Simple USB Speaker Project

[Frederick]

Simple Speaker Project

Hello, I'm new to these forums, but I'd like to know if anyone could help me with some basic plans for an 8Ω speaker. I have electronics knowledge up to basic semiconductors, so I know what most DC and AC components are.

In my electronics class, we are building projects, and I was assigned to build a speaker. As I wasn't sure how to build a speaker of any kind, I joined this forum for some advice to get me started. Would anyone please be able to explain to me how Computer speakers work and how they are made?

Here's what I know:

• Normal "wall" voltage is 115VAC.
• I need an LED for a power indicator
• I need a 3.5mm TRS connector for signal input

What I don't know is:

• I want to have 2 speaker diaphragms; how would I line them up?
• How to make volume adjustable
• Do speakers normally use capacitors?
• I hear most speakers use 8 ohms of resistance, but where do you put that?

Thanks in advance to anyone who can help me out.

EDIT: I think I'll need to make it wall plug-in.

 

[magnatro]

you can use the audio amplifier IC TDA 2822,
it has got 1 watt rms output(max) and a good noise reduction capabilities..
reply..

 

[transistor495]

USB powered speakers are not a big deal. They provide 5V at a maximum of 500mA(self powered hubs). So I assume we can get a maximum of 2Watts of audio power through it. TDA2822M is the best option here so that you'll get a stereo operation too. No need to go for series or parallel speakers. LED indicator, Vol ctrl and rest of the options are pretty easy to include.

 

[Hero999]

With a 5V supply and an 8Ω load, the maximum power will be no more than 250mW per channel, unless you bridge it.

The output of your sound card can probably directly drive a pair of 8Ω speakers to nearly that volume so why bother?

 

[Frederick]

The reason I wanted to go with a USB speaker is because the goal of the project I'm doing is to make something efficient that runs on as low of voltage as possible. 115V is obviously higher than 5V, so I chose the one with the lower voltage.

Also, I'm not sure exactly how to go about adding a volume control. If I just added a potentiometer, wouldn't the changing resistance alter both volume and pitch?

 

[transistor495]

You need a stereo pot(with 6 terminals) if you're using TDA2822. In that the center terminals(wiper) is the output that going to IC. It won't alter any pitch.

 

[audioguru]

Because you selected a supply voltage as low as only 5V then the amplifier will produce only 0.25W into 8 ohms per channel at clipping. A cheap clock radio will be louder.

My pretty good pc speakers are driven with 3W per channel at clipping. For little 3" speakers their magnets are huge and they have a rubber surround. Their enclosures are pretty big so they produce bass sounds well.

 

[Hero999]

Have you tried connecting the speakers directly to the sound card output?

I did that awhile ago with an old 486 because I didn't have a set of amplified speakers, it worked perfectly and it was loud enough for me.

Just buy a cheap pair of unamplfied walkman speakers and plug the jack into your sound card. I know you like the idea of powering it from a USB port but it really isn't worth it.

 

[Frederick]

Honestly, I've never built a speaker before, and don't really know where to begin. I changed the thread title and first post, due to the fact that many people here say it would be more difficult and less effective to run a speaker on a USB port.

 

[audioguru]

Have you determined how much power you want and selected suitable speaker drivers?
Mono or stereo?
Do you have plans for speaker enclosures that match the spec's of the speaker drivers?
Have you found the schematic of amplifiers with enough output power?
Have you selected parts for a power supply for the stereo amplifier?

 

[Frederick]

Once again, I only know the first couple steps to building computer speakers. I have drivers that I believe will work, and I wish to create as simple a speaker as possible, so it'll probably be mono, with drivers in one case, one for higher frequencies and a larger one for lower frequencies. I received my drivers from my electronics class' back room, so I don't know specs they have for encasement. My only knowledge on the encasing of drivers is that one must minimize external vibrations and to get shielded drivers to not distort the computer screen.

