Headphone amplifier hiss

Thread Starter

191Brian

Joined Dec 31, 2021
5
Hi

I have a Sony micro hifi with hiss on the headphone output that is not effected by the volume control. There is no hiss on the speaker output.

The headphones are driven by a TDA2822 IC circuit Screenshot 2021-12-30 110651.png
As the hiss is only on the headphone output and not effected by the volume control I am assuming this circuit is the source as I am new to circuit theory any pointers where I should be looking, could it be a faulty IC ?

Brian ...
 

Alec_t

Joined Sep 17, 2013
12,822
That points to the culprit being something which is common to both channels, such as the IC itself, the power supply to the IC or its grounding.
I'd be looking to check/resolder components connected to pins 2 and 4. Might be worth replacing the main decoupling cap (C567? 220μ?) on pin 2. Good luck.
 

BobTPH

Joined Jun 5, 2013
4,943
It could be a mismatch between the type headphones you are using and the type of headphones the amp is designed for.

What happens when you turn up the volume? Does it jump too quickly to a very loud level? If this is the case, the headphones are the wrong type.

Edit: My question about turning up the volume was meant t o be where there is an input signal, does the loudness of the music go up very quickly?

Bob
 
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Thread Starter

191Brian

Joined Dec 31, 2021
5
It could be a mismatch between the type headphones you are using and the type of headphones the amp is designed for.

What happens when you turn up the volume? Does it jump too quickly to a very loud level? If this is the case, the headphones are the wrong type.

Edit: My question about turning up the volume was meant t o be where there is an input signal, does the loudness of the music go up very quickly?

Bob
Hi Bob

The volume control has no effect on the hiss.

Brian ...
 

BobTPH

Joined Jun 5, 2013
4,943
I am not talking about the volume if the hiss. I am talking about volume of the music you are listening to. If you play some music, does the the volume go uo to max quickly, before the volume is turned up very far?

If so, that means the headphones are the wrong type, i.e. too sensitive.

Bob
 

LowQCab

Joined Nov 6, 2012
2,060
Levels, Levels, Levels ...........
Does the Amp get loud enough to hurt your Ears before any noticeable distortion occurs ?,
if so, You need to "pad-down" the Output with a Resistor-Voltage-Divider on each Output.

You may be surprised to find that You can cut the maximum Output Voltage by
minus ~4X, without affecting normal operation,
( with the exception that the Volume-Control will need to be set higher than You may be used to ),
this will put the hiss so far down that your Ears probably can't detect its presence.
.
.
.
 

Thread Starter

191Brian

Joined Dec 31, 2021
5
I see a headphone mute on the output connector. Is that available and if so does muting the headphones remove the hiss?
Hi

I don't think there is a mute option anywhere I assumed that mute was to mute the speaker amp when headphones are plugged in.
Here is more of the circuit diagram.
Screenshot 2021-12-31 210206.png
 

MrSalts

Joined Apr 2, 2020
1,762
The TDA2822 is like many 1990s era power amplifier chips, it is noisy and a mild background hiss is a "feature" of this chip.
 

sghioto

Joined Dec 31, 2017
3,296
I see a mute for the speaker amp through diode D506. Yes you are correct when the headphones are plugged the 9 volt voltage powering the TDA2822 is fed back through diode D507 to the speaker mute line.
I agree with MrSalts the problem is most likely inherent in the TDA2822 since it has a fixed gain of 39db (89)
Is this problem something new or has it always been the case?
 
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MrSalts

Joined Apr 2, 2020
1,762
Also, the "feedback" capacitors c577 and c578 could be causing the issue to some degree. The tda2822 amp triangles already contain the current limiting and feedback resistors internal to the tda2822. So the capacitors mentioned are not really feedback, they are going all the way back to input and may be creating a relaxation oscillator. The schematic around that TDA2822 looks quite different than the recommendation in the datasheet.
I would try clipping out or desoldering the two caps I mentioned as a test.
 

MisterBill2

Joined Jan 23, 2018
11,925
The two negative feedback capacitors between the outputs and the inverting inputs probably are intended to prevent the hiss, but the very large value electrolytic caps are preventing them from delivering adequate feedback. At least that is what I see. So the feedback to inhibit the hiss is being bypassed. A poor design indeed. Also, no DC connection to the inverting inputs. a normal opamp needs that. So there you have another guess, with a bit of logic behind it. Negative feedback seldom causes oscillation.
 

MrSalts

Joined Apr 2, 2020
1,762
Also, no DC connection to the inverting inputs. a normal opamp needs that. So there you have another guess, with a bit of logic behind it. Negative feedback seldom causes oscillation.
It is not a "normal op amp". If you look at the datasheet, the triangle simply represents one "amplifier module" and the gain is preset internally with input resistor and feedback resistor are inside the two amplifiers in the package.
 
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