PC speakers amp with TEA2025

[autir]

The speakers of my PC started to act strangely, and I thought that the time had come to redo some solderings. I opened the speaker containing the amplifier and saw the circuit. It is based on the TEA2025 from ST. I desoldered all components and I'm about to rebuild the circuit on a new board.

The circuit was nothing like the circuit proposed in the TEA2025's datasheet.

I attach it here in case anyone can explain why have they altered all these things.

note: Input connects to R3 and R4, output goes to C3 and C4. R7 and R8 are optional and used when connecting headphones.

Edit: I corrected the error with C12, R3 and R4 in my circuit. Several times :.

View attachment 37531

 

[audioguru2]

Hi Autir,
ST's circuit is very similar to yours.
You don't have pin 16 connected to VCC and your C12's negative wire should go to ground.
Your circuit has R5 and R6 added to reduce the gain, and R1 and R2 added to reduce the pop when supply voltage is applied with the volume control low.
The much higher value of C1 and C2 in your circuit charge slower so help reduce the pop. ;D

View attachment 37525

 

[autir]

Yes, now I see that I transferred the Vcc part of the circuit to Electronics Workbench with this error

 

[audioguru2]

C1 and C2 in my circuit are 100nF, just as in the datasheet.

C1 and C2 in your circuit are 100uF, the bootstrap capacitors.
I confused C1 and C2 in your circuit. The datasheet's input caps C1 and C2 are a low value. Your circuit has them (C7 and C8) much larger probably to charge slowly and reduce pop.

Why are my C5 and C6 100 and not 150 nF?

It is best to use 150nF as recommended by ST Micro.

Why is my C12 10uF and not 100uF?

It doesn't matter since your power supply probably has at least 1000uF in parallel.

Why are my C9 and C10 47 and not 100uF?

Because your circuit has R5 and R6 in series with them so their value can be lower.

Why in my circuit the resistors in input.left and input.out (R3, R4) are in series and not connected to the ground?

They are in series with the low impedance headphones to reduce their level. If the headphones were connected directly to the output of the amp they would blow your ears off just before they fried.

Does it play a role that the input of the circuit receives signal from a computer soundcard line-in and not from a "real" line-in? do these two differ?

The outputof a computer's soundcard is a real line output to the inputs of your amp. ;D

 

[autir]

It doesn't matter since your power supply probably has at least 1000uF in parallel.

The PSU powering this circuit was a linear transistor - two diodes - capacitor...
Ok, C12 will be changed to 100uF.

Because your circuit has R5 and R6 in series with them so their value can be lower.

Shouldn't I replace them with two 100uF caps and omit the resistors? Fewer components are always welcome

 

[audioguru2]

Shouldn't I replace them with two 100uF caps and omit the resistors? Fewer components are always welcome.

You have a very good point. ;D
Since R5 and R6 are 330 ohms and are in series with 50 ohm resistors inside the IC, they reduce the gain from 201 down to only 27. At first I thought it was good, because it also reduces the noise and distortion and extends the high frequency response. But after reading details in the datasheet whwere ST Micro don't recommend gain less than 63 then you are correct to omit R5 and R6 and use 100uF caps in your circuit. Other power amplifier ICs break into very high frequency oscillation on part of the waveform if their gain is set too low.

They are in series with the low impedance headphones to reduce their level. If the headphones were connected directly to the output of the amp they would blow your ears off just before they fried.

R7 and R8 are there to protect my ears. I was referring to input.left and input.right (R3 and R4).You don't show the value of the R3 and R4 pots in your circuit so I don't know how much input attenuation they cause when combined with the 30k input impedance of the IC. Since their 3rd terminal isn't grounded then they are a variable resistor. ;D

 

[autir]

Once more you are right

 

[audioguru2]

Hi Autir,
Now that R3 and R4 are drawn correctly they appear to be simple volume controls. When the slider is grounded then the amp input gets no signal.

The amp had less gain than recommended by ST Micro with its R5 and R6 added, and had R1, R2, R3 and R4 to reduce the gain even more. Actually, R1 and R2 in your circuit don't do much, they reduce the level only a small amount and aren't needed.

Make the amp like recommended by ST Micro and include your R3 and R4 on the inputs to reduce the gain to where it was before. ;D

 

[autir]

I have modified the circuit according to your corrections. Now the only difference from ST's circuit are capacitors C7 and C8 which are 1 instead of 0.22 uF.
In reality R1 and R2 (the only resistors in the new circuit) are a 6-pin linear potentiometer, chassis-mounted.
-> Does its value - 10kOhm - play a role?
-> Is there a special oil I can use to lubricate it so as to minimise noise when adjusting it?

View attachment 37535

 

[audioguru2]

Hi Autir,
1uF for C7 and C8 in your circuit might reduce "POP" sound when it is turned on.

