tda7240 amp project

[audioguru]

The pcb shown on the datasheet from ST Micro is very fuzzy. The speaker is called RL and is shown connected to pin 5 and pin 7 of the IC and to R1 and R2.

 

[aalauzier]

Alright, that is how i have it, one lead of the resistor is connected to the speaker out wire and to pin 5, all in a bit of solder. This is also doubled for pin 7. I was looking at the schematic and pcb, and it seems that the 1uf cap from the input is reversed, that the input goes in throught the negative end and goes into the IC through the positive end.

 

[audioguru]

Alright, that is how i have it, one lead of the resistor is connected to the speaker out wire and to pin 5, all in a bit of solder. This is also doubled for pin 7. I was looking at the schematic and pcb, and it seems that the 1uf cap from the input is reversed, that the input goes in throught the negative end and goes into the IC through the positive end.

The input capacitor polarity is not reversed, it is normal because the signal source is probably at 0VDC and the input pin on the IC is at half the positive supply voltage.

 

[aalauzier]

Alright, that is good to know, i will flip it around and see how it works, because it looked like it was two different ways from the schematic and the pcb

 

[aalauzier]

I changed the 1 uf cap to a slightly larger one, and also oriented it correctly, but the amp started buzzing loudly when power was applied, so i took it off a tried again, yet it still, buzzed. When I disconnected the speaker output wire, it stopped buzzing, so i tried flipping all of the axial caps around, because I thought they might have been reversed, yet still no change. The ground is one big rail of solder, is that a problem? Should they be separated somehow? I have never been taught any of this, so I don't know much about it, this forum is my main source of knowledge, sorry if I am being frustrating.

 

[audioguru]

Buzzing in an amplifier usually comes from a power supply that is overloaded or that does not have a big enough filter capacitor.
What do you have for a power supply? (its voltage, its max current and its output filter capacitor value)

 

[aalauzier]

This is the power supply I am using, I am also using a 220uf cap the one specified in the schematic.

 

[audioguru]

Your 12VDC/1A power supply has a max output of 12W. It might be enough for the amplifier to drive an 8 ohm speaker to 7W and make an additional 5W of heat. The power supply does not have enough output current for the amplifier to drive a 4 ohm speaker.

The buzzing is not coming from the power supply.

Does the input use a shielded audio cable? If the input cable is disconnected and the buzzing goes away then the buzzing is caused by the unshielded input cable.

 

[aalauzier]

Alright, how big of a power supply do you recommend? Thanks for the advice.

 

[audioguru]

Alright, how big of a power supply do you recommend?

Unless you say the impedance of your speaker then we are just guessing.
An 8 ohm speaker needs a fairly low power supply current.
A 4 ohm speaker needs double the power supply current.
The TDA7240A amplifier IC cannot drive 2 ohms.

If your amplifier is stereo then it needs double the current from the power supply.

 

[aalauzier]

I am using an 8 ohm, 50 watt speaker. I also replaced all of the input wire (4 pieces) with some shielded wire I had, it is basically a solid core wire with a thick, clear outer coating on it. Is this the right stuff? Because the buzzing went down a bit, but it is still there, and it gets louder if i get my hand close to touch any part of the circuit, even the pot. It is also starting to overheat too fast. It might be the 2 22uf caps, i will try flipping them around, and see if that helps, since they are axial and I might not be getting the orientation right.

 

[audioguru]

Since you don't know anything about shielded audio cable nor about the polarity of capacitors then maybe you should do gardening instead of trying to make an amplifier.

Shielded audio cable has a center signal conductor that is insulated then is surrounded by a grounded conductor in the shape of a flexible tube. It is coaxial flexible cable. The shield prevents pickup of mains hum and other interference.

