NFB and waveform

[pjd3]

Hi there,

So, I was trying to explain to an associate "technologist" at work what NFB was in guitar amps and how it is captured and used, and what its resultant benefits are.

During my explanation he asked a question I wasn't able to answer on the spot.

He noticed that the + terminal on a speaker is likely only the positive portion of a waveform that was referenced to a ground or zero volts, that place where the speaker is neither being pushed forward or backwards, and how the lack of a negative portion of the signal would effect the quality of the NFB, that only the positive portion of the waveform is being "negatively fed back" into the inverting side of the phase inverter.

I had to tell him I honestly had not thought about that, what the actual waveform is that the PI is "seeing" upon feedback.

Are you able to describe that element of the process?

Thanks. I was a bit embarrassed that after installing a few adjustable NFB on my amps this had never occurred to me.

Best,
Phil D

[Snicksound]

The + terminal is not referenced to ground or 0V (that would require a 3rd wire), it's referenced to the - terminal.

These are conventions we use to make sure the speakers are wired in phase.

It works like any other AC signal. The signal is not split between positive and negative swings (e.g. the phase inverter in the amp does not split into positive and negative swings, it creates two versions of the signal with opposing polarity)

The difference between the + and - pin is your signal, and it swings both ways

[Snicksound]

Wanted to add, in the context of NFB, we typically reference the negative side to ground. Therefore the - becomes the 0V GND reference, so on negative swings the "+" side simply swings negative.

Like I said, we call it positive by convention. It can carry a negative signal as well, seeing that we're talking about AC

[Snicksound]

Fun fact: if you flip the wires around, it sounds exactly the same (unless you do it to only one speaker in a multiple speaker cab)

[R.G.]

The convention is that if you momentarily connect a battery (dc...) across a speaker, if the "+" terminal of the battery is connected to the "+" terminal of the speaker, the cone moves outward. If you flip the battery over, so the battery "-" terminal connects to the speaker "+" terminal, the cone moves inward.

The + and - are polarity markings. They do not imply that the voltages on the speaker must be that polarity. In fact, speakers really need to be driven with AC-only voltages.

So speaker feedback from the speaker "+" terminal, with the speaker "-" terminal connected to 0V/common/ground will be an AC voltage. And amplifiers are designed with this in mind.

[pjd3]

Hello all and thanks for coming by,

I can see that my misnomer was thinking that there was a zero V ground reference, as if there were an isolated + and an isolated - with a ground reference in there somewhere. I'm surprised I fell for that one for a while there, as there are only 2 terminals on a speaker!

The technologist where I work was assuming a ground reference and directed the conversation as if there was one.

But yes, the fog has lifted and the speaker polarity world has come back into view.

Thanks everyone!

Best,
\Phil D

[LOUDthud]

Wait until he figures out that the input to the power amp and the feedback waveforms are IN PHASE. Have your answer ready !

[nuke]

Hi there,

So, I was trying to explain to an associate "technologist" at work what NFB was in guitar amps and how it is captured and used, and what its resultant benefits are.

During my explanation he asked a question I wasn't able to answer on the spot.

He noticed that the + terminal on a speaker is likely only the positive portion of a waveform that was referenced to a ground or zero volts, that place where the speaker is neither being pushed forward or backwards, and how the lack of a negative portion of the signal would effect the quality of the NFB, that only the positive portion of the waveform is being "negatively fed back" into the inverting side of the phase inverter.

I had to tell him I honestly had not thought about that, what the actual waveform is that the PI is "seeing" upon feedback.

Are you able to describe that element of the process?

Thanks. I was a bit embarrassed that after installing a few adjustable NFB on my amps this had never occurred to me.

Best,
Phil D

Let's expand on that a little bit.

For most guitar amplifiers, there's a "hot" terminal and a ground terminal.

But for higher power amps, like bass amps, they are often bi-amped. Meaning there are two power amplifiers driving the speaker, one amplifier on the "+" terminal and an identical amplifier, inverted in phase, driving the "-" terminal to the speaker.

Internally, the power amps are referenced to the ground or mid-point of the power supply.

Now, here's the mind blowing part: Each power amp has it's own NFB to linearize that stage! Neither amp "knows" which phase it is driving.