Sustain and Saturation for Distortion Pedal

Yggda · January 10, 2016, 10:10:06 AM

Hey Folks,

I made the circuit, assembled it and have been surprised when I heard "distortion noise" which was far better than expected. However nothing is ideal...especially with the first time project. So I have got the problems:

1)Sometimes sound disappears for microseconds while playing. Can that be the saturation of the OpAmp?

2) Is it possible somehow to sustain a distortion not changing design too much...make lower threshold for clipping diodes and longer distortion sound? I have tried to put resistor between diodes and C9 capacitor, however didnt hear the difference.

Thank you very much!

duck_arse · January 11, 2016, 09:45:41 AM

Yggda, hello and welcome.

use the search facility here, look for "blocking distortion". reduce C17, C4, C9, see what happens.

as for clipping threasholds, try some Germanium or Schottky diodes, lower forward voltages. also try a low value resistor or pot between the bottom of the diodes and ground, a "diode lift".

Yggda · January 12, 2016, 12:06:26 PM

Thank you duck_arse,

I will try to change capacitors.

Yggda · January 13, 2016, 12:49:30 PM

Tried to use different capacitors...however all the time was the same problem. Change 5Mohm resistor with 2Mohm one and sounds much more better

ElectricDruid · January 13, 2016, 02:57:12 PM

Could it be op-amp latch up if you're driving it to the rails? You say it saturates.

Several common or garden varieties of op-amp will 'hang' when pushed to the rails, and it takes them a little while to recover. Could this be what you're hearing?

The solution is to use diodes or similar to provide the clipping and distortion characteristic, rather than force the op-amp to its limit. That said, there are plenty of amps that distort more 'gracefully'. MC4558 won't latch up, for example.

HTH,
Tom

Yggda · January 14, 2016, 07:45:49 AM

Hi Tom,

I think it can be that.
I will try to record the sound with the different OpAmp gain level and another OpAmp as well

mth5044 · January 14, 2016, 08:38:13 AM

Are the positive inputs connected together as shown? Or are they connected to a voltage?

Yggda · January 14, 2016, 08:50:53 AM

They are connected to virtual ground Vs+/2

mth5044 · January 14, 2016, 11:49:43 AM

Ah sorry, those little red dots are very hard to see!

Yggda · January 16, 2016, 06:56:16 AM

Hey,

I said that with 2Mohm sounds better - there were no total silence in the sound in comparing with 5Mohm, however sound is still "wavy".
http://vocaroo.com/i/s0QFXGEISAeR - 2Mohm
http://vocaroo.com/i/s098pZpEPIFd - 5Mohm

duck_arse · January 16, 2016, 08:45:11 AM

yggda - what happens to your sound if you put some series resistance between C4 and R14, maybe 10k or more? anything?

ElectricDruid · January 16, 2016, 12:32:38 PM

If it is caused by latch up, you're going to need to make more significant changes than turning 5M into 2M.

Assuming a 1V input signal (fairly typical) you've got a gain of 5 on the first op-amp, so we'll say 5V output at that point. That should be ok on a 9V supply. But then that hot signal goes to the second op-amp where you've got an enormous gain of roughly 20M/20K = x1000. That 5V is trying to be 5000V. That's plenty to drive the op-amp into the rails and latch it up. Even x2 would probably cause the problem given the 5V input from the first stage.

Why not move your clipping diodes back into the feedback loop (in parallel with R14) instead? That'll prevent the op-amp from hitting the rails. At the moment, most of the clipping you're getting is from the op-amp anyway, so they're not doing much where they are now.

HTH,
Tom

Yggda · January 16, 2016, 03:15:08 PM

duck_arse and Tom

Thank you! - will try it. By the way, how can this, possibly, affect the equalizer?

I have only triedthe model with diodes in the feedback in the Multisim software - it said that will not work))

Yotam-A · January 17, 2016, 02:46:19 PM

Like said before, put clipping diodes on the second opamp (just add blue LEDs. They sound awesome, about 2.7V voltage drop).
And I spot a serious loading issue - are you driving the pedal from a guitar or from a buffered/low impedance source?
If it's from a buffer/low impedance source, then no problem. But if you drive it from a guitar, you really load the pickup, and lose frequencies and volume.
Try to put a buffer before the first opamp, or change its wiring from inverting to non-inverting.
Right now your input impedance is about 10k - most guitar-related stuff are 1M or higher (or at least 100 times higher than the pickup impedance).

Yggda · January 18, 2016, 10:46:25 AM

Thanks Yotam-A,

should I change the resistance of R14 as well to less than 1Mohm?

About impedance, I used this datasheet, and it says that Input impedance is 10¹³ Ohm... what does this mean ?

ElectricDruid · January 20, 2016, 10:29:54 AM

Quote from: Yggda on January 18, 2016, 10:46:25 AM
About impedance, I used this datasheet, and it says that Input impedance is 10¹³ Ohm... what does this mean ?

It means that the impedance of the actual op-amp itself is astronomically large. The PCB and even the air humidity start to have an effect on such ridiculous figures. Suffice to say - it's big enough to ignore.

Much more important is the impedance of the *circuit*. For an inverting op-amp, like your first stage, that's equal to the input resistor, since the op-amp's -ve input acts as a "virtual ground summing node". This means practically that you can regard the 10K input resistor as being between your signal and ground - e.g. it sets the input impedance.

It's hard to design a inverting amplifier with a large impedance (say 1M) because then you need a resistor as large or larger for the feedback resistor. Noise starts to be a problem with such large values. This is why you mostly see non-inverting op-amps as the initial buffer stages in guitar pedals. One specific case of "non-inverting" is the op-amp unity buffer, which provides no gain, and is recognised by having the output fed back to the -ve input.

HTH,
Tom

Yggda · January 23, 2016, 07:49:45 AM

Yeeeeah, now I understand why i hear mostly low frequencies

Yotam-A · January 23, 2016, 04:20:59 PM

Exactly what ElectricDruid said. Just put a simple buffer in-front of the first opamp (even a single BJT is enough, JFET is a better option tough - a few M ohms input impedance).

ElectricDruid · January 23, 2016, 05:21:42 PM

Quote from: Yotam-A on January 23, 2016, 04:20:59 PM
Exactly what ElectricDruid said. Just put a simple buffer in-front of the first opamp (even a single BJT is enough, JFET is a better option tough - a few M ohms input impedance).

Or replace the inverting amp you have with a non-inverting buffer. Something like the "Non-inverting unity gain buffer" about half-way down this page:

http://stompville.co.uk/?p=470

You'd be wanting R1 to be 1MOhm, and C to be maybe 100n.

HTH,
Tom

Yggda · January 24, 2016, 08:53:46 AM

Thank you guys,

will try unity gain later

I have made a "Non-inverting amplifier alternative" instead of my 1 one. 2nd Next after Tom suggested on this page http://stompville.co.uk/?p=470.

R_b = 2.4 MOhm, R_f = 50 kOhm, R_g = 10 kOhm, C1 = 2.2 uF, C2 = 0.3 uF.

R14 = 2.4 MOhm with 2 parallel diodes. No diodes after OpAmp.

Also, put 50 kOhm between C4 and R14.

Now I have high impedance and I can control frequencies with the equalizer. However still have that wavy sound in the output. Tried different OpAmps - still the same. Hopefully unity gain will help.

Thank you for your help.

News:

Sustain and Saturation for Distortion Pedal