Increasing 386 output

[Dr.EM]

I have some 386s now to experiment with, and I thought of this. In the diagram, you can see that one 386 acts as a non-inverting amplifier, while the other acts as an inverting one. My intention with this is that because the signals are fluctuating in opposite directions, the voltage between the two amps is effectively doubled, rather than the voltage just going from one amp to ground.

Would anyone like to start the ball rolling as to why this would'nt work :lol:

 

[Nigel Goodwin]

I have some 386s now to experiment with, and I thought of this. In the diagram, you can see that one 386 acts as a non-inverting amplifier, while the other acts as an inverting one. My intention with this is that because the signals are fluctuating in opposite directions, the voltage between the two amps is effectively doubled, rather than the voltage just going from one amp to ground.

Would anyone like to start the ball rolling as to why this would'nt work :lol:

It's a common configuration, called a 'bridged amplifier', however your design seems lacking in some of the usual parts - and you've got spurious speaker coupling capacitors, these aren't needed, the speaker should be DC coupled.

Try checking the datasheet for the LM386, most poweramp IC datasheets give examples of bridging.

 

[audioguru]

Hi Dr. EM,
The little LM386 isn't designed to provide the massive output current required for bridging. When the voltage is doubled across a load, then the current must double too. If they don't melt, you might get a little more power bridged into a 16 ohm speaker, but definately not an 8 ohm speaker.

Look at the TDA2002 car radio amplifier. It is designed to drive a 2 ohm load, so bridged into 4 ohms its current is the same but the voltage is doubled so its output power is nearly 4 times as much as a single one driving 4 ohms. It has a proper high idle current to avoid crossover distortion and a proper heatsink for the high current job. :lol:

 

[Roff]

Hi Dr. EM,
The little LM386 isn't designed to provide the massive output current required for bridging. When the voltage is doubled across a load, then the current must double too. If they don't melt, you might get a little more power bridged into a 16 ohm speaker, but definately not an 8 ohm speaker.

Look at the TDA2002 car radio amplifier. It is designed to drive a 2 ohm load, so bridged into 4 ohms its current is the same but the voltage is doubled so its output power is nearly 4 times as much as a single one driving 4 ohms. It has a proper high idle current to avoid crossover distortion and a proper heatsink for the high current job. :lol:

Yeah, but... but... but... with a low voltage supply, a bridging amp allows you to get more power to the speaker without necessarily overloading the LM386. No???

 

[audioguru]

Hi Ron But But,
An audio amp is supposed to have a strong power supply that doesn't sag under load and a very low output impedance so that its output voltage remains about the same with an 8 ohm or 4 ohm load.

Look at my attachment and select a low supply voltage like 6V.
The output voltage remains about the same with a 16 ohm or 8 ohm load. But at 4 ohms, the output voltage is much less.

In a bridged amp with an 8 ohm load, each amp must supply the same current as a single amp would provide into 4 ohms.

Let's calculate output power:
1) Single amp into 8 ohms: 4Vp-p = 1.414V RMS = 0.25W.
2) Single amp into 4 ohms: 2.8Vp-p = 0.99V RMS = 0.245W, it is less but it is supposed to be 0.5W.
3) Bridged amps into 8 ohms: 2.8V X 2 = 5.6Vp-p = 1.98V RMS = 0.49W, it is supposed to be 1W. The bridged amps are working very hard and the power loss goes into heat. :lol:

 

[Dr.EM]

Ok, thanks for all your help there. I had a 386 running at 13.5v supply into a 4ohm load the other day, and it didn't even get slightly hot. I might try this bridging thing, minus those caps, just so long as I don't leave it on too long :lol:

 

[audioguru]

Hi Dr. EM,
An LM386 with a 13.5V supply and a 4 ohm load will produce an output at clipping of a whopping 300mW, and a dissipation of 1.4W. Its internal temperature will exceed 150 degrees C, its limit if it is continuously operating at clipping for longer than a few moments.

With an 8 ohm load its output is double at 600mW, its dissipation is 1.1W and its internal temp is 110 degrees.

With a 16 ohm load, its output is nearly double again at 1W, its dissipation is 700mW and its temp is reasonable.

If you bridge two of them with a 13.5V supply and a 4 ohm load, their output will still be only a whopping 300mW at clipping, and you'll melt the poor little guys. :lol:

 

[Nigel Goodwin]

Ok, thanks for all your help there. I had a 386 running at 13.5v supply into a 4ohm load the other day, and it didn't even get slightly hot. I might try this bridging thing, minus those caps, just so long as I don't leave it on too long :lol:

With a bridged amplifier each amp effectively feeds half the speaker - so your speaker needs to be twice the impedance of that for a single amplifier.

There's nothing 'magic' about a bridged amplifer, it doesn't provide any more power than two single amplifiers - it just provides it into a different impedance!.

Basic rule of thumb - twice the power into twice the impedance!.

For an example, imagine a two channel amplifier that outputs 4W per channel into 4 ohms (stereo) - so that's a total of 8W into two 4 ohm speakers.

If you now bridge the amplifier, and connect the two speakers in series, then you get 8W (mono) into the resultant 8 ohm load. So EXACTLY the same power - 8W total into two 4 ohm speakers.

Where the advantage comes is if you only have ONE 8 ohm speaker, in bridged mode you can get the 8W, feeding it off just one channel would only give 2W.

 

[Dr.EM]

Audioguru, yeah, it did get pretty distorted pretty quick, but it seriously wasn't even warm after a few minutes. Thats the reason I wanted to try this bridging, to get more output. I kind of want to make a really quirky pair of powered speakers, and I have an idea for powering them that might shock...

