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boost converter noise

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Newbie level 5
Jun 7, 2015
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i am building a portable speaker and i am planing to use a 12 volt battery and step up the voltage to 25 volts for my amplifier to get more power out of it but i have a problem , when i connect a boost converter to the amplifier i get a weird high frequency noise that is independent of the amplifier volume. i am relatively new to electronics so i would like an simple explanation . also i know that a car amplifier uses a boost converter to step up the voltage but some how it get s rid of the noise. is there some kind of a simple filter i can build to get rid of this noise , or is there another way to do it ?
this is the amp that i am using -

and this is a boost converter that i use - **broken link removed**

and the battery is 12 volt lithium ion that i made myself from recycled laptop batteries.
please help me that noise is the only thin stopping me from finishing my portable speaker :p

You would do better with a bridge amplifier than boosting the voltage further but it should still work.

The problem is to do with the wiring. That kind of voltage booster draws it's power from the battery as short, high current spikes and that causes a voltage to be dropped along the wiring. It is also a class-D amplifier which itself draws fast current spikes. Your problem comes from letting the spikes enter the input connections. Although the wiring may join the correct points together, the order in which things are connected in the chain of point back to the power supply is also very important. The noise you hear is almost certainly due to the amplifier not only having the audio signal you want but the power line spikes as well because the input ground itself has noise on it instead of being the 'clean' reference it should be.

Things to check, without seeing your construction I can't make specific comments:
1. all the power and ground connections, between the battery and booster and amplifier should be thick wire. The thicker it is, the lower it resistance will be and it's the resistance that causes the voltage drop.
2. wire the voltage booster directly back to the battery.
3. wire the amplifier to the voltage booster (not back to the battery)
4. wire any signal wires only to the input connector, try to keep all the screens on signal wires joined only to the one point at the signal input connector.

thanks for your answer . i will wire everything up and post a picture here for you to see.the first boost converter that i used got shorted because i accidentally placed it on metal surface it was this boost converter - **broken link removed**
i ordered a new one right away. as soon as it arrives i will post a picture here of how is everything wired.

The new boost converter you ordered is the cheapest boost converter that I have ever seen. Usually cheap devices have poor spec's.
Its datasheet does not say its operating frequency which is probably within the audio range. But maybe it oscillates above audio frequencies but shuts off when its voltage is high enough then the voltage drops and it turns on again, then off, then on over and over at an audio frequency.

A car amplifier with a built in boost converter is designed to have the boost converter operate at a frequency above audible frequencies.

If your Lithium rechargeable battery has 3 series cells then it is 12.6V when fully charged, then its voltage quickly drops to 11.1V where is slowly drops as it runs down. When its voltage reaches about 9.6V then the battery is ruined. do you have a low voltage cutoff circuit to protect the battery?
the boost converter that i orderd is this one **broken link removed**
perhaps its a better one i don t know .. do you have any recomendation on what should i use ?
i got the battery covered with this protection circuit
i ve seen that boost converters don t use a lot of components maybe i can build a simple custom boost converter that has a fixed boost voltage , and ofcourse doesn t make noise .

You are buying cheap Chinese junk from ebay. The newest voltage converter you ordered is a total confusion. Its title says "Step Up Step Down" but its spec's say "Buck" but does not say Boost and has a 1V dropout which is probably a step down converter that does not boost the voltage.
There is a Chinese company selling a photo of a protection circuit that looks identical to a real circuit and some cheap batteries have it. Maybe the protection circuit from ebay is the fake one.

You did not say if you will use the amplifier with two speakers in stereo or as a bridged amplifier driving a single speaker. You also did not say the speaker impedance. You also did not say how much output power you are expecting.
isaac456, somethig else to consider: If the amplifier is exepected to produce 100W, and lets say it has an efficiency of 80%, it has to consume around 125W fom your 25V supply. That means it draws around 5 Amps. Next, consider the boost circuit probably has an efficiency of 80% so the current it needs is ((Vout/Vin)/0.8) * Iout which means it could draw 13 Amps. That's quite likely to be far more than your battery can provide. Those circuits are designed be run from big lead acid batteries.

The little 325mAh/70C "7.4V" Li-Po battery for my radio controlled electric airplanes can provide 325mA x 70= 22.75A for about 30 seconds. It weighs 23 grams (less than one ounce) complete with its tiny connector.
My recycled laptop Li-Ion cells have trouble producing 4A and two 18650 cells making "7.4V" weigh 100 grams.
You might need a wagon to carry a huge 12V car battery.

The amplifier is rated for 100W but that is when it is clipping like mad with 10% distortion. It is actually about 60W before clipping.
audioguru ,i asked the seller about the boost converter .. it boost s voltage and now i am waiting for him to supply me with more information about the board, most important the switching frequency of the boost converter . i tested the protection board and it works . i am planing to wire the speaker something like this

- - - Updated - - -

betwixt , i tested the amplifier with a 19v laptop power supply and the voltmeter showed about 0.5 to 0.7 amps before it starts clipping because i removed most of the low frequency tones which draw most current. so the objective of this speaker is not to have low frequency response but to be loud with mid and high frequency sound to use less current

- - - Updated - - -

when i am going to take the speaker out with me i will use a equalizer to remove low frequency tones to preserve the battery life. i am doing this project on a pretty low budget as you can conclude from parts that i buy for right now the recycled battery will do just fine i will make the back panel of the speaker removable so i can always add new features to it.
it seems to me that we got little bit of the subject here , the main reason for this thread was to solve the boost converter noise so i will appreciate from all of you to concentrate on the main subject
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It's performance will probably be about the same. You can't hear frequencies higher than about 18KHz, the upper limit varies from one person to the next. The problem is twofold though, firstly there may be other frequencies as well as the main switching rate and secondly, the Class-D amplifier also uses a high frequency switching signal. Individually, both are outside hearing range but together you may get a 'beat', the difference between the two. The difference will be lower frequency and may fall in the hearing range. For example the booster may be 150KHz and the amplifier 145KHz so you get a 5KHz (audible) interference between the two. It is almost certain that both frequency sources are 'free runnng', meaning they are approximate and may drift with time and temperature or even be slightly different each time you switch on.


good point brian but how do i get rid of that noise ?

You get rid of the noise by turning off the problem and design a circuit that has no noise.
Music without bass frequencies sounds "cheap and tinny". The converter might use an off and on voltage regulator like I described earlier that operates at an audio frequency.
The Chinglish in the instructions is horrible.

it will have as much bass as i want it to have at a certain point because of the equalizer .. but i don t know anything about circuit design

You should be able to eliminate almost all the noise by careful power and signal wiring. Remember that each stage adds it's own noise and power goes in at one and and your small audio signal at the other. You have to prevent the drop in the power wiring from reaching the signal wires. The way to do that is to realize that a signal is only 'clean' (noise free) when referenced to it own ground point so you need to connect the grounds in sequence back to the battery.

Try this: wire it all up as nomal but instead of applying your audio source, directly link the input to it's ground connection. You should notice the noises vanish. All the grounds are electrically joined together but now link the audio input to the power input ground at the amplifier. Notice the difference. Then again by connecting it to the battery ground side. Although there are wires linking all those points together, you should be able to hear the amplified voltage drop along the wires. You will probably even notice a difference in noise between ends of the same wire.


brian ,
thank you for your suggestion i will wire everything as you said .. posting the results as soon as my parts arrive

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