putting a resistor in series with a speaker to increase the impedence - Page 1
Forum section: electronics / beginners

author: Adhith
author_rank: Frequent Contributor
timestamp: July 24, 2018, 04:44:21 pm
content: 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??

author: HB9EVI
author_rank: Frequent Contributor
timestamp: July 24, 2018, 05:06:17 pm
content: you can add a resistor, but you can also just keep the output level low enough to not exceed the rating of the amp

author: JS
author_rank: Frequent Contributor
timestamp: July 24, 2018, 05:32:51 pm
content: 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

author: Adhith
author_rank: Frequent Contributor
timestamp: July 24, 2018, 07:02:17 pm
content: Quote from: HB9EVI on July 24, 2018, 05:06:17 pmThe 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.

author: Adhith
author_rank: Frequent Contributor
timestamp: July 24, 2018, 07:06:38 pm
content: Quote from: JS on July 24, 2018, 05:32:51 pmThank 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??

author: janoc
author_rank: Super Contributor
timestamp: July 24, 2018, 07:13:44 pm
content: Quote from: Adhith on July 24, 2018, 07:02:17 pmA 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.

author: Adhith
author_rank: Frequent Contributor
timestamp: July 24, 2018, 07:19:44 pm
content: Quote from: HB9EVI on July 24, 2018, 05:06:17 pmThank 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??

author: Jr460
author_rank: Regular Contributor
timestamp: July 24, 2018, 07:21:18 pm
content: Quote from: Adhith on July 24, 2018, 07:06:38 pm8 ohm is very common, 16 is also not hard to get from OEMs.

author: VinzC
author_rank: Frequent Contributor
timestamp: July 24, 2018, 07:24:22 pm
content: Seeon "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.

author: Audioguru
author_rank: Super Contributor
timestamp: July 24, 2018, 08:58:59 pm
content: 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.

author: JS
author_rank: Frequent Contributor
timestamp: July 25, 2018, 04:49:09 am
content: Quote from: Adhith on July 24, 2018, 07:06:38 pmNo, 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

author: bson
author_rank: Unknown
timestamp: July 25, 2018, 05:02:03 am
content: Quote from: Adhith on July 24, 2018, 07:02:17 pmThis 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.

author: janoc
author_rank: Super Contributor
timestamp: July 25, 2018, 01:55:39 pm
content: Quote from: Adhith on July 24, 2018, 07:19:44 pmThere 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.htmJust 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.

author: Adhith
author_rank: Frequent Contributor
timestamp: July 25, 2018, 03:16:19 pm
content: Thank you janoc. I'll refer the links that you have suggested

author: Zero999
author_rank: Super Contributor
timestamp: July 25, 2018, 04:26:12 pm
content: 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.pdfTry 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.

author: JS
author_rank: Frequent Contributor
timestamp: July 25, 2018, 04:39:04 pm
content: NTC is what's used to limit inrush, no need for resistor and bypassing as this devices do all for youJS

author: Zero999
author_rank: Super Contributor
timestamp: July 25, 2018, 04:45:59 pm
content: Quote from: JS on July 25, 2018, 04:39:04 pmI'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.

author: Seekonk
author_rank: Super Contributor
timestamp: July 25, 2018, 04:51:32 pm
content: 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.

author: Zero999
author_rank: Super Contributor
timestamp: July 26, 2018, 07:47:41 am
content: Quote from: Seekonk on July 25, 2018, 04:51:32 pmHow would that help?The capacitor would just bypass the resistor at AC, causing the thump to happen anyway. The resistor won't do anything!

author: Adhith
author_rank: Frequent Contributor
timestamp: July 26, 2018, 08:45:23 am
content: Quote from: Hero999 on July 25, 2018, 04:26:12 pmThank 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).Quote from: Hero999 on July 25, 2018, 04:26:12 pmI 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??

author: Zero999
author_rank: Super Contributor
timestamp: July 26, 2018, 11:16:52 am
content: 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.

author: Adhith
author_rank: Frequent Contributor
timestamp: July 26, 2018, 01:01:25 pm
content: Quote from: Hero999 on July 26, 2018, 11:16:52 amOk 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??

author: Zero999
author_rank: Super Contributor
timestamp: July 26, 2018, 01:07:54 pm
content: Quote from: Adhith on July 26, 2018, 01:01:25 pmThe 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.

author: janoc
author_rank: Super Contributor
timestamp: July 26, 2018, 01:56:35 pm
content: Quote from: Adhith on July 26, 2018, 08:45:23 amThat 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 supplymust be ableto 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.

author: johnkenyon
author_rank: Regular Contributor
timestamp: July 27, 2018, 12:16:55 am
content: Quote from: Hero999 on July 26, 2018, 11:16:52 amThree 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.

author: james_s
author_rank: Super Contributor
timestamp: July 27, 2018, 03:31:38 am
content: YMMV, but in my own experience it works fine to drive a 4 Ohm speaker from an amplifier designed for 8 Ohm speakers. This may not always hold true but I can't think of a case where I've had any trouble with it so long as the volume is kept at a reasonable level.Something else you can do is drive a pair of 4 Ohm speakers in series on each channel.

author: Zero999
author_rank: Super Contributor
timestamp: July 27, 2018, 07:58:23 am
content: Quote from: johnkenyon on July 27, 2018, 12:16:55 am1) The problem with that is it takes a finite time for a normally closed relay's contacts to open and it will need to break the full current surge, so will be less reliable, than a normally open contact.2 & 3) Good ideas, but it might be difficult/impractical to break the connection between the reservoir capacitor and amplifier, if the PCB is already made.Quote from: james_s on July 27, 2018, 03:31:38 amYes an amplifier designed of a 4R speaker should be fine with an 8R load.I wouldn't recommend connecting speakers in series, because it will result in a poorer damping factor and more peaks and troughs in the frequency response.

author: Adhith
author_rank: Frequent Contributor
timestamp: July 27, 2018, 02:54:05 pm
content: Thank you every one once again for all those sincere and detailed explanation. I'll build one such circuit and let you guys know about the result.

author: Audioguru
author_rank: Super Contributor
timestamp: July 27, 2018, 03:59:29 pm
content: Quote from: Hero999 on July 27, 2018, 07:58:23 amPlayers of electric guitars like that awful sound and call it "effects".