• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Help me find the right amplifier


Well-Known Member
Most Helpful Member
Bass is usually played on both stereo channels at the same time. So two channels (mono) make +3dB (double the output power) which sounds a little louder than the single output you had before.

If only one channel plays parts of the stereo music then adding a channel with a signal to a channel with no signal gives you 50% on each channel. Then the overall loudness is the same as with one channel playing.


Well-Known Member
Most Helpful Member
how does mixing stereo into mono with 2 resistors affect loudness and bass ?
The mixing will affect maximum loudness. It depends on the resistors used and the input impedance of the amplifier. You can measure it with a potentiometer, a scope and signal generator. You feed a signal throuh a variable resistor at 0 ohms and until the signal is 1/2 the value. Take the pot out of the circuit and measure it's resistance. That's your input impedance.

Typical consumer values is 10K. using 10K to mix it, you lose half the signal, but audio is not linear in loudness, so it won't be 1/2 as loud. it depends on the output Z too of what's driving the 10K resistor.
Is there any way to mix it with minimal losses ? Because loudness of this little bluetooth speakers is, apart from bass and portability, the most important thing. Does half the signal mean half the power, as in -3dB which is about 23% perceived loudness ? What if i use resistor of lesser value ?

Btw, stupid question but .. the easiest option with simply shorting the pins under the pot to mix into mono ... what if instead of simply shorting this two pins, i add a resistor between them ? Would that have any effect at all ?

If it cannot be done i will only have 3 choices:

1) "shorting" the pins on the pot, which means at max volume there is no resistance from the pot, hoping that the receiver will survive that (is there any data on how possible it is to actualy kill the receiver this way ?)
2) find a mono bluetooth amplifier or mono amplifier without bluetooth and mono receiver
3) simply living with the fact that stereo will not play properly. Me personaly it doesn't bother at all, i listened to 100s of songs and only noticed this problem on probably 3 songs and for a few seconds. It might bother my friends though. So an easy solution to this is to replace the tweeter with a full range speaker capped at 150Hz and enclose this full range to protect it from the woofer. This way, on either channel the stereo part of the song will be played, the difference will be much smaller than it is now where woofer covers 0 -3000Hz and tweeter covers 3000-20000Hz (which means that when part of the song is only played over tweeter, you basicaly hear silence .. )


Well-Known Member
Most Helpful Member
-3dB is half the power. It sounds only A LITTLE less loud Since the sensitivity of our hearing is logarithmic, then -10dB or 1/10th the power sounds half as loud.
Unless you have an original signal and switch it to one that is -3dB and back and forth, then most people will not notice the small drop in volume.

If the maximum undistorted volume is too low then use a higher supply voltage for the amplifier so you can turn up the volume control.
I am already at the limit, i am using 25V voltage with 4 ohm speakers, its 40W dayton woofer and as it is, the max measured watt usage was 45W, so i am near limits here.

What about those 3 questions i made? For instance, using resistor of 10k nets half less signal strength which is -3dB. What if i use resistor of 4k or 2k ?


Well-Known Member
Most Helpful Member
Don't worry about it - just use the two 4k7 resistors in place of the pot, it will be fine.

If the left and right channels are equal, the signals are identical and you get full volume (or near enough so).

At the other extreme, where a signal only appears on one channel, the two resistors in series cause the level to be halved.
But, that is exactly what the overall output would have been with a full stereo setup - one channel full volume and one no volume, so average half.
Awesome, thank you both for your help, without you i wouldn't even know where to begin.

But i still have to ask .. let's say i mix it into mono with resistors and then drive each mono channel through a pot (which i would desolder first) ... let's say the pot was turned on all the way. Wouldn't that in effect be the same as if resistor wasn't there in the first place and that would give me the max volume without any major losses (if we ignore the fact that surely pot has some small resistance even if it is turned all the way). And when the pot would be at like 50%, surely, the 2 resistors we used for mono mixing and resistance from pot would combine, but at that point we do not care about it .. if we wan't it louder we just put the pot to like 60%. And like i said, when we want all out, maxximum volume, we just turn pot to 100% and hence its like there is no pot, almost. Am i understanding this right ?


