# Volt/Amp dependent RLC band pass -> Loudspeaker expander using lightbulbs

#### maxbauer

Jul 6, 2018
4
hey everyone, my name is max and I like to tinker with amps and speakers

I want to create a input energy dependent frequency shunt network which goes in parallel with a loudspeaker,

at low volumes a certain midrange frequency should be shunted off the loudspeaker, the shunt effect should go down as the power from the amp goes up, and more midrange signal reach the loudspeaker

Series RLC variable resistance meaning:

at low voltage/current from the amplifier, the shunt network impedance should be low and reduce a certain audio band that reaches the speaker, lets say center f 500Hz, with a bandwidth of 300Hz , -25db

as the energy coming from the amp goes up, the shunt network impedance should go up and eliminate less! signal and deliver more to the speaker,

center f and bandwidth should stay the same as energy goes up or down

my idea is to use a LC band pass resonance network in series with a lightbulb, as the energy increases, the lightbulb heats up, resistance increases, which raises the impedance of the shunt network, and directs more energy to the loudspeaker

A cold 100W bulb reads around 40ohms, it goes up tenfold when hot. I imagine 3 bulbs in parallel to 13ohm cold.

I realize the following issue,

The lightbulb resistance apparently does not influences the center frequency but the bandwidth!, at cold 10ohm the bandwidth is around 300Hz, at hot 100ohm the bandwidth goes up to around 3000Hz, (which I do not want)

but:

The 16ohm loudspeaker resistance will practically be in parallel with the lightbulb resistance, would this deliver a stable low resistance to keep the bandwidth at a narrow range?

Also the damping factor of the shunt network goes from 0.3 at 10ohms, up to 3 at 100ohms, can someone explain the damping factor for me? can this be translated in a total volume loss in db ? or in voltage or current?

Is this whole thing even a possible approach or are there other issues with this idea?

thank you for any help in advance max

#### duke37

Jan 9, 2011
5,364
Welcome Max

If the amplifier has a low output impedance, then the speaker voltage and output will not change, just the bulb will light at the resonant frequency

#### maxbauer

Jul 6, 2018
4
hey duke!

okay, well if the bulb lights at resonant frequency does that mean substantial energy at that resonance is taken away from the speaker and burned in the bulb, or will there not be a substantial drop in volume at that frequency in the speaker?

edit: would there be another passive solution for the genereal idea presented?

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#### duke37

Jan 9, 2011
5,364
If the amplifier has a low output impedance, then it will produce more power at the resonant frequency to light the bulb. The energy comes from the amplifier. which maintains a constant voltage.

If you raise the output impedance of the amplifier, say with a 5Ω resistor, then the speaker volume will drop at resonance.

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#### maxbauer

Jul 6, 2018
4
If its a Tube Output Stage It uses an outputTransformer to Match the speaker impedance to the Tubes which Need to See a couple thousand Ohms, does this Qualify AS Low Output?

If the amplifier has a low output impedance, then it will produce more power at the resonant frequency to loght the bulb. The energy comes from the amplifier. which maintains a constant voltage.

If you raise the output impedance of the amplifier, say with a 5Ω resistor, then the speaker volume will drop at resonance.

#### duke37

Jan 9, 2011
5,364
A solid state amplifier will have lots of negative feedback and a very low output impedance.

A valve amplifier without feedback will have a high output impedance but quality amplifiers will have negative feedback to reduce distortion and this will reduce the output impedance.

There is a vast difference between valve output impedance, for pentodes it could be 1MΩ and optimum load line (a few kΩ) to get the power out. Some overvoltage protection is necessay if there is little or no feedback.

#### maxbauer

Jul 6, 2018
4
its going to be a novelty sound effect thing for guitar tube amps, which have less NFB than HIFI amps, guess I will have to give it a try

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