A Level Project - Powering Two Amp/Speaker Setups

[Morley93]

Hi there, I have a query regarding an A Level project that I've been working on.

Basically, I've made a speaker system with two stacked speaker enclosures that can rotate in order to be positioned independently of one another. The system takes a single 3.5mm jack input and each enclosure has its own integrated amplifer and speaker setup.

Currently, I have trailed 4 wires (2 positives and 2 negatives) out the back of the unit in order to provide power to the amplifiers. I twist the corresponding wires together and connect them up to a test bench style power supply. The speakers both sound great when this is correctly configured, but I'm looking for a more convenient way of powering the amplifiers.

The amplifiers output upto 18W and take 8-20V DC. The packaging states that their max current draw is 800mA. Speaker compatibility is listed as any 4-16ohm loudspeaker. The speakers I've been using are 2 x 4ohm car speakers with a nominal output of 20W.

I also have a switching mains power adapter which can go upto 15V and is rated at over 3A. I have a matching socket for this power adapter. When I wire the socket upto the wires coming out the back and attempt to power the unit, I hear both speakers click in and can feel that they're receiving power, but when I attempt to play music, there is no sound. When I try measuring the voltage going into the power wires, there is only a tiny voltage (the maximum I've seen is 4V).

My teacher thinks that maybe the amplifiers are trying to pull too much current and the voltage is being throttled as a result. Due to this, we've been thinking of experimenting with some kind of current regulation circuit, but as of yet we've been unable to come up with a solution and we're unsure whether it would solve the problem anyway.

Does anyone have any ideas what I need to do to get this working? Sorry if I've been a bit vague, I can supply more details on the exact components I have if required.

Thanks

John

 

[Harald Kapp]

Let me see whether I understand this:
Apart from the rotating stuff and whatnot, you have
- a workbench power supply attached to an amplifier attached to a speaker
This setup works.
- When you replace the workbench power supply by the 15 V mains adapter (all other wiring stays the same) you hear a click, but no sound.

A few ideas:
1) is the polarity of the mains adapter correct (this may sound silly, but one never knows)?
2) are the wires from the mains adapter to the amplifier thick enough to carry the current without noticeable loss? (measure the voltage at the mains adapter and at the amplifier under load to find out whether there is a voltage drop along the wires).
3) Have you measured the current that flows into the amplifiers? What happens if you remove the speakers and put power to the amplifiers only?
4) Do you have an oscilloscope to monitor the waveform of the switch mode power supply's output? Some switch mode supplies, especially cheap ones, do not output a good DC voltage but have a lot of high frequency (HF) switching noise superimposed on top of the DC. This HF may cause malfunction of the amplifiers. Try a good electrolytic capacitor (e.g. 1000µF 25 V) across the voltage terminals of the amplifers. Observe polarity!

Harald

 

[Aniruddh]

Switch Mode Power Supplies are not successfull in audio circuits , as the instantanious power for the audio system varies as per the track.
Better you try the Linear Power supply using step down transformer and regulation circuit. since that can supply extra instantnious current to the audio system.

 

[Harald Kapp]

Switch Mode Power Supplies are not successfull in audio circuits , as the instantanious power for the audio system varies as per the track.
Better you try the Linear Power supply using step down transformer and regulation circuit. since that can supply extra instantnious current to the audio system.

That is why I proposed the additional capacitor.

 

[Morley93]

Thanks for the replies.

Let me see whether I understand this:
Apart from the rotating stuff and whatnot, you have
- a workbench power supply attached to an amplifier attached to a speaker
This setup works.
- When you replace the workbench power supply by the 15 V mains adapter (all other wiring stays the same) you hear a click, but no sound.

Yes, that's exactly the issue.

1) is the polarity of the mains adapter correct (this may sound silly, but one never knows)?

The adapter does have one of those push-fit connectors, I'll try it the other way.

2) are the wires from the mains adapter to the amplifier thick enough to carry the current without noticeable loss? (measure the voltage at the mains adapter and at the amplifier under load to find out whether there is a voltage drop along the wires).

I doubt that this is the problem, as I used the same wires to connect directly to the workbench power supply, but I'll take some measurements anyway.

3) Have you measured the current that flows into the amplifiers? What happens if you remove the speakers and put power to the amplifiers only?

I have tried powering the unit with only a single speaker connected with the same result, but I will give it a go with no speakers and try to take some measurements.

4) Do you have an oscilloscope to monitor the waveform of the switch mode power supply's output? Some switch mode supplies, especially cheap ones, do not output a good DC voltage but have a lot of high frequency (HF) switching noise superimposed on top of the DC. This HF may cause malfunction of the amplifiers. Try a good electrolytic capacitor (e.g. 1000µF 25 V) across the voltage terminals of the amplifers. Observe polarity!

I'll see if I can get hold of an oscilloscope. Unfortunately, the project is currently still at college while I'm working on it, so progress is quite slow, but I will update with progress. Thanks!

 

[BobK]

Don't just switch the polarity of the power! Check it, and get it correct. If you connect it wrong you could destroy the amplifier. Your voltmeter should tell you the correct polarity.

Do you know that the switching power supply is in working order? What voltage does it measure when not connected to the amplifier?

Bob

 

[Harald Kapp]

Do you know that the switching power supply is in working order?

Yes, good idea. Just another checkpoint along the route to locating the problem.

You can even check the switching supply by applying a resistor (e.g. somewhere around 15 Ohm, 15 W) across the output of the supply and measure current and voltage. A thungsten light bulb rated 12V, 15 W plus a 3 Ohm resistor 5W in series can do.

Harald