Well-Known Member
Most Helpful Member
The "mixing" resistors and the overall resistance of the pot then act as a permanent divider and reduce the total volume. With 4K7 mixing resistors feeding the 10K pot, the voltage would be around 2/3 the original level.

Putting the resistors after the pot would hopefully have minimal effect.

But; if you have decided now that you want to keep the pot, just try bridging the to wiper terminal like I originally suggested and see if there are any detrimental effects.

If it works OK forget the extra resistors!
Alright, i will do that. And if it doesn't work, then we will work out another solution. The only part that bothers me is that i read a lot about this mixing online and i read that when only 1 of the channels plays .. and there is no resistance (aka pot at max volume), then ur basicaly shorting the receiver terminals and the receiver can die in time. That would be a big problem since i give or sell this speakers away and well, if 20 people come to me after a year saying the receiver died, well that is a problem for me. But then again, this periods of music when there is only 1 channel playing ... i mean i found like 3 songs so far of the music i listen to and its only seconds long. I think i will risk 1 of the amplifiers, i mean its 4$ per amp ... and just short the pot and play only 1 channel straight for a few days and see how it goes :)


Well-Known Member
Most Helpful Member
If you want to idiot-proof, just put the mixing resistors after the pot, like I suggested earlier.
The amp should be rather higher input impedance so it will cause rather less attenuation.

You can also use lower value resistors, eg. 2k2 or 1k.

I cannot find any datasheet for the bluetooth IC on that audio module, though i am struggling to read the number...
I do not think u will find it ... the one i use is nameless chinese amp ... that is why i was so surprised, i put it against many other amps with good chips and it is just wonderful. Its WUZHI something. Btw, the 2x100W uses 3110 chip or something like that.

I will think about putting the resistors after the pot. I will have to check if i have any 1k resistors, sadly i ordered like 100 4k7 resistors. If i use 1k instead of 4k7, in theory it should be tiny bit louder, right ?

I have another question. Now that we located receiver output under the pot ... i was wondering .. this amp has no 3.5mm jack, i am making a 3.5mm jack for my friend and he would like 3.5mm jack. I can order this amp again, there is a version of it with 3.5mm jack included. But my friend will have to wait 2 months for it to arrive. Is there any chance of somehow soldering 3.5mm jack directly to receiver output, so that it would work through either bluetooth or 3.5mm jack ? If that would produce problems, i could even add buttons that would allow either 3.5mm or bluetooth to be connected to amp, but never both ?


Well-Known Member
Most Helpful Member
All you need for that is a stereo 3.5mm socket that has break contacts on both tip and ring.
See the diagram at the bottom of page 2 of this datasheet for an example:

You would remove the pot and connect the socket between the top pot pads (signals from the amp) and the actual pot top, the clockwise terminals. Reconnect ground and wipers to the board.

The bluetooth audio would go to the middle two pins on the jack diagram, the break contacts.
The feed back out to the pot from the next two out, the contacts that connect to a plug.
And ground to ground.

When an aux lead is plugged in, that connects to the amp and the bluetooth audio is disconnected.

Another example socket:

Again, if you wanted mono, add the mixing resistors between the pot wiper pins and amp inputs.
Alright, thank you for the replies.

I have another project to finish but right after that, i will do a couple of speakers again. First project will be a big 15L bluetooth speaker. So i will have a couple of questions about it.

1) its a 40W speaker, SB Acoustics SB20PFC30-4 8". I am trying to figure out whether i need to increase battery capacity as opposed to the previous speakers. The previous speakers were using Dayton Audio TCP115-4 4" 40W woofer. So they are both 40W, will be driven by same amplifier.The SB Acoustics is double the diameter, 8" vs 4" and while they are both 40W, for some reason i simply think that the 8" consumes more electricity simply because moving a bigger part uses more electricity. I am guessing i am wrong ? Am i safe in keeping the same battery capacity which proved more than enough for the smaller Dayton 40W system ?

2) I decided to skip the pot in this system to make things simpler for me and i decided i will only include pot when the buyer expresses the need for it. Because it is simply not used since people control volume with mobile phones and it is just confusing to use 2 separate volume knobs (2 weeks ago i was listening to speaker outside and after listening to it for 1 hour, thinking that its not as loud as it used to be, i found out that knob wasnt at 100%, even though i dont remember touching it. Pissed the crap out of me, my next speaker will not have this pot). So i will just use the simple mixing method into mono with resistors. I only have 4K7 resistors but i have tons of them. So i was thinking, can i just use 4 of them in parallel to get about 1175 Ohm resistance per channel ? I understand that even using 4K7 resistor i will probably not notice the difference in volume but i would simply like to lose as little as possible since the buyer wants absolute maxximum possible loudness above all else. So it would really be stupid for me spending tons of time searching for the right speaker to get every last % of volume out of it ... and then losing 3dB (23% !!) loudness.

3) I do not have the 3.5mm connector like you linked. I do have the cable extension and plan to use that. I will get back to you with a schematic so u can double check it before i connect it.


Well-Known Member
Most Helpful Member
The 8" woofer will be noticeably louder (+5.7dB) than the 4" woofer. Then people might turn down the volume and use less battery power.
At a distance of 2m the sensitivity of the 8" woofer is 86.5dB at 2W. Then 89.5dB at 4W. Then 99.5dB at 40W.

With 4.7k mixing resistors can you turn up the volume of the phone so that the amplifier output is clipping? if so, then most people will not turn it up higher.
A while ago i did some graphs in winISD, trying to figure out if it would make more sense to use 2x 4" TCP115-4 or 1x SC 8". The reason was that the price was very close. So in 15L enclosure, this is the graph that it gave me (in the attachment). Sadly i haven't have the opportunity to test it yet but i will. I got a dB meter that is supposed to have range of 30-18.000Hz. I do not know how accurate it is in this kind of usage but we will see.

I haven't had the time to test any mixing of stereo into mono, right now i am fully working on another interesting project (a control system that will take care of edible mushroom farm and save its owners hours and hours of work), but i have to make sure it is perfect and until it is, i cant work on anything else. I do know that this amp with pot, when both pot and phone are at 100%, in some songs you can hear a very slight distortion but not enough to be annoying. I do not know if it is because of less boost or some other thing but i remember on TDA7492P there was very bad distortion, but again, it could be that i was driving a full range in low frequency mode. But that is part of the reason i am so in love with this Wuzhi 2x25W .. i can let anyone connect to it and not worry that he will put some song to loud and it will distort the hell out of it. I will have to test it and get back to you.

It would still be good to know how the resistor value will affect it. For instance using 4.7K vs 2K vs 1K, what will change, what will remain the same. Also i am very interesting if this pot really has no resistance when on 100% or is there still some small resistance.


Well-Known Member
Most Helpful Member
The little 4" speaker is a mid-bass that cuts frequencies below about 90Hz. The 8" speaker is a much more sensitive real woofer that produces deep low frequencies. Two little speakers do not equal one larger speaker.

Like modern American or European amplifier ICs, the Wuzhi amplifier IC outputs are bridged (each speaker wire is driven with power) so with a 24V supply it produces about 25W into 8 ohms per channel or almost 50W into 4 ohms.
The TDA7492 amplifier IC is old and its speakers have one wire grounded so its output power is much less than the Wuzhi unless it has a much higher supply voltage (up to 66V).

ANY amplifier distorts like hell if its volume is turned up beyond clipping. Usually an amplifier has more gain than is needed most of the time so that low levels can be made loud enough. If the Wuzhi hardly ever distorts at max volume then low levels will always be low.
Instead of making a new topic, i will post some questions here. I didn't have the chance to test the stereo mixing and/or button placement yet.

We usualy make a crossover for tweeter and woofer so each cover their own part of frequency graph. I would like to understand what happens if we do not do a proper crossover and woofer/full range both cover certain part of frequency. Just for the sake of simplicity lets asume that woofer and full range both play at same loudness. Meaning both playing vs only 1 playing adds +3dB (~25% increase). I will give you a simple example:

Woofer: covers 50-4000Hz
Full Range: covers 150-18.000Hz

So my question is, what is the effect of this ?

1) Does this mean the frequency at 150-4000Hz that overlap are amplified ?
2) Does this mean that the frequencies at 150-4000Hz cancel eachother and actualy it plays less loud ?
3) How does it affect the quality.

I hope you can help me understand how this works to better understand the reasons behind the crossovers we all use.

Latest threads

EE World Online Articles