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Help me find the right amplifier

Hello,

I am making some ultra cheap bluetooth speakers for my friends. The amplifier i use right now is WUSHI ZK-502L. I am madly in love with this amp, its 4$, has a working knob, no annoying chinese woman voice, managed to drive them up to 65W per channel on 3Ohm speakers, no distortion at max volume, etc. Bluetooth coverage is great, no idle hiss, etc. The link to the amp is below. The only problem is, i am using a woofer + tweeter setup and this is a stereo amplifier. Which means rarely on those stereo expressed songs, part of the song will be played only on tweeter or woofer which means very strange sound indeed. If possible, i would love to change this amp into 2x mono output, by mixing the stereo signal into mono between receiver and amp chip. The problem is, this thing is pretty small and i dont know if it can be done.

LINK TO WONDERFULL WUSHI AMP

If not, i need help picking up either another bluetooth amp or just use receiver + amp separately. I need it to give me 50W at 4 ohms and i will suply it with around 24-26V. I would like the price of combined to be around 5$ +/- 2$. The problem is that all bluetooth amps of around 50W are stereo. Which means separate receiver and amplifier. There are tons of good mono amps without bluetooth so that is no problem. The problem is the receiver. First of all, unlike amps, i dont know how to judge them, test them, how to know which is rubbish and which is good. And second, very important ... wushi has no chinese voice when connecting bluetooth, it just has beeps. I just got some tiny bluetooth receivers and it has this poor chinese woman trying to speak english, telling me what is happening and it is just, i dont even know if it is funny or sad.

So if anyone played around with receivers and amps and has some good experience with some gear, please share. And keep the price in mind. To be honest finding good amplifier without bluetooth for cheap wont be a problem so maybe concentrate more on some good bluetooth receiver without the poor chinese woman.
 

rjenkinsgb

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Most Helpful Member
The simple solution is to directly bridge the two wipers of the volume pot.
That will mix both stereo channels and give duplicates of the mix on both amp outputs.

There is a chance of some distortion at maximum volume setting when the two bluetooth receiver outputs become directly shorted, so it's not ideal; but it's the only simple option I can think of without dissecting a board and tracing the circuit out.
 
My god, fantastic, i completely forgot about the pot - the two stereo signals go through the pot. I will need some help identifying which wires to connect. Also what about this: i take both signals from the pot, mix them into a single mono signal using resistors. Then i just duplicate this mono signal and feed it into each channel that goes through the pot
 

rjenkinsgb

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Most Helpful Member
At anything much below full volume, the upper halves of the pot tracks act as the mixing resistors, equal values from each channel.
 
I am not sure what that means, i am pretty naked when it comes to amps. Could u help me trace the pins i need to bridge ? Then i can test it and see how it affects bass, loudness and distortion
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Can you post a picture of the under side of one of the boards so teh pot pin layout & tracks are visible?

A volume pot, at anything other than an end position, can be thought of as two resistors in series.
One above the wiper (the input / clockwise end to the wiper) and one below, wiper to ground.

As you turn the pot, the wiper position (and ratio of the two resistances) varies so you get a different division ratio from the incoming audio to the output.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Then there is no resistance above the wiper, so the two channel inputs would be directly shorted together.

That may do nothing bad, or it may cause some distortion if the outputs of the bluetooth receiver are fighting each other with different signals.
The simple thing is just to try it and see how it sounds!
 

rjenkinsgb

Well-Known Member
Most Helpful Member
The pot has two rows of three pins for the two potentiometer channels, plus two separate pins for the on/off switch.
The wipers are the middle pins on the rows of three - just bridge them with a blog of solder.

I have some amp boards with the same style pot; I've just added a solder bridge on one to demonstrate where it would go on those boards of yours:

IMG_1582.JPG
 
Thank you so much !! I will test it tomorrow. I was also wondering ... this knob is nice and all but its pretty useless since you have volume control on mobile phone, i never actualy got to use the knob. So i was thinking, if i remove the knob completely and then properly mix stereo into mono using resistors like audioguru explained ... would that produce better result as far as distortions goes and other problems ?
 

rjenkinsgb

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Most Helpful Member
If you do not need the pot, you could remove it completely and add a single resistor, eg. 4K7, from the clockwise end (nearest the PCB edge) to the centre hole on each channel then link the two centre pins as I have.
That would provide channel mixing with minimal attenuation.

Lower resistors may be OK, down to 1K probably.
 
Ok, i will make a drawing before i actualy go and connect things. Would it be possible for you to tell me what each of the pins is ?

As for the theory behind stereo into mono mixing, i bought 4.7K resistors as audioguru adviced. I made a simple scheme mixing stereo into mono and would like to ask you of i did it correctly. On the scheme on left, 2 stereo signals come from receiver, get mixed into mono, then separate mono signal goes into each input in amplifier. On scheme u can see separate inputs for left and right, but in reality most amps i saw had only 3 input pins: LEFT, RIGHT and GROUND, meaning u would connect ground of left and right into a same ground pin.
 

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rjenkinsgb

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Most Helpful Member
Yes, that is exactly right.

To do that one one of the amp boards you have, you just need to remove the pot, add the two resistors and a couple of links.

I've drawn the connections over a part of your original photo; red are links (or a solder bridge / folded wire under the pot position).
Yellow and green show the PCB pads where the resistors would go, through the holes the pot fitted in.

The long red link is to replace the pot on/off power switch.
I've tried to enhance the contrast on the photo a bit to make the pot connections stand out more

Be careful desoldering the pot, it looks like there are topside connections to the PCB pads & pulling any tracks / pads could break them.
When I modded the boards I have, removing a similar pot, I got the bulk off with a solder sucker then the last traces with wick, checking each pin would move around in the hole before trying to lift the pot out.


bluetooth_amp_mod.jpg
 
Thank you, at first i was confused at the picture but it is very simple. The green is 1 channel and yellow is 1 channel. Removing the pot i lose contact on those channels so obviously i have to connect them, but connecting them with resistor protects them from shorting when 1 channel is playing and 1 is silent. Then i connect both of them together to get mono with short red. And the long red again needs to be connected in order to there be any contact. What about those other 2 pins, dummy pins ?

As for desoldering, i will be careful yes, i might even use a heatgun and measure with thermometer gun to make sure i never go over 150C. Luckily i had 1 of those bad experiences on a cheap amplifier months ago when i tried to remove a button and i took some PCB together with button. Well actualy, cheap amplifier, it was a 4$ just like this one, but it was a 2x6W. My god, this amplifier screams value ...

I will let you know as soon as i remove the pot and test it. Wish me luck.
 

rjenkinsgb

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Most Helpful Member
Spot on correct.

The other two pins are the grounds at the "bottom end" of the volume control pots.

As both the audio source and amp are part of the same board and the grounds between them are already connected, you can just ignore those pads.

Just found a perfect illustration on google:

 

rjenkinsgb

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Most Helpful Member
For clarity, when I mention the top or bottom of the pot, it's relating to how a volume control is commonly drawn in a schematic diagram, like this; top end input (clockwise terminal), wiper output (middle terminal) and bottom end (anticlockwise terminal) grounded.

That also shows how it can be considered two resistors, if it's not at one extreme end; the parts of the resistance track above and below the wiper.

The fixed resistor mod in effect removes the part below the wiper and replaces the part above it with a 4k7 fixed resistor, in each channel.

 
Thank you, its easier to understand because i know how potenciometer works. Before i found out about step-ups and step-downs i just bought a cheap potenciometer, it was 0.5W if i remember, got bunch of them. I wanted to use them to control fans. So i connected some strong fan to it (12V, somewhere between 0.5 and 1.5A, cant remember). It was a very good effect, the pot was glowing red and smoke coming out. That was my first lesson learned regarding pots :)

Btw since i am unsoldering the pot ... couldn't i just combine both, that is, first drive signals through pot the same way i am doing now and after the pot just mix the signals into mono and feed them to pins going into the amp ?
 

rjenkinsgb

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Most Helpful Member
Yes, but that may lower the maximum level & if the pot is anything less that full up, just bridging the two wiper terminals does exactly the same thing..
 
The problem with pot is, this speakers are always used outside where people tend to see the pot and go, woo, what does that do. So on a friday party night, sooner or later the pot will be turned all the way and i read that it can actualy damage the receiver. As for lowering the maximum level .. if the pot is turned on all the way, isn't that basicly almost 0 resistance ?

I am just trying to figure out the best way to do this. The easiest obviously is shorting the 2 terminals under the pot, you get mono + pot. But then again, the term "shorting" doesn't sound good and i dont really use pot, so i like that option with 2 resistors. But some friends might enjoy the pot while having the mono signal.

Btw, if we forget about the pot for a second .. how does mixing stereo into mono with 2 resistors affect loudness and bass ? Should it in theory be the same as it was before ? Also .. will some songs sound strange since usualy when stereo comes into play, only 1 speaker will play that part so 50% power, but if both speakers play the same part that is 100% or double the power or +3dB AKA +23%
 

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