Tone circuit with op amp

[icydash]

Hey guys, i renovated my previous tone circuit and added an op amp (LM741) in for gain. I also added in two switches which'll be what the DPDT swtich controls and an LED. For some reason my LED doesn't light up though when the circuit is closed, which boggles me lol but everything else seems to work. I just wanted your input on changes/ideas/better op amps to use/ etc ...and maybe why the LED doesn't light up?

[icydash]

upon looking it over it seems that there is only like a very small millivolt droppage across the LED which is not enough to turn it on, i just dont understand why because +9v should be going that way from the battery ;/

[DuncanM]

That works? OK...

The bypass switching is still strange - when bypassed, the input (your generator) is not only driving your scope but also the OUTPUT of the 741....
The "better" way is to move the switch so that:
the "pole" of the switch connects to your scope (output),
one leg of the switch connects to the output of the 741
the other leg of the switch connects to the input (the junction of R1 and C1).

Also, isn't either R5 or R7 redundant? (or is it like that to make the pot behave like a log taper?  )

As far as the LED is concerned -    why not just through a 4.7k to ground - why to pin 7 of the 741? Most unusual - it boggles me too...

And what about output decoupling to remove DC?
How is the 741 powered (from the battery I'd guess - or are you using a positive and negative bench supply?).

[Gilles C]

You also have to connect the 9V battery and the pin 4 of the opamp to ground because right it is floating.

You always have to work with the voltages and the signals in relation to ground.

Gillesw

[icydash]

I made most of those changes, i still need to add in a capacitor after the op amp stage to get rid of DC offset right? what size would be good? and also, where do i do the 4.7k resistor/diode to ground? after the opamp?

[icydash]

And as far as R7 and R5, without the R7, when the potentiometer is at 0% or 0 ohm resistence, there's just a short to ground, and basically you get ridiculous output.

[DuncanM]

And as far as R7 and R5, without the R7, when the potentiometer is at 0% or 0 ohm resistence, there's just a short to ground, and basically you get ridiculous output.

If you remove R7 then the bottom of the pot will not be connected to anything - so the pot acts like a variable resistor.
You also might have to add a capacitor in series with R5 (between R5 and ground)...

As far as the LED side goes - have a look at some schematics on generalguitargadgets.com to see how others manage to do it. It is entirely separate from the op-amp...

And the decoupling capacitor should be big to allow bass frequencies through - the exact low frequency cutoff will depend on the input impedance of the following stage. Try a 10uF electrolytic with the +ve towards pin 6 of the 741.

[icydash]

Well the next stage would be the amp. So what would the input impedance of the amp be?

That's all the pedal will be, tone controls and a volume, its a simple basic pedal, its my first one

[icydash]

The updated circuit: should be good but i still havnt yet figured out how to add an LED indicator to this...what do u think now?

[R.G.]

The LED doesn't light in the first circuit because there's not enough current going through it to light it.

The second circuit will not work because the opamp input is biased to the negative power supply. The input common mode range on a 741 ends a couple of volts up from the negative power supply. You'd need a rail to rail input opamp for signal to flow through the opamp. And then only the positive half of the signal would go through, because the output of the opamp is also biased to ground.

This ain't going to get any better until you construct a synthetic ground in the middle of the 9V battery, perhaps with two resistors and a capacitor. See the article in circuit sweepings at GEO on how to select the parts for that circuit. Then connect the "ground" end of your tone stack to the Vbias you've just made. Also take the "ground" end of R5/33K to that bias point. Now it will pass signal.

But it will pop loudly when you flip the bypass switch because of cap leakage on the input and output caps. To cure that, take a 1M or larger resistor from the outboard ends of the input and output circuits to the real ground as shown on the schemo.

So far, so good.

But the circuit sucks tone because the input impedance is too low. I would put another opamp section in front of it, set up as a high impedance buffer. Now it will not suck treble.

With the buffer, you can permanently attach the input to the signal in without degradation. So you only need the output-side of your bypass switch. Use what is now the front end of your bypass switch to switch the LED.

Read "The Technology of Bypasses" at GEO for "buffered bypass".

[R.G.]

