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A Stretch

Martaine2005

May 12, 2015
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There are a couple of ways to check the op amps on their own.
Power them Vcc as normal. Connect inv and non inv to ground then check output. It should 0V.
Leave inv at ground and put a voltage on non inv, say 5V, the output should now be 5V. This is a ‘follower’ or ‘buffer’.
This should show whether the op amps are functioning or not.
There might be better ways that others can explain.

Martin
 

Martaine2005

May 12, 2015
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Not necessarily, depending on inherent input offset.
Very true, nothing is perfect, this is the real world.
Might be a couple of millivolts off. But keeping it simple for me and the OP, it gives a general way of testing.

Martin
 

Audioguru

Sep 24, 2016
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The old 4558 opamp has an open-loop DC gain of At least 20,000 times. Its inputs do not work properly (common mode input voltage range) if they are within 3V from a supply voltage. The input offset voltage is typically 2mV then the output might try to be at least +40V if the inputs are connected to the negative supply that is ground in this circuit.

In this circuit, the opamp is a DC follower since the 500k gain control pot connects its output to its inverting input. R32 and R33 divide the supply voltage in half and feed it to the non-inverting input, then the output DC voltage should be the same as the voltage divider plus or minus a few millivolts.
The TL081 opamp had a wrong output voltage of much higher than half the supply voltage. Does this opamp produce that same problem??

Since the TL081 opamp produced the same problem of no distortion then this circuit must have a common problem with the gain control not increasing the AC gain up to 501 times.
 

SparkyCal

Mar 11, 2020
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The old 4558 opamp has an open-loop DC gain of At least 20,000 times. Its inputs do not work properly (common mode input voltage range) if they are within 3V from a supply voltage. The input offset voltage is typically 2mV then the output might try to be at least +40V if the inputs are connected to the negative supply that is ground in this circuit.

In this circuit, the opamp is a DC follower since the 500k gain control pot connects its output to its inverting input. R32 and R33 divide the supply voltage in half and feed it to the non-inverting input, then the output DC voltage should be the same as the voltage divider plus or minus a few millivolts.
The TL081 opamp had a wrong output voltage of much higher than half the supply voltage. Does this opamp produce that same problem??

Since the TL081 opamp produced the same problem of no distortion then this circuit must have a common problem with the gain control not increasing the AC gain up to 501 times.

since this was to be my foray into building an op amp pedal, I am happy to abandon this circuit. Does anyone know of a circuit that uses the op amp I haVe (assuming it is not the problem), that actually works? I will search as well
 

Audioguru

Sep 24, 2016
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The circuit you have (without the horrible passive tone control) will work fine if it is built properly. If the opamp output measures half the supply voltage then the opamp is probably fine. Another circuit was shown to be exactly the same.

When the gain control is adjusted to be zero ohms then the AC gain is 1 and there will be no distortion produced by the diodes.
When the gain control is adjusted to be 500k ohms then since resistor R46 is 1k at the inverting input of the opamp and has a 220nF (224 or 0.22uF) capacitor coupling it to ground then the AC gain is a whopping 501 times and the distortion should be extreme.
 

SparkyCal

Mar 11, 2020
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We may be onto something.The Gain pot reads A500K on it. The funny thing is, when I turn it counter or clockwise, it has no effect whatsoever. The center and right side terminals are connected together (shorted). The left terminal is connected to Pin 2 and the center/right terminal is connected to pin 1. nafter ot goes through the 2 diodes and C31, that is).

So, maybe it is the pot?
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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The pot is easy to check with your multi-meter ohms function. Clip the meter leads between the two end leads of the pot. One of these end leads should already be connected to the center (wiper) lead. Measure the change in resistance when the pot shaft is rotated between its two extremes. In one extreme the resistance should be very close to zero. In the other extreme it will measure more resistance, but maybe not as much as you would expect, because one of the diodes is likely to become forward-biased by the battery used by the multi-meter ohms function when the pot shaft is rotated out of the "zero" resistance position.

A more reliable way to check the pot is to remove it from the circuit, leaving the center lead connected to one of the two end leads. Measure the resistance between the two end leads as the pot shaft is rotated between its stops. You should see zero ohms (or nearly) at one extreme and 500,000 ohms at the other extreme.
 

Audioguru

Sep 24, 2016
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The gain control pot does not "go through the diodes and C31". They are all in parallel, not in series. C31 is only 100pF and the circuit should work with or without it.
 

SparkyCal

Mar 11, 2020
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Sorry for my loose language- Yes, they are in parallel.

I eliminated the ton control, in order to avoid further complications and because most believe it is not needed.

I attached a pic of the project although I know you will not be able to use it. It is just for a visual.

