4558 Basic Schematics

[sbirkenstock]

Hi there,
I have quite some experience in building valve amps, but almost none with stompboxes.
I could not find a very simple based on an 4558 opamp on the web. At least not a complete one. And I spent quite a while looking for one.
If you know one, please send me a link or the schematic.

So I tried to build a primitive booster first.
I ground V- (Pin 4), put 9V on V+ (Pin .
I took two 22k resistors from 9v to ground and get 4,5V in the middle that I connect to +In A (Pin 3) via a 10k resistor.
The signal connects to Pin 3 via a 0.022 cap.
The output (Pin 1) I connect to a 250k Poti via another 0.022 cap,
the viper of the pot goes to the amp input, the other side to ground.
Pins 5 and 6 (Inputs B) are grounded.

When I turn it on, the 9V voltage drops to about 2V (Bias to about 1V) and I get some hum, but no signal.
Guess I should still measure 9V if it is turned on?

With Pin 2 I was not sure what to do.
These three things I tried:
1) Ground it
2) connect to output via a 100k resistor
3) connect to Pin 3 via a 100k resistor
The problem remains the same with each.


Any help highly appreciated :-)
best regards,
Stephan

[Kipper4]

Welcome Sbirkenstock.

Take a look here at the bottom of the page and see how it compares to your circuit.

http://www.muzique.com/lab/boost.htm

Some other stuff


http://www.generalguitargadgets.com/how-to-build-it/technical-help/articles/design-distortion/

I hope this helps

[rutabaga bob]

Welcome to the forum!!!

[balkanizeyou]

Hello,
It appears that you are trying to build a simple linear amplifier using a dual op-amp. "How op-amps work" is a very wide and broad topic, but in a lot of simple signal amplifying really it is enough to know how an op-amp based inverting amplifier and non-inverting amplifier work. Those two, along with voltage follower, comparator and relaxation oscillator maybe, form basic "building blocks" that cover most uses for us stompbox builders. You can see them also used as rectifiers, active filters etc.

The funny thing is that you can derive the behaviour of most of those "blocks" by just knowing how the voltage divider works (so, just ohm's law) and one simple rule - op-amp tries to do everything in it's power to keep it's inputs at the same voltage. It just happens that "everything in it's power" means manipulating it's output, because that's the only thing the op-amp can do. There are of course a lot of limitations to op-amps that exist in the real world, unfortunately (like for example a finite differential gain, it's inability to swing fully rail-to-rail, finite common mode rejection ratio, problems with offset voltage, even finite input impedance can be a problem sometimes etc), but this simplified model can be surprisingly useful for a big chunk of analysis.

Take a look at the non-inverting amplifier setup for example (I'm going to assume that we're using a dual power supply here):



Using those two simple guidelines we can deduce the output voltage - the op-amp manipulates it's output so that the voltage at the inverting input (indicated by the "-" sign) is the same as the input voltage. Assuming we're not getting into the saturation region, it means that we know V_(-)=V_in. Obviously the other end of the R_g resistor is at ground, also R_g together with R_f form a voltage divider. Knowing the two resistances in a voltage divider and two of three voltages, we can easily deduce the third voltage, which in this case is (1+R_f/R_g)*V_in.


It gets a little more complicated when you get to the single supply setup, but not by much. Obviously the op-amp output cannot swing below ground in this configuration (and in fact most op-amps can get only as low as 1V or 1.5V from it's power rails), so to achieve maximum possible swing you want to bias it somehow around half the supply voltage. How do you achieve this? You can simply add the 4.5V DC component to your input by coupling the input through a capacitor and then coupling the 4.5V DC through a resistor big enough not to load the input (make sure you don't form an unwanted high-pass filter that cuts your audio frequencies. Think for example 100nF and 1M. Overkill, but it never hurt no-one). You can see that in the muzique link Kipper posted below.


Then instead of connecting the R_g to ground, you connect it to the same 4.5V point in the circuit. Or else, you connect the 4.5V to the ground through some big-ass capacitor (say, 100uF) to make the ground and 4.5V have basically the same "AC potential", and then you add a big capacitor between the R_g and ground - if you choose the capacitor big enough, then for audio signals it's essentially the same as connecting it directly to ground because they offer almost no impedance in this frequency range, but if you choose it carefully, you can get some audible highpass filtering there.

This 4.5v point is often called a "virtual ground" for the op-amp for obvious reasons.

The easiest way to get the 4.5V point is of course the voltage divider - there's a smart article on R.G's website about the choice of resistors in there, but for most single opamp uses you can just throw a pair of 10k's in series from 9v to ground and take the 4.5V voltage from the middle point.

Take for an example MXR micro amp - it uses all of the techniques mentioned above, and adds some others, like subtle LPF cap (I'm too lazy to do the math at the moment, but it looks like it cuts the radio-and-above frequencies in the feedback loop) and anti-pop pulldown resistors.

It would be helpful if you could draw a schematic of what exactly you built.

And the thing about the unused op-amp - if you're using just one opamp of two in one package, the best thing to do IMO is connect it's non-invering input to virtual ground (or ground in the dual-supply case) and connect it's output to it's inverting input - saves you some power, and prevents some nasty weird stuff from happening when the output saturates.

Hope that helps a bit.
Mikołaj

[duck_arse]

what they said ^ sbirkenstock. also, the 4558 is a fairly ordinary part, very similar to many other ordinary type numbers. generally speaking, for a basic audio circuit, anything designed for a TL072 or TL082 or LF353 or LM1458 or LM833 or NE5532 or or or .... can be used with/for/instead of a 4558.

and then you check the datasheets, to see why there are so many different parts.

[GibsonGM]

For one thing, the 4558 will be noisier than say a TL072...but you may not even notice, since we're guitarists and playing 'down here' in the low frequencies, and not up in the GHz where such things are very important!

Welcome!

[sbirkenstock]

Thank you guys!
Got my first booster going now :-)
And thank you Mikolaj for your long explanation, especially for what to do with the unused second opamp.
Very helpful.
Best regards,

Stephan