Opamp Question

[Microtone]

I am looking at attempting to build an opamp muff fuzz.

I see it uses both of the ICs integrated in the package.

Can anyone shortly explain to me whether it makes a difference if one uses two separate identical opamps for this purpose or whether it has to be built using just one opamp package, using both ICs.

[antonis]

The later saves you space, soldering time and money..

P.S.
IMHO, the only reason for using single op-amps in a more than one configuration is the need for DC offset null..

[Clint Eastwood]

An obvious reason for using single op amps is that you have lots of them, and you would rather use those than go buy dual op amps.
The only reason I can think of, apart from what Antonis said, that dual op amps are better is that they could be very closely matched, which can be an advantage in certain very specific applications. But for general guitar related purposes, I rely on my stock of ancient 741's.

[MrStab]

Dual op-amps are also much more ubiquitous in different types.

The only real reasons to use single op-amps are if you're using expensive high-speed op-amps, you got really drunk one night and bought loads by mistake, you're building a RAT or if you're into masochism.

To terminate an unused section of a dual/quad op-amp properly, short the inverting input and output together (as if it was a buffer) and tie the non-inverting input to half the supply voltage (usually 4.5V, aka. Vref, in Pedal Land).

[amptramp]

Individual op amps are justified when the layout of the circuit using a dual op amp would result in oscillation due to feedback.  Think of a couple of gain-of-20 stages in series.  If the layout allows the signal with the combined gain of 400 to go from the output to the input, you will have no end of trouble trying to keep it stable.  You would be able to get much greater separation of traces on the board (or whatever type of interconnect you use) if you have two separate packages.

[Mark Hammer]

It can sometimes be the case that what is demanded, or caused, by part of a circuit, impacts on other parts in an undesirable way.  For example, there are countless threads on this site about audible "ticking" in this or that modulation pedal (tremolo, phaser, chorus, flanger, etc.).  The ticking stems from the sudden current demands of LFOs that begin with a square wave.  There are cures for that, but they are often best implemented by what one does to the specific op-amps used to produce that square wave, without affecting any others in the audio path.  In which case, the circuit will function best if the LFO portion is its own separate chip, and not combined with anything else in, say, a quad op-amp to save space.

Basically, it's a "divide and conquer" strategy.

[Microtone]

Wow! Thank you all for your awesome and clearly very knowledgeable input.

I fear I have waded in out of my depth....

This is what I did. In my infinite wisdom I thought it would be a good idea to use the JLCPCB online PCB creator to create and fabricate a prototype board with surface mount components of an opamp muff fuzz.

The online application is very easy to use. I think I have used all correct values and connected up all the paths correctly. Except the board that I received through the mail doesn't work. D'oh!

There are these three points where I think it may have gone wrong:

1. I chose schottky diodes as 1N34As were not available.

2. Instead of using just one dual JRC4558 opamp, I incorporated two into the schematic: the first opamp connected just at pins 4, 5, 6 and 7; and the second opamp connected just at pins 1, 2, 3 and 8.

3. I followed the Beavis audio schematic available online, which uses a 10 uF capacitor closest to the 9V supply. I noticed that other schematics online show a 10 pF capacitor at this point.

Can any of you sleuths decode my idiocy and offer me a clue as to where I went wrong?

[marcelomd]

Can any of you sleuths decode my idiocy and offer me a clue as to where I went wrong?

Sure!
Can you post the schematic, layout and any pictures of the board/assembly/final product?

[duck_arse]

2. Instead of using just one dual JRC4558 opamp, I incorporated two into the schematic: the first opamp connected just at pins 4, 5, 6 and 7; and the second opamp connected just at pins 1, 2, 3 and 8.

very much this. each opamp package of 8 pins needs a V+ supply and a ground supply [in this inst]. so both IC's need pin 8 to V+ and pin 4 to ground. we can go into the other correct things missing at a later stage.

[Microtone]

very much this. each opamp package of 8 pins needs a V+ supply and a ground supply [in this inst]. so both IC's need pin 8 to V+ and pin 4 to ground. we can go into the other correct things missing at a later stage.

This!!!!

Thank you so much for spotting this schoolboy error on my part. Having no electronics training, I'm kind of painting by numbers when following schematics. You've hit the nail on the head here, with your superior expertise. I thank you for taking the time to clarify that for me in words of one sylable!

