stereo to mono problem

[SentinelAeon]

Hello, i am trying to convert stereo into mono on my bluetooth amplifier/receiver, since i am using a woofer + tweeter setup. I included a picture of how i solder the pins together. Right now the pot is up to a maximum, which means the pins are effectively shorted together. I know ur supposed to use a resistor but i am looking for a max SPL, even if the amplifier dies after a few months. The amplifier chip itself is TPA3116D2. The amplifier board is ZK-502C hifi.

Problem 1: Even when the pins are shorted together, i get on average about 4-5dB less SPL than i would otherwise and i dont understand why since i am not using any resistors and pot doesnt offer any resistance.

Problem 2: When pins are shorted together, the speaker amplifier will shut down and restart if the pot is over 80% volume. This happens even if the speakers are not connected ! I am really wondering why.

Question: Is there any way to get around this since i need every single last dB i can get, so i cant lose even 1dB, let alone 3dB.

 

[audioguru]

The TPA3116 amplifier produces 65W with fairly low distortion into a 4 ohm load per channel when your battery is fully charged at 25.2V. But the max for the 8" woofer is only 50W and the tweeter max is 30W. If you reduce the battery to 5 cells and 21V fully charged then the output with fairly low distortion into a 4 ohm load per channel is 47W then the woofer will survive. The tweeter will not survive unless you attenuate its input to 30W with a 1 ohm/10W series resistor and redesign the crossover parts for a 5 ohms tweeter.

With a second-order crossover, the woofer and tweeter haver a phase shift that cancels sounds at the crossover frequency. Then the two speakers must be connected out-of-phase.

You do not have a 2 ohms load. The woofer is 4 ohms at low frequencies and the tweeter with a 1 ohm series resistor is 5 ohms at high frequencies. Two of the channels in parallel will sound exactly the same loudness as one channel.

Here is a schematic: using one amplifier or two paralleled amplifiers:

 

[rjenkinsgb]

@rjenkinsgb: Can you verify my last post, thats how i should connect the speakers ? Would you advice parallel or series connection of woofer and tweeter ? I need to know that to design the crossover.

Correct in principle - but just parallel the signals at the screw terminals, not the IC pins. That way it keeps the two sets of output filters in parallel, so no increase in maximum current through them.

I'd keep to a parallel speaker system, with the appropriate crossover. Turning the volume pot down if the output is too high is far easier than redesigning the crossover if you went with series and the output was too low..

 

[SentinelAeon]

rjenkinsgb Ok, reading from datasheet, it says it produces 100W into 2 ohm load. If i connect the speakers in parallel i will be at 2 ohms. I will connect the same way i connect now ... each speaker into 1 channel. Do i need to reverse the polarity if i do that ?

audioguru The tweeter is 30W you say, but why do i need a resistor if i am doing a crossover ? Since the tweeter will only play high frequencies i wont get nearly 30W into it, at least thats how i remember.

 

[audioguru]

When the battery voltage drops to 21V then your woofer plays most audio frequencies and its 4 ohms will produce 50W max before it is overdriven which adds distortion power.
50W/21V= 2.28A RMS which is 3.37A peak. Can your battery and its wiring produce that much current?

Almost all home HiFi speakers are 2-way or 3-way and are 8 ohms. All the woofers, midrange and tweeters in them are also 8 ohms. Your speaker uses a 4 ohms woofer and a 4 ohms tweeter then it is 4 ohms, not 2 ohms.

I agree that a tweeter does not play loud continuous tones for long durations like a woofer does but the tweeter still plays the full power of the amplifier for short durations and when playing clipping distortion that has frequencies above the audio hearing range.

I think your amplifier shuts down when the input volume control is turned up and the amplifiers are paralleled because the paralleling has something wrong causing the amplifier outputs to fight each other (not synced) which would also reduce the max output level just before they shut down.
I bet the amplifier does not shut down when the volume control is turned up, the amplifier outputs are not paralleled and the speakers are not connected.

In my post #21 I showed the tweeter and woofer connected with opposite phase which is needed when the crossover is second-order. Opposite phase connection is needed even if the crossover has one paralleled amplifier or two separate amplifiers.

I think that two separate amplifiers can produce louder sounds than a paralleled amplifier because with a separate amplifiers system, clipping distortion in the woofer's amplifier is not played in the tweeter's amplifier.

 

[rjenkinsgb]

I bet the amplifier does not shut down when the volume control is turned up, the amplifier outputs are not paralleled and the speakers are not connected.

