[SOLVED] Connecting stereo to mono

[Darius051]

Hello
First I'm not in electronics so much and please have this in mind if you going to give me some advice.:wink:
I have this little amp:



VMA2012 - 2x3W Class D Audio Amplifier

and I need to connect only ONE speaker to it because my project requires it. I have read some proposals here before I decided to ask for help but I'm not sure how to do it right. The speaker that I'm going to connect is this one:

**broken link removed**

and I understand that it is very risky to connect it to the outputs in such a way that both channels are involved. So the right way is to make some adjustments from the input side. So I did just connect the R and L channels together after the 10K resistors but then I had some distorted sound in the upper mid range freq. and now I would be happy if someone could explain what and where to add "something" that it sounds as expected.

Thanks :wink:

 

[Audioguru]

The tiny speaker produces no bass sounds below 150Hz.
I could not find a manufacturer's name nor datasheet for the little stereo amplifier IC.

The two channels of the amplifier are already bridged so their outputs cannot be combined for higher output power in mono.
With a 5.5V supply, it produces 1.5W into 4 ohms per channel with low distortion at low frequencies. Reviews say it distorts higher frequencies.
With a 5.5V supply, it might produce 3W per channel into 4 ohms per channel with high distortion.

But since your tiny speaker is 8 ohms then the little amplifier will feed it a maximum of only 0.8W with low distortion and maybe 1.6W with high distortion.

 

[Darius051]

Thank you Guru for this quick reply. I have been scratching my head for days about this problem. I have to mention that at Dealextreme it says 2*8ohm so I thought I have the right speaker. However I have a 4 ohm speaker of the same kind and it is not a problem to connect. I have tried the 4 ohm like this at my own risk and it worked:



but there was some hiss like noise in the speaker while not playing so maybe it is not the safest way to use this amp. I have connected two 8 ohm speakers as designed and they worked fine.
So my question is what shall I do to make the input summed to a mono signal before it enters the amp? Thnx

 

[Audioguru]

DealExtreme do not know anything about this amplifier. They simply buy them from China and sell them. They lied about 3Wx2 high quality sound with 2*8 Ohm speakers and do not say the resulting amount of distortion (it is 3W into 4 ohms with very high distortion). Instead their detailed specifications only say the size and weight.

Read its reviews, it has noise and distortion. it is a lousy low power amplifier.

With a 5.5V supply then a perfect (impossible) bridged amplifier (this one has each channel bridged) has a maximum output of 11.0V peak-to-peak which is 3.89V RMS. Then the maximum output is only (3.89V squared)/8 ohms= 1.89W RMS. But the amplifier is not perfect, it has some loss so its maximum output might be only 1.5W with an unknown amount of distortion.

You have the amplifier inputs in stereo and have an 8 ohm speaker connected differentially to one wire of each output. So when the input is the same on both channels (mono) the maximum output into the speaker is 1.5W with an unknown amount of distortion. When there is a signal on one channel then the output will be a maximum of only 0.37W which is bad because it should be 0.75W if the amplifier inputs are properly connected in mono.

3W sounds only a little louder than 1.5W because our hearing sensitivity to loudness is logarithmic. You might not notice the difference. 10 times the power sounds twice as loud.

 

[KlausST]

Hi,

this is not possible with class D amplifiers.
Reason: Often they use phase shifted PWM signals and independent spread spectrum, this makes it impossible to use the "digital" signals.

**********

One possibility i see (not tested): if you use low pass filters on the outputs and connect both filtered signals to your speaker.

**********

Is speaker1 and speaker2 connected to real speakers?

Klaus

 

[Audioguru]

One possibility i see (not tested): if you use low pass filters on the outputs and connect both filtered signals to your speaker.

Sure, chop off all the high audio frequencies then the noise, distortion and most musical sounds will disappear!

 

[KlausST]

Hi,

@Audioguru:
nomen est omen....
Therfore i think you know, that usually class D amplifier use the speaker´s internal inductivity to get "audible" signals by low pass filtering the digital PWM signals.

Many datasheets/application notes of class D amplifiers show to use of additional low pass filters. With that the speakers see analog signals instead of digital signals.
Often those filters are used to lower EMI. (Speaker cables as tramsitting antenna for HF signal...)

And i completely agree with you about the quality of this amplifier....

Klaus

 

[Audioguru]

The duty-cycle of a class-D amplifier produces the low frequency audio. The high frequencies of the PWM waste power in a speaker and might produce IM distortion.
Texas Instruments and others make some filterless class-D audio amplifier ICs.

