Help me find the right amplifier

[audioguru]

2 speakers in phase increase the loudness +3dB which is a little louder. 10 speakers will sound twice as loud since our hearing's sensitivity to loudness is logarithmic. The small increase in loudness between 150Hz and 4kHz will affect the tone making it sound a little "nasal".

A woofer has a "cone breakup" peak in the upper midrange that a crossover eliminates. The midrange might sound boomy at its resonant frequency that a crossover eliminates. A real tweeter is destroyed by the high cone movement caused by powerful low frequencies.

Don't you have the woofer playing one of the stereo channels and the full range playing the other? Then listeners probably will not hear both channels mixed into mono unless they can hear both speakers.

 

[SentinelAeon]

I am asking for this 1 friend because he wants woofer + full range as opossed to woofer + tweeter as i usualy do. And i am wondering where i should cap the full range at. Capping it at 4000Hz is quite useless - i listened to some songs of mine where i eliminated frequencies under 4000Hz and all i heard was hiss. So i am wondering where i should cap the full range. I would like it to be as low as possible. Also i am wondering whether i should just leave the amplifier as it is in stereo, or mix it into mono.

 

[audioguru]

Adding a series capacitor to a speaker to reduce its output at low frequencies still allows it to resonate since at resonance its impedance is high. A speaker at resonance usually sounds bad (boomy) unless it has the extremely low output impedance of the amplifier's direct connection to damp the resonance.

The datasheet of the full range speaker will show a curve of its impedance vs frequencies for you to calculate calculating a cutoff frequency of maybe 1.5kHz.

 

[SentinelAeon]

Another example;

https://www.farnell.com/datasheets/2579914.pdf

Just wanted to say i am finaly going to test mixing stereo into mono on this amplifier i have. I decided for this one, i will not use the pot at all because it is not needed. So i will just use 2 4K7 resistors. I attached a picture you drew, just to verify that all is well. So i am guessing that the bottom 2 pins are either going to the receiver or amplifier and the middle 2 are going to 1 of them. So i put a resistor between resistor and amplifier on each channel to prevent current going from 1 channel to the other when 1 plays and the other one is silent. And obviously i link both of them together to get mono.

 

[audioguru]

I show the two resistors and a jumper wire to convert stereo to mono.
The volume control had an on-off switch that I show as a jumper wire.

 

[SentinelAeon]

Ahh, i see, so the 2 bottom pins go to the amplifier. Thank you. What about 2 pins at the top ? Do we even use them ?

 

[audioguru]

The bottom pins of the volume control come from the Bluetooth receiver.
The two pins at the top of the volume control are ground (0V) connections. The middle pins (sliders) connect to them when the volume is turned down all the way. See post #17.
The middle pins are the sliders that connect to the amplifier inputs which is why I jumpered them together for mono.

 

[SentinelAeon]

Thank you ! And the resistors are here to protect each channel when for instance only L channel is playing, there is no resistance on R channel and the backflow to R can damage the receiver ? And if i use 4K7 resistor it will prevent that. And the lower the value of resistors i use, the less losses i will have but more chance of flow backflow to the channel that isnt playing ?

 

[audioguru]

An antique amplifier with vacuum tubes always need a load because its output transformer will cause very high voltages without a load.
A modern solid state amplifier does not always need a load because it does not have an output transformer.
The load is parallel with the output and to ground.

The 4.7k mixing resistors are in series with the output of the bluetooth receiver. Whenever the stereo signal cause one channel to go positive and cause the other channel to go negative then each channel will have the 4.7k mixing resistor as its load to ground. We do not know if the Bluetooth receiver can drive a load as low as 4.7k ohms.

Instead of guessing if the Bluetooth receiver can drive a load resistance as low as 4.7k then simply measure the resistance of the volume control that you are removing. For each set of 3 pins measure between the outer pins. Use mixing resistor values that have the same resistance.

 

[SentinelAeon]

Well, i have some good news and some bad news. The good news is that i managed to get the mono to work. The bad news is i managed to completely obliterate the amplifier in the process, no way i can give anyone this amp in a speaker. But it was all good fun. The pot is unbelivably hard to get off. I first used a heatgun and then put solder over all 6 pins on the right and slowly got it off, but the 2 pins on the far side were almost impossible to get. I wish i was a little more patient because i already had it off .. but in the end i managed to pull off 1 of the lines. To make things worse, it didnt take solder. So i had to literaly take a gob of solder and encase it with it. In the end, mono works, i tested it. I didnt have enough time to test the quality and SPL of this setup. But anyway, this wont do, if i cant find a way to get the pot off in a safe way. Maybe i could just cut it off with a rotary tool. Or ... just short the L and R .. and glue the pot at like 90% volume. I will need more data on how that will work.

 

[audioguru]

I use a temperature controlled soldering iron that never gets too hot.
Then to remove a soldered part I put a little hot rosin-core solder on the joint then heat it again and use my solder sucker to slurp the solder off.
The solder sucker has a spring-loaded plunger and a button to activate it.

 

[gophert]

My favorite way is to add a little bit of solder to the joint then use a piece of good quality solder wick on it then hold my soldering iron on it for 2-3 seconds and most is gone. A little wiggle of the part snd it comes loose.
Be sure to use a temp controller iron. Mine is very old and uses a tips made of alloys that demagnetize at 600, 700 or 800°F (depending on the top). The magnet releases when the tip phase transfers to the non-magnetizable phase occurs. Low tech electrically, higher tech materials. It's nearly 50-years old, works like a charm, tips are still available from the original manufacturer.

