Author Topic: putting a resistor in series with a speaker to increase the impedence  (Read 6223 times)

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Offline AdhithTopic starter

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hello everyone...
I'm building a speaker setup using the 25w+25w amplifier board based on the class D amplifier IC  TDA7492P. The amp board also has an inbuilt Bluetooth receiver module and also I'm using a 4ohm driver at the output. The description about the amp board says that it is ideal for 8 ohm drivers even though it could work with 6 and 4 ohms without problem. Since 4 ohm was on the borders of limit and since Im powering the amp board with a 4S1P battery pack I'm thinking of using a higher ohm driver to reduce the total power consumption.
So what my question is, is there any way to increase the impedence with out changing the  4ohm driver that i'm currently using. does a 4 ohm resistor in series with the driver work?? if so what should be the wattage rating of it??
 

Offline HB9EVI

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you can add a resistor, but you can also just keep the output level low enough to not exceed the rating of the amp
 
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Offline JS

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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

If I don't know how it works, I prefer not to turn it on.
 
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Offline AdhithTopic starter

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you can add a resistor, but you can also just keep the output level low enough to not exceed the rating of the amp
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. This peak current is making the problem of the bms to shut down(maybe it was kind of confusing as a short due to the high current draw of 1.8A). So the board is not at all even getting started up at the first place since  the bms turns off before it.
But if the drivers are disconnected during startup and connected after the first two seconds of startup, then there is no problem and everything works smoothly.
This board does have this thing of drawing too much current at the start up since i have  found some articles about the same on internet.
« Last Edit: July 24, 2018, 07:04:30 pm by Adhith »
 

Offline AdhithTopic starter

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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
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??
 

Online janoc

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you can add a resistor, but you can also just keep the output level low enough to not exceed the rating of the amp
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. This peak current is making the problem of the bms to shut down(maybe it was kind of confusing as a short due to the high current draw of 1.8A). So the board is not at all even getting started up at the first place since  the bms turns off before it.
But if the drivers are disconnected during startup and connected after the first two seconds of startup, then there is no problem and everything works smoothly.
This board does have this thing of drawing too much current at the start up since i have  found some articles about the same on internet.

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.
« Last Edit: July 24, 2018, 07:15:58 pm by janoc »
 

Offline AdhithTopic starter

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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.
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??
 

Offline Jr460

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So are you saying that the better option is finding another driver of some higher impedance like 8 ohm??

8 ohm is very common, 16 is also not hard to get from OEMs.
 
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Offline VinzC

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See on "impedance matching" and some of the common preconceptions. As mentioned earlier, adding a resistor will at least dissipate half of the power in the resistor, among other unwanted effects. Note: Paul McGowan is the CEO of Psaudio.
 
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Offline Audioguru

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The datasheet of the European TDA7492P says that it can drive a 6 ohm or 8 ohm speaker but not a 4 ohm speaker unless the supply voltage is reduced because its maximum allowed peak output current and its heating must not be exceeded. The Chinese module you have says different things, maybe its TDA7492P is a defective Chinese copy.
Your 6 cells battery is 16.8V when fully charged and the output from the amplifier into a 6 ohms speaker is 16W so its peak voltage will be about 12V into 4 ohms which needs a peak current of only 3A. The Chinese heatsink might be too small for the output to be 16W into 6 ohms and much too small for more than 16W into 4 ohms.

Your battery and its BMS are too small for only one channel of this two channels amplifier.

Adding a resistor in series with a speaker ruins the excellent damping from the amplifier. Then audio sounds ring like a bell and low frequencies sound boomy like a bongo drum and higher frequencies shriek.

 
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Offline JS

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Re: putting a resistor in series with a speaker to increase the impedence
« Reply #10 on: July 25, 2018, 04:49:09 am »
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
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??
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

If I don't know how it works, I prefer not to turn it on.
 
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Offline bson

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Re: putting a resistor in series with a speaker to increase the impedence
« Reply #11 on: July 25, 2018, 05:02:03 am »
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.
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.
 
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Online janoc

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Re: putting a resistor in series with a speaker to increase the impedence
« Reply #12 on: July 25, 2018, 01:55:39 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.
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??

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.

 
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Offline AdhithTopic starter

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Re: putting a resistor in series with a speaker to increase the impedence
« Reply #13 on: July 25, 2018, 03:16:19 pm »
Thank you janoc. I'll refer the links that you have suggested
 

Online Zero999

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Re: putting a resistor in series with a speaker to increase the impedence
« Reply #14 on: July 25, 2018, 04:26:12 pm »
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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Offline JS

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Re: putting a resistor in series with a speaker to increase the impedence
« Reply #15 on: July 25, 2018, 04:39:04 pm »
NTC is what's used to limit inrush, no need for resistor and bypassing as this devices do all for you

JS

If I don't know how it works, I prefer not to turn it on.
 

Online Zero999

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Re: putting a resistor in series with a speaker to increase the impedence
« Reply #16 on: July 25, 2018, 04:45:59 pm »
NTC is what's used to limit inrush, no need for resistor and bypassing as this devices do all for you

JS
I'm doubtful an NTC resistor will work in this application. If this were for a motor or incandescent lamp, I'd agree, but I'm not sure it'll work here.

An audio amplifier will draw huge peak currents, along with a very low quiescent current, so I don't think the resistor will heat up enough to reduce its value low enough, to power the circuit. I think the value of the NTC resistor will stay high, after power on, causing the supply voltage to collapse, resulting in lots of distortion, as soon as the beat drops.
 

