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TOPIC: Circuit Understanding

Circuit Understanding 2 years 4 months ago #916

  • khettid
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Hi everybody! Nice project!

I have some question on understanding the circuit:

1. How do I scale RV1? Do I have to check the amplitude of the signal first? Because I want to use it in my FX loop and I donot know the output level of my FX Send. My understanding from MXR Micro Amp is, that it is one of the control knobs, so in this case the Level, but if so, wouldn't it be mentioned in the interface section of the circuit? And why are there only 3 knobs on the pedal if there are RV1 to RV4?

2. Is the +3.3V supplied by the +3.3 pin of the arduino to the whole circuit? So I just connect for example the T1 and the RV2 to this pin?

I would be grateful for your answer.
Kind regards
FX Loop Guy
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Circuit Understanding 2 years 4 months ago #918

  • Ray
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Hi,
1. How do I scale RV1? Do I have to check the amplitude of the signal first? Because I want to use it in my FX loop and I donot know the output level of my FX Send. My understanding from MXR Micro Amp is, that it is one of the control knobs, so in this case the Level, but if so, wouldn't it be mentioned in the interface section of the circuit? And why are there only 3 knobs on the pedal if there are RV1 to RV4?

Check the software utilities topic, there is a code that helps you to set the VR1 pot, check it out for all the info:
www.electrosmash.com/forum/software-peda...3-software-utilities

The VR1 trimmer will set the input gain of the input stage (between 1 and 20) once you set it for your guitar (or input device) you wont need to move it. I usually adjust it by ear, when it sounds distorted I turn it a bit down and that is all.
The other 3 potentiometers are programmable, so the arduino read its value and use it to make different operations in different pedals. They are not in the "signal chain", they are only used by arduino.
2. Is the +3.3V supplied by the +3.3 pin of the arduino to the whole circuit? So I just connect for example the T1 and the RV2 to this pin?

Yes, the 3Ve comes from the Arduino board, and I dont understand what do you mean by "I just connect for example the T1 and the RV2 to this pin", could you explain it a bit more? :)
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Circuit Understanding 2 years 4 months ago #922

  • khettid
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Thank you, you were already very helpful!! :-)

Another question about the circuit: The IN_BUFFERED from the Input Stage will be connected to the IN_BUFFERED from the Output Stage, right?
The VR1 trimmer will set the input gain of the input stage (between 1 and 20) once you set it for your guitar (or input device) you wont need to move it. I usually adjust it by ear, when it sounds distorted I turn it a bit down and that is all.
But then I can only go louder, can't I? Because if the signal is for some reason let's say 4 V and I need to attenuate it, I cannot do it in this configuration, because the lowest amplification factor is 1.
What makes sense to me, is that the signal needs to be amplified to the full voltage range of the Arduino pins, to get the most precise signal processing possible. But wouldn't this configuration always boost the input signal, bypassing the Arduino via IN_BUFFERED, as well? So that you always have a boosted signal?
(As I mentioned above, I am not sure, if I got the purpose of the IN_BUFFERED correctly. I understand it as a switchable bypass of the Arduino, to add the "raw" input signal to the modified input signal on the output, e.g. for a delay effect.)
Yes, the 3Ve comes from the Arduino board, and I dont understand what do you mean by "I just connect for example the T1 and the RV2 to this pin", could you explain it a bit more?
You already made it clear to me, thanks :-)
I was just wondering, if there is some other voltage source of 3 V where I need to connect the T1 and RV2, or if it is the +3V3 pin of the Arduino.
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Circuit Understanding 2 years 4 months ago #924

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But then I can only go louder, can't I? Because if the signal is for some reason let's say 4 V and I need to attenuate it, I cannot do it in this configuration, because the lowest amplification factor is 1.
Yes, the lowest amplification value is 1, normally a guitar signal is 200mVpp, so it needs to be reasonably amplified to have a good signal to noise ratio at the input of the ADC.
If you have a hi level input signal (4Vpp) you would need a resistor divider at the input to attenuate this value to lets say 3Vpp and then use a gain of 1.
I was just wondering, if there is some other voltage source of 3 V where I need to connect the T1 and RV2, or if it is the +3V3 pin of the Arduino.
you can use the 3V3 of the arduino, yes.
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Circuit Understanding 1 year 7 months ago #1322

  • d4_ev1l
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Hello,

I've successfully built the pedashield from scratch, uploaded the chorus program and attached a sine wave pattern to the input and my scope to the ADC input (opamp output)

I noticed there's some serious clipping going on. And adjusting RV1 isn,t changing anything to the input signal.
Is this normal or should i look for an error in my input stage opamp?

output stage seems to be working fine. Mixer too.

i'll attach the scope image and the image when i simulate the input amplifiers
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Circuit Understanding 1 year 7 months ago #1326

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Hi, which signal levels are you using at the input of the pedal?
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Circuit Understanding 1 year 7 months ago #1327

  • d4_ev1l
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1vpp or 0.35Vrms sine wave.

normally the amplification factor of the first opamp should be adjustable from 0.2 to 21 with the RV1, no?

(function generator is on the left of the scope image)
Last Edit: 1 year 7 months ago by d4_ev1l.
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Circuit Understanding 1 year 7 months ago #1329

  • Ray
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1vpp or 0.35Vrms sine wave.
1Vpeak to peak is a bit high as the pedal was designed for guitar signals that usually go from 200mVpp to 500mVpp, BUT you are right, the input stage has a gain factor from 1 to 21 so using 1Vpp and x1 as a gain factor should not clip the signal.

I was asked trying to find the source of clipping, it would be ideal if you are clipping the signal before the Arduino DUE or after the Arduino DUE.
Could you measure at R9 and R10 (before Arduino) and on C11 (after Arduino) to see where the clipping starts?

NOTE: Make sure that you dont measure on the DUE DACs pins directly, they will be damaged.
Last Edit: 1 year 7 months ago by Ray.
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Circuit Understanding 1 year 7 months ago #1335

  • d4_ev1l
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Hello Ray,

It's definitely at the input stage that the opamp goes awol.

i measured on the inverting and non-inverting outputs.
i've attached my results in attachment.

Have you encountered similar problems with the input stage?
What is the best way to debug this one?
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Circuit Understanding 1 year 7 months ago #1337

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Yes, your input stage is clipping.
It could be possible that some of the resistors that you are using on that op-amp are not the right ones. With minimum gain, the op-amp should be buffering (gain=1) and the inverting and non-inverting signal should be equal.
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