
So this would essentially work like a variable slope lowpass filter on the left half of the cut/boost knob? That would be fantastic!
I've got a lull in projects for a couple of days, so I can do a bit of breadboarding and layout on this now. What's the feature set for the whole thing? 1590A is probably going to be a tradeoff between features and size. I'd say that just a three control 1-band PEQ could be done pretty easily. But, and here's where it gets a bit interesting, because of the topology of the controls and bands, it's pretty easy to set this up so that one PCB gives multiple options. You could have a single board that could support one or two PEQ bands, a PEQ band and a high shelf, etc. The slick part is that if you don't want that band, then you don't populate that section of the PCB and it won't impact the sections of the circuit you do want at all. The same is true of setting a fixed Q or having a Q control in place, fixed frequency, ...
I mentioned the possibility of making the high shelf a movable frequency. I haven't tried that with a transistor as the buffer for the capacitive multiplier. I'd need to breadboard that for sure. Using the transistors would be helpful for making the thing modular. There is another possibility with just putting three cap selections on a toggle. This could be just as useful for what you are looking for. I'm guessing from what you mentioned, that you are looking to do some pickup emulation with the EQ. If that's the case, then doing the high shelf with the frequency selection caps on a toggle and a variable slope could be useful. Although, some testing with the variable slope would be needed to see how much we can really get from it and how much it adds to what's already available from the boost/cut pot.
The version with one three control PEQ and one two knob, one toggle high shelf (with boost/cut pot, variable slope, and frequency selector toggle) might be doable on a PCB that could fit in a 1590A with selecting just the controls you want but still make it useful in a bigger build as a utility board. This version should only need one dual op-amp and a single transistor (and the passives).
How's this sound? Anything that definitely needs to go in or out of the idea?
This sounds fantastic. And I really appreciate the time you (have already) put into this.
The main thing would be controlable mids. It's usually good to have mid boosts before distortion, and mid cuts after.
As for the treble cut/boost this would be a bonus. Both can be useful before and after distortion, depending on the desired result. (Personally it's usually treble cut on either side for me.) A few switchable caps indeed may be all what is needed. Hm, variable slope (varying the R in the RC path to Vb) may not be too important as this overlaps somewhat with the boost/cut function anyway, not sure about that. All this wouldn't take much space on the board, the bigger part beeing pads for pots and a switch.
So that's probably the way to keep this thing small and still usable enough. I'm a bit bothered about the possible sweep range. If I remember right all designs I have seen so far seem to stick to a 10:1 ratio for C1:C2. But Keen uses other ratios. And he states "We only get about a 4.5:1 range". Maybe I need to wade through all the formulas he gives...
Edit: On the other hand, 400...1800 Hz should cover enough mids.
(Edit2: This http://sound.westhost.com/p28_fig1.gif shows (besides a treble shelf) how to turn a peak into a (bass) shelf. I wonder if something similar simple exists for treble?)
This is one of the projects I've had sitting in the back of my head and in the notebook for some time now. It's good to get motivated to actually work on them. My eventual goal is to get it into a guitar body, so I've got to figure out what's really needed.
I think the ESP schematic with the low shelf switch requires a dual power rail for the op-amp. When you switch in the shelf toggle, it's got a non-capacitive path to ground. Though, it might work to rearrange the whole thing to center around a Vref.
You could do a similar thing with the treble/high-shelf and you wouldn't have the problem with the ground path since there will always be the top cap in place. But, toggling to high shelf mode will take the op-amp or transistor completely out of the signal path, so I think it would need a DPDT so that you could ground the input of the amplifier. Pretty much, you'd want to toggle to a completely new path past the caps. It would work though, and you'd make one level pot do double duty. I need to pull out my notes on using the capacitive multiplier idea to move the high shelf frequency, but it might be possible to work that into the system for toggling between peak and shelf.
The thing with the ratio's and frequency sweep is that the caps essentially set the max Q (with the resonance pot at zero) and then the Q changes with frequency. So, you have to limit the frequency range to keep the Q in a reasonable range and figure how big you can let Q get in one range to make it usable across the whole sweep. I think there's some room to play here, depending on the usage scenario.
Quote from: RobA on September 27, 2014, 02:50:35 PM
This is one of the projects I've had sitting in the back of my head and in the notebook for some time now. It's good to get motivated to actually work on them. My eventual goal is to get it into a guitar body, so I've got to figure out what's really needed.