Ooops, sorry, forgot. Get a more modern opamp in there. TL072 works well.

[icydash]

Ok lol not gunna lie, i have no idea what your talking about with the synthetic ground stuff, i get the high impedence op amp buffer stage, i dont get how you mean to change the switching to add in the LED, and i get what you mean with the 1Mohm resistor to kill pops on when bypassing.

So basically, can you explain the synthetic ground and switching thing again? and what's GEO / whats the url to those tutorials?

thanks for the help!

[icydash]

ok i take that back, i tried adding a 1Mohm resistor after the output capacitor to ground and it just shunted my ability to have the volume control work. So i guess i don't get that either lol ...i understand stuff better by seeing it, any chance you could wip up a fast circuit diagram?

[icydash]

The whole idea behind this is for me to understand each piece individually. I want to understand the tone part, then i'm going to make a distortion pedal next, then eventually put the two together into a "superpedal"...but i'm trying to understand it in pieces.

[R.G.]

GEO =

http://www.geofex.com

Highly worthwhile reading, especially if you're going to build a superpedal. You may be interested in reading a LOT of it, especially the Guitar Effects FAQ, the "Technology of..." series, and the circuit sweepings.

On to bias voltages.

Opamps cannot produce output voltages more positive than their positive voltage supply nor more negative than the most negative voltage supply. In fact, most of the ordinary ones, like the 741, can only swing their output to within about 2V lower than the positive rail and 2V more positive than the negative rail. Their inputs tend to do ugly things when you get them outside the "input common mode range", which is datasheet talk for the range of input voltages you can put in without killing the inputs. Those vary, but on normal opamps - again, like the 741- it's a couple of volts less than the power supplies.

You're running your 741 from a 0V and +9V supply . So the output can only swing between 2V and +7V. The inputs should be in the same range for linear operations.

You have the + input tied to the most negative supply. It's common practice to construct a "synthetic ground" with a pair of series resistors between +9 and ground. Usually the resistors are the same value, making a voltage of half the power supply. This is a good place for the positive input to be biased. Since a couple of resistors is not a good low impedance ground, it's common practice to use a large capacitor from the junction of the two resistors to the real ground (0V) to "short" AC currents to ground. So the opamp + input is tied to +4.5V. If you move the resistor from the - input to ground over to the new +4.5V, then only the differences between +4.5 and the positive input will be amplified.

That is, you construct a bias voltage to move the opamp operation to a DC voltage level where it CAN work.

When you do this, you have to use capacitors on the input and output to block the +4.5V DC level. The input tone stack does this, and your output capacitor does this, so it's OK.

ON switching. Read "Technology of Bypasses" for buffered bypass. If you put a high enough impedance buffer on the input, you can just leave the input connected all the time. So you only need half of a DPDT hooked up with its pole on the output jack, and one throw to each of the input signal and the output of your effect. That leaves you the other half of the DPDT free to turn an LED on when the output jack is connected to the output of the effect.

Watchers - didn't TI have an article on single voltage operation of opamps downloadable?

[icydash]

Awesome, i'll get to some reading and then get back to ya i really appreciate all this help

[icydash]

aha ! i get it now thanks!

[icydash]

Hey, in looking at http://www.geofex.com/circuits/+9_to_-9.htm  ...do u think the setup with the MAX1044 as shown in the schematic is a good and easy way to bias the op amp instead of using a few resistors etc...?

[icydash]

...though, in that circuit, where would the synthetic ground be? Where the down arrows are at the bottom of the circuit legs?

[icydash]

Ok so i did some reading and looked at some circuits and this is what i have for my power supply circuit:



Just to double check, the 10uF capacitor is for shorting AC currents to ground right?

Does this look good? it seems to be what you were all talking about so it seems right to me....

And then i'd attatch the bottom of my tone stack to Vbias (Vref), the op amp +V input to the +9v, and the -V into the real ground...and the feedback loop resistor would attatch to Vbias (Vref) as well right....?

And then i should pass signal ok and everything should be wonderful? :-)

If this is right ill put it all together and post it for final commentary

OH yeah dont worry about the swtich/LED, thats part of my 3PDT on/off switch and controls the LED when its on.