 

[aalauzier]

The amplifier is working now, it is humming slightly and is not as loud as I expected. I added a bigger heatsink and a small fan that will cool it inside the enclosure. I checked the datasheet to see how hot it would get and it said it peaks at 150 C. It seems to be getting very hot, although it does not smell like it is burning and causing damage. The datasheet also showed a few changes that could be made with the caps, but I did not see anything that would change sound quality. It seems to amplify higher frequencies more than lower ones, even when the music is only a bass. What changes could I make to help amplify the lower frequencies, would caps on the speaker outputs work, depending on their size? I am sorry for my previous negligence, however i am not going to give up.

 

[audioguru]

I checked the datasheet to see how hot it would get and it said it peaks at 150 C.

The datasheet says that the max allowed chip temperature is 150 degrees C. The chip is inside the case where you cannot measure it, you can only calculate its temperature. the temperature can be anything, not a peak.
The size of the heatsink and how good is the airflow determine the chip temperature.

It seems to be getting very hot, although it does not smell like it is burning and causing damage.

It should not be hotter than just a little warm if it is not working hard. It will be very warm when it is working hard.

It seems to amplify higher frequencies more than lower ones, even when the music is only a bass.

The datasheet shows that its output is down slightly (-3dB) at 30Hz which is a very low sound frequency. It should sound like a woofer that is missing a tweeter. Maybe you hear the terrible high frequencies from the woofer that have a strong peak at about 5kHz. A woofer uses a crossover network that reduces sounds above about 3kHz so you don't hear the terrible peak.

A woofer needs an enclosure that matches its spec's. Without an enclosure then sounds from the rear come around the speaker and cancel low frequency sounds from the front.

What changes could I make to help amplify the lower frequencies, would caps on the speaker outputs work, depending on their size? I am sorry for my previous negligence, however i am not going to give up.

Maybe your speakers play bass sounds poorly.
Maybe your speakers do not have enclosure designed for them.
Maybe the values of your capacitors are wrong.
Maybe you have stereo speakers that are out-of-phase.

Capacitors that are in series with speakers reduce bass not boost bass.

 

[aalauzier]

Alright, thank you, I will work on some enclosures. Does this type of amp hum very slightly and is not audible when the music is playing? It also buzzes when I touch the POT. There are 2 pins in the back, are those used to ground the POT so it does not cause interference, or will it be fine when I have a cap over it. Thanks again for the help.

 

[audioguru]

Every name-brand speaker has a very detailed datasheet. Then an enclosure can be made to match those specs. Frequently a recommended enclosure design is on the datasheet. If the enclosure does not match the spec's of a woofer then there will be no bass or there will be no deep bass but instead there will be boomy upper bass.

An audio amp should produce no hum if it is built correctly.

If you connect the metal case of the volume control pot to the circuit's 0V then it won't hum when you touch it. We don't know which pot you have so we don't know what the two pins on the back are for. Maybe they are simply an on-off switch that are not even connected to the pot.

 

[aalauzier]

The pins on the pot are not for a switch, and I cannot find any information online about the speakers besides RoHs. Since the speaker output wires are not carrying an amplified signal, do they need to be insulated? And since the heatsink is connected to Pin 4 which is grounded, it also does not need to be connected to ground?

 

[audioguru]

The pins on the pot are not for a switch

If you have a mono (single channel) volume control then its pot has 3 pins all on one side.

The other two pins on the back

I think they are an on-off switch.

I cannot find any information online about the speakers besides RoHs

Then it is a cheap no-name-brand poor quality speaker.

Since the speaker output wires are not carrying an amplified signal, do they need to be insulated?

What? Of course the speaker wires have an amplified signal and they must be insulated.

since the heatsink is connected to Pin 4 which is grounded, it also does not need to be connected to ground?

All grounds on the schematic including pin 4 and the 0V of the power supply must be connected together.

 

[aalauzier]

That is not the pot I am using, although I have one of them. And I do not need to ground the heatsink?

 

[audioguru]

You should connect pin 4 to ground. The metal tab on the amplifier is internally connected to pin 4 and is bolted to the heatsink so the heatsink doesn't need to be connected to ground separately.