That's a nice goal. It could add new dimensions to a guitar. (The only question is: how many knobs can a guitar bear...)
Quote
I think the ESP schematic with the low shelf switch requires a dual power rail for the op-amp. When you switch in the shelf toggle, it's got a non-capacitive path to ground. Though, it might work to rearrange the whole thing to center around a Vref.
Not sure I get that. If you added that switch to the split-rail design, wouldn't that have the same effect, Vref being "relative ground"?
Quote
You could do a similar thing with the treble/high-shelf and you wouldn't have the problem with the ground path since there will always be the top cap in place. But, toggling to high shelf mode will take the op-amp or transistor completely out of the signal path, so I think it would need a DPDT so that you could ground the input of the amplifier. Pretty much, you'd want to toggle to a completely new path past the caps. It would work though, and you'd make one level pot do double duty. I need to pull out my notes on using the capacitive multiplier idea to move the high shelf frequency, but it might be possible to work that into the system for toggling between peak and shelf.
That would give a nice circuit. It could be customised to all sorts of specialized eqs, or one super flexible yet compact one. But I somehow have the feeling that we are talking about two (related but different) projects simultaneously...
Quote
The thing with the ratio's and frequency sweep is that the caps essentially set the max Q (with the resonance pot at zero) and then the Q changes with frequency. So, you have to limit the frequency range to keep the Q in a reasonable range and figure how big you can let Q get in one range to make it usable across the whole sweep. I think there's some room to play here, depending on the usage scenario.
OK, this confirms my gut feeling about the Q of the filter. I think the exact Q isn't critical. No extreme Q should be needed and a bit of change in Q would be acceptable, may even be musical (if it should turn out to change in the right direction...)
Perhaps we should not restrict the discussion to this gyrator based design? There are other bandpass filters that could be used (multiple feedback bandpass, inverting bandpass filter aka active Wien bridge bandpass). They all would need dual pots ideally but you can get away with making only one resistor variable and accept a bit more change of Q along the sweep.
Quote from: kothoma on September 27, 2014, 05:11:14 PM
[...]
Quote
I think the ESP schematic with the low shelf switch requires a dual power rail for the op-amp. When you switch in the shelf toggle, it's got a non-capacitive path to ground. Though, it might work to rearrange the whole thing to center around a Vref.
Not sure I get that. If you added that switch to the split-rail design, wouldn't that have the same effect, Vref being "relative ground"?
Yes, this should work. You just need to be a bit careful about the details and how you couple to previous stages (if they are there).
[...]
Quote
Quote
The thing with the ratio's and frequency sweep is that the caps essentially set the max Q (with the resonance pot at zero) and then the Q changes with frequency. So, you have to limit the frequency range to keep the Q in a reasonable range and figure how big you can let Q get in one range to make it usable across the whole sweep. I think there's some room to play here, depending on the usage scenario.
OK, this confirms my gut feeling about the Q of the filter. I think the exact Q isn't critical. No extreme Q should be needed and a bit of change in Q would be acceptable, may even be musical (if it should turn out to change in the right direction...)
Perhaps we should not restrict the discussion to this gyrator based design? There are other bandpass filters that could be used (multiple feedback bandpass, inverting bandpass filter aka active Wien bridge bandpass). They all would need dual pots ideally but you can get away with making only one resistor variable and accept a bit more change of Q along the sweep.
I think it would be possible to rig it so that the Q would be maximum at a particular point in the frequency sweep and then fall off from there in either direction. Maybe it could be done with the peak close to one edge or the other.
The weak spot of the gyrator version looks to me to be the Q control pot. It's not as powerful as other filter topologies. But, some of the advantages for this particular application are pretty nice -- like being able to drop one band or adding the shelving toggles, etc. without disturbing the overall layout.
If you have or can install LTSpice, I can do a simulation that'll let us look at the frequency responses for various settings and that might help guide the choices.
Quote from: RobA on September 27, 2014, 09:38:01 PM
If you have or can install LTSpice, I can do a simulation that'll let us look at the frequency responses for various settings and that might help guide the choices.
OK, just downloaded and installed LTspiceIV and successfully opened one of the example files.
I was tempted to do so in the past but always chickened out. Schematics done in LTspice always looked different and scary.
Good opportunity to finally jump right in.
I have only seen this now, so I may be a little late to the discussion ;)
I already have a 1590a design and some pcbs lying around as I recently did a custom build for a bass player using a single param eq as discussed here.
Its an OpAmp design based on RG Keens idea. As a standalone device it needed an additional adjustable gain stage.
I will post some details later.
Quote from: kothoma on September 28, 2014, 08:08:34 AM
Quote from: RobA on September 27, 2014, 09:38:01 PM
If you have or can install LTSpice, I can do a simulation that'll let us look at the frequency responses for various settings and that might help guide the choices.
OK, just downloaded and installed LTspiceIV and successfully opened one of the example files.
I was tempted to do so in the past but always chickened out. Schematics done in LTspice always looked different and scary.
Good opportunity to finally jump right in.
The schematic capture is a bit weird and limited. The standard parts are also limited, but OK for most of the stuff needed for basic modeling for effects circuits. The bigger issues are knowing the tricks to getting Spice to behave -- and I certainly don't know enough of those. Still, it works alright for getting ideas. I wouldn't trust anything without breadboarding and listening first though.
I adapted another circuit I had modeled to match the one from the RGK graphic you linked above. It's at http://rock.it-frog.com/Downloads/Code/PEQ_2Band_OPA.asc (http://rock.it-frog.com/Downloads/Code/PEQ_2Band_OPA.asc)
There are three sections in there. The first two are what the PEQ section would look like in either low shelf or PEQ mode. The third one is a high shelf made so that it can be frequency swept using a capacitive multiplier circuit.
NotesPots: I haven't seen pots in LTSpice (I could have missed them easily if they are there), so I model pots as resistor divider networks or just as a resistor for variable resistor pots. So, R2 and R22 form one pot. R1 is the resonance pot. Notice that R1 is set to 1Ω. That's because you can't set a resistor to 0Ω in LTSpice and have the simulation work. R26 is a combination of the frequency pot and the 51k resistor in the above schematic. (I used 47k) So, you can vary the frequency by moving this resistor from 47k to 1047k.
High shelf: C4 sets the base frequency in combination with the 8.2k resistors and R7. R5 is a variable pot and a 470Ω resistor combined. The total capacitance of the section is C4 * R5/R8 and that will move the two corner frequencies for the high shelf. R7 is also a 470Ω resistor in combination with a pot. The shelving filters are a bit weird because the slope is really set by moving the upper and lower corner frequencies and these are influenced by the values of the gain network resistors (R29 and R13) as well as the caps and resistors in the shelf filter network. They are a bit fiddly to work with. I went to the 8.2k resistors to get a range for these that's easier to work with.
PEQ to High shelf?: If you look at the high shelf and the PEQ sections, they are really pretty close to each other. Basically, you need to flip the position of R5 and C4 and then add a cap at the top above R7. The problems here are that I think that'll take a 3PDT and the pot that is part of R5 is very different from the high shelf to the PEQ (10k to 1M or so). So, if you want to do a combo high shelf and PEQ, I think it would need to go to a simple shelf with some fixed capacitor values on toggles.
If you happen to use Python, here are a couple of functions I use in an interactive session to look at cap values to play with. the caps function returns the two caps needed to match a desired Q and frequency. The second function returns the range of frequency and associated Q from the extreme settings of a 1M frequency pot.
from math import *
def caps(q, f):
c1 = 1.0/(q * 2.0 * pi * f * 470.0)
c2 = q/(2.0 * pi * f * 47.0e3)
return c1, c2
def f_q(c1, c2):
q2 = sqrt((c2 * 47.0e3)/(c1 * 470.0))
q1 = sqrt((c2 * 1047.0e3)/(c1 * 470.0))
f2 = 1.0/(2.0 * pi * sqrt(470.0 * 47.0e3 * c1 * c2))
f1 = 1.0/(2.0 * pi * sqrt(470.0 * 1047.0e3 * c1 * c2))
return f1, q1, f2, q2
The Q value for the PEQ is proportional to the square root of the frequency pot setting (plus the base 47k resistor), so it increases (a lot) as the frequency goes down. That's kind of the opposite of what I'd want it to do and puts a restriction on how high you can set the Q on the high end. The resonance control can help offset that though, so including it will give you a broader range of usable Q.
Hey, thanks for the file and the detailed explanations.
So I open the file, start Simulate/Run, probe the output and get two curves. I guess one is the fequency response and the other one is the phase? The third (high shelf) section is fully ccw (dialed to the + side of the opamp) on the R15/R20 pot, that is maximum cut. The other two are neutral. So the solid line is the frequency response and shows a treble cut, right?
The similarity/duality to a gyrator (and low shelf) is stunning. Maybe you can't make it switchable, but the board could be designed to choose either version. So with two sections you could build a two-band parametric eq, or a bass/treble tone control with sweepable working points, or any mixture of these. Or you only polulate one section. Nice. Now is the question if the three lower opamps can be replaced by transistors to support the flexibility.
Hm, how do I change values of components? [right click]
Yes the second line is the phase response and you've got what the controls are setup like correct as well.
Where you run into trouble with the transistor version is the emitter-ground connection. This usually goes through a smallish (1k-to-10k or so) resistor. The problem is that this connection also hits in the feedback path of the buffer through the 470Ω resistor. That causes a problem because you then have the path from ground to the inverting op-amp input of the main op-amp and that screws with the Vref. Maybe you could put a large value cap of say 22µF between the emitter resistor and ground. It'd alter the frequency response some, but maybe not enough to matter.
The selection of shelf versus PEQ can be made with jumpers. That just leave the option of not including a section to figure out. The op-amps are already setup as unity gain buffers, so if a section is not going to be used, we just need to connect the non-inverting input to ground and then it's safe to leave off that band. I think that should make it so that it can be done with a dual op-amp and no real downsides.
The low shelf/PEQ selection could be put on as a toggle. If the toggle is left off, it defaults to PEQ. To select a fixed low shelf, you just leave off the toggle and jumper the cap.
Another possible option is to jumper the frequency pots or resonance pots of either section.
It should make for a pretty flexible little board -- if it can be made small enough. Using SMD op-amps might help, but it's probably not going to get a whole lot of space savings. If these ideas sound good, I can do a layout and see how small it can be.
I think you've sorted out the switching/jumping options allright.
So two dual op amps then instead of one quad? More flexibility in the layout?
Now I'm really curious how small this can be made! And I like the 2-for-1 deal.
Quote from: kothoma on September 28, 2014, 03:06:49 PM
...
So two dual op amps then instead of one quad? More flexibility in the layout?
Good question. I tend to like duals over quads because of more options available for types of op-amps and selecting for purpose. It also does help in doing some layouts. In this case though, I can't think of any reason to not use the same op-amp in each position and since it'll all be off board pots, there probably won't be much help from using duals for the layout.
I'll try it with a quad and PTH first and see what I can do with that.
But, I'm going to do a bit of breadboarding first. I'll let you know if that verifies the concepts.
Not sure if you want all the fun for yourself, so I added a spoiler :-)
Only the boost/attenuate and the volume pot are outside. Q and Freq adjustment are inside via trimmers.
SPOILER
-----------------------------------
Schematic & Layout:
http://diy.thcustom.com/wp-content/uploads/2014/09/1-band-parameq_V1.0.pdf (http://diy.thcustom.com/wp-content/uploads/2014/09/1-band-parameq_V1.0.pdf)
One of the prototypes:
No spoiler at all, Thomas. Nice to see how it could be done. Using the fourth opamp as a booster can be useful in many situations. Your layout shows that there's not much space to waste.
Our litte discussion here has slowly drifted towards a 2-band parametric EQ. Will you make your board available in your shop? It could come in handy.
Quote from: RobA on September 28, 2014, 04:35:10 PM
But, I'm going to do a bit of breadboarding first. I'll let you know if that verifies the concepts.
Thanks, and take your time. Meanwhile I'm having great fun playing with values in your LTspice model!
Quote from: kothoma on September 28, 2014, 05:52:50 PM
No spoiler at all, Thomas. Nice to see how it could be done. Using the fourth opamp as a booster can be useful in many situations. Your layout shows that there's not much space to waste.
Our litte discussion here has slowly drifted towards a 2-band parametric EQ. Will you make your board available in your shop? It could come in handy.
I will list the latest board revision (including volume boost) in the shop next week.
If you want one (or two) of the first revision (without volume boost) let me know and I will just send it to you.
Quote from: Thomas_H on September 28, 2014, 06:11:41 PM
If you want one (or two) of the first revision (without volume boost) let me know and I will just send it to you.
PM'd. Thanks a lot!
Quote from: Thomas_H on September 28, 2014, 05:28:19 PM
Not sure if you want all the fun for yourself, so I added a spoiler :-)
Only the boost/attenuate and the volume pot are outside. Q and Freq adjustment are inside via trimmers.
SPOILER
-----------------------------------
Schematic & Layout:
http://diy.thcustom.com/wp-content/uploads/2014/09/1-band-parameq_V1.0.pdf (http://diy.thcustom.com/wp-content/uploads/2014/09/1-band-parameq_V1.0.pdf)
One of the prototypes:
The volume control is a good idea. I've noticed while playing with the graphical EQ circuit, that there are definitely EQ settings where you could use some volume boost or cut to keep the overall perceived volume where you want it. I'm not sure about the right way to do this for a Swiss army knife utility board though. In many cases, it'll be handled be the volume control of the main circuit, so it'll need to be optional. I'll think about this. It could be that a simple BJT output gain stage with a preceding volume pot could do the job. If you want to skip the volume section, you leave off all the associated parts to the amplifier and take the output of the volume pot input pad. If you do want the volume, you take it from the output of the final cap. That would add more size to the PCB though.
Quote from: kothoma on September 28, 2014, 05:54:22 PM
... Meanwhile I'm having great fun playing with values in your LTspice model!
I've found that it can be a really useful tool for getting ideas worked out even with its limitations. It can be fun to play with too.
Quote from: RobA on September 29, 2014, 04:37:52 PM
The volume control is a good idea. [...] there are definitely EQ settings where you could use some volume boost or cut to keep the overall perceived volume where you want it. [...] It could be that a simple BJT output gain stage with a preceding volume pot could do the job. [...] That would add more size to the PCB though.
Hm, how about avoiding an additonal gain recovery stage by making the input buffer (optionally) into a small (fixed) gain stage and only tack on the volume pot at the end?
Quote from: kothoma on September 29, 2014, 04:49:18 PM
Quote from: RobA on September 29, 2014, 04:37:52 PM
The volume control is a good idea. [...] there are definitely EQ settings where you could use some volume boost or cut to keep the overall perceived volume where you want it. [...] It could be that a simple BJT output gain stage with a preceding volume pot could do the job. [...] That would add more size to the PCB though.
Hm, how about avoiding an additonal gain recovery stage by making the input buffer (optionally) into a small (fixed) gain stage and only tack on the volume pot at the end?
That's a possibility and I'll give it a test too. The concern I have with doing it that way would be that adding the gain before the EQ and the cut after the EQ might cause distortion in the EQ op-amp with some boost settings.
I've done a bit of breadboarding on this now, and it does look like all the modular possibilities will work. The one thing that's still a bit of a question is where to put the gain. The cleanest place circuit wise is to add a bit of gain on the first buffer op-amp. But, you've got to be pretty careful here. Using the TS922, I can push a gain of about 6 times without distortion, that's using some pretty hot P90's. OPA2134's had distortion at a gain of around 3 times, but they tend to not be too happy in single-supply circuits to begin with. I'd say that if you only want a gain of two to four or so, this would be an OK spot to do it. (BTW, without distortion means I can push a really high Q on the PEQ band and peg the high shelf and have the added gain from the first stage with out distortion.)
If you are driving the EQ hard from a preceding stage, it could end up with trouble, but then you probably wouldn't want any gain here to begin with. The cute thing about doing the circuit this way is that if you jumper the feedback resistor and leave off the resistor and cap to ground, then you've got a straight unity gain buffer and everything is cool.
I'm going to think about the circuit just a bit more and then I'll layout a PCB for it and we can give it a try.
Quote from: RobA on October 02, 2014, 03:15:38 AM
I've done a bit of breadboarding on this now, and it does look like all the modular possibilities will work. The one thing that's still a bit of a question is where to put the gain. The cleanest place circuit wise is to add a bit of gain on the first buffer op-amp. But, you've got to be pretty careful here. Using the TS922, I can push a gain of about 6 times without distortion, that's using some pretty hot P90's. OPA2134's had distortion at a gain of around 3 times, but they tend to not be too happy in single-supply circuits to begin with. I'd say that if you only want a gain of two to four or so, this would be an OK spot to do it. (BTW, without distortion means I can push a really high Q on the PEQ band and peg the high shelf and have the added gain from the first stage with out distortion.)