If you had bothered to read the full thread, the amp module shuts down when configured for stereo, with nothing at the outputs paralleled.

All it need is the volume pot wipers to be paralleled and the volume set to max - even with no output loads at all.

It is the BLUETOOTH IC that is shutting down, not the power amp.

I believe the Bluetooth IC is actually a headphone receiver or similar, and the cross-channel short occurring when the volume is at max with bridged wipers is overloading it.

 

[audioguru]

I agree that the Bluetooth IC is shutting down when its stereo outputs are shorted together when it plays ping-pong stereo at high output level.

Since every dB is wanted then I think this little amplified speaker will have the amplifier clipping like mad then something will fail soon.

 

[SentinelAeon]

Ok this is seriously getting out of hand, there is so much information that simply doesn't relate to what i am doing while the important questions are still unanswered so i cannot begin working on it and i stated that i am in a time hurry. Can we please stick to what i actualy need to complete this project and keep the theory and whatifs for some other time.

rjenkinsgb The problem is that i do not know what running the amp in mono mode actualy does and i need this information because the person helping me with the crossover is waiting for that information. All i can say is that the datasheet says that in this mode, the amplifier will produce 100W at 2 ohms. And since i am using 2x 4 ohm speakers in parallel, this is indeed 2 ohms. Please check the images included and verify that this is indeed correct.

 

[audioguru]

Your speaker with crossover network is 4 ohms and will be powered with 50W. It is not 2 ohms and will not produce 100W.

The woofer and tweeter must connect to the output of the crossover network and not to the amplifier.
The input of the crossover network must connect to the output of the amplifier, as shown.

Your tweeter is wrongly shown connected directly to the output of the amplifier, then it will be destroyed by the low frequencies. The tweeter must be fed from the output of the highpass filter in the crossover network.
The 30W tweeter will soon fail when fed 50W or more from the amplifier unless it is attenuated with a 1 ohm series resistor. Then the highpass filter must be designed for 5 ohms but not 4 ohms.

Your woofer is shown wrongly connected directly to the output of the amplifier without being fed from the lowpass filter in the crossover network. Most woofers sound bad when playing high frequencies. The tweeter plus woofer high frequencies add and subtract sounding even worse.

Your woofer and tweeter are wrongly shown with the same phasing which produces cancellation if the crossover network is designed including a lowpass filter for the woofer. With a filter for the woofer, before I showed correct opposing phasing.

 

[SentinelAeon]

Ok now you are just ignoring the images i posted. Did u see the left picture where i shown how i changed the amplifier into mono mode ? And i stated that DATASHEET says that in this mode the amplifier is 100W 2 ohm ? The LEFT POSITIVE and RIGHT POSITIVE are connected together inside amplifier now, which means no matter on which terminal i connect, the speakers will be parallel. 2x 4 ohms in parallel is 2 ohms. And i know i need to connect the crossover, i stated that i have the guy working on it but first i need an answer.


Please let rjenkinsgb answer this one since he is the one that helped me run the amplifier in mono mode.

 

[audioguru]

You correctly show the amplifier wired for mono mode with its two amplifiers in parallel. But you do not have a stereo to mono converter to feed its input.

The crossover network separates your two speakers then they are not in parallel and do not produce a 2 ohms load.
Two 4 ohm woofers in parallel are a 2 ohms load.

I had a car that was made with a 2 ohms sub-woofer. It produced so much good bass that I never turned it up to clipping then it had plenty of "headroom" and no distortion.

 

[SentinelAeon]

Sure i have the stereo to mono converter to feed its input. I bridged the receiver outputs and used a 1K resistor on each line to protect it.

I am not sure how the crossover will disconect the two speakers. All i know is that i want the 100W. That is the whole purpose of doing this. If i only get 50W then i might aswell not bother doing it. I want 100W and then i will limit the power by a pot.

 

[rjenkinsgb]

Only ever draw the mono-connected amp as a two terminal block, to avoid confusion with the stereo version.
Both "right" are terminal 1, both "left" are terminal 2.
NOT right + to left + etc. as you seem to be drawing the connection diagrams.

Polarity is irrelevant. The relative phasing between the two speakers matters, to suit the crossover.

To get more power to the same speakers, you need higher voltage.
To get more power with the same voltage, you need lower speaker impedance.

Using the amp configured for mono may? give less distortion at higher powers as the current is spread over both sets of outputs.

