How does this addition to the op-amp effect so much?

[maor]

Hello everyone, I have been recently working on my own effect and I noticed how something very specific changed the entire response of the op-amp from nothing to actually working but I don't understand why.
http://schematicheaven.com/effects/boss_ds1_dist.pdf
In this schematic, after the low pass filter in the input and the second cap, theres a connection to ½Vcc via a 1M Ohm resistor, and when its connected the amp works like it should but without it it doesn't work at all.
My question is why does that happen?, Whats the logic here?

(also, my simulated "input" is 120mV at 440Hz)

Thanks in advance,
Maor.

 

[Nigel Goodwin]

Fairly obviously it provides the bias for the opamp, the two 10K's provide a 'spilt-supply' - you can't use it with no bias.

 

[maor]

Bias?, explain please.

 

[Nigel Goodwin]

Bias?, explain please.

I suggest you do some reading on how opamps work, it's fairly essential to know.

 

[maor]

Damn the things they teach me at school.

Alright, I'll look up a bit.

 

[Speakerguy]

Also look for the term "virtual ground".

Basically, a dual supply opamp will have V+ , V- , and ground. Usually +15V, -15V and 0V for audio op amps.

Single supply op amps only have V+ and ground. Signals need to be biased around V+ divided by two. It's called a virtual ground, and they can be made with resistor dividers or by integrated circuits made specially for the task, like the TLE2426:

https://www.electro-tech-online.com/custompdfs/2010/05/tle2426.pdf

Virtual Ground Circuits

That should be plenty to get you started.

 

[MrAl]

Bias?, explain please.

Hi there,


Very simply put, the op amp can not work down to zero volts dc on the input when the signal input is AC because the AC signal swings below ground as well as above. By 'biasing' the op amp input to 2.5vdc through the 1M resistor, the AC can go plus and minus and the op amp sees that plus and minus relative to 2.5vdc instead of 0.00vdc.
For example, with a signal of 0.1v peak AC input, the op amp sees down as low as 2.5 minus 0.1v and up to 2.5 plus 0.1v, which is fine for normal operation. Without that bias, the op amp would see plus 0.1v down to -0.1v and that minus voltage is not correct for the op amp as most op amps need to have the signal between the postive and negative supplies (positive and ground in this case). With the 2.5v bias, the op amp sees a comfortable 2.4v to 2.6v varying AC signal.
If we biased it at only 1v then it would see 0.9 to 1.1v, which is still comfortable for such small input signals.

 

[audioguru]

Most 741 opamps will not work from a supply as low as 5V. The inputs do not work if they are within 3V from the positive or negative supply. This circuit does not have a negative supply so the input must be at least 3V more positive than ground for the inputs to work. The two 10k resistors set the input DC voltage of the opamp at half the supply voltage so that both inputs work properly.

 

[maor]

Wow thanks a lot guys, this really helped me, now I have some questions to ask my electronics teacher.

 

[MrAl]

Hello again,


Yes, op amps like the 741 present another problem for using in circuits like this. The input voltage for linear operation has to be significantly higher than the minus supply voltage and significantly lower than the positive supply voltage. The different could be as much as 8 volts in some units, so at least a plus and minus 15 volt supply is usually recommended.
The LM358 on the other hand is a little more easy to get along with. It's inputs will work down to the minus supply rail, and up to the positive supply rail less 1.5 volts.
That's a better op amp to start with, although it does not have built in offset adjustment pins like the 741 does.