Hi all!
I built this distortion that I found at:
http://www.generalguitargadgets.com/richardo/distortion/index.html
It works great except for the tone control which does nothing that I can discern. I thought the value of the capacitor was a bit low for the cutoff to be in a reasonable range so I did the math and tried some other (larger) values, even as high as 0.1 uF but still it doesn't seem to do anything at all. I checked my wiring Several times to make sure it's all correct.
Here's the circuit with the tone control section circled in red:
It looks like it should work to me, but maybe I'm missing something. Any help or info about this would be greatly appreciated.
Thanks,
Ken
I built this distortion that I found at:
http://www.generalguitargadgets.com/richardo/distortion/index.html
It works great except for the tone control which does nothing that I can discern. I thought the value of the capacitor was a bit low for the cutoff to be in a reasonable range so I did the math and tried some other (larger) values, even as high as 0.1 uF but still it doesn't seem to do anything at all. I checked my wiring Several times to make sure it's all correct.
Here's the circuit with the tone control section circled in red:
An externally hosted image should be here but it was not working when we last tested it.
It looks like it should work to me, but maybe I'm missing something. Any help or info about this would be greatly appreciated.
Thanks,
Ken
Yes, I know it forms a low-pass filter. The problem is - it doesn't do anything. I replaced C6 with a 0.1 uF capacitor, which should produce a cutoff frequency of around 15 Hz but still no effect at all.
I will try the 10k resistor between the 10 uF and the top of the tone pot and see if that has any effect. Even if it doesn't I'll leave it in. The circuit needs the impedance there.
Thanks,
Ken
I will try the 10k resistor between the 10 uF and the top of the tone pot and see if that has any effect. Even if it doesn't I'll leave it in. The circuit needs the impedance there.
Thanks,
Ken
op amps tend to have very low output impedances, especially with feedback. you might get the impression from a data sheet that a particular op amp has a 5k output impedance. but when negative feedback is added, this figure drops to a few hundred ohms, because the negative feedback causes the op amp to compensate for the internal losses of the op amp. with a wider bandwidth op amp, this effect extends higher in the spectrum. so this circuit might have worked just fine with a RC4558, because the output impedance of a 4558 is high above 1khz. but it might not work properly with a NE5532, because the output impedance above 1khz is much lower.
Thanks Uncle Jed!
I hadn't thought about how various opamps may differ. I just thought the author accidentally left a resistor out.
I used a 741, because as the catalog says, "sometimes you don't want fidelity." lol! It works fine with the 10K in place.
I substituted a 0.047uF cap for the little 1nF in the design (at C6) for a bit more effect. Also, C4, at 50nF cuts far too high (about 3.2 kHz) - no guts at all. I put a 1uF cap (15 Hz rolloff) in its place and it sounds much better. Btw, both those values assume the Drive is all the way down. With it all the way up the 50nF rolls off at 31.5 Hz, and the 1 uF rolls off at 1.5 Hz. That's pretty low but I didn't think it would hurt anything. Seems to work okay.
I hadn't thought about how various opamps may differ. I just thought the author accidentally left a resistor out.
I used a 741, because as the catalog says, "sometimes you don't want fidelity." lol! It works fine with the 10K in place.
I substituted a 0.047uF cap for the little 1nF in the design (at C6) for a bit more effect. Also, C4, at 50nF cuts far too high (about 3.2 kHz) - no guts at all. I put a 1uF cap (15 Hz rolloff) in its place and it sounds much better. Btw, both those values assume the Drive is all the way down. With it all the way up the 50nF rolls off at 31.5 Hz, and the 1 uF rolls off at 1.5 Hz. That's pretty low but I didn't think it would hurt anything. Seems to work okay.
An old thread - but I have a question related to the overall effect of the treble rolloff of the cct posted at the top of this thread.
I have a 10k resistor fitted as suggested (between the Opamp output & the top of the variable resitor)- I'm using the same capacitor as taken from my guitar's original (passive) treble roll off circuit (unfortunately, I don't have the actual capacitor value to hand)...my issue is the treble rolloff heard when used in the above active cct is minimal in comparison to the amount of treble the same capacitor rolled off in the guitar's passive circuit? (I'm using a 22k variable resistor where R6 is in that cct)
So why is it....
Passive control cct - rolloff is substantial.
Active Opamp output - rolloff is very little
What can I do to get more treble rolloff in the above type of cct?
I have a 10k resistor fitted as suggested (between the Opamp output & the top of the variable resitor)- I'm using the same capacitor as taken from my guitar's original (passive) treble roll off circuit (unfortunately, I don't have the actual capacitor value to hand)...my issue is the treble rolloff heard when used in the above active cct is minimal in comparison to the amount of treble the same capacitor rolled off in the guitar's passive circuit? (I'm using a 22k variable resistor where R6 is in that cct)
So why is it....
Passive control cct - rolloff is substantial.
Active Opamp output - rolloff is very little
What can I do to get more treble rolloff in the above type of cct?
Many thanks....upping the capacitance helped (it seems there was something line a 4nf cap in there - 47nf has a lot more impact on shunt the treble away!)
I have a 22k *linear* pot in there at the moment (ie where R6 is in the diagram referenced further up this thread), all the rolloff seems to be in the final 3rd, which makes me think I should probably drop a 10K pot in there - question is would log or linear be better (for an audibly smoother tailoff along the whole of the turn - as opposed to just the last part of the turn).