 

[audioguru]

Sentinel, Your speaker system is 4 ohms with 50W of output. If you connect the amplifiers in parallel and use two of the woofer, tweeter and crossover speaker systems in parallel then the speaker system will be 2 ohms and the max output will produce 100W which sounds only a little louder (+3dB) than 50W.

The crossover network passes low frequencies to the woofer and blocks high frequencies to prevent it from shrieking and producing too much sound level of high frequencies.
The crossover network passes high frequencies to the tweeter and prevents low frequencies from exploding it.
Again you wrongly connected the woofer and tweeter without a crossover network You also connected then in-phase when the second-order crossover network needs them to be out-of-phase to avoid cancellation of some sounds.

 

[audioguru]

JRW, the TPA3116 stereo amplifier uses its own special way of paralleling (Mono Mode, PBTL) its two amplifiers:
1) Connect INPL and INNL directly to Ground to set Mono Mode on power-up).
2) Connect OUTPR and OUTNR together for the positive speaker terminal and OUTNL and OUTPL together for the negative speaker terminal.
3) Mono audio input is fed to the right channel input.

 

[rjenkinsgb]

1) Connect INPL and INNL directly to Ground to set Mono Mode on power-up).
2) Connect OUTPR and OUTNR together for the positive speaker terminal and OUTNL and OUTPL together for the negative speaker terminal.
3) Mono audio input is fed to the right channel input.

Again, if you had read the thread, that is exactly what I have said repeatedly.

It still needs the output filters - and the filters must be paralleled to stand the higher current. So paralleling the output terminals (correctly) seems the optimal approach.

 

[SentinelAeon]

+3dB is almost 25% more SPL. That is not little, even if it was 3% i would be motivated to use it. I still don't understand why i am losing about 4-5dB when i bridge the mono with resistors. I understand that those 2 resistors reduce the signal a bit. But that was tested with 10 ohm resistors !!! While people said if i use 10K resistor i will halve the signal which is 3dB. So how am i losing 4-5dB with only 10 ohm resistor, which is a fraction of a 10K resistor. And that is the motivation why i was tempted by those 100W. I need dB, every single decimal point of dB. Period. I think i might just have to use that boost option for additional 4dB. I thought using mono 100W would give me more power. But it just seems that if i use mono, i will always be about 3-4dB behind if i used a stereo. And that kind of sucks :/

 

[audioguru]

Decibel measurements are logarithmic, not linear.
The sensitivity of our hearing is logarithmic. +3dB is 1.414 times the signal voltage but double the power because when you increase the voltage then it increases the current so the resulting power increase is double (1.414 x 1.414= 2).

90dB is loud and 93dB is double the power of 90dB but is only a little louder. 100dB is 10dB louder than 90dB but is 10 times the power and sounds only twice as loud.

You added resistors as a stereo to mono mixer but the Bluetooth output is probably overloaded with 10 ohm resistors and 10k resistors reduced the level too much. Try 1k resistors.

 

[rjenkinsgb]

So how am i losing 4-5dB with only 10 ohm resistor, which is a fraction of a 10K resistor.

As I said earlier - any sound on one channel only, is attenuated to half level by the mixing, no matter how it is done.

Only sounds centred in the stereo image are unaffected or minimally affected & everything else is attenuated to some extent, depending on the proportion on each channel & phasing etc.

Too high value resistors just make the effect worse.

If the BT output is intended for headphone, 100 Ohms on each channel should avoid shutdown and overload, with minimal effect on volume.

That's why I suggested making it stereo. A bit more expensive but better sound and more volume.

 

[SentinelAeon]

That's why I suggested making it stereo. A bit more expensive but better sound and more volume.

The problem with stereo is that with 2 same speakers i will never get that bass in such a small enclosure. In a big party speaker that might work, but certanly wont work in a small 3L portable speaker that i also use. There i have 1 TCP115-4, if i add another one, i get +3dB, but lose probably 30Hz on -3dB, plus double the power usage.

 

[audioguru]

The datasheet for the little 4" TCP115-4 mid-bass woofer shows that its max allowed power is only 40W.
Its free air resonance is 54Hz and in a 3L sealed enclosure it will probably resonate with a boomy peak at 80Hz with a sharp dropoff at frequencies less than 70Hz. It is too small to produce 30Hz unless a bass-boost circuit is used.

Bass-boost actually plays low frequencies at normal levels then reduces the level of all higher frequencies.