Using $1 stereo amp in mono mode

[SammyT]

Hi, first post here

The speakers on my laptop are terrible so I'm thinking of making a small amp and speaker unit, mainly for watching things like YouTube. Since it won't be for music so much as dialog I don't need stereo.

There's these $1 amps on eBay: http://www.ebay.com/itm/2-Channel-M...728?pt=LH_DefaultDomain_0&hash=item5419730ef0

How could I use both channels on that to power a single speaker?

 

[KrisBlueNZ]

Hi there and welcome to Electronics Point

That board uses a PAM8403 stereo BTL output IC. You can't connect the two outputs of that amplifier together. You would have to use only one channel, and mix the left and right audio signals (from the wires from the headphone plug) into a single signal to use as the input for that channel. This is pretty easy to do - probably just one wire added to the board.

When you have the board, upload a good clear photo of the top side and I or someone else here will annotate it to show you what you need to do.

The available output power depends on the supply voltage so for best results, you really need to provide 5V to the board. Even a drop to 4.5V will have some effect on the maximum output power you get. Also, the board can deliver more power into a 4Ω speaker than into an 8Ω one.

 

[SammyT]

Thanks a lot Kris.

What you said is kind of what I was thinking - using just one side of the amp.

I was hoping there might be a way of combining the two channels of the amp so I'd get more power. But for what I'm thinking of maybe one channel would be ok.

As far as the 5V power supply, I was going to get that from a USB port on the laptop.

 

[KrisBlueNZ]

OK, cool. Well, you can get a bit more power if you use two or more speakers. You can feed the signal into both channels of the amplifier, and each output can take 1x 4Ω speaker or 2x 8Ω speakers. But if you want a single speaker, make it a 4Ω speaker and use just one channel.

So, upload the photo when you have the board and we'll go from there

 

[BobK]

If you are using the USB port on your computer to power the speaker it can only draw 100mA, which makes 0.5W. You computer may be able to deliver more than that, but there is no guarantee, and it might just shut down, or reduce the voltage. You would be better off to run it off a USB wall wart.

Bob

 

[SammyT]

Thanks Kris and Bob. I'd rather not go with the wall wart - more wires.

I have a USB light that plugs in my iBook and it's quite dim. That made me wonder if the USB port would even have enough power for a small amp.

I have a couple of powered USB speakers - a Music Bullet and a Vivatar that's wireless. What I'd like is one that's wooden, has a bit bigger speaker and a big volume knob on it. I could just gut the Vivatar and use it.

 

[Gryd3]

If you are using the USB port on your computer to power the speaker it can only draw 100mA, which makes 0.5W. You computer may be able to deliver more than that, but there is no guarantee, and it might just shut down, or reduce the voltage. You would be better off to run it off a USB wall wart.

Bob

I had thought that 500mA is the maximum load that USB 1.1 or USB 2 can support.
http://en.wikipedia.org/wiki/USB#Power

 

[BobK]

Yep, but only if can talk USB protocol and negotiate for it, and even then, it might not be granted. For something that is just drawing power and not connecting via USB, the max is 100mA on USB1 and 2 and 150 on USB3.

From your link:

A unit load is defined as 100 mA in USB 1.x and 2.0, and 150 mA in USB 3.0. A device may draw a maximum of five unit loads (500 mA) from a port in USB 1.x and 2.0, or six unit loads (900 mA) in USB 3.0. There are two types of device: low-power and high-power. A low-power device (such as a USB HID) draws at most one-unit load, with minimum operating voltage of 4.4 V in USB 2.0, and 4 V in USB 3.0. A high-power device draws, at most, the maximum number of unit loads the standard permits. Every device functions initially as low-power (including high-power functions during their low-power enumeration phases), but may request high-power, and get it if available on the providing bus.[

Bob

 

[SammyT]

Interesting comments - thanks everyone.

I'll order one of those cheap amps and see what it does. My volume needs are on the low end.

On the Wikipedia page that Gryd3 mentioned they show a Y-shaped USB cable that has two plugs one end, so it can plug into two USB ports. I have one that was from an external USB 2.0 enclosure, I was wondering if there's any hope for getting more power using that.

