I'm Back- sort of

[SparkyCal]

Sparky Cal, the schematic of your distortion making pedal circuit shows a single opamp with the pin numbers of a single opamp.
BUT you are using a dual opamp with different pin numbers. That is probably why your circuit does not work.

No, because that’s one of the first things I learned here. I always remap the pins but I will double check

 

[SparkyCal]

Here is the Pin map for the UA741CP

1- null
2 Inverting
3 non inverting
4 negative Ground)
5. Offset
6 Output
7 Voltage +
8 do not connect

I mapped the pins against the schematic

 

[SparkyCal]

I just double checked. I think the pin mappings are the same between the one on the schematic, and the one I am using. Have a look:

 

[bertus]

Hello Sparky,

Wich opamp are you actualy using?
You mentioned the TL032 is one of your older threads.
The TL032 is a dual opamp.

Bertus

 

[Audioguru]

The output voltage of an old 741 opamp cannot be higher than about 1V less than its supply voltage when it has no DC load. Your supply is 9V and you said the output pin 6 was 8.9V which is impossible.

The DC output voltage is controlled by the inputs voltages. Measure them. The + input should be near +4.5V and the - input should also be near +4.5V. Then the output will also be +4.5V so that it can swing up and down with the signals.

 

[SparkyCal]

I am using the 741 op amp.
Thanks for the measurements suggestions. I will report in once I have had a chance to follow up

thank you

 

[SparkyCal]

Sorry. Audio guru. When you say it has no DC load, I had the power supply hooked up to power the circuit while measuring. Is that wrong

 

[Audioguru]

You measured the output voltage of the opamp when it has no load. The voltage you measured is too high to be possible. Because something went wrong to cause the voltage to go almost to +9V when it should have been +4.5V.

The output voltage of an opamp will be reduced when it has a load. The datasheet for a 741 opamp shows that some of them have a 3V voltage loss when the output current is 1.2mA and have a 5V voltage loss when the output current is 5mA.

 

[SparkyCal]

Hi Audioguru- I am not sure I understood. Do I use my multimeter to make the measurements, while the circuit IS connected to the power supply at 9V.

or

Do I use my multimeter to make the measurements, while the circuit IS NOT connected to the power supply ? I am thinking it should be connected, otherwise, I would not get any measurements.

To be clear, this is the way I did it. I hooked up the + and - rails to the + and - of the power supply that fed a 9volt supply.

Then I put the black lead from my multimeter, on the negative terminal of the circuit, and moved my positive test lead, to the points in the circuit you suggested I measure. Did I do that right? Maybe I am getting confused with the term "load". I interpret that to mean, voltage being supplied to the circuit.

 

[Audioguru]

Obviously, voltage is measured when the opamp is powered.
With your 9V supply, the maximum output of a 741 opamp is about 8.0V but you measured 8.9V so something is almost shorted to the +9V, especially since the output should have been +4.5V.

 

[SparkyCal]

I think I've made some progress.

Based on the comments here, I decided to take my multi meter and follow each connection to the op amp to make sure there are no shorts.

By doing so, I discovered that Pin 2 was not connected to anything. I guess I was doing this project in increments, and when I got called away, I must have thought I'd already wired it. Anyway.....

Here are some measurements,. with the circuit being powered by 9 volts and with Pin 2 now being connected.

My negative lead from the multi meter is on the negative rail.

I then put my positive multi meter lead to the following pins and got the following readings:

Pin 2. 2.204

Pin 3 1.88

Pin 6. 2.22

Pin 7 8.96

Does this sound reasonable?

 

[Audioguru]

Your voltages are wrong.
I think the voltages are wrong because the 741 opamp is a fake or is defective and has way more input bias current than the original TL081 opamp.. Oh, maybe your voltmeter is an old one with a low resistance?
Pin 3 is the +input that is fed half (+4.5V) the supply voltage (+9.0V). Yours is not even half the voltage it should be.
Pin 2 is the -input with DC negative feedback from the output so it should be the same voltage as pin 3.
Pin 6 is the output and with the DC negative feedback its voltage should also be the same as the voltage on pin 3.

The original circuit used a TL081 opamp that has an input bias current that is 1/2500th of the massive input bias current of an old 741 opamp. Then the 1M resistor that gives the 4.5V to the +input produces a maximum of 0.5V error.

 

[SparkyCal]

Thanks. Let me check the rest of it and then swap in another 741 OP Amp and see if it makes a difference.

 

[Harald Kapp]

ou measured the output voltage of the opamp when it has no load.

Hi Audioguru- I am not sure I understood. Do I use my multimeter to make the measurements, while the circuit IS connected to the power supply at 9V.

"Load" is the impedance (resistance) attached to the output of the amplifier. It is not the power supply. Therefore "no load" and "with power supplied" are two entirely different things. When @Audioguru asked you to measure without load, that meant to remove e.g. C3:


Do not remove the 500 kΩ gain potentiometer. You need the feedback from output to input to keep the opamp within its operating range (around 1/2 × V_bat). If you removed the feedback, the opamp would try go to either max. or min. output voltage. It would be restricted by the diodes D1 and D2 and then stay fixed at ~ 1/2 × V_bat ± V_diode (4.5 V ± 0.7 V).

 

[SparkyCal]

Thanks. I have not removed the 500k pot, but I had not paid attention to where it was set at. I will rerun tests when I get a chance and report in.

 

[bertus]

Hello,

The 500K pot should not be the problem.
You can not remove the 500K pot, as the gainloop will be open.
If you want some "safety" use a 1K resistor in series with the 500K pot.
In that case the gain can be adjusted between 2 and 502 times.

Bertus

 

[Audioguru]

You are using an old 741 opamp in a circuit designed for a newer TL081 Fet-inputs opamp that has no input bias current.
The datasheet for an old 741C opamp says that its maximum input bias current is 500nA. Then the 1M input resistor would cause the voltage at pin3 to be 4.0V instead of 4.5V. A 741A opamp or a typical 741C opamp would have an input bias current of 80nA and cause a voltage drop of only 0.08V or less. The voltage you measured at pin3 is much too low. That would cause the other voltages to be too low.

A simple test to see if the opamp input bias current is too high would be to parallel the 1M resistor feeding 4.5V to opamp pin3 with about 10k ohms. Then measure the voltage at pin3 again to see if the voltage becomes 4.5V at pin3. A guitar pickup needs the opamp to have an input resistor of 1M or more and an opamp with low input bias current.

If you accidently connected the +9V to the output of the opamp when the output measured 8.9V then that opamp is probably fried.

 

[Audioguru]

I just noticed that the small value of 0.22uF for C28 produces a -3dB cutoff at 727Hz and cuts all lower frequencies. No bass.

 

[Harald Kapp]

You are using an old 741 opamp in a circuit designed for a newer TL081 Fet-inputs opamp that has no input bias current.

@SparkyCal : A good hint. Opamps can be very different (and they are). Using another type as the one used originally in a circuit can lead to unexpected results.

 

[bertus]

Hello,

What other opamps do you have?
The LM741 is a very old opamp with not the best properties.
You mentioned that you got the following opamps:
UA741
J4558D
LF353N
The 4558 is a dual opamp with transistor inputs.
The LF353 is also a dual opamp, but with fet inputs.

Bertus