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

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New Member
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)


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
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schmitt trigger

Well-Known Member
Interesting proposal, but to effectively work, you have to add a resistor in series between the opamp output and what you label "in". You may find that to obtain selectivity the series resistor will be large and most of the amplifier's power will be lost there.

Another problem that I find is that speaker impedance is not constant with frequency.

I would suggest that any signal conditioning that you wish to perform, to be done at the preamp level. You will be able to use a number of opamp configurations that will allow you much greater flexibility and capabilities.
Then you apply the modified signal to a power amplifier.


New Member
its going to be a novelty effect for (tube) guitar amps, between power stage and speaker.
the idea is: guitar strummed lightly, lightbulb rather cool - low resistance, midrange energy dissipated and sound from speaker thins
guitar hit hard, output goes up, lightbulb resistance increases, more midrange goes to the speaker and sound fattens
tube amps do have high output impedance which helps,
but I guess in the end its really a trial and error and kind of hard to predict how its going to perform ... :)


New Member
ok here is the updated version:

the reactive mid shunt involves several lightbulbs in parallel (which will be switchable to adjust)
for a total resistance of just a couple ohms - cold.
energy from amp increases - bulb gets warm, resistance goes up, mid shunt becomes less efficient

the mid-expander network is supposed to work in conjunction with the mid-shunt,
its basically just an open gate at this moment, might improve it with diodes maybe?
energy from amp goes up and this route becomes less resistive letting more mids to the speaker

the compressor network also involves several switchable bulbs in parallel, to compress the highs and lows only
which can be a desireable sound effect on a guitar amp,

I notice now there is a minor fault, the highs and lows in the compressor network should be -200Hz and +750Hz of course,
and not 250/1500, or there would be a gap in the frequencies let through to the speaker.

the desired sound effect is, a more bass heavy and sparkling top end sound when played soft/clean, hit it hard and the mids
shall be more prominent and highs and lows compressed, for a fat midrangy sound, very reactive and dynamic to the playing style.

here is a good tool I found : https://rf-tools.com/lc-filter/

the values of the components are just to fill in the blanks basically, I could not as of yet work out ways to combine these filters and calculate
numbers that make sense so far... if someone could assist would be highly highly appreciated!!! ;):)

the big unknown is how much the lamps resistance will increase from lets say the amps output goes from 5w up to 30w or higher
(these low wattages are perfectly normal in guitar tube amps even for big rock bands)


I dont have membership at circuit lab so I cannot run the circuit that I designed in that program, its rather expensive haha, or maybe
it would be worth it?


Well-Known Member
Most Helpful Member
the big unknown is how much the lamps resistance will increase
Because guitar sounds have a brief peak followed by a longer sustain/decay level, I think the bulb heating will be affected mainly by the sustain/decay level. I'm no musician, but I would guess that level could be only a small fraction of the peak, i.e much less than 30W. Do you have any figures for typical sustain levels?
Here's an LTspice simulation of your circuit. The yellow traces and blue traces are repectively for lamp resistances (a) of 5 Ohms each and (b) 50 Ohms each. I assumed the signal source impedance was matched to the 16 Ohm speaker impedance. The simulation is unrealistic because the two lamps won't heat equally and, as pointed out above, speaker impedance is not constant.
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New Member
hey alec, yep you are hitting it on the nail! A common 100w tube amp may be idling at 20w, depending on the bias of the tubes, unless you crank it up fully it might go
to 50w or 75w usually. The sharp peaks are supposed to go away, but yes it really comes down to how warm the lightbulbs get and stay! I added zener diodes in series with the midrange filter to the spaker, they should have a higher resistance putting more pressure on the bulb and then open up when the bulbs resistance increases, zeners are actually quite smooth in a certain range and not a sharp cutoff at all, but its hard to simulate that stuff,

heres a graph from circuitlab, the top one is with the bulbs at 5, the bottom with bulbs at 25ohm. also the graphs change quite a bit depending on whether its a voltage or a current source.



but I would guess that level could be only a small fraction of the peak, i.e much less than 30W. Do you have any figures for typical sustain levels?
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