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

Discussion in 'Analog & Mixed-Signal Design' started by maxbauer, Jul 6, 2018.

1. ### maxbauer Thread Starter New Member

Jul 6, 2018
1
0
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

2. ### Nykolas Member

Aug 27, 2013
108
36
The low output impedance of an amplifier makes this approach quite impractical (huge coils and capacitors), but possible. The amp wouldn't like it much either.
Why not an NTC/PTC resistor network (an L-pad) between amp and speaker? This way you can control impedances.
Both lamp and NTC/PTC resistor network circuits have delays in their attack and decay times do to temperature lag. It would be better to do this with an active circuit in the pre-amp. E

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3. ### Max Bauer New Member

Jul 6, 2018
14
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hy nykolas, yes its very much a trial and error and unsure how its going to perform.

this will be a kind of a novelty guitar effect, strum the guitar lightly, the bulb is rather cool - low resistance,
the shunt network eats up some energy of the midrange which its tuned to, the sound from the speaker thins

hit the strings hard, the bulb heats up and more signal be directed to the speaker and the sound fattens up.

it should be as simple and smoothly operating as possible which is what the bulb is for, no manual operation needed.

preferably used with a tube guitar amp as they have high output impedance, it involves just a few parts so
I should just try and report

4. ### danadak Well-Known Member

Mar 10, 2018
1,438
293
Assuming the bulbs are consistent build to build.

Depending on topology the change in R can affect either or both BW and center
freq. BW because of the loss applied to the "tank", center F because a series RC
can be translated into parallel effective, and R controls the reflected C. Makes sense
as I raise R in a series RC the network becomes more dominated by R and less by
C. In the limit C effectively disappears.

https://www.eetimes.com/document.asp?doc_id=1278134

http://www.phys.ufl.edu/~majewski/n...ries to Parallel Impedance Transformation.pdf

Regards, Dana.

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5. ### -live wire- Active Member

Dec 22, 2017
800
70
Why not replace your incandescent bulb with an LED that will be 10 times more efficient? You can use a capacitor network to get the same effect with it slowly turning on. I'm sorry, but I just can stand to see people abuse power and use such inefficient things.

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6. ### Max Bauer New Member

Jul 6, 2018
14
0
hy dana, appreciate your input! concerning changing the BW, in the simulation yes the BW increases substantially as R increases, in the same time the damping factor goes down alot also, so maybe the effect of the filter drops enough so it wont be noticeable when R rises?
center f stayed the same the way one calculator showed, I am no expert on this by any means so I could be wrong.

7. ### Max Bauer New Member

Jul 6, 2018
14
0
hey live, haha I get your point thing is, guitarists love century old guitars and tube amps which are by themselves the pinnacle of inefficiency haha

can you explain how it would work with a LED and cap network for me ? I have looked into LEDs and have a hard time figuring out which one to use, those which can handle the potential power at this point open at rather high voltages no? maybe I dont understand how it works, take me through it if you will

8. ### danadak Well-Known Member

Mar 10, 2018
1,438
293
I was not paying attention. You have a series resonant circuit, so changing R
will not affect F but will affect Q (losses) hence BW.

One way of having BW stay constant is to design filter with both a fixed
Rfixed and the Rbulb, such that Rfixed >> Rbulb. That way changes in
Rbulb will not have appreciable impact on BW. But this causes less energy
to be diverted from speaker at low volumes.

I think a control loop, one that measures freq content and then suppresses
in band from speaker makes most sense. That would be an OpAmp based
filter followed by a RMS detector (to measure inband energy) driving a MOSFET
in series with the network. MOSFET acting as a variable power R.

Regards, Dana.

Do

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9. ### -live wire- Active Member

Dec 22, 2017
800
70
I am kind of thinking you drive a power LED with constant current almost up to it's max rating. You have an op-amp controlling the current. There is a capacitor that gets charged slowly through a resistor, with a charge voltage dependent on the sound level. There is also a discharge resistor. The voltage of the capacitor controls the op amp controlling the LED. This will be far more efficient and allows for a lot more customization.

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10. ### Max Bauer New Member

Jul 6, 2018
14
0
hey Dana, this is going the active route no? I would rather try the simple passive way first,
check out my updated circuit down below

11. ### Max Bauer New Member

Jul 6, 2018
14
0
hey live, this requires an active power source no? check out my updated version down below please

12. ### Max Bauer New Member

Jul 6, 2018
14
0
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?

13. ### danadak Well-Known Member

Mar 10, 2018
1,438
293
If you have a bench supply with DC you could characterize the bulbs
R by simple measurements.

Clever idea, and current topology looks like less impact on BW variance.

Could sim in LTC by creating a current or voltage controlled R possibly ?
Or just run multiple sims to get response with various fixed bulb R's. Or
do a sweep of Rbulb in component sweep capability of spice.

LTC is a freebee.

http://www.analog.com/en/design-center/design-tools-and-calculators.html

Controlled Rs -

http://www.ecircuitcenter.com/Circuits/vc_resistor1/vc_resistor1.htm

Regards, Dana.

Last edited: Jul 9, 2018
14. ### danadak Well-Known Member

Mar 10, 2018
1,438
293
Sim at 5 ohms each bulb -

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15. ### danadak Well-Known Member

Mar 10, 2018
1,438
293
2 ohms each bulb

16. ### danadak Well-Known Member

Mar 10, 2018
1,438
293
Here is sim file, attached.

Regards, Dana.

File size:
1.4 KB
Views:
1
17. ### Max Bauer New Member

Jul 6, 2018
14
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thanks mate! I am right now registering at circuit lab, the cheapest membership is just a couple bucks per month, will get back later with some graphs and see how it compares with spice and other programs!!

I am also looking into zeners to add in series with the mid-freq gate to the speaker to make it more dynamic

18. ### Alec_t AAC Fanatic!

Sep 17, 2013
8,610
1,956
The sim circuit doesn't match the post #12 circuit.

19. ### danadak Well-Known Member

Mar 10, 2018
1,438
293
I did have a wiring error, here is 2 ohms -

And 5 ohms

20. ### Alec_t AAC Fanatic!

Sep 17, 2013
8,610
1,956
Danadak, the TS has C3 connected directly to V1.