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Simple 7W RMS Audio Amplifier Recommendations?

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TheGuy

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Hi all,

I've been looking into how to make a simple amplifier for a laptop speaker system, as I can't find a kit (yes, I'm a newbie) that would suit my needs.

I need to build an amplifier that can supply roughly 5-7W per channel, and can run off my laptop's 19V power brick. It'll be 2.1, as the system has got a bass speaker to fill in the low range, which makes things a little bit more complicated (to my eyes, anyway).

My idea was to have a stereo amp power the mid/tweeters, and have that signal also mixed down into mono (see below) and going into another amplifier (this one mono, and more powerful) for the bass speaker.

Obviously, the bass speaker needs to have only low frequencies pass through (as well as being mixed). I'll be doing this by using a simple (and probably inefficient) way:

Code:
left channel from stereo amp-----\/\/\/\----
                                   1k       |
                                            |-------------------- positive to mono amp
                                            |         |+
right channel from stereo amp----\/\/\/\----        10uF
                                   1k                 |-
                                                      |
                         ground --------------------------------- ground

It does work really well though, and the frequency cut off is PERFECT for the bass speaker I'm using.

So, basically I'm wondering if anyone has any simple circuit recommendations for both a stereo, and a mono amp that can run off 19v? The last time I made a circuit from scratch it didn't work properly, which is why I'm looking for something simple. :) Thanks!
 
Did you Google for "Audio Amplifier Kits"?
 
It's unlikely that you will be able to buy all the parts and build such an amp for less than the cost of most of those kits. For example this kit is only $20.
 
Also check the minimum load resistance that your LINE OUT outputs from your laptop will drive; the 1K resistors look a little too low...
 
It's unlikely that you will be able to buy all the parts and build such an amp for less than the cost of most of those kits. For example this kit is only $20.

Hard to get something cheaper than $30 over here across the pond. :) That same one is £20 ($35).

Also check the minimum load resistance that your LINE OUT outputs from your laptop will drive; the 1K resistors look a little too low...

I initially used 10k resistors, but that reduced the volume too much for my test amp, so I tried 1k resistors. It sounded fine, and didn't seem to cause any mixing before the signal went through the resistors. Would there be an audible difference if 1K was too low? Only the stereo amplifier would be directly connected to the laptop; the mono one will run from the same outputs as the mid/tweeters.

Edit: Regarding kits, there seem to be quite a few available (such as this one: **broken link removed**) which can run off 18v. Would a 1v increase be damaging? Or is it possible to reduce the voltage using a couple of diodes?
 
Last edited:
Hard to get something cheaper than $30 over here across the pond. :) That same one is £20 ($35).

I initially used 10k resistors, but that reduced the volume too much for my test amp, so I tried 1k resistors. It sounded fine, and didn't seem to cause any mixing before the signal went through the resistors. Would there be an audible difference if 1K was too low? Only the stereo amplifier would be directly connected to the laptop; the mono one will run from the same outputs as the mid/tweeters.
I think my argument still holds. By the time you buy all the parts plus a vector board to mount it you will likely spend more than the cost of a kit.

The reason the 10kΩ resistors gave a low level signal was that it rolled off all signals above the 3.2Hz (-3dB) rolloff point (with a 10µF cap). Try the 10KΩ resistors with a 1µF cap. That still will roll off anything above 32Hz.

Typically that roll off point is set closer to 100Hz for most subwoofers.
 
I think my argument still holds. By the time you buy all the parts plus a vector board to mount it you will likely spend more than the cost of a kit.

Alight, you may be right. I've been looking around and I've found two which might be ideal. The stereo one looks to be the same one you mentioned.

**broken link removed**
**broken link removed**

About £25 for both. The stereo one will run off 19v, but the mono one states 8-18v. Do you think a one volt increase will be too much for it?

I've also found **broken link removed** on ebay. If it were disassembled, it could be ideal. But 3x 20w RMS per channel seems almost too good to be true for £11! Do you think they've somehow over rated its RMS rating?

The reason the 10kΩ resistors gave a low level signal was that it rolled off all signals above the 3.2Hz (-3dB) rolloff point (with a 10µF cap). Try the 10KΩ resistors with a 1µF cap. That still will roll off anything above 32Hz.

Typically that roll off point is set closer to 100Hz for most subwoofers.

Hmm, 32Hz is too low for the speaker I'm using. What configuration would I need for, say, a 350Hz roll off?
 
Alight, you may be right. I've been looking around and I've found two which might be ideal. The stereo one looks to be the same one you mentioned.

**broken link removed**
**broken link removed**

About £25 for both. The stereo one will run off 19v, but the mono one states 8-18v. Do you think a one volt increase will be too much for it?

I've also found **broken link removed** on ebay. If it were disassembled, it could be ideal. But 3x 20w RMS per channel seems almost too good to be true for £11! Do you think they've somehow over rated its RMS rating?

Hmm, 32Hz is too low for the speaker I'm using. What configuration would I need for, say, a 350Hz roll off?
I would not run an amp above its maximum ratings. You usually want at least a few volt margin for reliability.

I can't access ebay at the moment so can't advise you on that particular unit. But the old saying is that "If it's too good to be true then it probably isn't".

350 Hz is a little high for a subwoofer since you will hear some of the low voice frequencies coming from it.

The formula for the -3dB roll-off frequency of a 1st order filter is 1/(2*pi*R*C) = f. Using two 10kΩ resistors gives a parallel equivalent of 5kΩ. Then for 350Hz roll-off, C = 1/(2*pi*350*5k) = .09µF. To use the more common value 0.1µF, change the resistor values to 9.09kΩ.
 
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