Electronics > Beginners
putting a resistor in series with a speaker to increase the impedence
JS:
--- Quote from: Adhith on July 24, 2018, 07:06:38 pm ---
--- Quote from: JS on July 24, 2018, 05:32:51 pm ---If you add a resistor you will just drop on efficency and peak power capability. Also damping factor will drop dramatically.
With 4S you have about 15V nominal, into the 4Ω it's baerly 25W rms if the tpa goes close to rails, as it should, with music the average power consumption will be much lower, like under 10W with a reasonable THD. But. You could get 800W PMPO if you want to sell it.
Adding the resistor makes no sense, higher impedance coils makes better response and efficency for higher voltage situations, but adding a resistor in series only makes those numbers worse.
JS
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Thank you very much for your reply. So I understood that adding resistor is a bad choice. So are you saying that the better option is finding another driver of some higher impedance like 8 ohm??
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No, I actually think 4Ω is the best option, as it usially is in low voltage situations. With 8Ω you will only get about 7W RMS instead of the intended 25.
JS
bson:
--- Quote from: Adhith on July 24, 2018, 07:02:17 pm ---The reason that I'm planning to higher the impedence is because, during startup the amp instantly consumes a peak current of 1.8A for 2 seconds and then lowers the current draw to a very low level.
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This isn't just an inrush current? That won't be affected by speaker impedance at all. On the other hand, if you have large DC spikes on the output during power-on those need to be addressed independently or you will eventually destroy the speaker coils.
janoc:
--- Quote from: Adhith on July 24, 2018, 07:19:44 pm ---
--- Quote from: HB9EVI on July 24, 2018, 05:06:17 pm ---A common solution for these problems is a relay that will connect the speakers only after a few seconds delay - once all the capacitors are charged and the amp has stabilized. It also prevents the nasty "THUMP" from the speakers on power up which could damage them.
Something like a 555 timer in a monostable mode + a transistor to drive a small relay would do the job just fine.
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Thank you very much for you help Sir. Since I'm a beginner could you help me with a suitable diagram?? maybe a circuit that connects the speaker after 4 or 5 seconds??
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There are plenty of these designs online, search for "speaker delay circuit", "speaker protection circuit", "anti-pop circuit", etc.
E.g. something like this one (didn't test it myself):
http://www.redcircuits.com/Page171.htm
Just a transistor/darlington and a few passives. The large capacitor will slowly charge up making the transistor eventually turn on and energize the relay connecting the speakers.
If your amplifier has a microcontroller for something, you can use that for a delay too. There are many ways how to build this.
Adhith:
Thank you janoc. I'll refer the links that you have suggested
Zero999:
Why not just use a 555 timer?
This should be unnecessary for the TDA7492P, which has mute and standby inputs, that can be used to delay the turn on and avoid any audible clicks, during power on.
In this case, I think this has nothing to do with the speakers. You'll probably find the board still draws a huge inrush current, even with the speakers disconnected. Refer to the circuit on page 5 of the data sheet. It has a 1000µF capacitor (C32, next to J2) across the power rail, which is probably causing the problem.
https://www.st.com/resource/en/datasheet/tda7492p.pdf
Try adding a 22R resistor in series with the amplifier board, for the first second, after applying the power, then bypass it with a wire link. It will limit the current to a safe level and stop the BMS shutting down. A more permanent solution is to use a timer relay (either a 555 & relay or you might be able to find a module) to bypass the resistor, after 0.1s to 1s, of the power being applied.
The above solution will only work at low power levels. You need a more powerful BMS which is capable of providing the amplifier, with the current it needs, otherwise this will happen again, when the volume is turned up too high and there's lots of bass in the music. Your amplifier needs a power supply capable of providing 6A peak, to stand any chance of being able to work at full power. Even changing to an 8R speaker will still cause the amplifier to draw more current, with the volume turned up, than your current BMS can provide.
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