Laptop extra internal speaker + subwoofer

Thread Starter

fesanand

Joined Oct 8, 2017
5
After a new acquisition, my old hp 6720s laptop is used as electronics dummy tester. I don’t mind to put it under certain risk…

It has one internal speaker with two wires (red and black), so far I have installed another internal speaker(B) jumping the red and black wires in parallel, the two speakers works flawlessly even connected to one single channel.



I bought an internal laptop Subwoofer(S) to install inside my laptop, I guess that they are quite power demanding, so I acquired a mini 5V amplifier PAM8403 board (Stereo). I’ll power the amplifier jumping to one of the USB ports (+5v -5v).

Soundwise the input of the amplifier is 2 channel L + R + GND. So what would be the best way to connect the two laptop red and black sound wires?

Option 1:

Laptop red wire to amplifier R

Laptop black wire to amplifier GND



Option 2:

Laptop red wire to Amplifier R and L

Laptop black wire to amplifier GND



(The output of the amplifier is two speakers but I’ll connect the 2ndspeaker(B) and the Subwoofer(S) )



Thanks!
 

Dodgydave

Joined Jun 22, 2012
8,471
Yes you can connect the L+R together to one of the laptop speakers Red wire, both the channels will give the same sound.
 

PeteHL

Joined Dec 17, 2014
258
You should wire the two mid-range speakers to the board as you show in your post #2. Usually (-) of the output of an amplifier is ground, so those can be connected (shorted) without a problem. However, even if you are feeding a mono input signal to both input terminals of the amp (which you won't be), you can't connect (short) the positive (+) output terminals of the amp.

Also, you are not being clear about how you will connect the "sub-woofer" to the output terminals of the PAM8403.

The signals put out by a sound card are almost always stereo. So the sub-woofer will cause the reproduction to be unbalanced if connected to only one channel of the power amp.
 

Thread Starter

fesanand

Joined Oct 8, 2017
5
The hp 6720s was built with a single mono speaker, with two wires. No stereo at all.

The Subwoofer (HP HDX 16) is quite tall, barely fits inside the optical bay. I guess requires lots of power(relatively) and must be connected to the amp output.
This is my idea of connection, correct me if I'm wrong...
 
Last edited:

PeteHL

Joined Dec 17, 2014
258
The problem with that arrangement (your post #5) I think would be that for a volume control setting of the power amp of the laptop making the original speaker loud enough, then you are over-driving speaker B. Or setting the volume control of the laptop so speaker B is at the right level, and then no sound is coming from the original speaker. This is assuming that the PAM8403 produces voltage gain. So if possible what you could do is in some way make the voltage gain of the PAM8403 equal to unity or somewhat above unity gain.

Not having experience with using power supply from a USB port, I'm guessing, but I think that perhaps the supply accessed at the USB port doesn't supply as much current as the PAM8403 would require. I'm sure someone else here at the AAC could answer that question.

Another possibility that might be easier would be to connect original and B speaker in parallel to the output terminals of the laptop's power amp (which you say you already have successfully tried) and use the PAM8403 to drive the sub-woofer only. To make use of both channels of the PAM8403, it could be setup to drive the sub-woofer in bridge mode (possibly). Is the sub-woofer just a raw driver?
 

Thread Starter

fesanand

Joined Oct 8, 2017
5
Tomorrow is the day I receive the PAM8403.
All the wiring is ready and its overlength allows to comfortably try different combinations or even monitor current required with my multimeter from the usb port (Max 0.5A I guess).
I understand your point,the unbalance, and why to have a speaker consuming power if I'm not able to hear it. I'll start trying your configuration(the last one).
I'll also try only 1 speaker and 1 subwoofer to each amp output but is not my preferred choice.
The subwoofer is a raw one, removed from its original plastic case. Will be installed correctly to let the cone move freely. Unfortunately the subwoofer has not been labelled with ohms, no impedance information available.
I'll post results.
 

PeteHL

Joined Dec 17, 2014
258
Tomorrow is the day I receive the PAM8403.
All the wiring is ready and its overlength allows to comfortably try different combinations or even monitor current required with my multimeter from the usb port (Max 0.5A I guess).
.
If it were me, I'd try to find out how much current the power supply connected to the USB port can deliver, and how much current the PAM8403 draws, before testing. Like I said, I'm not at all familiar with power supplies connected to a USB port, but I would think that you could possibly ruin the laptop's supply by drawing too much current.
 

Audioguru

Joined Dec 20, 2007
11,251
A speaker system with a subwoofer and smaller high frequency speakers uses a crossover network (passive or active) to feed only low frequencies to the subwoofer and feed only high frequencies to the high frequency speaker so that low frequencies will not destroy it.
A subwoofer usually sounds awful when producing high frequencies, like a shriek sound at about 5kHz.

The PAM8403 produces only 2W per channel (3W with horrible clipping) into 4 ohms. If the speakers are 8 ohms then their power will be a little more than 1W per channel.

You cannot "bridge" the two channels in the PAM8403 because they are already bridged.
 

Thread Starter

fesanand

Joined Oct 8, 2017
5
The problem with that arrangement (your post #5). Or setting the volume control of the laptop so speaker B is at the right level, and then no sound is coming from the original speaker. This is assuming that the PAM8403 produces voltage gain.
That is exactly what happened, the subwoofer amplified was louder to the level that the original speaker was imperceptible.

A speaker system with a subwoofer and smaller high frequency speakers uses a crossover network (passive or active) to feed only low frequencies to the subwoofer and feed only high frequencies to the high frequency speaker so that low frequencies will not destroy it.
A subwoofer usually sounds awful when producing high frequencies, like a shriek sound at about 5kHz.
You're right about I need to feed it with low frequencies, actually the Subwoofer sounds like is a normal speaker with higher quality than the original-speaker(4 times smaller) with no awful or shriek sound at high frequencies.

So I decided to add a "passive low pass filter" 1K resitor and 1uF electrolytic cap before the amp input. The good: this lowers the voltage signal and now speaker and subwoofer(amplified) are audible. The bad: getting there, bass is now present, sounds well most of the time, but distortion appears sometimes which is annoying.

I might need a better filtering, I love to work with arduinos(5V) but I read that they are useless for sound filtering.
I might need an active low pass filter working with 5v... Would it make a difference compared with the passive filter? Any other recommendation?

PS: Powerwise 5v usb source, no problems so far.
 

Audioguru

Joined Dec 20, 2007
11,251
Your series 1k resistor then a 1uF capacitor to ground when fed from the original amplifier speaker output cuts frequencies above 160Hz at a gradual rate of -6dB per octave and should be fine and you do not need an active filter. But the original amplifier might cut bass since the original speaker could not produce bass sounds.

The PAM8403 amplifier produces 2W into 4 ohms per channel with low distortion or about only 1.2W into 8 ohms. If you turn up the volume then of course the amplifier will produce distortion.
 
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