So, I added a couple of tiny wires onto the SMD ICs, a live to the 8th pin on the first IC, and a ground to the fourth pin on the second IC and....

IT WORKS!!!!

I'm sure you all know they joy of troubleshooting a circuit and getting it to work. What a marvelous feeling.

And I love the sound of these little muff fuzz circuits. The Schottky diodes don't seem to hinder it at all. In fact I like the tone a lot.

I read somewhere online that the opamp muff fuzzes are 'unremarkable' late 70s fuzz boxes. Having played through this little thing all evening, I am starting to feel that is rather harsh criciticism!

Thanks again to everyone who took the time to give me a helping hand here. You rock!

[ElectricDruid]

Glad it's working, that's great news.

Now Duck will want to step back in and tell you about the other things you should have done 

[Microtone]

Glad it's working, that's great news.

Now Duck will want to step back in and tell you about the other things you should have done 

I'm all ears and eager to learn!

[antonis]

Duck will appear only after schematic and build pics appearance..!! 

[GibsonGM]

Glad it's working, that's great news.

Now Duck will want to step back in and tell you about the other things you should have done 

I'm all ears and eager to learn!

with your superior expertise

Please...BE CAREFUL with that...

[duck_arse]

it's all in the syllables, fellas.


well sir, you now have an unterminated opamp in each package. they will get up to hijinks and shenannigans unless you distract them with voltages of some sort. because we do do things right hereabouts, we connect each of the free non-inverting (+) inputs to a voltage, and each inverting input (-) to its output. then ignore them.

if you look on your nearby datasheet for the opamp you are using ...... you will see a spec for "Common-Mode Input Voltage Range", which is the range of input voltage that produces reliable repeatable IC behaviour. usually it excludes 0V and +V supply, so we just use the V/2, Vref, bias voltage, whatever is on your schem (which we are yet to see). there is many plenty examples of circuits to be found that don't follow the correct termination method, but we point them out whenever they appear for discussion.

so now it's back to you - we want to see your circuit diagram with the correct and current connections, all of them, and some photos of the built thing. and as for those other blokes, they just too lazy to type is all.

[antonis]

and as for those other blokes, they just too lazy to type is all.

https://www.ti.com/lit/an/sboa204a/sboa204a.pdf?ts=1692796714906&ref_url=https%253A%252F%252Fwww.google.com%252F
https://www.analog.com/en/technical-articles/avoiding-noise-and-power-problems-with-unused-op-amps.html
https://www.electronicproducts.com/properly-terminating-an-unused-op-amp/

[Microtone]

it's all in the syllables, fellas.


well sir, you now have an unterminated opamp in each package. they will get up to hijinks and shenannigans unless you distract them with voltages of some sort. because we do do things right hereabouts, we connect each of the free non-inverting (+) inputs to a voltage, and each inverting input (-) to its output. then ignore them.

if you look on your nearby datasheet for the opamp you are using ...... you will see a spec for "Common-Mode Input Voltage Range", which is the range of input voltage that produces reliable repeatable IC behaviour. usually it excludes 0V and +V supply, so we just use the V/2, Vref, bias voltage, whatever is on your schem (which we are yet to see). there is many plenty examples of circuits to be found that don't follow the correct termination method, but we point them out whenever they appear for discussion.

so now it's back to you - we want to see your circuit diagram with the correct and current connections, all of them, and some photos of the built thing. and as for those other blokes, they just too lazy to type is all.

Thank you very much for that explanation. I appreciate you taking the time to school this noob!
I feel better informed already!
So , as promised.  Here is the schematic I used, which was just the first one I found on the web.
And a pic of the board. (Provided I can work out how to successfully upload all that here!!)

[Microtone]

and as for those other blokes, they just too lazy to type is all.

https://www.ti.com/lit/an/sboa204a/sboa204a.pdf?ts=1692796714906&ref_url=https%253A%252F%252Fwww.google.com%252F
https://www.analog.com/en/technical-articles/avoiding-noise-and-power-problems-with-unused-op-amps.html
https://www.electronicproducts.com/properly-terminating-an-unused-op-amp/

Many thanks for these three articles. They look exactly what I need. A little bedtime reading for me.
Now if I can just work out how to upload a video here, I can let you guys know how this things sounds!!

[Microtone]

Creamy fuzz tones!