 

[BobK]

I expect that many implementation of USB do not enforce the 100mA (or 150mA) limit, which is why people get away with making things that take power form a USB port.

Bob

 

[SammyT]

I ordered one (see pic). It was $1.24 USD and free shipping from Hong Kong, pretty cheap entertainment!

 

[KrisBlueNZ]

I expect that many implementation of USB do not enforce the 100mA (or 150mA) limit, which is why people get away with making things that take power form a USB port.

I don't think there's any expectation that the host will "enforce" the limit anyway. The slave is supposed to obey the limit to avoid overloading the host's power supply, but if it doesn't, the host isn't expected to disconnect it or take any other action, AFAIK. The host may have a resettable thermal fuse but that would be rated to suit the negotiated maximum, i.e. at least 500 mA.

 

[SammyT]

I got the amp yesterday, and today had a chance to do a mock up... it works! Roughly, it's about like a small handheld radio that's ok at low volumes and obnoxious after 1/2 way. I went through some parts I had and found a pair of 3" speakers from a Sylvania tv. They are lightweight speakers with real small magnets and are 8Ω not 4Ω so I was concerned they'd be low in volume but they aren't. This is for watching documentaries, how-to videos, interviews... which is what I like to do before I go to sleep, and the iBook's speakers don't cut it.

There's zero problem with powering it from the USB port, works fine. For the volume I need the volume control on the amp is 1/3 of the way up. That's perfect. It's very clear and when I unplug the input and crank it full there's no noise problem.

The main problem is that the sound sucks. The sound is cold, small and the ratio of low end to high is out to lunch, WAY too much treble. The problem is the speakers, not the amp. As soon as I settle on the speakers I'll make a wood cabinet and the wood will change the sound totally, but the speakers need to be closer than what I have now. To make it small likely the big speaker will face forward and the small one will be on the top of the speakerbox.

Since I really only want mono what I'm thinking now is to bridge the left and right channels at the input and use one side of the amp to power a larger speaker and the other side to power a smaller speaker, maybe even one of the 3" ones. It would probably sound better if I could make the smaller speaker get less power. I wouldn't mind having a passive low pass tone control on the input because the sound is like an upside down triangle if you know what I mean.

This isn't a serious project, more just having fun and learning. I have a S5 tube amp kit I put together and a 70's Harmon Kardon stereo amp that sound great and I'm not expecting to rival those.

Probably the most amazing thing for me is that I can get a little amp that actually is clear for about the price of a chocolate bar.

BTW I had to get a new soldering iron today and Radio Shack has a coupon on their site that gives you $10 off of a $25 purchase, instore or online.

 

[KrisBlueNZ]

Good!

Speaker drivers never sound very good without some kind of enclosure.

 

[SammyT]

No, they don't, they sound real thin and have no warmth.

Kris, you'd mentioned uploading a pic of the pc board before, so I've attached one. There was zero documentation that came with it, but it's pretty simple. There were 9 soldering connections needed, from left to right:

right speaker -
right speaker +
left speaker +
left speaker -

power - (the black wire in a USB cable)
power + (the red wire in a USB cable)

left input + (white wire)
ground from input (black wire)
right input + (red wire)

The three inputs are from the laptop's headphone jack - tip (left input +), ring (right input +) and sleeve (ground).

 

[KrisBlueNZ]

Right, that was only so I could tell you how to link both channels together, but that's obvious - just connect the L and R inputs together and feed them from a mono source. To convert a stereo headphone output to mono, just feed each signal through a resistor of around 100Ω to a common point and connect that to the L and R inputs on the board. (And connect the headphone cable screen to G on the board, of course.)

 

[SammyT]

Why are the resistors needed, and wouldn't that make the final volume a little lower?

 

[KrisBlueNZ]

It will reduce the volume a tiny bit. But you shouldn't tie the left and right headphone output signals together directly. It's not likely to do any harm but it might increase distortion.

 

[SammyT]

I've seen in some passive summing circiuts, like summing line level audio signals together, where they used 10kΩ resistors for that. Are you suggesting the higher value, 100Ω because the headphone output is a higher output, since it's meant to drive headphones?

 

[KrisBlueNZ]

100Ω is lower than 10 kΩ. I recommend that value because there will be less signal attenuation. That's the main reason.