This amplifier might use a Chinese copy of a Tripath class-T amplifier IC. Tripath went bankrupt a few years ago for lying about their output power.

 

[KlausST]

Hi,

yes i know the filterless TI types.
here a picture of my test pcb with TAS5611. I always used the filters (you see the L) called "BD Modulation filters".

Without the filters the whole digital HF is transmitted with the cables. And without the filters one can not see any anlog signal waveform.

Klaus

 

[Darius051]

Ok guys, as I thought this went out of my knowledge level. Let's forget this amp. and the whole idea of stereo to mono story.:-|

I put it in a simple question. Is there a mono amplifier on the market with stereo input that runs in the 3-6V voltage range and that has something like 2-4W power?

Or better, where can I buy just the same amp. as I have now but with only one channel? It doesn't matter if it sounds lousy and distorted because it is of no importance.
Thank you very much if you could help me with this like you helped me to realize that this vma2012 could not be converted from stereo to mono in a easy way.:wink:

 

[Audioguru]

A simple stereo to mono converter is two 4.7k ohms resistors connected together. The left channel is at one resistor, the right channel is at the second resistor and their junction gives mono.

A 3V power supply is used for an earphones amplifier, not for a speaker amplifier.

If you have a 6V supply with four AA alkaline batteries then it produces only 4V when the batteries are almost finished. An ordinary amplifier powered from only 4V has a peak-to-peak output of maybe 2.5V which is 0.88V RMS. Then the amplifier produces a maximum undistorted output of only 0.1W into 8 ohms. A cheap clock radio produces about 0.5W.
If you have a 6V battery supply that drops to 4V and it powers a bridged amplifier then its peak-to-peak output is about 6V which is 2.1V RMS. Then the amplifier produces a maximum undistorted output of 0.56W.
The output of an amplifier is about 1.8 times more into 4 ohms if the higher current does not destroy it.

You want 4W into 8 ohms from a bridged amplifier? Then it needs a supply that is about 12VDC.
Years ago there were 76 bridged amplifier ICs made for cars. They produced about 14W into 4 ohms per channel when the car battery was charging at 13.8V. Today most are gone.
National Semi was recently purchased by Texas Instruments and I think their LM4950 bridged amplifier is still available. With a 12V supply it produces about 4W into 8 ohms.
Texas Instruments make many little class-D amplifiers that might be what you need.

 

[KlausST]

Hi,

I did the question before " are there real speakers on both channels".

If the answer is "no", then you simply can add both stereo signals at the input side.

With car audio and linear BTL amplifiers you can use one channel for right speaker, one channel for left speaker
?.. and additionally one speaker connected to left+ and the other to right- ( or vice versa)

Klaus

Sorry for getting off topic before.

 

[Darius051]

Thanks Guru for the resistor thing. As I can see from the picture above every input has one 10k resistor before entering the amp. I have bridged these channels after the resistors to see what happens and it worked but there were some distortions in the mid frequencies. So if I follow your instructions I don't know how to get now 4.7k when these 10k are welded on the board and they are hard to get off.;-)
Klaus, sorry for not answering. If you asked me if I have connected real two speakers on these channels the answer is yes and they worked fine. But I assumed that they were 8 ohm and now Guru explained that they are in fact 4 ohms so I have to try with new speakers. And as for this power thing. It is not needed to be 2-3W the 0,5W are just fine. I was referring to these false data that are misleading the viewer when coming to sites like dealextreme etc. So I hope you understand, I just used this description to describe what I need.
Thanks again...;-)

 

[Audioguru]

Since NOBODY knows any details about the Chinese amplifier then the input resistors might be used for voltage gain. Then the existing resistors CANNOT be connected together to make mono. Adding two series 10k resistors to convert stereo to mono is used at the input of an ordinary mono amplifier that does not already have a series input resistor.

I did not explain that your speakers are 4 ohms. Your sales sheet says they are 8 ohms. You said you tried connecting a 4 ohm speaker oddly to both channels together and it worked. But the amplifier is designed to drive one 4 ohm or 8 ohm speaker per channel and its channels are already bridged so they cannot be connected together.

Many speaker and amplifier ratings say false data. RMS Power is continuous true power. Peak Power is a false number that is simply 1.414 times higher. Music Power or Maximum Power is peak power but only for a brief moment before the supply voltage sags or before the amplifier gets too hot so the false number is higher again.