Or would a 22k log probably do me alright?
I have a 22k *linear* pot in there at the moment (ie where R6 is in the diagram referenced further up this thread), all the rolloff seems to be in the final 3rd, which makes me think I should probably drop a 10K pot in there - question is would log or linear be better (for an audibly smoother tailoff along the whole of the turn - as opposed to just the last part of the turn).
Or would a 22k log probably do me alright?
Thanks - after I made my post, I dug around my component pile & actually found a 10k log...so I'll try that in tonight.
My logic goes that if there's 22k linera in there now, but I don't actually hear any treble rollof off until the last third of the pot rotation, then that must mean it's having an audible affect around the 7-8K region....therefore a 10K would seems a good option - but I don't know whether log or linear would be better with respect to hearing the the effect of the capacitor as the pot travels through its whole arc.
My logic goes that if there's 22k linera in there now, but I don't actually hear any treble rollof off until the last third of the pot rotation, then that must mean it's having an audible affect around the 7-8K region....therefore a 10K would seems a good option - but I don't know whether log or linear would be better with respect to hearing the the effect of the capacitor as the pot travels through its whole arc.
Filter pots should work similarly to volume pots for which a log taper is perfect, so let us know how it works. I'm also curious as to the value of the capacitor you're using.
(I found this link useful when trying to identify some with markings that were a bit arcane: http://www.kpsec.freeuk.com/components/capac.htm)
(I found this link useful when trying to identify some with markings that were a bit arcane: http://www.kpsec.freeuk.com/components/capac.htm)
Thanks for the URL - until yesterday I couldn't measure capacitance - but now I have a meter that does 
My circuit had been using the (passive) tone filter capacitor from another guitar I broke up - it didn't have much effect - it traspires it was only 4nf.
Late last night I tried a 47nf cap in there - it's much better!
What I have is a guitar with piezo pickups & magnetic pickups.....I can belnd actively between the two. The piezos have a lot of high frequency content in comparison to the magnetic pickups. What I've found is that the 47nf doesn't seem to have as much effect on the piezo output compared to the magnetic pickups. The magnetic sounds very muffled with the 47nf fully kicked in (as you'd expect - & as per a standard passive tone effect/sound), whereas with the piezos, the sound still seems very clear (hard to put this into words).
As, I'd like to have the option for a bit more effect on the piezos, I therefore intend upping the ante & trying say 80nf cap in there - probably not until over the weekend now though - I'll report back on this & the 10K Log pot results.
My circuit had been using the (passive) tone filter capacitor from another guitar I broke up - it didn't have much effect - it traspires it was only 4nf.
Late last night I tried a 47nf cap in there - it's much better!
What I have is a guitar with piezo pickups & magnetic pickups.....I can belnd actively between the two. The piezos have a lot of high frequency content in comparison to the magnetic pickups. What I've found is that the 47nf doesn't seem to have as much effect on the piezo output compared to the magnetic pickups. The magnetic sounds very muffled with the 47nf fully kicked in (as you'd expect - & as per a standard passive tone effect/sound), whereas with the piezos, the sound still seems very clear (hard to put this into words).
As, I'd like to have the option for a bit more effect on the piezos, I therefore intend upping the ante & trying say 80nf cap in there - probably not until over the weekend now though - I'll report back on this & the 10K Log pot results.
the reason the cap in your guitar has more effect in the guitar is that the guitar pickups have a high source impedance, like 50k or higher. the source impedance (even with the 10k resistor after the op amp) in the circuit above is pretty low compared to the pickup impedance. at a particular frequency, the cap might have a 5k reactance. in comparison to a 50k source impedance, that cuts the output 10db. with a source impedance of 10k, that's only a 3db cut.
quick rule of thumb if your resistor is 1/2 the value, double the capacitance. the time constant of the circuit is 0.67*R*C. the output impedance of the op amp going into the tone circuit is part of the RC network if it is connected directly to the capacitor. in the guitar the coil resistance and the coil impedance are both part of the time constant. since the coil impedance is a complex quantity which changes with frequency, generally guitar designers get close with a simple calculation using the coil resistance and the potentiometer resistance, then fine tune the value of the cap by trial and error. with op amps, the output impedance also changes with frequency, but in a very predictable manner, so a designer can usually figure out what value for the cap on paper.
next time i design and build an onboard preamp for a guitar, i'm going to include a Negative Impedance Convertor at the output. this is a device made with an op amp that counteracts the voltage loss with low impedance loads. so it does'nt matter if you plug in to a high impedance input (most amplifiers) or a low impedance (such as a low Z balanced input connected by an adapter without a matching transformer). but that will be AFTER the tone control circuit. driving a capacitor with an NIC would make an oscillator.
next time i design and build an onboard preamp for a guitar, i'm going to include a Negative Impedance Convertor at the output. this is a device made with an op amp that counteracts the voltage loss with low impedance loads. so it does'nt matter if you plug in to a high impedance input (most amplifiers) or a low impedance (such as a low Z balanced input connected by an adapter without a matching transformer). but that will be AFTER the tone control circuit. driving a capacitor with an NIC would make an oscillator.
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