22 W Audio Amplifier

[davecoles1]

Hi,

I have some questions that I couldn't find the answers to on these pages.

It's regarding the amplifier found at:

http://www.electronics-lab.com/projects/audio/006/

It says at full power the amp will draw 5 amps of current which seems pretty high to me! If I use a supply rated at say 1.2 A, what will happen as the amplifier tries to draw more than 1.2 A of current? Will it kill the supply? Or will the supply just limit the current and therefore the resulting volume?

Also, if I wanted to add a volume control, what's the best (but not too complex) way of doing that? Put a potentiometer across the inputs to the op-amps?

Any answers would be greatly appreciated.

Thanks, Dave  :)

[audioguru]

Hi Dave,
Welcome to our forum.
The TDA1554 supplies 22W RMS each into two 4 ohm speakers at 10% distortion with a 14.4V supply. It gets hot so additional power heats it. So 44W will go to the speakers and about 22W will heat it. The total power is 66W and the max current at 14.4V is 4.6A.

If you use only 12V then the max power output and the current are much less.
If you use 8 ohm speakers then the max power and current are also less.

A 12V, 1.2A supply is not enough unless your speakers are 8 ohms. Then each speaker will get a max of only 7W each and the IC will heat with about 7W. The total power is 21W and the max current at 12V is 1.75W. Amplifiers never play music and voices at max volume continuously so your 1.2A supply might be OK.
It depends on your supply what happens to it when it is overloaded. It might limit the current or blow a fuse or blow itself up.

Connect a volume control to the Right-in signal and ground wires and to the Left-in signal and ground wires. Use a stereo volume control then it will have only a single knob.

[audioguru]

u can use the same ic in 4 channel 11watts per channel

That is into 2 ohm speakers, at terribly clipped 10% distortion and with a very high 14.4V supply.
It is a little less than 4W per channel into 4 ohm speakers just before clipping (low distortion) with a reasonable 13.2V battery.

[davecoles1]

Hello again,

Thanks so much for the replies, I'm never quite sure if I'll get a reply or not  :)

So you're saying the way to go may be with 8 ohm drivers? If so I'm glad I didn't order the 4 ohms yesterday! 7 watts should be enough - the drivers I'm looking at have a sensitivity of 86 dB so theoretically 7 W should give another 8 dB which should be loud enough for the application I'm looking at. And 7 W dissipated as heat sounds a lot more agreeable  :)

I could always just put an ammeter on the input and see how much current it'll draw couldn't I?

Does all this sound sensible?

Thanks again,
Dave

[audioguru]

Each of your two 8 ohm speakers will have a max power of only 5W at clipping and will produce a max sound pressure of 93dB at 1m from a speaker. The max average sound pressure from music will be only 83dB at 1m which isn't very loud. 

[davecoles1]

O dear. What am I to do? I just wanted to build a nice small amplifier to power a couple of full range drivers to have somewhere like the computer or the kitchen or something.

Why do all the amplifiers seem to need such a high voltage supply? I would have thought 12 V would be a nice and common voltage supply to work from.

But then again, I don't really know much about this kind of thing!

[audioguru]

My computer speakers are 3" drivers, 4 ohms, and are driven by an ordinary amp, not a bridged one like the TDA1554.
Their power supply is 12VDC/1A and they are nice and loud when they are right in front of me. They are each getting about 3.5W at clipping and have huge magnets for such tiny speakers so they must be very sensitive. They are too small to make deep, powerful bass.

[davecoles1]

So what is a bridged amplifier? I thought it was to do with the fact that it could drive two channels (stereo).

Maybe I need to look at some different drivers...

Thanks so much for all the help

[audioguru]

The TDA1554 has 4 power amplifiers inside. It can be used as 4 amplifiers or as 2 bridged amplifiers.

An ordinary amplifier drives one end of a speaker and the other end is grounded. The speaker gets the supply voltage less losses as its peak-to-peak voltage.
A bridged amplifier is two amplifiers that each drive their end of the speaker in opposition. The speaker gets nearly double the supply voltage less losses as its peak-to-peak voltage.
When the voltage is nearly doubled then the current is also nearly doubled. Power is volts x amps so the power is nearly 4 times as much.

[davecoles1]

I see

Thanks  :)

[mugsby]

10% THD is awful, maybe you should check out the LM3886

[audioguru]

10% THD is awful

All amplifiers can make 10% distortion when their volume control is turned up too loud making the amplifier clip. But then the power number is bigger.

With a 14.4V supply and operating bridged into a 4 ohm speaker, the TDA1554 has 22W output per channel with 10% distortion (s square-wave) or at 17W has only 0.5% distortion (slightly clipped). The datasheet doesn't show it but I suspect at 15W its distortion is only 0.1% or less (not clipped).