My first amp: constant white noise problem

[simsim]

I just finished my first amp, based on TDA1558Q, "2 X 22 W or 4 X 11 W Single-ended Car Radio Power Amplifier"

It sounds great but the problem is that if I connect it to anything else than my old ipod (classic white model with metal back) there is white noise/static at even volume, very noticable when you stop the music or play soft. So far I tried at different friends houses with an ipod shuffle, portable cd player and 2 laptops.

I rebuilt it on breadboard and tried both the 2x22W (Bridged) and 2x11W circuit, same result. I must be doing something wrong. What could be the source of the noise?




TDA1558Q data

 

[AndrewT]

high impedance on the input?

 

[gfiandy]

Does it do it with nothing attached?

If yes then it is probably as AndrewT has suggested and needs a lower input impedance. You could try a 10K resistor to ground.

If it is not present when no input is connected then this may be due to quite high gain of this amplfer amplifying noise on the ouput of your source. Strange that an iPod would be better than a CD player though.

The other possibility is that it is unstable and the instability is creating noise in the audio range (if it is unstable it is likely that the oscillations will be at several hundred KHz if not into the MHz. If this is the cause the it is likely that you need to improve you power supply decoupling and check that none of the input signlas are tracked close tot he output signals or near the power supplies.

Regards,
Andrew

 

[simsim]

Thanks for replying so quick!

I'm new to this so it's really good to get advice. I'm going to try that thing with the impedance, the amp is very loud, too loud actually and if you pull out the cable from the source there's a terrible sound

This evening I breadboarded another amplifier, using two LM1875 and the suggested circuit from the datasheet. This one sounded perfect, no noise, so I think I will finish this one before I go back to the 1558Q. On this new amp build I noticed something to do with your second suggestion, when I moved the input voltage to a strip on the opposite side of the breadboard, a distortion I heard earlier totally disappeared.

I used another power supply for the new one, the LM1875 needs at least 14 volt I think, the old wall wart I used for the 1558 didn't suffice.

I'll report back

 

[TheMG]

Yup, definitely way too much gain then.

Looks to me like you're going to have to add a couple resistors or a pot to attenuate the signal coming into the TDA1558, as the chip has a fixed gain.

 

[simsim]

yeah, I actually had a volume pot in the first build, hastely added, which might have solved the noise issue, the problem was that I naively thought you could use a pot on the supply voltage to regulate the volume, I suppose I heard somewhere you shouldn't interfere with the input signal I think my next idea would have been to put it just before the speakers. But then I googled it.

 

[TheMG]

Simplest and most effective way to implement a pot for passive vol control is to tie one end to the audio source, other end to ground, and the wiper (center connector) to the amplifier input.

 

[simsim]

Thanks, I wasn't quite sure what to do with that 3rd connector on the pot

Well, the pot doesn't help so much with the 1558 it turns out. when I increase the resistance the music lowers but instead you get this other horrible loud noise, like when you pull the connector from a guitar sometimes. That makes you almost miss the gentle hissing white noise. That loud noise is there when there is nothing connected to the amplifier too. Only when I short out input and ground there is beautiful silence. I probably misread the schematic somehow and I'm thinking I'll leave it and go back to it when I know more.

 

[TheMG]

You did set it up like in the picture below, right?

What value of pot did you use?

 

[simsim]

Since you asked what value I used I decided to try some other values. Turned out to be the solution! I had been using 100K pot but I found out that with 2k pots it worked much better. Thanks!

 

[AndrewT]

sounds like there is a resistor missing at the input.
I wonder if there is anything else missing?

 

[simsim]

Well, knowledge on my part is missing of course , but apart from that there's only 4 components in the whole 1558q amp not counting the chip. There is no resistor in the datasheet circuit for the BLT. I'm pretty happy with it now, it was my first project. Only thing I notice is that the speakers in the schematic says 4 ohm and mine are 8.

I also finished my lm1875 amp and I'm looking for a nice box to put it in.
Thanks again.

 

[AndrewT]

there's only 4 components in the whole 1558q amp not counting the chip. There is no resistor in the datasheet circuit for the BLT.

then some components are certainly missing.

 

[simsim]

I realise now when I look at the picture that I always assumed the resistors drawn inside the chip were illustrations of its internal workings, not external components to be added. It's very unclear if that's what they mean.

This is the circuit I built:

[IMGDEAD]http://img187.imageshack.us/img187/3741/1558qml1.jpg[/IMGDEAD]