Hello. I am trying to modify the 6 band EQ at http://www.generalguitargadgets.com/index.php?option=com_content&task=view&id=46&Itemid=26 to be a seven band. I primarily play bass, so I want to use their lower range (31.3hz) up through about 2khz. I read the frequencies of guitar notes, and if I'm not mistaken, 1300hz is around the highest note guitars can play. I've read where a lot of people use 6khz in their EQ's which has confused me, so if anyone can clear that up, I would be really thankful.
Also, I'm having some trouble reading the schematic. I need to work in a tl071 (I only bought seven 5k pots, so a tl072 would be a waste, right?), and I'm having a hard time figuring out how the signal path works. It seems to me that the 5k pots change whether or not the majority of the output of each opamp is going to the cut or boost sections, but it also looks like the sound travels from the cut section to the inputs of each opamp as well. This isn't exactly crucial for me to understand before I work in another opamp (mostly with jumper wires, I'm assuming), but it is sure confusing the hell out of me. If someone could walk me through the signal path, I would really appreciate it.
Another thing: Is there any point to making this circuit true bypass? It seems like I'm building the eq because I want to use it all the time, not some of the time. Any thoughts on that?
Thanks in advance.
If anyone can point me to other posts, that would be great too. I don't need a whole response if it has already been said.
I'm not the best person to help you with the electronics side of things, others could give you a much better answer than myself..
but in terms of your statement "I read the frequencies of guitar notes, and if I'm not mistaken, 1300hz is around the highest note guitars can play. I've read where a lot of people use 6khz in their EQ's which has confused me, so if anyone can clear that up, I would be really thankful." -
The highest normally tuned guitar note may be at 1300hz (I haven't verified this but I'm assuming you have checked or know), but this is only the fundamental frequency. Instruments don't produce sine waves, the produce a sound wave with a fundamental frequency (1300hz = 1.3khz in your case), plus added harmonics. These harmonics don't necessarily determine the main pitch of the sound, but they change the sound (timbre, if you like). Like you might find a bright sounding acoustic guitar's high E string is the same fundamental frequency as an electric guitar with the treble rolled off - but the acoustic's harmonics might extend strongly up to 10khz or so, lending it a brighter sound. In comparison, the same note on the electric has the same fundamental, but the harmonics may start becoming very weak around 4 or 5khz. So an EQ that goes up to 6khz will have more control over this brightness, even though it's well above the highest fundamental frequency you're likely to encounter.
Sorry if I over-explained... but in the same respect, if you need further clarification just yell out.
Thanks
Jonathan
Ok, I will walk you through the circuit.
Graphic EQ's are pretty simple.
Signal goes in and is boosted by the first opamp, with a 250K pot for gain.
Then it goes through a 10K resistor and a 15pF low pass filter to filter out UHF, this helps the EQ sections by minimising bandwidth the opamps have to deal with.
Next we have a bunch of opamps set up as gyrators to simulate inductors, this forms an LCR (inductor capacitor resistor) filter. The center frequency of the LCR filter controls the frequency of boost/cut.
The pots are attached between the + and - inputs of the output opamp. Putting the wiper at one end lowers the gain at the center frequency of the simulated LCR filter, at the other end it increases the gain at the center frequency.
The filtered signal is then buffered through the output opamp to the output.
Some Maths.
Center Frequency(hz) = 1/(2*pi*SQRT(L*CA))
L = 1K*220K*CB
I will have a similar EQ project up on my page soon but using transistors for the gyrators instead of opamps.
As has been danced around here, EQs have multiple purposes. Certainly one of them IS to adjust the relative levels of note fundamentals such that no single note stands out too much. At the same time, EQs are also intended to:
a) adjust the relative contribution of harmonic content of certain ranges of fundamentals
b) adjust the harmonic content and fundamentals to compensate for speaker characteristics
c) adjust the harmonic content and fundamentals to compensate for room characteristics
d) adjust the harmonic content and fundamentals to compensate for acquired noise of different types
So, even if your guitar doesn't go down to 60hz, the hum you may have picked up does. And even if your guitar does not go out to 5khz, the hiss you may have picked up does. And even if there are no wolf-tones whatsoever on your guitar, the speaker or cab may well have an incorrigible resonance in the 150hz zone or the 3khz zone.
There are many ways for the signal to be "unequal".
Using the capacitors values on the schematic, what makes most sense for a bass guitar? I bought seven pots (a little dumb in hindsight) so I can do seven different frequencies. Bass is my main instrument, so it will be the focus. I'm assuming the seven lowest are probably the right ones, but I'll hold off on building it until I get some more information from people who actually know what they're doing.
Would it be outside of my capabilities (seven focus frequencies) to also make this accessible for guitar?
Quote from: nelson on September 02, 2008, 12:42:21 PM
I will have a similar EQ project up on my page soon but using transistors for the gyrators instead of opamps.
If it uses similar potentiometer values, I might wait to see what your ideas are. I just figured this one was the simplest to build. How soon do you expect to have that up?
Thanks for all the great help, I understand this a lot better now.