I haven't found a schematic for an audio amp because I didn't know I needed one.

No, I haven't found parts for the amp I didn't know I needed to build.

 

[audioguru]

If you computer's sound card does not have a power amplifier then you must make one.
Since you don't have spec's for the speaker drivers then you must guess at how much power they will survive. You also must guess at the size of the enclosure. If the woofer is cheap and small then it will not produce low bass frequencies. If the enclosure is too small then the speaker will not produce low bass frequencies.

Your mono speaker will be a two-way type with a tweeter for high frequencies and a woofer for low frequencies. You must make a crossover network that keeps the low frequencies from destroying the delicate tweeter and to keep high frequencies out of the woofer.

 

[Frederick]

Thanks for helping!

My computer's sound card does indeed have an audio amp.

I've found an enclosure that looks like it'll be big enough for my drivers, but I'm not sure if I can find a "woofer" driver wider than about 6" on my budget (Which just happens to be $20 or so)

I just learned about high-pass and low-pass filters last month, so I knew I'd need to do something with that.

One more question I have is thus: How exactly does the speaker receive signal and power? Most speakers I've seen both plug into a wall and use a TRS connector (the lime green plug). I'm fairly certain that the signal comes from the TRS and the voltage comes from the plug-in, but where do they fit into the circuit?

 

[audioguru]

Computer speakers have built-in stereo (two channels) power amplifiers. the tRS plug is their input and a wall-wart AC/DC adapter powers them.
The power amplifier on your sound card might or might not have enough output power to drive your speaker loud enough.

 

[transistor495]

True USB speakers requires no extra connection to the PC audio port. Power+data is provided by the USB port itself. It may expensive as the speaker contains D/A converters and interfaces for communicating with the PC. In addition to that a power monitor is also included to prevent any attempt to draw more power through USB(5V,500mA max. allowed).

 

[Frederick]

True USB speakers requires no extra connection to the PC audio port. Power+data is provided by the USB port itself. It may expensive as the speaker contains D/A converters and interfaces for communicating with the PC. In addition to that a power monitor is also included to prevent any attempt to draw more power through USB(5V,500mA max. allowed).

I decided to do a normal, non-USB speaker, for cost and efficiency reasons.

How much voltage/current should this "wall-wart" be supplying?

 

[transistor495]

You need to give more info about your requirement. How much power needed, speaker spec.s if you've got any already, any old power supply you've access to -I'm asking in terms of cost effectiveness.

 

[audioguru]

True USB speakers requires no extra connection to the PC audio port. Power+data is provided by the USB port itself. It may expensive as the speaker contains D/A converters and interfaces for communicating with the PC. In addition to that a power monitor is also included to prevent any attempt to draw more power through USB(5V,500mA max. allowed).

I have never seen a true USB speaker. If its supply is 5V at 500mA and its amplifier is linear with 60% efficiency then its output power is only 1.5W.

 

[Hero999]

I'm not meaning to be pushy but have you tried what I originally suggested?

Connect a pair of 8Ω speakers to a headphone jack and plug it into the output of your sound card, it won't be very loud but it might be good enough for you.

Then if it isn't loud enough, you can have a go at building your own amplifier.

 

[Frederick]

@hero999:
By plugging it into the sound card, do you mean that I would be doing that by plugging the speaker only into the headphone jack, or attaching it in some way to my sound card? I tried just plugging my 8Ω resistors into the headphone jack, but to no avail. Not even a whisper.

@audioguru
Here's an example of a solely USB run speaker. It's not loud, but it apparently gets the job done.

@transistor495
I've already gotten a tweeter, but I'm still looking for a suitable larger speaker (should I go for a midrange or a woofer?) The tweeter's rated at 8Ω & .25 Watts.

For power supplies, I have an AC/DC power supply that can put out up to 24V. I'm not quite sure what you mean by asking if I know how much power I need. Wouldn't I not know that until I had both of my drivers?