The value of pots R1 and R2 in your new circuit doesn't affect this amplifier. It is the load on the source that is feeding them. Most sources can drive 10k.

Pots are noisy when you adjust them if they are corroded or dirty inside, or if they have DC across them. You can temporarily clean their insides with "Contact Cleaner" spray. If your source has DC then use 1uF coupling capacitors to feed the pots.
Frequently an old electrolytic capacitor will develop leakage current. If it couples to a pot then noise will be produced when the pot is adjusted. It should be replaced. ;D

 

[autir]

You mean this lovely product, Philips Contact Cleaner, they sell in electronics shops? Now it has 1001 uses  ;D

One last question: Could you please check the polarity of capacitors C7 and C8? There is something suspicious...

 

[audioguru2]

He, he. I didn't know Philips made a contact lens cleaner! ;D ;D It will do your job.

If you look on the datasheet for the TEA2025, it shows a DC voltage of a whopping +0.04V on both inputs. Therefore the polarity of the C7 and C8 electrolytic caps are correct. ;D

 

[autir]

I have good news and bad news.

The good news is that it works. I was very surprised to listen to the music coming from the speakers. It is the first circuit I have ever built - breadboard prototypes don't count - and soldering on the stripboard was a new painful experience. I was pretty sure that I must had done something wrong, you know...

The bad news is that from the speakers there comes a constant humming which does not depend on the volume of the input sound. The circuit is grounded both to the signal ground and to the power ground. If I disconnect the circuit's ground from the signal's, the humming decreases. If I touch the earth cable with my fingers, the humming stops.
edit: Almost forgot. There exist some high frequency oscillations, too. frequency is constantly increasing. When I touch the ground everything stops.
Anything to suggest?

 

[audioguru2]

Hi Autir,
Your good news is very good. You did a fine job making a stereo amplifier. ;D

The hum in your bad news should be easy to fix.
1) Use shielded audio cables to the inputs of your amp, not a headphones extension cord.
2) Your amplifier probably doesn't hum when nothing is connected to its inputs, right?
Then the shields of the audio cables should be the only path between the ground of your music source and the ground of your amp. The case and audio ground of a computer might be connected to the ground wire of the mains but your amp shouldn't be.
Maybe you bolted input audio connectors of your amp to its metal case that is connected to the ground wire of the mains. Then insulate the connectors from the case (plastic connectors are available). ;D

 

[autir]

My amp hums even with no input.

The case and audio ground of a computer might be connected to the ground wire of the mains but your amp shouldn't be.

Why? What happens then?
Both in the old circuit and in ST's datasheet all grounds are connected: input signal ground, output signal ground, power supply ground. So is the case in my circuit.
The power supply I used for the test was an LM7805-based circuit.
My output cables are shielded audio ones. My input cables are wires from UTP cable.
I have not soldered any connectors yet. Used cables with crocodile clips to connect to power, input, and output.

 

[audioguru2]

Hi Autir,
Only a few cm of unshielded wire will pickup hum from the mains since your amp circuit has a lot of gain. Your amp's output cables to the speakers don't need shielding but the wiring from the pots to the IC must be shielded or inside a metal box connected to the circuit's ground. Wires with crocodile clips make great antennas for hum pickup. It is also best to keep the power supply's transformer, its wires and mains cord away from the amp circuit.
I don't know what "UTP" cable is, but the audio cables from your computer (or any other source) to the inputs of your amp must be shielded audio cables to avoid hum pickup.

Most tuners, tape decks, CD players or MP3 players aren't grounded to the ground wire of the mains so they don't cause a "ground loop" if the amplifier is grounded to the mains ground wire. A ground loop is more than one ground connection between audio components and causes hum. Your computer is grounded to the ground wire of the mains. Disconnect your amp's ground from the mains ground wire and connect your amp's ground and the metal case to the input grounds only.

 

[autir]

A big mistake from my part: Whenever you saw "power ground" in my last two posts, read "power neutral" instead. So the neutral cable of the LM7805 is connected (via capacitors, circuit etc.) both to my PC line-out ground and to the output speakers' ground. I have not grounded my LM7805's circuit.

 

[audioguru2]

Let's call your circuit's "power neutral" its ground. Earth is an earthed ground that your computer has through its mains cord. Since your amp makes hum when it doesn't have an input then its input wires and parts are unshielded antennas for mains pickup.
Maybe your power transformer is too close to the amp circuit and its magnetic field is inductively coupled to the input wires or parts of the amp, or its mains wires are capacitively coupled through the air. 

 

[autir]

It works. Perfect 

It was the unshielded copper wire which, in conjuction with the thousands  of crocodile clips, caused the hum.
I put it in a box and added all the necessary connectors.
One of these days I'm going to borrow a digital camera from a friend and take some snapshots.

Audioguru, thanks. Without your help this would not have been possible.