# Summing each volume-controlled output from R, L and C

Discussion in 'The Projects Forum' started by fez, Dec 6, 2009.

1. ### fez Thread Starter Member

Dec 6, 2009
47
1
Long post. Please bear with me. I'll try to convey efficiently.

So I have a project due, its supposed to be a three band mixer + equilizer.
So what we came up with was to take a song, digitally separate it into its three bands of Hi, Mid and Low, send those bands individually via 3 mp3 players, sum + amplify the voltages via a summing amplifier. Then have the signal filtered/separated via a RLC in series, capture the voltage differences across each component in the RLC (to get three different frequency bands) (same as the ones given or different, doesn't matter), have those bands volume controlled (the EQ-ing), sum them, into a power amplifier, and hear through a speaker.

We've made 2/3 of the circuit, I think. Things are fine till the RLC bit. here is figure 1.The output from the summing amp is coming from the left.

Now, I have tested, through a speaker as well as the oscilloscope, the voltages being received. I have tested across 12, 34, 56, and 16 as well. All is well, the R has the band frequencies accross it, the L has the highs and the C has the lows. And all of them are being received across 16. If its any help, the RMS of the output is around 2 to 2.5 V. (I keep changing value of Rf on the summing amp for calibration)

On paper, this is what I had in mind:-

...to use a voltage divider method to receive volume-controlled (EQ-ed) outputs, lay the outputs in series on a wire which heads to the power amp or speaker or whatever.

When I actually got around to doing this on the breadboard, I got not output across 16! (Can't remember about 12 or 34 or 56. I think it was zero output as well)

I thought, perhaps I don't have the variable resistors connected right or something. So, I took the wires at 2, 4 and 6, changed all their connections from the middle pin to the right pin of the their respective var. resistors. But still no signal.

Then I removed the variable resistors alltogether, so that 123456 all connect directly to the wire on which the RLC lies. Still nothing.

I was forced to conclude that I had the concept wrong. So, my questions:

1. What is wrong with my concept?
Allow me to explain the reasoning behind it. We have studied, in our first circuit analysis + theory course, about phasors, filters, KVL KCL, Nodal and Mesh analysis, transfer functions, two port networks, wye wye and delta circuits, and other stuff.
Ever since studying two port networks I started seeing things as "boxes"/blackboxes. I started to compartmentalize in viewing circuits, and it made sense to me. Input -> blackbox - > output. When I made this project on paper I concluded midway that any output of potential difference is a possible candidate for a "source" in another circuit. I mean, its just like having two blackboxed cascaded, the output from the first is the input or source for the second. These two things, along with the circuit analysis we did in the start of the course, convinced me that having two or more output voltages in series is just like having two or more batteries in series! What do batteries in series do? They add up, ofcourse! I mean, consider the power supply unit itself. What does it do? It provides you an output voltage. On paper, you show the power supply unit as a power supply or as a battery, but what IS the power supply unit, really? It's a blackbox itself! It has its own circuit inside of it! In the end, the power supply itself is giving you a voltage-divided output from some complex circuit inside of it.
So I was convinced that I could several voltage outputs in series and I'd get a sum.

2. What should I do? Please suggest a circuit. I need to get the voltages across the components, have the individual voltages controlled, and sum them. I am not willing for another another summer amplifier, we need to conserve space and equipment for several reasons.

I should say that I still don't understand HOW a summer amplifier works, though. I know what an inverting op amp in negative feedback does (though I lack the reasoning behind that too), but the summer amplifier escapes me. I am under the impression that the "sum" is generated at the inverting input of the op amp, which is then amplified by it. But the "sum" fails to make sense to me since the input voltages seem to be in parallel! And don't they lose the input voltages altogether across the resistors (R1, R2 and R3 in the picture) anyway??!!

2. ### Audioguru New Member

Dec 20, 2007
9,411
896
Your circuits with the coils and capacitors do not show an input and do not show outputs.

The inverting opamp with many inputs is a "perfect" summer. Its inputs do not affect each other.