If you are driving the EQ hard from a preceding stage, it could end up with trouble, but then you probably wouldn't want any gain here to begin with. The cute thing about doing the circuit this way is that if you jumper the feedback resistor and leave off the resistor and cap to ground, then you've got a straight unity gain buffer and everything is cool.
I'm going to think about the circuit just a bit more and then I'll layout a PCB for it and we can give it a try.
That's good news. I'd say the gain is only needed to allow for unity gain not as a boost per se. So it's mostly needed on heavy cut settings. At least it's a nice option to have that can easily be left off as you said. A gain of up to 4 times should suffice. And one could always add a little booster board at the end if needed. I think you're on the right way. I'm really excited to see how much space all this will take.
Quote from: kothoma on October 02, 2014, 08:22:34 AM
... I'm really excited to see how much space all this will take.
Well, I'm going to do a bit more playing with the layout, but right now, it's looking like 35mm by 45mm as a lower limit. I might actually be able to save a bit more space by deleting all the unneeded pot connections, but it isn't likely to get much smaller than that using PTH parts. I could probably get smaller by going to SMD resistors and op-amp and using a mix of SMD and PTH caps. But even there, the number of off board connections is a limiting factor. One good thing with going to some SMD parts is that it opens up the use of the TS924 which is only available easily in SMD now.
What do you think? How open are you to using SMD parts? Is the size OK if I can get it to around 40mm by 35mm using PTH parts?
I think this size is quite acceptable. Compare it to the number of pots that could be attached to it.
And at least two of them (frequency, cut/boost) would always be present or one would choose a simpler circuit.
I'm not a big fan of SMDs. As yet. SMD JFETs are somewhat unavoidable but soldering them makes me sweat. Maybe I could work my nerve up to ICs. But those little resistors are really intimidating.
How about SO14 pads in parallel within DIP14 pads?
Quote from: kothoma on October 07, 2014, 04:47:38 PM
I think this size is quite acceptable. Compare it to the number of pots that could be attached to it.
And at least two of them (frequency, cut/boost) would always be present or one would choose a simpler circuit.
I'm not a big fan of SMDs. As yet. SMD JFETs are somewhat unavoidable but soldering them makes me sweat. Maybe I could work my nerve up to ICs. But those little resistors are really intimidating.
How about SO14 pads in parallel within DIP14 pads?
OK, I'll try to stick with PTH and maybe add the embedded SO-14 into the DIP-14 if I can make it work and not cause the size to increase.
The SO sizes really aren't too bad once you get used to them. I've been avoiding the really tiny resistors and caps. I use the 1206 size which then also works with the 0204 mini-MELF resistors which I prefer in the audio paths. The size of those isn't too bad at all. But you do have to develop a technique with the MELF's or they tend to roll away and disappear.
Hm, looking at this
(http://s002.radikal.ru/i199/1010/7f/02ee554d7d65.jpg)
it's probably not possible to get it inside. At least it would be a tight fit.
Quote from: kothoma on October 08, 2014, 08:00:22 AM
Hm, looking at this
(http://s002.radikal.ru/i199/1010/7f/02ee554d7d65.jpg)
it's probably not possible to get it inside. At least it would be a tight fit.
No, you can't get the SOIC-14 inside the DIP-14. In the end, I did two different layouts. The first is all PTH and the second uses SMD IC, resistors and some caps. I left the electrolytic power cap and the actual filter caps as PTH though.
But, before I place the order for the test boards at OSH, it would be nice if someone checked over my schematic. There are some weird things that can happen with the high shelf/PEQ circuit. As was talked about above, you can make it as either a high shelf or a PEQ. There are two jumpers that select which one is going to be used. Then the rest of it is set by which parts are filled with a jumper and which with parts. There's blue text on the schematic that describes what needs to be done for each.
Here's the schematic,
http://rock.it-frog.com/Downloads/Docs/PEQ_Schematic.pdf (http://rock.it-frog.com/Downloads/Docs/PEQ_Schematic.pdf)
Quote from: RobA on October 11, 2014, 10:55:15 PM
Here's the schematic,
http://rock.it-frog.com/Downloads/Docs/PEQ_Schematic.pdf (http://rock.it-frog.com/Downloads/Docs/PEQ_Schematic.pdf)
I looked over it and compared it to the sources discussed and to the Spice sim. Looks good to me. I see you've omitted the pulldown resistors.
