Help with audio amp D:

[Sourcegeek]

Hi.. I'm trying to make a lasershow with Hard Drive Galvo's, so...
Today I got a 18W Sound Amp (TDA2030A) and i connected all... At the start it was very nice, but when I touched the Amp, the hard drive started to move so fast, then i discovered that it occurs when I touch everywhere in the circuit =/

I dont know what I did bad.. Can you help me? Or it's normal? =/

Thanks

 

[Bionic-Badger]

It's probably amplifying noise in the circuit, including what comes from your body. Put some decoupling capacitors on the DC power supply (the DC power supply between power and ground) and try to isolate the inputs to the amplifier as best as you can.

Read the section on PCB layout in this op-amp guide as well. It has some nice info about analog circuits and how to deal with them.

 

[Sourcegeek]

Ok, the solution was connecting the negative of the power source to the ground.
Now, i got another problem =/


Ok, i have the circuit of two channels (attached)
Please note that one amp is connected to the left and the other to the right signal of the PC (the signal generator I used in the program is one channel) and the LED's are the Hard Drives

I did the circuit and all good, but, the signal isn't separated =/
I mean.. I make a left signal in the PC and the two hard drives are moving, not only the left one..
What i did bad in the circuit? I think all is right

Im desesperated

Thanks

 

[Bionic-Badger]

It looks like you don't know how to work with op-amps. Try reading the guide I posted above and get a feel for basic circuits and op-amp theory. It has a nice section about both near the beginning. You also need to tie your grounds together for anything to work, but first you need to make a usable amplification circuit (read that guide).

 

[Sourcegeek]

Ok, I'm going to read the guide, but, can you help me with this problem? Please, I will need the laser in a few days

 

[Bionic-Badger]

Well first make it a proper negative feedback amplifier, but you need to read the guide to learn how.

 

[Sourcegeek]

The negative feedback amp is when you combine the negative input to the output, right?

 

[Bionic-Badger]

Yeah, that's negative feedback. If you just outright attach leads to the inputs of the differential amplifier it's going to just jump directly to the highest and lowest values because of the amplification. The feedback reduces the gain to controllable levels.

 

[Sourcegeek]

The PDF is soooo large
So, i found this circuit on the internet, i think it works; http://www.elexp.com/tips/comp2_f3.gif
What do you think?

PD; I'm not sure, but i think i found too a circuit on the page 271 of the PDF. If it is, i can't do it because I only have some resistors

Thanks for all your help!

 

[Bionic-Badger]

Yes, that's a standard op-amp circuit, but you really need to understand what it's doing, which is what the first chapter or two of that PDF explains. It's not like we can just give you values here to put for your resistors and stuff, you need to figure it out yourself with the hardware you have.

 

[Sourcegeek]

I read the first chapters of the PDF and i did another circuit (attached), but I have more troubles...

When I connect all to the electric source, all the 9V goes directly to the Hard Drives without any audio signal going to the amp input... I mean, 0V in the non-inverting input and 9V in the Hard Drives without audio signal!
How it is possible? What i did bad? An Electric Ing told me that the circuit is good =/

 

[Bionic-Badger]

You need to read how to use those amplifiers. The EE person probably doesn't even know what you're trying to do and is just verifying that your circuit can do something even if it is not what you want.

Your amplifiers need negative feedback in order to turn your input signal into something useful at the output. Without negative feedback the output of your amplifier will "rail" based on the inputs, in other words it'll jump to the highest voltage it can provide (9V in your case) if the difference between the + and - inputs of the amplifier is positive. It'll also jump to the lowest value possible (0V in your case) if the difference between the + and - inputs is negative.

Read how to set the gain of your amplifiers in the book I linked you to. It's the most important thing you need to know about how to work with these amplifiers.

 

[Sourcegeek]

Yes but there is another problem when I use negative feedback and not ground;
If i make on the PC a left signal, the voltaje goes to the two hard drives, not only to the left one =/
I need a bipolar source?

 

[123splat]

It looks like and sounds like you have/had the amps up and running originally. You were picking up stray AC when you touched the circuit (like Badger said), and over driving the amps. Badger's fix was good. Your statement of your second problem ("signal isn't seperated") is not a problem, but your appliation of the input signal(s): you only have one input (looks like a 5V sine) feeding into two amp circuits. Both amps see an in-phase input and will track that signal on both outputs. Try phase split/shift for one of the inputs, or use two, unsyncronized, input signals. Also would be a good idea to duplicate the Test circuit or the typical application example in the data sheet for the 3020 (can't go wrong there).

 

[Bionic-Badger]

Sourcegeek: These amplifiers work by multiplying their gain by the difference between the + and - inputs. This gain is usually in the thousands if not even more. They're designed to work using feedback, otherwise even the tiniest voltage difference will send the output to one extreme or the other.

Read chapter 3 of that guide. You're not going to get anything useful out of your amps if you don't use feedback in them. Also, if you're not going to learn the basics, I'm not going bother helping you further.

 

[Sourcegeek]

I read the chapter you told me and i did this (attached). I think it works, but tell me what you think