Controlling the amplitude of the output signal from a pedal?

[YurkshireLad]

I rebuilt the circuit again, and have:

OUT = 4.62V DC
-ve = 4.62V DC
+ve 2.27V DC

This time it might be right, though I'll have to double check later.

Now that I've rebuilt it again, most of my problems seem to come from wiring up the gain loop and biasing the input, i.e. the ordering of pins in a breadboard column:

https://cdn.shopify.com/s/files/1/0176/3274/files/Breadboard_Remarked_grande.png?15033584625641436291

[GibsonGM]

It's traditional to go thru the pins, in order, Yurk...from the top looking down, left side down, across and up the right side.   So if we know you're using a TL071, you say "Pin 7 = 9V" and we know it's ok    If you measure DC voltage with it in-circuit, don't short the pins together.

The 4.62 is quite encouraging, tho if "-ve" and "+ve" are the inverting and non-inverting inputs, they should be nearly the same...can you post the voltages in this order?   This is the pinout for MOST single opamps, disregard if you're using something else, only posted for PIN NUMBERING, see data sheet for what each pin is on YOUR opamp (tho likely the same ins/outs/power).

Watch those breadboard 'rows and columns'   

EDIT: Here is how the pins run, looking down from top, just to be clear:

NOTCH

1  8
2  7
3  6
4  5

[Rob Strand]

The 4.62 is quite encouraging, tho if "-ve" and "+ve" are the inverting and non-inverting inputs, they should be nearly the same...can you post the voltages in this order?

Multimeter loading can cause the +input to read low.  If the -input is a Vcc/2 and the opamp output is at Vcc/2 then when you see a low voltage on the +input you would have some confidence it's caused by DMM loading.  After that you could analyse the circuit with an imagined 1M resistor to ground representing the DMM (assuming a 1M input for a cheaper DMM).  For example if the circuit has a 1M input resistor to Vcc/2 and you put a 1M DMM on the +input:  the 1M and DMM form a divide by 2 voltage divider, so you would expect 4.5/2 = 2.25V.   Which looks very much like what is happening here.

[YurkshireLad]

Thanks again.

I must apologize as I forgot to point out that the schematic uses a TL071 but I'm actually using a TL082. The TL071 was the only op amp symbol I had with the power supply pins and simulation data.

[GibsonGM]

Ok. If we account for DMM loading, you seem to be ok...I always forget that, my meter is like 7.5Meg.  If you're using a dual opamp, you want to configure the 2nd half of the opamp as a voltage follower so it doesn't oscillate, but deal with that after you make this work!  You are assembling this in real life to test, right?

   I guess my pin numbering picture went away...I'll replace it with a generic description for clarity.

[YurkshireLad]

Yes, I'm building as I go to learn.

[GibsonGM]

Does it work now, audibly?

[YurkshireLad]

I haven't had time to try it with the guitar amp yet.

[GibsonGM]

Ok, let us know. You may be at the stage where "IT'S TIME" ha ha.  I typically assemble things like this on breadboard, no meter, and just plug it in. If it doesn't work, I look at the parts/connections (scratching my head) til I see what's wrong.   Meter is LAST in my system, but your measurement may vary ("YMMV")...your method will come in time     

Once you do a few builds, you'll probably do less 'theory' too, and go for known values, put together the 'blocks' like gain stages, and spend more time thinking about what diodes for clipping, what you want for tone control(s) and so on. 

[YurkshireLad]

So close, yet no guitar solo. The output signal to my guitar amp sounds like a mixture of a clean signal and a distorted signal. But the distorted signal is very quiet while the clean signal is very load. Will re-investigate tomorrow.

[GibsonGM]

Shouldn't be distorted, no...for simplicity you can leave out the 1k/10u cap in the feedback loop, build a basic buffer to start.   Then it's just connecting pins 1+2 as the feedback path, input cap to pin 3 and to output cap from 1 as well.   Maybe it's the breadboard rows/columns making you mis-place parts  

clean mixed into a 'processed' signal suggest maybe you have input also going to ' - in' when it shouldn't be...be sure to 'straddle' the breadboard center line with the opamp....

Not sure if you know this, could be just from another topic I was on earlier, but chips go like this:


*Edit: modified for pinout, OP using TL082 NOT TL071.

[YurkshireLad]

What, the pins go down!!?? 

[GibsonGM]

Yes, Down!!    ha ha.   Sometimes the error we're making is that simple!   Earlier someone was discussing confusion about what rows/columns are connected, and I didn't want to go back thru this post to make sure it wasn't you    Since of course, there isn't connection across the center strip, but elsewhere there is.

Note I DID go back to remind myself that you have a TL082, so I updated my last pin-post there to be correct for this dual opamp!   

I'm taking this to be the current schematic, pin #'s for the dual opamp.   Try building this and see if it works for you; it does on this end.  I took out the filter network in the feedback loop (1k/10u), and put the 10u as output cap - 47u is quite large.  Added 100k load resistor to the end so the output's not floating.  Note only C1's right side is connected, with the 1M "Vr" resistor, to pin 3 node. 

[YurkshireLad]

I don't know if you can make out the connections here, but hopefully it shows what I have for the op amp. You can't see the connection from pin 4 to ground in the picture.

The orange wire on the left is the input, the orange wire on the right is the output.

[GibsonGM]

Edit....where is the bias voltage that should be going to pin 5, the "+ in" pin?  And, is there an input cap we can't see on the orange 'input' wire? Output cap after the output wire??   BAD distortion without the caps!  C1 and C3 in my schematic.

[YurkshireLad]

There are input and output caps but I couldn't get them all in the picture; it was impossible to really see any detail when I pulled the camera (phone) further out.

The resistor you identified is supposed to be R4 in my schematic at https://postimg.cc/XXJNxZ9b. Isn't the gain loop basically a voltage divider?

[GibsonGM]

Yes, the feedback loop is a divider.  What I'm trying to get you to do is make a voltage follower first off the schematic I posted above, with the bias voltage there and all....then you can insert the resistors needed to add gain      I don't see the 1meg bias resistor in your photo, arrow points to the "+" input, no bias voltage that I can see...can't read those d@mn blue resistors, either, he he.   

[YurkshireLad]

Yeah the bias resistor is off the top of the picture. I'll try to take a picture of the whole shebang but it won't be pretty!

[GibsonGM]

Ok.  I don't see it connected, perhaps you have things like " input >  Cap > bias resistor > over to chip  which would be fine.     Provided the parts are in the right places, it WILL work - laws of physics!!

[YurkshireLad]

It's probably really hard to see what's going on. I haven't had time to change anything today.