So, with the bridging, your saying it might be easiest to just use 2 amps and 2 speakers essentially? Obviously, I could use a higher powered chip, but i'm sticking with these for now :lol:

 

[Nigel Goodwin]

Audioguru, yeah, it did get pretty distorted pretty quick, but it seriously wasn't even warm after a few minutes. Thats the reason I wanted to try this bridging, to get more output. I kind of want to make a really quirky pair of powered speakers, and I have an idea for powering them that might shock...

So, with the bridging, your saying it might be easiest to just use 2 amps and 2 speakers essentially? Obviously, I could use a higher powered chip, but i'm sticking with these for now :lol:

What impedance speaker are you trying to use?, according to Audioguru's figures a 16 ohm one would provide best power, so if you bridge two amplifiers you would need a 32 ohm speaker to give twice that power.

Have you considered using a better device than the 386?, it's really pretty puny!.

 

[audioguru]

Hi Dr. EM,
The little LM386 is a low power amp that doesn't like a low impedance load and its resulting high output current. Bridging them doubles the output current and makes it worse.

Here is a real amplifier IC that will provide about 50W RMS at low distortion operating bridged into a 4 ohm load with your 13.5V supply. Your supply will need to provide it with about 6A.
It operates in class-H, with a built-in voltage doubler. It switches supplies from normal 13.5V for low power output for increased efficiency, to the doubled supply voltage for high power output.
They claim 70W, but at 14.4V and with an awful-sounding 10% distortion overdrive. https://www.electro-tech-online.com/custompdfs/2005/06/TDA1562Q_ST_SD_2.pdf

 

[audioguru]

I'm using a little 8-pin MC34119 bridged amps IC in my 9V battery powered low distortion phase-shift oscillator to provide a high output voltage. The IC is designed to drive a 32 ohm speaker in a speakerphone with fairly high distortion. In my oscillator application, with a load of 600 ohms or more, its distortion is hardly measureable. :lol:

The MC34119 works pretty well in my son's modified (very loud) alarm clock too.

 

[Dr.EM]

Cool, can't find that one over here though. I think I will try and use the LM384N chip, which supplies 5watts apparently. I want to use it in a portable speaker system, but will need to be able to get the needed 22volts or so from some batteries. A DC/DC convertor chip would be neat, but it would cost a lot for that power.

 

[Nigel Goodwin]

How about telling us what supply voltage you have available?, what speaker impedance you want to use?, and we could make suggestions for suitable circuits.

 

[audioguru]

Hi Dr. EM,
The LM384 is in a standard DIL package so can't dissipate heat very well. It can produce 5W at an awful-sounding 10% distortion into an 8 ohm speaker with a 22V supply and with an unusual DIL heatsink for 30 seconds until it melts. It produces about 4W at clipping where distortion is 0.25%, but will be operating at its absolute maximum temp.

Besides its poor power dissipating ability, it has a high loss, because it clips at 16Vp-p with a 22V supply.

There are many amp ICs in packages that are easily heatsinked and are made for car radios. About 14W per channel into a 4 ohm speaker at low distortion is common.
I know! TI has a few class-D amp ICs and one produces 200W with a small heatsink. **broken link removed**
Another produces 20 Whats with a reasonable supply voltage and without a heatsink. You solder the bottom of its tiny surface-mount package directly to the pcb somehow, after connecting its many tiny pins. :lol:

 

[Thunderchild]

hey pal what u wasting ur time at just get a TDA2002 and with less components and complication u can have 5 watts or even 6 and an output current of up to 3.5 amps so quit the crazy figuring and use it on something like bridging two TDA2002 or ever the TDA2050 that i thinks is 18 watts(per chip). another method is to use power transistors for amp boost but i don't know how to figure the component values but i supose it is basically a voltage folower configuration (also known as common emmiter)
p.s. i thout u meant 80386 processors at first. try to be plainer especialy in the thread tittle tkx

 

[joecool85]

Wow...no offense you guys, but you don't know much about bridging the lm386's eh?

It's been done, many a time, and done very well.

http://runoffgroove.com/littlegem.html

It works real well as a practice amp for guitar.

 

[audioguru]

Yeah man,
We don't know much about bridging two puny little LM386 amps.
I figure that a 4 ohm speaker like in the article will represent the same current as a 2 ohm speaker with a single IC. The amps will both be seriously overloaded and produce a voltage swing of about only 1.5Vp-p from one, or 3Vp-p when bridged. 3Vp-p is only 0.28W into the 4 ohm speaker. A single LM386 will produce 0.245W in a 4 ohm speaker. You won't hear the small difference of only 1dB.

 

[joecool85]

Regardless of hearing a difference or not, its totally possible and they won't overheat driving a 4ohm load when bridged. That was my point.

**EDIT**
And besides that, a lm386 chip will out out around 500mw running at 9volts and pushing an 8ohm load.

https://www.electro-tech-online.com/custompdfs/2006/01/LM386.pdf

 

[Nigel Goodwin]

Regardless of hearing a difference or not, its totally possible and they won't overheat driving a 4ohm load when bridged. That was my point.

The bridged circuit WILL overheat feeding a 4 ohm load, unless it's only run at low volume - in a bridge circuit each amp effectively sees half the load - so with a 4 ohm speaker each amp is feeding 2 ohms, which is too low for a 386.

Also, as there's no feedback in the design, the two halves of the amps aren't balanced - so distortion will be far higher than a single amp, however that's probably not a concern for a guitar?.

With a minimum 8 ohm load, and if you're not bothered about high distortion, then it is possible - but it's a pretty poor idea!.