Here is where I am at:

Tone control removed
Gain Control works now. I can hear the difference when I turn it clockwise and then back down.

The volume pot works but it is reversed. The volume gets lower as I turn it clockwise- likley i just have the wires reversed.

BUT...the major problem is still NO distortion. Maybe this particular chip has different pin arrangements? I am following this data-sheet https://components101.com/ics/lm4558-dual-op-amp-pinout-datasheet and I have also shorted Pin 6 to pin 7

Pin 1 go the - of the battery measures .18 v

Pin 1 to the + of the battery flashes a measurement of 6.66v on and off
 

Audioguru

Sep 24, 2016
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Pin 1 go the - of the battery measures .18 v
Pin 1 to the + of the battery flashes a measurement of 6.66v on and off
No. Pin1 is the opamp output and it should measure half the supply voltage.
Pin4 is circuit ground and the minus terminal of the battery.
Pin8 is the positive supply voltage.
Pin2 is the inverting input.
Pin3 is the non-inverting input.
With pin6 shorted to pin7 then pin5 should be connected to ground to disable the second opamp.
 

Audioguru

Sep 24, 2016
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If your guitar has a volume control on it then it must be turned up to produce a high signal level. Then the gain control on the new circuit will control how much distortion is produced by the diodes.
 

SparkyCal

Mar 11, 2020
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Hi folks:

i owe it to you to tell you that I am backing away from this project for a while. No matter how hard I try, I can't make the op amp produce distortion. Maybe I will but a different brand op amp next time I'm around the parts store.

Thank-you for all of your help. I am sure it is just me (or a bad part). as I say, I do not consider this a failure, but a learning step.

Maybe as I try other circuits and learn more, i will come back and try this one again.

Thank-you again!!!!!
 

SparkyCal

Mar 11, 2020
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I am switching gears and pursuing this circuit:

https://www.diystompboxes.com/smfforum/index.php?board=33.0

Here are my reasons:

1. the explanation is very detailed. It is right down to put this pin in this hole on the breadboard. And it also explains the thinking along the way. It also shows how the actual parts look, in addition to the breadboard view.

2. Because of the detail in this tutorial, and because I ordered the LM741U Op amps (which came today) specifically for this purpose, i thought I'd try it.

I respect AudioGuru's advice that this op amp is very outdated, but I already received it and one has to start somewhere. This circuit works on a 9 volt battery, so let's see what happens. i'll report back.
 

SparkyCal

Mar 11, 2020
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Thank-you Bertus. Very helpful as always. A bit of context, and then a status update. The gentleman who created the original circuit I was trying to get to work (I beleive his name is Brian Wampler) was kind enough to give me some guidance through an email exchange. Understandably, he suggested I go to DIY Stompboxes to learn more. That's where I found the new circuit.

I built it on a breadboard and it worked perfectly. However, it was short lived and then reverted to the same old problem - no distortion. But before that happened, I clearly heard distortion with just the op amp operating, and then different distortion when i added the three diodes. So, I thought- great- finally. But then something happened and I lost all the distortion. So it may be one of two things:

1. The breadboard method got unstable- easy fix- move to PCB. and/OR

2. The capacitors I am using are not the right ones. Let me explain:

I have the right diodes (I purchased them specifically for this- thanks for the substitution suggestion though Bertus). I have the right resistors.(pretty easy to find those). But the capacitors are confusing. I have film, ceramic and electrolytic capacitors. I am not sure what type to use when. I wish the gentleman who created this, would have been specific in that way. I note he uses ceramics, it looks like electrolytic are also used and i have to assume films. But I don't know for which and, as I have learned, polarity is important when using electrolytics.

Could it be that although i am using ther right capacitance values, using the incorrect type of capacitor may have caused the circuit to stop working and maybe even damaged the op amp??

Ideally, i'd like to clarify this second question on capacitors before I proceed to a PCB attempt.

Now that Bertus has made it easy by providing the direct links to each step, if anyone has any other advice, I'm all ears.

Plus, my wife is starting to wonder what the heck is taking so long and why i am working on the same thing over and over ;-). Today she said, "are you back down in that hole" (my basement. lol)
 
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Audioguru

Sep 24, 2016
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When the battery voltage becomes too low for the the old 741 opamp then it stops working. The old 741 opamp was designed 52 years ago to use only a 30V power supply. Maybe your opamp stops working when the battery voltage drops below 8V or 7V.
Try the circuit when it is powered from 12V.
 

SparkyCal

Mar 11, 2020
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Ok..I will try that and report back once I have had a chance to do so. Thank-you.
 

SparkyCal

Mar 11, 2020
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I wonder what would happen if i drop a 4558D in the same circuit but make sure the pins are re-mapped. Might that be better?
 
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