Controlling the amplitude of the output signal from a pedal?

[YurkshireLad]

Thanks again - some excellent information I need to try to understand. 

[GibsonGM]

You'll get it in time. Do a lot of reading, and look at the common schematics - you'll start to notice things that occur constantly. I don't have a lot of the deep theory on semiconductors, but I'm pretty good at copying! he he. 

[YurkshireLad]

I borrowed the power supply from the DS-1 and I made various changes that were suggested. I installed the "sub" and '"sym" files for the TL072 into LTSpice. The guitar input signal is about 200mV p2p and the output of GAIN_OUTPUT is just under 400mV p2p. The input impedance is still low I think, and I've also lost a lot of gain for some reason.

[YurkshireLad]

If I revert to this, then I get the gain of 10 as I would expect. I'm not sure why there's a difference.

[Rob Strand]

You need to wire the ground side of the 1M to the 4.5V point, not the opamp + input to 4.5V.

This form is preferred compared to your second circuit because it picks up less noise from the power rails.

I borrowed the power supply from the DS-1 and I made various changes that were suggested. I installed the "sub" and '"sym" files for the TL072 into LTSpice. The guitar input signal is about 200mV p2p and the output of GAIN_OUTPUT is just under 400mV p2p. The input impedance is still low I think, and I've also lost a lot of gain for some reason.

[FiveseveN]

Also you're shorting all your signal to ground through C6 otherwise.

[GibsonGM]

At C6, add a 1 Meg resistor from 4.5 bias point to the opamp input. Remove the 1Meg to ground there now.   Without some additional resistance feeding the opamp, the signal just goes back to the power supply and shorts to ground thru C6, like 5/7 is saying.   

[YurkshireLad]

Thanks all. I'll try to update it later today.

[YurkshireLad]

At C6, add a 1 Meg resistor from 4.5 bias point to the opamp input. Remove the 1Meg to ground there now.   Without some additional resistance feeding the opamp, the signal just goes back to the power supply and shorts to ground thru C6, like 5/7 is saying.   

It sounds so obvious when explained like this. I wouldn't have thought of this, but it's something you gain with experience and knowledge. I think FiveseveN was also saying this and I didn't understand what he meant, but now I do. The output from GAIN_OUTPUT now has a gain of 11 once again. Thanks!

[Vivek]

Fantastic progress !

Do post your latest schematic

With frequency response graph (some of your caps could be too big)

And wave shape for various amplitudes of input signal

[Vivek]

Did you try CTRL R and CTRL E with Opamp selected, to change its orientation ?

[YurkshireLad]

You could be right. I need to understand how to view a frequency response graph. I'll check other pedals for examples of capacitor values.

[antonis]

All of them are 1st order High-Pass Filters and obey the general formula f = 1/2*π*R*C..
e.g. 1μF & 1M from a HPF of 0.16Hz cut-off frequency (subsonic..)

An optional Low-Pass Filter should be formed by a cap in parallel with R4..

[YurkshireLad]

I haven't modified any values yet, but this is what I have so far.



The simulation results are:



Though I'm not sure why the output signal is "doubled". You can see the output signal is rendered twice.

[Vivek]

Why doubled beats me

Can you post your ASC file here ?

It is done by opening up the ASC with a text editor, copying it

using CODE button here that looks like a #

and pasting into the CODE area


Alternatively you could email it to me.

I can add commands for frequency response etc for you and sent it back to you.

[Rob Strand]

Though I'm not sure why the output signal is "doubled". You can see the output signal is rendered twice.

Why doubled beats me

I was going to suggest setting the time step to something smaller like 0.1ms.

I believe you can (manually) load previous waveforms and compare them against the current simulation but I doubt it's happening here.   If you can exit LTspice, do sim, and still see the double lines then it's not previously saved stuff.

[Vivek]

So far, I only know of STEP command to be able to get a retrace of same data (but with different parameters). But I could not see a STEP command in the graphic

Feature to compare last waveform to current waveform would be great. How does that one work ?

For time between Data, I like to begin with 100 data points per cycle as the minimum, and sometimes going up much higher.

[antonis]

I haven't modified any values yet, but this is what I have so far.

No need for 3 X 1M bias configuration resistors..
(make R7 & R8 10k to 47k, say..)

[Vivek]

No need for 3 X 1M bias configuration resistors..
(make R7 & R8 10k to 47k, say..)

Excellent suggestion on R7 and R8 !

Any comments on C4 = 47uF ?

and C3 = 10uF ?

[antonis]

C3 : Anywhere between 10μF (16Hz) and 1μF (160Hz) should be fine..
(clearly it's a matter of taste..)

C4's value is strongly dependent on what comes next.. (volume control, next effect input impedance, etc)
10μF should be a safe value for following impedances down to 1K or so..