Problem building regards weird noise (TDA7240)

[guitar89]

I was building amplifier based on chip TDA 7240A, but encounter a weird noise.
It only happen when i connect to my music source (netbook), but doesn't happen that when i directly connect to my speaker unit (without amplifier).

It notice when i'm not using AC (use battery power) on my netbook, that noise doesn't happen. This also same for my previously build discrete amplifier.

But if i use any commercial brought amplifier, no such problem. So i guess need to incorporate something to make this noise disappear ?

hm.... i decided to record the noise using my netbook's in-built mic, so its not good quality. Attached in following link :

**broken link removed**
At the first 4 second, it was the ambient noise (the amplifier is turn off) and was raining (doesn't sounds like raining is it my mic sucks).
After the first 4 second, i turn on the mic amplifier, with AC powered netbook as input. (without any music on, just plug in), the noise is obviously heard, but the pitch is different in real world (my lousy mic......) Is it oscillation ?

About at 12 second, I plug out the AC charger, thus on battery powered only. That makes the noise goes away, and speaker is nice with just a little hum (acceptable and can't heard by the mic).

At 22 second, I plug in the AC charger into netbook again, thus the noise came out again. About at 28 second, I remove the input TRS jack, thus no noise heard. At 30 second, I plug in the input jack to netbook, thus making noise again.

Any advise how to remove such noise from my amplifier (discrete amp noise is different)?

 

[audioguru]

The amplifier is producing a "motorboating" sound. It is caused when the output of the amplifier draws current which causes the supply voltage to sag which is amplified by the mic preamp or amp. The supply must have better voltage regulation or the mic preamp or input bias of the amplifier must be properly filtered to prevent it.

Please post the schematic of the preamp and amp circuits showing the power supply filtering.

 

[guitar89]

**broken link removed**
I have build based on this link, although my 470uF in the picture is 2 x 220uF, others mostly the same. (I use point to point solder technique, which uses point by point proto board)

As for the power supply, i'm using a computer power supply (capable to supply to 14A of 12V)

There is no preamp in my design, only this power amp. I can't quite get what need to do X.x

 

[guitar89]

also when the input' hot and ground are shorted, the weird noise change into low pitch similar noise. (just tested)
Adding resistor between hot and ground doesn't change anything.

 

[cachehiker]

That's the same circuit as the example given on the datasheet.

https://www.electro-tech-online.com/custompdfs/2011/10/CD00010656.pdf

I also suspect the motoring sound is coming from the power supply but that it has nothing to do with feedback through a preamp stage. A lightly loaded 14A switched mode supply may well be operating in burst mode (as opposed to continuous mode) and merely pulsing on and off at some frequency in the 20-20KHz range as the output capacitors discharge at a rate that's relatively slow.

 

[4pyros]

That's the same circuit as the example given on the datasheet.

https://www.electro-tech-online.com/custompdfs/2011/10/CD00010656-1.pdf

I also suspect the motoring sound is coming from the power supply but that it has nothing to do with feedback through a preamp stage. A lightly loaded 14A switched mode supply may well be operating in burst mode (as opposed to continuous mode) and merely pulsing on and off at some frequency in the 20-20KHz range as the output capacitors discharge at a rate that's relatively slow.

Add some 12 volt lamps to the power supply and see if that helps. A computer power supply may not be the best for an amp.

 

[guitar89]

yeah, i agree that an computer power supply is not best for audio amplifier, but I'm in testing period (using it temporarily).

I heard that normally this condition occur regarding problem of power supply, but why is it only happen when the music input source connected to AC ? (netbook using AC charger trigger this problem)

I particularly can't understand why this happen, and how to solve it. I don't have a 12V now lamp, but i will put Multimeter across the terminal and see what current consumption and voltage fluctuation may have.


About the power supply, i don't think i drive it until 14A ....... maybe just few ampere, but will adding a reservoir capacitor across the terminal will help (above 100uF), since it will smoothen the pulse in burst mode. And the 'Total Quiescent Current' is only about 120mA, most probably won't cause the power supply to 'motorboat' because of that. (my recorded sound wasn't playing any sound)

I personally think that somehow the current is leak to the Ac ground when connected to AC charger through Input ground, is it possible ?

