Feedback loops...what are they?

[ckim715]

Effects loops and feedback loops. I often see them in modulation/delay/reverb pedals. What are they/what do they do? I know what an effects loop is in an amp, but not sure how that would apply to a pedal. Thanks!

[jtn191]

They work pretty much the same way as an effects loop in an amp. The benefit is that you could engage a series of pedals with just one stomp.

Like Reverb send->phaser->delay-->reverb return if you want your reverb phased and delayed everytime it's on

[stecykmi]

^ that's an effects loop.

a feedback loop is a much more general term for the concept of feedback in physics/engineering and is unrelated to effects loops. A common example might be the feedback resistor in an op-amp circuit.

It's quite a complex phenomenon to understand in depth, but basically the output of a circuit keeps rest of the circuit under control by preventing the gain from getting too high. To further elaborate, if you don't don't connect the feedback resistor in an opamp, it will quickly saturate (ie the output will go to it's max or min, usually somewhat less than the supply voltages). So the feedback resistor send some information back from the output to the input to control the gain, thereby making the circuit stable. You can make the gain frequency dependent by adding capacitors (and/or inductors, although that's more rare), which is why you often see a cap in the feedback loop, but consider this an advanced topic.

I should note that this is negative feedback. Positive feedback can actually makes the gain go up, but also generally makes the system unstable (usually not good because information is lost).

For more information, try reading the wikipedia page on opamps. The feedback resistor is generally labeled "R2". Specifically, notice the gain equation for an op-amp in the inverting configuration:

Vout / Vin = - R2 / R1

This means that if you want your output signal to be 2x as large as your input, the ratio needs to be 2 : 1 (for example 10kohm / 5kohms). This is known as gain. Now notice if you set R2 = infinity (meaning disconnected), the gain is huge (infinite, in fact). There is no way a regular op amp can supply infinite gain (which can't actually exist), so R2 is used to control the output.

also note that feedback is not limited to electrical systems, almost any form of energy in any type of system can utilize feedback (mechanical, chemical, biological [which is debatably a combination of the three others], etc.)

[jimmybjj]

Thanks for that explanation Misha.

[ckim715]

So essentially, and I am probably dumbing this down, a feedback loop is a resistor that limits the amount of gain that is passed through to output by taking some of the signal that is passed through the opamp, hitting a resistor, and re-sending that signal into the input of the opamp? If so, wouldn't the gain just stack over and over again?

<- not an electrical engineer by any stretch of the imagination.

Thanks!

[TNblueshawk]

Dang, I need an explanation for the explanation Misha  Thanks for the info as I have not read it in those terms before.

[JakeFuzz]

One of my control systems professors had the best analogy for feedback that I still use when thinking about these kinds of things. If you are driving a car on the freeway, you know you want to drive in the lines (hopefully  ). We know that we can control the car by moving the steering wheel clockwise or counterclockwise. But if we cannot see the lines on the road how can we know that we are driving inside the lines? If you were trying to drive blind you wouldn't know if you were turning the steering wheel too much or too little and would probably end up crashing into a tree (unstable system). Our eyes (and ears for those of you who like to hit the "wake up" bumps) are our feedback loop. They provide our brains with real-time information about the car and how it is traveling with respect to the road (output). If we adjust the input (steering wheel movement) our eyes will tell us if our input is too large or too small so then we can continually adjust the angle of the steering wheel using our sensory feedback until the car is going straight. So a feedback loop takes information from the system output, processes and uses it to change the input signal until the output is what we would like to be (matches the input signal).   

[ckim715]

So if I am understanding this correctly, hypotethically if I wanted to increase gain, I can lower the value of the feedback resistor and let more gain "pass through"?

[JakeFuzz]

I think you would have to examine it on a case by case basis. The feedback loop is very general and you probably can't apply that rule to every circuit. Is there a specific circuit you were looking at?

[stecykmi]

In general, particularly for opamp circuits, increasing the feedback resistor increases the gain, so the opposite of your earlier thought. This is counter-intuitive I suppose, but you can think of it like adding less of a negative number, ie a net gain. This makes sense when you consider the fact that it's negative feedback.

It's also worth noting that if the feedback resistor is shorted (ie resistance = 0), you can make the gain really low, a minimum of 1 for the non-inverting configuration (Vin = Vout, or a buffer) or even 0 for the inverting config. Analog mixers often make use of this rather than pots because it offers advantages of keeping low impedance output to that circuit block, but I digress.

It's worth noting that playing with the feedback resistor is a common mod for pedal DIY'ers to get more or less gain out of opamp based effects.

[ckim715]

I think I"m starting to understand...grasping at straws, but making baby step progress.

I didn't have any specific circuit in mind, I was just trying to get a general grasp of different aspects of circuitry in general.