Using $1 stereo amp in mono mode

[SammyT]

100Ω is lower than 10 kΩ...

Yes it is Sorry - it's getting late!

I found a real beefy 4" speaker, can't recall what it's from, maybe a Hotspot type stage monitor and it sounds a lot better, actually can hear some bottom. I also found another 3" speaker that sounds better so I'm thinking now just to use those two in stereo because they sound good that way. I know it's wacky having two mismatched speakers but it's all in one box so normal stereo it's not.

Kris, just noticed your Down Under flag I'm in Hawaii no wonder we're the only ones awake.

 

[KrisBlueNZ]

I found a real beefy 4" speaker, can't recall what it's from, maybe a Hotspot type stage monitor and it sounds a lot better, actually can hear some bottom. I also found another 3" speaker that sounds better so I'm thinking now just to use those two in stereo because they sound good that way. I know it's wacky having two mismatched speakers but it's all in one box so normal stereo it's not.

Yep, that seems a bit odd, but hey, it's your project right?

Kris, just noticed your Down Under flag I'm in Hawaii no wonder we're the only ones awake.

We also have several members in Australia, including two moderators. I'm about to hit the sack anyway.

 

[SammyT]

I stayed up last night and watched some YouTube stuff with the mocked up amp providing audio. In the end, I like using just one speaker best, it sounds purer. Having two speakers in the same box is a bit like dropping two pebbles in water - the waves run into each other and create weirdness that detracts more than adds.

I've had amps that you could run forever with no speakers attached, and others that would blow if there was no load. The manufacturer of this $1.24 amp say it will blow without a load. In my mockups, the flimsy speaker wires kept breaking so there were several times I know I was running this thing with no load for maybe 30 seconds, so I know it it does have some resilience to that.

Questions: if I want to run just one speaker, can I use a 10Ω resistor as a dummy load on the unused channel's output? In an amp this small, is a 1/4 watt resistor ok or would a 1/2 watt be better? Does it matter that the speaker on one channel is 8Ω (or say 4Ω) and the dummy load is a 10Ω resistor?

 

[KrisBlueNZ]

No, that amp won't blow up without a load! I don't know why the seller said that. Perhaps he heard that some amps need a load and decided to play it safe by telling you that. But you can put a load - say a 33Ω 1/4W resistor - across the unused output if you want. And disconnect the signal from the input of that channel, to save a bit of power.

Re putting two speakers in the same cabinet: try reversing the wires to one speaker and see if that improves the sound. Both speakers need to be in phase, that is, both cones need to move outwards at the same time, and inwards at the same time. If one of them is reversed, they'll move in opposite directions and partly cancel each other out!

 

[SammyT]

Thanks Kris. I'm familiar with the phase issue and have tested polarity with 9V batteries. With this amp, I noticed that when you turn it on (the volume knob has an off switch at full counterclockwise) that it made a pop and both speakers went outwards. Plus I did observe the plus and minus marks on the speakers, although awhile back I think it was JBL whose speakers were the opposite than everyone else's.

I'm going to go with the one speaker, I suppose I could put a jack on the other side to add on another speaker for big parties

Kris, what makes it that this amp can run with no load? Is it just the way the PAM8403 IC is designed? Making an amp that can blow when there's no load seems like a booby trap.

 

[KrisBlueNZ]

Ah, that's good that you knew about phasing. From memory, I think speakers are usually marked so the cone moves outwards towards the listener when you apply the voltage with the marked polarity, but you can't rely on that, especially if you're mixing manufacturers; you need to test each speaker with a AA cell or similar. Also I wouldn't rely on the pop being of the same polarity for both channels. It probably is, but I wouldn't rely on that.

Yes, the way the chip is designed means that its output pins do pretty much the same thing regardless of loading (unless they're overloaded by a speaker with too low resistance). It's a Class D amplifier, which means its outputs are constantly switching between GND and VCC at a high frequency. What is, or isn't, connected to the outputs doesn't make much difference.

The exception is mentioned in the data sheet: if external LC (inductor-capacitor) filtering is used, it's possible that the output stage could be damaged because of the voltage-generating characteristics of inductors. But that circuit doesn't use external LC filters, and removing the speaker reduces the inductance, so there's no problem.

Amplifiers that do require a load aren't designed deliberately to have that characteristic. It's a by-product of other design decisions that were made for other reasons, and the designer decided the benefits outweighed the disadvantage.

 

[SammyT]

Thanks I'll just skip the dummy load resistor. I've be listening to it for about 45 min with only one speaker and no dummy load and it still sounds like a $1.24 amp! It sounds way better than the laptop's speakers, even without an speaker enclosure. I wonder why Apple didn't put an amp of this quality in the laptop originally. I'm cutting up some scrap wood to make a wiki enclosure.

 

[BobK]

You could safely go down to 33Ω resistors for typical headphones, which are 32Ω. This would give you the same sound level as a mono amp without the resistor.

Bob

 

[BobK]

Thanks I'll just skip the dummy load resistor. I've be listening to it for about 45 min with only one speaker and no dummy load and it still sounds like a $1.24 amp! It sounds way better than the laptop's speakers, even without an speaker enclosure. I wonder why Apple didn't put an amp of this quality in the laptop originally. I'm cutting up some scrap wood to make a wiki enclosure.

It is not likely that the amp is inferior, more likely the speaker drivers, which have to be quite small.

Bob

 

[KrisBlueNZ]

Yes it will be the drivers. My mother's laptop, an Asus, has a 2.5 mm socket on the side for connection to an external speaker (just a driver, mounted facing downwards in a cylindrical plastic enclosure), called "SonicMaster". The amp is inside the laptop. And the external speaker is very loud and clear and has a reasonable bass response too.