 

[4pyros]

test with a car battery

 

[guitar89]

wow.... good idea to connect to a car battery, but my car's battery is not exposed and available to use, except i go dig it out (don't like it).

But how can I use the battery without causing mess that irreversible ?

 

[4pyros]

But how can I use the battery without causing mess that irreversible ?

Put it in a box and bring it inside.

 

[guitar89]

tried with input ground disconnected, the 'motorboat' sound still persist if i'm using AC charger.

Also measure using my DMM, but seems nothing much except for stable voltage drop due to large load(happen when i tune up the music a louder)

Tried connect directly from input connection to speaker unit (bypass amplifier), the sound doesn't change even if i connect to AC charger or using battery.

Also experiment using either input's hot or cold only, the 'motorboating' sound still there, but different pitch.

 

[cachehiker]

I heard that normally this condition occur regarding problem of power supply, but why is it only happen when the music input source connected to AC ? (netbook using AC charger trigger this problem)

...

And the 'Total Quiescent Current' is only about 120mA, most probably won't cause the power supply to 'motorboat' because of that. (my recorded sound wasn't playing any sound)

I personally think that somehow the current is leak to the Ac ground when connected to AC charger through Input ground, is it possible ?

It's called a ground loop and that it's only there when two power supplies with earth grounds are connected.

I'm also guessing this ground loop is probably picking up the burst mode frequency.

And the switched mode supplies I've been working with lately (assuming they're 14A and not 4A) would be in burst mode for any loads below about 1-2 Amps. Methinks 120mA definitely qualifies as less than 1-2 Amps and adding a 100uF cap would merely change the burst mode frequency.

 

[guitar89]

It's called a ground loop and that it's only there when two power supplies with earth grounds are connected.

I'm also guessing this ground loop is probably picking up the burst mode frequency.

And the switched mode supplies I've been working with lately (assuming they're 14A and not 4A) would be in burst mode for any loads below about 1-2 Amps. Methinks 120mA definitely qualifies as less than 1-2 Amps and adding a 100uF cap would merely change the burst mode frequency.

I guess you are right that it was ground loop problem, any idea to solve that easily ?
(i see isolation transformer solution..... very expensive, probably multiply few times my project's cost....)

hm... now i tried to connect a high capacitor as reservoir (2.2mF) is quite big for this amp i think, but overkill won't be problem.
The problem is still persist, but its ONLY a little bit help it, ALMOST no different from the 1st record. (but slightly better, just SLIGHTLY)

**broken link removed**

This time, i'm not making it standby, but playing music.
About after 8 second, I connect the AC charger. Thus the heavy ''''music'''' came out !
about at 22 second, I disconnect it, thus nice music again. After few seconds, I unplug the input, there is a little buzz (hard to listen from the recording), it was worst when i didn't use the reservoir capacitor. Any idea to solve these problem ?

 

[DemonicSandwich]

Cigarette lighter plug perhaps?

 

[guitar89]

wah... nice idea ! going to find some old used device using cigarette plug to try.

 

[guitar89]

yay, I have achieve improvement of the noise, recorded and attached link.

**broken link removed**

The configuration is using a attenuation VR (volume control) between computer output and amplifier input.
Before using this, my computer must configured volume to 5% for normal audio, 15%-20% for booming. above those will deaf my ear.
But now I'm using 100% of computer volume, and control using VR, the noise is significantly reduce ! (difference between 5% and 100% noise)
The record sound is connected to AC charger all the time, just changing VR knob.
With music, its not obvious but audible, obvious when quiet transition. (heard as microwave oven sound) at high volume, the noise doesn't hears like motorboating with music on, just like distortion.

**broken link removed**

While this is sound heard when adjust knob with no music playing, but connected to computer. with low volume and high volume, the noise is different pattern and sound.


it seems not the problem of ground loop (since changing volume does change my noise volume) :

Ground Loops

The single most common "noise" problem we encounter, manifested as a constant 60Hz tone that does not vary with volume, ground loops are an especially common source of noise and complaints. Checking for ground loops can be a bit challenging.

And I checked that the Amplifier side, only chassis are connected to Safety Earth, no further connection. what possibly could this noise be ?