 

[rjenkinsgb]

My favorite way is to add a little bit of solder to the joint then use a piece of good quality solder wick on it

My exact technique also; or alternate solder wick with a solder sucker.

Ensure each/every pin is totally clear and can be moved around in the PCB hole, before removing the components.

I've removed a number of those exact stereo switched pots from amp boards when rebuilding my house audio to stereo; they are a bit fiddly but not as bad as many ICs.

 

[SentinelAeon]

Sadly all i have is a heatgun and simple welder. I ordered the desoldering wick. The biggest problem is that i need to desolder 8 pins at once and my solder isnt nearly big enough to contact them all at once.

As for right now, i need a mono amp in a manner of days. So lets say i leave the pot and just join the middle pins. That way the pot would serve as resistor on each channel. I will glue the pot at the right place and it will be hidden inside the box so it cannot be adjusted. My question is, how do i determine the right position of the pot. 80% ? 90% More/less ? Or should i maybe measure the resistance between the bottom pin (receiver) and middle pin (exit from pot going to amplifier) and just turn it until value between this two pins is about 4K ?

 

[audioguru]

The amplifier pcb appears to be double sided then the pins for the volume control might pass through to the other side then wick will not work.
You remove solder from each pin separately, heating all of them at the same time will damage the pcb.

Many years ago when I was a kid I had a soldering gun. It damaged everything I used it on because it could get red hot which is way too hot for soldering. I got my temperature controlled soldering iron when I got my first job many years ago and it still works perfectly and is still sold.

Your idea of adjusting the volume control for 4k ohms on each channel, joining the wipers together then glueing it is good. Try it.

 

[rjenkinsgb]

The biggest problem is that i need to desolder 8 pins at once

That's the problem; you need to do one pin at a time, remove all the solder and ensure it is free in the hole, then do the next.

Don't you even have a "solder sucker" type tool?

Draper SCS Solder Sucker - Machine Mart - Machine Mart

www.machinemart.co.uk

 

[audioguru]

My solder sucker might slurp ketchup better than I can.

 

[SentinelAeon]

That's the problem; you need to do one pin at a time, remove all the solder and ensure it is free in the hole, then do the next.

Don't you even have a "solder sucker" type tool?

Draper SCS Solder Sucker - Machine Mart - Machine Mart

www.machinemart.co.uk

Sadly, i dont. I never had the need for one till now and i buy tools slowly as the need arises. I have no electronic background, i am a different kind of engineer by degree (sometimes i think i should have gone to electro instead of computer science (programming) ), so i learn as i go. I learned a lot since i first came here, from audioguru and others. I am really happy for the help i get, on other forums mostly i got laughed at for my lack of audio knowledge.

Anyway back to the topic. I measured the resistance of the pot, it goes from 0 to 10K ohms, which makes sense since, well, it is a 10K pot. So i set the pot to 4.5K, soldered the middle joints together and did AC voltage testing on the speaker terminals. Before at 100Hz i got about 7V, now i got about 3.5V. So 4.5K is out of the question, but i read that people used anywhere between 300 and 1K ohms for this. I might settle for 1K but only because the speaker is not for me. At 1K ohms i get about 6V instead of 7V and while i can hear the difference, the fact that on the other channel i will have 2 full ranges blasting from 200 or 500 to 18K .. i think combined the max SPL will be the same. Again, this is for my friend, based on his wishes (i explained to him pros and cons .. if i was building speaker for myself, i would keep the current woofer on left, tweeter on right, stereo and keep every last 0.1dB i can, this is the most important thing to me, out of 100s of songs i played on the speaker, i only noticed the stereo thing twice and even then it was just for a part that lasts few seconds ... remember, usage of speaker in places where people talk very loud, music around is playing, you need SPL, people are drinking, no one cares about stereo quality).

Anyway, this loss of dB when mixing stereo into mono is really bothering me, simply knowing i am not using the full potential. So i am thinking about the alternatives. I really like this amplifier but by the looks of it, i will not be able to mod it in a way to get the same SPL, not with my tools and knowledge. Since i am playing mono now, it would be great to be able to somehow join the two channels. If now each of my channels is capable of 50W when in stereo ... if i could use both as single mono, then with the resistor losses i would still get about 80-90W combined ...

As for finding a nice mono amplifier in my price range .. i turned aliexpress upside down. I could find plenty of very nice cheap mono amplifiers that lack the bluetooth receiver. And receivers are cheap but the ones i tried have the horrible chinese woman performing an audio genocide on english language. It is stressing me out because i have some friends waiting for my speaker, they loved the one i use and asked me if i can make 1 for them. But while i dont care about the stereo thing, i dont want to sell them a speaker and have them message me later saying that there is that 1 special song which is completely out of whack because i dont have a proper stereo or mono setup.

If any of you has any idea how to go forward with this problem, it would be awesome

 

[rjenkinsgb]

How about this? It supposedly uses a TPA3116, which is a TI device with full documentation available.

https://www.amazon.com/Bluetooth-Amplifier-DAMGOO-Technique-Password/dp/B07XG33WPN

https://www.ti.com/lit/ds/symlink/tpa3116d2.pdf?ts=1602885939007&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FTPA3116D2

 

[audioguru]

I think it is pure luck and amazing that the amplifier produces its maximum undistorted power when your phone and the amplifier volume controls are at maximum. Surely the maximum output level of each phone brand is different. The phone's volume control allows weak levels to be turned up then normal or high levels are turned down. Your phone/amplifier combination has both volume controls at or near maximum then weak levels cannot be turned up and some phones will not produce a high enough level.