Offline Seekonk

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Re: putting a resistor in series with a speaker to increase the impedence
« Reply #17 on: July 25, 2018, 04:51:32 pm »
If it is just a thump that worries you, Add that resistor in series with the speaker and just add a 470uF capacitor in parallel with that.  Soles the low resistance DC startup without changing audio much.
 

Online Zero999

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Re: putting a resistor in series with a speaker to increase the impedence
« Reply #18 on: July 26, 2018, 07:47:41 am »
If it is just a thump that worries you, Add that resistor in series with the speaker and just add a 470uF capacitor in parallel with that.  Soles the low resistance DC startup without changing audio much.
How would that help?

The capacitor would just bypass the resistor at AC, causing the thump to happen anyway. The resistor won't do anything!
 

Offline AdhithTopic starter

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Re: putting a resistor in series with a speaker to increase the impedence
« Reply #19 on: July 26, 2018, 08:45:23 am »
Why not just use a 555 timer?

Thank you very much Sir for your reply. I'm trying to solve this issue with a 555 timer which you have suggested in my other thread( 555 timer circuit for 5 seconds).

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.

I had connected an ammeter across the amplifier board to see the current draw. As I mentioned before the current draw was about 1.8A at the start up and then reducing considerably. But when I disconnected one speaker the current draw is only about 1A and the BMS is driving the amp smoothly without any trouble. So I guess there is some rush of current to the speaker during start up or I'm totally wrong??
Another thing about the amp board is that, it plays a music at the startup of the amp and the BMS is shutting down just before that music is played. So does this means that the startup music is drawing all this current??
 

Online Zero999

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Re: putting a resistor in series with a speaker to increase the impedence
« Reply #20 on: July 26, 2018, 11:16:52 am »
The mute function can be used, as described in the TDA7492P data sheet. Failing that, use the timer relay circuit to bypass the current limiting resistor, once the surge has passed. 5 seconds is probably much longer than necessary. 1 second or less will do.
« Last Edit: July 26, 2018, 11:20:13 am by Hero999 »
 

Offline AdhithTopic starter

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Re: putting a resistor in series with a speaker to increase the impedence
« Reply #21 on: July 26, 2018, 01:01:25 pm »
The mute function can be used, as described in the TDA7492P data sheet. Failing that, use the timer relay circuit to bypass the current limiting resistor, once the surge has passed. 5 seconds is probably much longer than necessary. 1 second or less will do.

Ok got it. but one small thing what would be the wattage rating of the current limiting resistor??
Also would it be wrong to use a dpdt relay to cutoff both the left and right audio output to the drivers??
 

Online Zero999

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Re: putting a resistor in series with a speaker to increase the impedence
« Reply #22 on: July 26, 2018, 01:07:54 pm »
The mute function can be used, as described in the TDA7492P data sheet. Failing that, use the timer relay circuit to bypass the current limiting resistor, once the surge has passed. 5 seconds is probably much longer than necessary. 1 second or less will do.

Ok got it. but one small thing what would be the wattage rating of the current limiting resistor??
Also would it be wrong to use a dpdt relay to cutoff both the left and right audio output to the drivers??
The resistor is only used for a short period of time, so can be a fairly low power rating like 1W.

You could add another relay to cut power to the speakers, but it shouldn't be necessary for this circuit, which should limit the inrush current to the amplifier, to a safe level.
 
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Online janoc

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Re: putting a resistor in series with a speaker to increase the impedence
« Reply #23 on: July 26, 2018, 01:56:35 pm »
Another thing about the amp board is that, it plays a music at the startup of the amp and the BMS is shutting down just before that music is played. So does this means that the startup music is drawing all this current??

That just means that your BMS/power supply is not suitable (is way too weak) for powering this amplifier. The amplifier has a low quiescent current (when not playing anything after the initial inrush) but then when you actually attempt to play sound on it, it needs to get the energy from somewhere. The same when there are any signal peaks (e.g. a big drum note). Then the current through the circuit will jump even an order of magnitude higher than the average current of the amplifier for a short moment. Your power supply must be able to handle this otherwise the amplifier will clip severely or you will burn something out.

You must fix this before you try to do anything else because it is meaningless to troubleshoot issues with inrush current and what not when  your power supply will not be able to handle the power requirements of your amplifier when actually in use anyway.

BTW, I wouldn't trust what your meter shows you on the inrush current - multimeters are not great at measuring short transient events, especially if you have a cheap meter that makes only 2-3 measurements per second. There could well be a high current spike and you will miss it because it falls between two measurement cycles of your meter. A better way to measure things like this would be using an oscilloscope, e.g. by measuring the voltage drop across a small value shunt resistor.

 
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Offline johnkenyon

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Re: putting a resistor in series with a speaker to increase the impedence
« Reply #24 on: July 27, 2018, 12:16:55 am »
The mute function can be used, as described in the TDA7492P data sheet. Failing that, use the timer relay circuit to bypass the current limiting resistor, once the surge has passed. 5 seconds is probably much longer than necessary. 1 second or less will do.


Three ideas:

1) Use the suggested circuit, but bypass the current limiting resistor with a normally closed relay contact, and have a timer which energises it for 5 seconds.

2) Put a normally closed relay contact on the power rail between the reservoir capacitor and the load and drive the relay for 5 seconds at power up time.
That way, you have 5 seconds of the battery feeding capacitor only, followed by the battery topping off the capacitor and powering the load.

3) Have the battery circuit permanently connected to the reservoir capacitor and switch the amp on afterwards.
Same as 2 above, but the relay contacts are replaced by the on/off switch - no timer required.

 
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