 

[SammyT]

You could safely go down to 33Ω resistors for typical headphones, which are 32Ω. This would give you the same sound level as a mono amp without the resistor.

Bob

How is using summing resistors any different than turning the laptop's output volume down?

I've been doing some mocking up and testing with 100Ω resistors, and to me it actually sounds better to not use them and just turn the Mac's volume down, which I'd instinctively do because it's louder when you sum the channels. Aren't the summing resistors just a preventative thing so that someone doesn't turn everything full blast and then complain that it sounds funny?

 

[KrisBlueNZ]

Resistors mix the left and right channels together. You need to do this - for music, at least - because instruments are placed at different positions on the soundstage and listening to only one channel may mean you don't hear some instruments, and others are louder than they should be.

You shouldn't mix the left and right channels together by shorting them. It's unlikely to do any harm but it may cause distortion in the amplifiers in the laptop.

Mixing the channels together loses a bit of volume but the difference shouldn't be noticeable. Typically only 1~2 dB depending on how the audio is mixed.

 

[SammyT]

The 100Ω resistors actually did make a noticeable drop in volume in this tiny amp, enough that I had to run the amp a little high (around 1/2 way) whereas it does seem to have a sweet spot at about 1/3 volume, like a lot of amps.

Solution: don't know why I didn't think of this before, go to the Sound panel in Systems Preferences and hard pan the sound hard left. Now I don't have the volume drop and I get both channels. Plus I could put a jack on it to power a second speaker.

 

[KrisBlueNZ]

I would not expect that panning the sound hard left would mix the left and right channels together evenly. Check with a test signal that has audio in each channel separately.

 

[Gryd3]

The 100Ω resistors actually did make a noticeable drop in volume in this tiny amp, enough that I had to run the amp a little high (around 1/2 way) whereas it does seem to have a sweet spot at about 1/3 volume, like a lot of amps.

Solution: don't know why I didn't think of this before, go to the Sound panel in Systems Preferences and hard pan the sound hard left. Now I don't have the volume drop and I get both channels. Plus I could put a jack on it to power a second speaker.

Modifying the 'pan' in the music player, or the volume properties will not mix the left/right sound channel.
You could however attempt to change the 'type' of output. Many windows based machines have the ability to pick stereo/mono/surround outputs... give those a try. Additionally some music players have the ability to blend the Left and Right audio channel together (usually with plug-ins) which would help you avoid using resistors to mix channels.
I do certainly agree that you should not simply drop a channel though.. I have music that plays wonderfully in stereo, but you miss lyrics or some instruments if you only listen to one channel . (Problem with using only 1 ear-bud on my iPod I'm afraid...)

Last resort solution is to reprocess all of your music / audio for Mono playback... not ideal, but would get the job done.

 

[SammyT]

You're right, panning hard left in Apple's Sound panel won't work, all you'd get would be left coming out left. But there's a Mac freebie called Soundflower that gives you a real nice mixer on the Mac's output. It's like a mixer in an audio program like Cubase, with the left and right channels on a mixer, so you could pan both the left and right channels hard left, and that would be mono on the left. The mixer also has compressors, limiters and eq so you can get even the built in speakers to sound way clearer. You can leave it on and everything you play, like YouTube will go through Soundflower and then out the Mac. Apple should really have that on all Macs... at least a eq and limiter.

But the main thing for my project was to listen to dialog type videos, not music, where there's very little panning so even if I just listen to what's on the left side it will almost always be fine. So I'm just running my amp as a "left only" model, leaving the option open for adding on a second speaker for the right side later.

I'm happy with it. If there was anything to add it would be a low pass filter so you could turn the amp up without cranking up the highs. I like that it's wood and doesn't sound like plastic and that it's got a big volume knob compared to the computer's awkward volume controls.

My woodworking is crap - don't look too close. The worst part of the whole thing was dealing with flimsy wires that would break if flexed more than twice. So it wouldn't stink or be covered in plastic, I stained it with some strawberry and blackberry jam my sister in law sent.

I really appreciate everyone's time and giving me advice on this. I learned a lot and have a whole bunch of leads to learn from in the next while. Thanks!

 

[KrisBlueNZ]

Great! I'm really glad you're happy.

 

[MichaelZ]

Hello,
I read this whole thread and I have a question. I got the same board - PAM8403 - with the intention of connecting a subwoofer to my JVC component system. The system has a mono output for the subwoofer which requires an amp if I want to use it with the passive subwoofer. The PAM8403 board has stereo input and output and so I wanted to check whether I can combine the input signal by bridging L and R inputs and then use L+ and R- output to get the maximum volume out of this amp. Could someone tell me if this is advisable.
Michael

 

[Audioguru]

The amplifier has bridged outputs that already have a "+" amp and a "-" amp. You cannot get more power than it already produces. If you connect a speaker to L+ and R- then the speaker produces half the left channel level mixed with half the right channel level. Half plus half= 1 so you gain nothing.
You can mix the inputs with resistors and use one output channel to drive a speaker to its maximum output power.

Don't believe the 3W rating. That is into a 4 ohm speaker with horrible 10% clipping distortion. The datasheet shows that it clips at 1.2W into 8 ohms or 2.2W into 4 ohms. The power is so low that you must wear the subwoofer on your ears to hear it.

 

[MichaelZ]

Thank you. That makes sense. I'm not very familiar with electronics, so please bear with me...
The subwoofer says 3ohms - if it matters. So I have:
+ GRD -, on the input. Do I connect my mono input as:
+ bridged with -, and GRD as the second wire or
+ briged with GRD, and - as the second wire? Does it make a difference?

Would bridging L_out+ with R_out+ and L_out- with L_out+ (sorry, R_out-) increase the overall output?

Michael