An amplifier rating is supposed to say how much true continuous RMS power it produces into a certain impedance with the amount of distortion it has. An amount of distortion that is 0.1% or less is pretty good. Many amplifiers state their power output when the distortion is horrible at 10% then their power output number is falsely much higher but some people play acid rock with the amplifier distorting like that.

 

[KlausST]

Hi,

Then the existing resistors CANNOT be connected together to make mono.

True.

To be on the safe side you need four resistors 20k each.
* from IN_left to AMP_left
* from IN_right to AMP_left
* from IN_left to AMP_right
* from IN_right to AMP_right

Then both amplifiers work with the same input impedance (10k) and both amplify the same (mono) signal.

**********
@audioguru:
You are so right about the power ratings and distortions....It really is horrible.
Some are very creative with their names, like "super short peak pulse power" and giving a value of 300W for a car amplifier. Inside often is a TDAxxx BTL chip with max. 14.4V working voltage...

Klaus

 

[Audioguru]

@audioguru:
You are so right about the power ratings and distortions....It really is horrible.
Some are very creative with their names, like "super short peak pulse power" and giving a value of 300W for a car amplifier. Inside often is a TDAxxx BTL chip with max. 14.4V working voltage...

A couple of years ago I bought a Sylvania 2.1 powered speaker system for $24.99 as a "special buy" at a local store. It sounds great, looks good and is fairly loud. I use it for the TV in my computer room and the digital cable TV converter also plays many FM radio stations.

In the ad it says 150W. On the box and owner's manual it says 75W RMS. On the power label it says 75W (so it is 100% efficient!). Inside it has a small power transformer labelled 9VAC/1.1A (9.9W). The amplifier is probably 50% to 60% efficient but might be a modern class-D switcher that is 90% efficient.

Its amplifiers are two 16 pins DIL ICs with a small heatsink glued on.
One IC is probably a stereo amplifier feeding 1.5W to 2.0W real Watts to each 4 ohm satellite speaker.
The other IC is probably bridged and feeds 4.0W to 5.0W real Watts to the 6 ohm sub-woofer.
So the total real output power is 5.5W to 7W.

Each satellite speaker has two 1.5" 2 ohm speakers with foam surrounds and the "soft-dome tweeter" is not a speaker but is actually a vent covered with a soft dome.
The sub-woofer is a 5" long-throw speaker with a foam surround in a pretty big wooden ported enclosure. It lights up with 6 blue LEDs.

The amplifier and power supply are built into the sub-woofer enclosure and has a volume control and bass and treble tone controls.

I have played it loudly for hours and it doesn't smoke. I have never heard its amplifiers clipping. I tested its frequency response with my 68 years old ears and it plays from 40Hz to 18kHz smoothly and very well.


The woofer became intermittent. I looked inside and saw many glued-together connectors for the bass and volume controls and I saw that the "5 inch woofer" is not a speaker, it is a vent cone and the real woofer is deep inside and is tiny.

 

[Darius051]

Hi,



True.

To be on the safe side you need four resistors 20k each.
* from IN_left to AMP_left
* from IN_right to AMP_left
* from IN_left to AMP_right
* from IN_right to AMP_right

Then both amplifiers work with the same input impedance (10k) and both amplify the same (mono) signal.

**********
@audioguru:
You are so right about the power ratings and distortions....It really is horrible.
Some are very creative with their names, like "super short peak pulse power" and giving a value of 300W for a car amplifier. Inside often is a TDAxxx BTL chip with max. 14.4V working voltage...

Klaus

Thnx Klaus :wink:

Is this what you had in mind?

 

[Audioguru]

Why bother connecting additional resistors to the input of the unused channel? Simply connect its existing 10k input resistor to ground. Then do not connect its output to anything.
The wanted channel input gets a series 20k resistor from the left signal source and another series 20k resistor from the right signal source. Its existing 10k resistor is not connected to anything.

We do not know if the inputs need a DC path to ground. Maybe they already have a DC path to ground. We also do not know if the signal source has a grounded voltage or if it floats with a coupling capacitor.

 

[KlausST]

Hi,

Exactely.
Just to be sure. Both 10k resistors are not needed.

Klaus

 

[Darius051]

Hi
I'm proud to announce that the advice Klaus gave me is running smoothly without any distortions and with a big grin on my face..
Here is how I connected the resistors. Bare in mind that the original 10k are very small and hard to mess around without making some damage on the board in my skill level so I did the whole work on the input connector side..:wink:

So Klaus, thanks again and the speaker that I have connected is this 4 ohm one: **broken link removed**..:wink: