Some stupid questions on RC filter; A high distortion arrangement with no diode.

[kit102]

This is my understanding on the above circuit.
C1 and R1 form a loww pass filter with cut-off Frequency of 16K
C3 and R7 form a loww pass filter with cut-off Frequency of 1K
output of op-amp having gain about 30

Question1: what are the C4 (100pF) and the P1 (100k Ohm Variable resistor) do in the feedback loop? According to some source, they form a high pass filter.It really confuse me since they are both connected to the -input and the output of op-amp, it means that signal will combine again at the -input point even the signal is being filter out by the C4 and P1.

Question 2: do the C3 and R7 really do the job in the circuit? I found nothing different by just hearing the output sound.

Question 3: if i need the output sound having frequency range of, say 1k~16k, do i need to put a lowpass filter before the output jack? Since there is already a lowpass filter with cut-off freq. of 16k at the input, i guess there is no need a lowpass filter at the output. However, i am not sure that will the op amp produce signal with freq. higher than 16k, even its input signal has been limit to 16k  and below.


An interesting result : when i switch the pin2 of 100k ohm variable resistor to position P2 as shown in the above diagram, there will be a very high distortion output!! It seems that we can do a high distortion pedal without any diode and transistor, but with a single op-amp. Can anyone tell me what is the theory working behind this??

[bioroids]

An interesting result : when i connect the pin2 of P1 to a caps as shown in the above diagram, there will be a very high distortion output!! It seems that we can do a high distortion pedal without any diode and transistor, but with a single op-amp. Can anyone tell me what is the theory working behind this??

hi!

You're running the opamp with no negative feedback (open loop), so the gain of it is the maximum gain posible for the opamp. You're amplifying the signal a lot and it hits the power supply limits, so gets squared. That's the high gain distortion!

Also C1 acts as a high pass filter, cutting lows.
Hey you have no bias resistor at the +input! What are the voltages at the pins of the opamp?

Luck

Miguel

[MartyMart]

I just tried a little design with no "bias" at + opamp input,
NOT a good idea ! it failed a few times ( see "Mr Crunch" thread )
Use two 470k r's one from a 4.5v VCC and one to ground
both connecting to + input.

Marty.

[kit102]

hi!

You're running the opamp with no negative feedback (open loop), so the gain of it is the maximum gain posible for the opamp. You're amplifying the signal a lot and it hits the power supply limits, so gets squared. That's the high gain distortion!

Also C1 acts as a high pass filter, cutting lows.
Hey you have no bias resistor at the +input! What are the voltages at the pins of the opamp?

Luck

Miguel

Sorry to confuse you guys. I  might have mistaken in drawing the diagram. I tried the 470n caps using the above diagram, but with a bias resistor (here i forgotted to include the bias resistor ). Then, I also tried the 470n caps with a simplified version of: take out C3, C4 and R7, then switch both + and - input of op-amp. no bias resistor. This version does sound similar to the first version in diagram with bias resistor.

Do you mean C1 itself form a low-cut filter? not C1 and R1? Then how do you estimate the cut-off freq ?
In my opinion, connect C and R in series should form a bandpass filter, C do the low-cut, and R do the high cut. Am i right?

is it a good idea to use this open-loop gain in circuit design?

I just tried a little design with no "bias" at + opamp input,
NOT a good idea ! it failed a few times ( see "Mr Crunch" thread )
Use two 470k r's one from a 4.5v VCC and one to ground
both connecting to + input.

Marty.

I tried a modified version of this: take out C3, C4 and R7, then switch both + and - input of op-amp. no bias resistor.
It did worked (got sound).It seems that not a good idea to feed the + input with no bias resistor, but not for the - input.

So what do you mean "failed"? No sound at all ? ( i guess should be no sound. sorry again for my mistake)

[MartyMart]

It would "work" for a time, then refused to "power up" after
a break ( input jack removed )
Also became intermittant and just produced silence !
With proper VCC/bias it works great
+ input "needs" a ref to ground to be stable

Marty.