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[on:]

I made an amp in my class that takes an mp3 player input and amplifies it to 2 speakers one for left and  one for right.

The amplifier chips are 2 TDA2005's and I hooked up a transformer to take my wall voltage down to 12 volts, then rectify it, then filter it with a 1000uF capacitor. Then on to the circuit itself.

And my problem is that the amp buzzes when I turn it on with or without music. Does anyone know what might be the issue here or some common causes? I know I haven't posted a schematic or anything but any common causes would be great.

edit: okay I think the problem might be that Im giving it 12v and the speaker impedance is only 6 ohms. The transformer is rated at 12v 1amp so could this be a problem? I  think the amplifier needs more than 1 amp am I right?

[Reply #1 on:]

Oh boy, you're in the world of audio sound issues   This is a pretty good read and discusses both your buzzing and humming:

http://www.ethanwiner.com/dimmers.html

--
The Rugged Motor Driver: two H-bridges, more power than an L298, fully protected

[Reply #2 on:]

1) You could have ripple (power line hum) in the power you are feeding the TDA2005's.  1000uF is a very low value for a power amplifier filter capacitor, one would expect something 10x to 50x larger than that.  Try putting a MUCH larger capacitance there.

2) You could be picking up hum due to inadequate shielding of the audio input (which is rather sensitive.) 

Try shorting the power amp audio input directly to ground right at the TDA2005 chip. That would be one good differential diagnosis for seeing whether the hum was internal (from the amp power supply) vs. external (picked up in the audio input wiring.)

Okay I did what you said about connecting the input to ground but that didn't help at all, but like I said the transformer is rated at 6 ohms and the circuit provides 12 volts. That would require 2 amps right? And my transformer is only capable or "rated" at 1 amp.  would this be the problem?

[Reply #3 on:]

I might go to 2200uF or 4700uF, but 1000uF could well be adequate, and there shouldn't be a lot of hum especially if it's just sitting there idling with no input.  At that point it shouldn't be drawing enough current to create significant ripple.  But if grounding the inputs doesn't help, I'm not sure what to think.  Maybe some weird ground loop gremlin is running around in there.  I don't think the transformer size is an issue either.  You might not achieve maximum power output, but it seems like it ought to work OK if you don't try to rattle the windows too much.  Maybe the buzz is a feature.  Are these the "Marshall" TDA2005s?   

Thinking about it a little more, it might be worth swapping out the filter capacitor, just on the off chance you've got a bad one.

[Reply #4 on:]

12v 1amp  = 12W, so you will definitely not get 20W out of it.
And 12V/6 ohm will need 2A as you indicated.

The buzz is going to be due to your production skills.  Can you post a schematic of what you have assembled?

[Reply #5 on:]

You are building something like this?

You definitely need a bigger power transmformer:

ABSOLUTE MAXIMUM RATINGS
Symbol Parameter Value
Vs Operating Supply Voltage  18 V
Vs  DC Supply Voltage  28 V
Vs  Peak Supply Voltage (for 50 ms)  40 V
Io  Output Peak Current (non repetitive t=0.1ms)  4.5 A
Io  Output Peak Current (repetitive f>10 Hz)  3.5 A


http://www.hobby-hour.com/electronics/tda2005-power-audio.php

[Reply #6 on:]

Would a regulated power supply make a difference?

[Reply #7 on:]

Sure would - still need one that can do 24V, 5 Amp to get nice 20W from each side.

http://www.mpja.com/prodinfo.asp?number=16015+PS maybe this

[Reply #8 on:]

okay well tomorrow I can go get the schematic  and scan and post it here, and Ill also look for a bigger transformer.

About the filtering and such, I checked on an oscilloscope when I was bread boarding the power circuit and the ripple was almost non existent! So I don't think theres a problem with the capacitor.

CrossRoads yes the circuit is constructed more or less from what I can remember the same way you showed in your image.

[Reply #9 on:]

Yes, but take that almost non-existengt ripple and multiply by your audio gain of 50dB per the data sheet, on the order of 10-20,000x, and now that ripple becomes pretty audible.

[Reply #10 on:]

Yes, but take that almost non-existengt ripple and multiply by your audio gain of 50dB per the data sheet, on the order of 10-20,000x, and now that ripple becomes pretty audible.

okay well I added a 6400uF capacitor instead of the 1000uf and no difference. I am pretty sure it is from the transformer because my class power supply provides 12 v at 2amps and it works without buzz.

So now  think I must find a 2amp  transformer OR, if anyone can guide me the right way ,  how to get the amp to not output as many watts  therefore reducing my current need?

[Reply #11 on:]

Reduce the gain of the amp - read the specs on the TDA2005 part, tells you which resistors to change to lower the gain.

[Reply #12 on:]

okay first off I will check the components and reduce the gain of the amp a bit. I'll also check  if the capacitors are not dried up and such because the ones I used might have been no good so ill check again.

Thanks for the info.

But heres another brain teaser you guys might be able to help me with:

I took apart my old 10 watt guitar amp to see the circuitry and now when I connect it and power it u all it does is hum and the rectifier gets VERY hot and transformer get VERY hot too. I'm not sure what I have done to the circuit while taking it out.

[Reply #13 on:]

Youv'e apparanty done something that is now drawing a lot of current.

Next time, try searching for schematics first, usually easier to determine what is going on that way.

[Reply #14 on:]

If the transformer is within 6 inches of the amp circuit try moving it further away - you may well be picking up its magnetic field in your input circuit.