Hm, I wonder if the second band could be done with less jumpers (and space).
There's this sequence ---C11-+-R10---RV6--- for the PEG with the midpoint (-+-) going to the non-inverting input (pin 12).
For the shelf this sequence is simply reversed with the same midpoint (and probably the pot has to be wired the other way).
So if the geometry of the layout permits you only have to jumper (swap) the start and end points.
Just an idea.
Edit. Or you could replace this sequence with the following: ---Rx---Cx-+-Ry---Cy--- with the C pads doubling as pot pads and polulate them like this:
PEG: Rx = jumper, Cx = C11, Ry = R10, Cy = RV6.
Shelf: Rx = R10, Cx = RV6, Ry = jumper, Cy = C12. (plus C10 = jumper, of course)
Just a crazy idea.
Thanks for the input on those. I'll check it out and see about altering the layout and how much it would get in return.
I took a look at the changes you suggested. The first one would be good but it doesn't get any reduction in size and I kinda prefer being able to label the jumpers to easily identify which configuration they go with.
Your second idea is ingenious, but it doesn't mesh well with my OCDish need to have the pots lined across the top of the PCB. In other words, the two together (pots acting as caps, and sitting on the top row of the layout) would actually end up making it bigger.
I've got the two layouts in at OSHPark. They end up being this big:
PTH 43mm x 36mm
SMD 43mm x 24mm
The width is set by putting the pots and toggles across the top row. At this point, I'll probably order the PTH version this week and do some testing on it to see how well it works. I am going to do a little looking into other parametric EQ topologies first and see if it might be better to go another direction.
Thanks for the update. I guess you're right, with all the different options clear labels and easy access to the pads are preferable.
The layout sizes seem very acceptable to me. I'm curious to hear what your testing shows.
Other topologies: MFB bandpass, inverting (Wien bridge) bandpass, and SVF come to mind. Any others?
Quote from: kothoma on October 20, 2014, 12:37:33 PM
Thanks for the update. I guess you're right, with all the different options clear labels and easy access to the pads are preferable.
The layout sizes seem very acceptable to me. I'm curious to hear what your testing shows.
Other topologies: MFB bandpass, inverting (Wien bridge) bandpass, and SVF come to mind. Any others?
I was going to start looking at the MFB type first. I don't know of others yet, but I need to do some research. What I'd like to find is something that would play well with either an active tilt EQ or a Baxandall.
You're certainly aware of the following fine pages:
http://www.electronics-tutorials.ws/filter/filter_7.html
http://sound.westhost.com/articles/active-filters.htm
http://www.rane.com/note122.html
http://www.sound.westhost.com/project150.htm (Wien Bridge Based Parametric Equaliser)
http://sound.westhost.com/articles/state-variable.htm
Quote from: kothoma on October 20, 2014, 01:13:28 PM
You're certainly aware of the following fine pages:
http://www.electronics-tutorials.ws/filter/filter_7.html
http://sound.westhost.com/articles/active-filters.htm
http://www.rane.com/note122.html
http://www.sound.westhost.com/project150.htm (Wien Bridge Based Parametric Equaliser)
http://sound.westhost.com/articles/state-variable.htm
Thanks! I have seen most of them, but you saved me tons of time by making it so I won't have to search through my disastrous mess of bookmarks to try and find them again :D.
I keep forgetting to mention that I've verified the PTH version of the two band parametric EQ. I've shared the project on OSHPark now. I'll do a build doc for it soon, but the link is here,
https://oshpark.com/shared_projects/3W6nzzYI (https://oshpark.com/shared_projects/3W6nzzYI)
This project corresponds to the schematic that I linked to above. I was playing around with it a couple of days ago and it's actually pretty usable as a pedal by itself if you populate all the knobs and switches.
Many thanks for sharing your 2-band parametric EQ (http://rock.it-frog.com/Downloads/Docs/PEQ_Schematic.pdf)!
I think it turned out great and very much appreciate your hard work that made this happen.
I missed the development, but I've been looking for something like this. Thanks for sharing!