Volume Related Amplifier Noise

Thread Starter

Daan Lageschaar

Joined Jul 8, 2019
36
Hi,

So I just got myself a cheap AliExpress amplifier, but at about half gain, it has quite a bit of 'hissing' noise, I allready tried lowering the input volume using a potentiometer and turning the gain all the way up, since there is no hissing sound when the gain is set to max, but that didn't help, since the hissing still persisted, so it must have something to do with the voltage it's trying to get?

Does anyone know what might cause this, and/or if there is a fix for this?
No problem if it can't be fixed since it cost only €4, but still, would be nice :p

this is the amp: https://aliexpress.com/item/4000175137078.html?spm=a2g0s.9042311.0.0.7c834c4d7kEkvV
this is the chip the amp uses: http://www.ti.com/lit/ds/symlink/tpa3118d2.pdf

Thanks in advance! :D
-Daan
 

DickCappels

Joined Aug 21, 2008
6,532
Searching for clues.
When you experience this, what is the input connected to?

If you short the inputs to ground does noise continue?
 

Audioguru again

Joined Oct 21, 2019
1,740
The Texas Instruments TPA3116D2 IC is an excellent amplifier with a noise level of only -102dB which is inaudible. Maybe the amplifier you bought uses a cheap Chinese copy or its input resistors might be noisy. I hope the the amplifier does not wrongly using its volume control as a gain control.
If you are using a switching power supply then it can add some hiss.
 

KeithWalker

Joined Jul 10, 2017
1,139
This is a class D amplifier so you are probably hearing switching noise. I can see from the illustration that there is a switching filter on each channel output. They state that you can use 4 ohm or 8 ohm speakers but the filter can only be optimized for a single output impedance. What impedance speakers are you using? You may get better results by using speakers with the other impedance.
Regards,
Keith
 

Thread Starter

Daan Lageschaar

Joined Jul 8, 2019
36
Thanks for all the replies :)

- the noise is always there, even when no input is connected
- I'm using 8 ohm speakers, but the hiss is also there when using 4 ohm speakers.
- i'm using a switching mode power supply (I think?) it's a old 19v laptop charger

I'll try connecting it to a PC power supply with 12v, but other than that I wouldn't know what else I could do
 

Audioguru again

Joined Oct 21, 2019
1,740
The amplifier IC switches at radio frequencies that are well above audio frequencies. The output filter is to avoid causing interference to nearby radios.

It looks like this cheap amplifier uses a heatsink that is much too small:
 

Attachments

schmitt trigger

Joined Jul 12, 2010
310
Since this amp module can work from 12 volts, try to power it from an automotive battery, and see if the problem goes away.

If so:
When you power a class-D amp from a switchmode supply, each operating at different frequencies, you may have heterodyning or intermodulation products. Meaning that a a pair of new frequencies, one the sum of the and the other the difference, are created.

Usually the difference frequency can fall within the audio range.
 

Thread Starter

Daan Lageschaar

Joined Jul 8, 2019
36
Since this amp module can work from 12 volts, try to power it from an automotive battery, and see if the problem goes away.

If so:
When you power a class-D amp from a switchmode supply, each operating at different frequencies, you may have heterodyning or intermodulation products. Meaning that a a pair of new frequencies, one the sum of the and the other the difference, are created.

Usually the difference frequency can fall within the audio range.
Hooked it up to a pc power supply (12v), and indeed, the hissing went away. Now the problem is where I could get a new power supply that supplies a decent amount of voltage (20v+) without causing interference....


EDIT: Apparantly I made a mistake, because I hooked it up to the psu again and the hissing is still present
 
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Audioguru again

Joined Oct 21, 2019
1,740
Look again at the very low cost for the amplifier and guess what they skimped on to make it so cheap.
Cheap amplifiers from China are usually no-name-brand and might be designed by somebody who does it wrong.
 

Thread Starter

Daan Lageschaar

Joined Jul 8, 2019
36
Look again at the very low cost for the amplifier and guess what they skimped on to make it so cheap.
Cheap amplifiers from China are usually no-name-brand and might be designed by somebody who does it wrong.
yeah could've expected that of course, I'm happy it even works at all :p
tried hooking it up to two 9v batteries in series btw, but the hissing was still there so doesn't seem to be a psu issue, so I guess I'll just have to live with it then
 

BobTPH

Joined Jun 5, 2013
2,497
I think @Audioguru might have got it right in his first post. The "volume" control is really a gain control designed to allow for a large range of input levels.

Do you get hiss when you feed the input from say an MP3 player and have the volume turned to a reasonable level?

If it hisses only at a setting that you would never use, it should not be a problem.

Bob
 

Thread Starter

Daan Lageschaar

Joined Jul 8, 2019
36
I think @Audioguru might have got it right in his first post. The "volume" control is really a gain control designed to allow for a large range of input levels.

Do you get hiss when you feed the input from say an MP3 player and have the volume turned to a reasonable level?

If it hisses only at a setting that you would never use, it should not be a problem.

Bob
don't really think it's a gain knob, because when you use google translate to translate those chinese letters next to it on the PCB it says volume knob (or they labeled it wrong), also, at max gain/volume there isn't any hissing, which wouldn't really be the case if it would be a gain knob, right? The hissing is there whilst playing audio, but you can't really hear it if the audio is loud enough
 

Thread Starter

Daan Lageschaar

Joined Jul 8, 2019
36
The hiss goes away with the volume control all the way up?

This might indicate a bad connection on the volume control.

Bob
EDIT: It just popped into my head again that when you lower the input volume using a different potentiometer (between source and amp) the hissing is still audible when you turn it up to max gain/volume. So it can't be a issue with the potentiometer being broken, right?

hmmm, so the volume knob might be broken in that case? any way I could measure that using a multimeter, don't have a scope (yet) unfortunately. It's a 2 channel B50K potentiometer like this one:
aliexpress.com/item/32860108972.html?spm=a2g0o.productlist.0.0.241792ae2E0m1U&s=p&ad_pvid=202004151509157372195610265150004497405_6&algo_pvid=d98bdb46-f47f-40e9-af61-15c22ef011cb&algo_expid=d98bdb46-f47f-40e9-af61-15c22ef011cb-5&btsid=0b0a187b15869885551345574eaab9&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_
 
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Audioguru again

Joined Oct 21, 2019
1,740
All 3 pins are connected on a volume control.
A volume control has the resistance connected between the signal input and ground. Then the slider slides up towards the input to make higher volume or slides down towards ground to make less volume.
 

Thread Starter

Daan Lageschaar

Joined Jul 8, 2019
36
All 3 pins are connected on a volume control.
A volume control has the resistance connected between the signal input and ground. Then the slider slides up towards the input to make higher volume or slides down towards ground to make less volume.
Ok I just found something out, when the input is connected, not playing audio, just connected to a phone, there is no hissing at max volume, but when there is no input connected there is hissing at max volume. Any clues as to why that might happen?
 

Audioguru again

Joined Oct 21, 2019
1,740
It is normal for hiss to be reduced when something (the phone) shorts the input of the amplifier.
The datasheet for this power amplifier does not show it but here is the amount of input noise voltage produced by an audio opamp with different source resistances:
 

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BobTPH

Joined Jun 5, 2013
2,497
So are you saying that there is no hiss if the phone is plugged into the input and it is not playing anything? If so, then I guess the hiss should not be a problem.

Bob
 
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