Help with simple op amp circuit

[Alistair Ballantyne]

Hello,

I made a simple LM 386 mono amplifier circuit with an MP3 player and 8 ohm speaker. It worked well.

To create a stereo amplifier I simply made two identical mono circuits and fed right and left jack plug inputs from the MP3 into each circuit, with a common ground.

I was surprised at how good it sounded!

I went online to se if I could improve on this clumsy stereo arrangement and discovered the following circuit.

I cannot get it to work. Before stripping the breadboard down for a third time I thought I would ask for experienced observation on the circuit. My question is should it work?

Also, my limited reading on the LM386 shows pin 3 as positive input and line 2 as negative input. The circuit below throws that on its head for reasons I don’t understand.

Also – is 47k resistance on the input lines not too large?

Is there a better (but beginner friendly) circuit arrangement with either the LM386 or some other op amp?

Appreciate your help.

 

[danadakk]

What do you observe in either channel on a scope on
output when you drive that channel ?

The pins on datasheet correct, 2 is inverting, 3 non inverting
inputs, 8 pin part.

The circuit approach I gather saves one LM386 when implementing
a bridge config for 2 channels, but seems power dissipated by middle
LM386 is doubled....

Regards, Dana.

 

[Alistair Ballantyne]

Thanks Dana - sorry I don't yet have a scope.

Is this what's called a closed loop circuit to more accurately control the gain of the amplifier?

In your view should this circuit work? I don't want to spend any more time trying to rewire if it is suspect in your opinion.

Many thanks
Alistair

 

[KlausST]

Hi,

Spend a fourth IC and wire it according Dana's circuit. You need it twice, because it shows only one channel in bridged mode.

Or better use a more modern circuit, with more power, higher efficiency, less noise, less distortion, less heat, better sound....

Klaus

 

[Alistair Ballantyne]

Thanks Klaus,

Yes I am about to wire up Dana's bridged circuit.
It will be interesting to compare with I have done so far.

On your second point - can you please give me any pointers to find a "more modern circuit"?
Many thanks,
Alistair

 

[danadakk]

Thanks Dana - sorry I don't yet have a scope.

Is this what's called a closed loop circuit to more accurately control the gain of the amplifier?

In your view should this circuit work? I don't want to spend any more time trying to rewire if it is suspect in your opinion.

Many thanks
Alistair

If you do not have a scope you can get Zeitnitz freebe which turns your
PC into a scope, spectrum analyzer, tone generator. But you have to protect your sound card
inputs to make sure you dont blow up your PC audio channel. Its good for audio freqs.

Soundcard Scope

Software projects of Christian Zeitnitz

www.zeitnitz.eu

Daqarta - Sound Card Input Range and Limiter Circuits

Project 154

ESP Audio Projects - PC Oscilloscope interface.

sound-au.com

Turn Your Computer’s Sound Card into a Scope

Use this free software tool to gain oscilloscope skills, analyze audio signals, and measure digital signals.

www.nutsvolts.com

--- Updated Mar 4, 2021 ---

Stereo Class D amps -

Class D Audio Amplifiers: What, Why, and How | Analog Devices

Class D amplifiers, first proposed in 1958, have become increasingly popular in recent years. Here’s some basic information.

www.analog.com

https://www.ti.com/audio-ic/amplifiers/speaker-amplifiers/products.html

https://www.ti.com/audio-ic/amplifiers/speaker-amplifiers/overview.html

Regards, Dana.

--- Updated Mar 4, 2021 ---

One comment on the simple bridge, its DC coupled, so any offset between the
two LM386's results in power and heat. Youc an eliminate by adding an offset
correction circuit, or AC coupling the speaker. AC coupling would have to use
non polar caps, or two polars back to back, eg. +--+ or -++- connection of two
caps in series.

Regards, Dana.

 

[Alistair Ballantyne]

Thanks Dana - a lot of useful information.

I did make the bridged circuit which you kindly forwarded. It worked for a while from an MP3 player but for some reason just stopped. As two LM386’s are used I thought it would provide a little more power to the speaker than the original circuit I built (below) but it didn’t seem to.

I will happily check out the other more up to date options.

Many thanks,

Alistair

 

[danadakk]

Did a LM386 overheat ?

In your schematic above you are directly loading output with 1000 uF,
curious why ?

Datasheet typical application shows -

If you are seeking higher power there are many solutions out there -

https://www.ti.com/audio-ic/amplifiers/speaker-amplifiers/overview.html

Tales From the Chip: LM386 Audio Amplifier

Tales From the Chip: LM386 Audio Amplifier: Audio is one of the most entertaining, time-consuming and (eventually) wallet-draining ways of learning about and falling in love with electronics. Reproducing, recording and amplifying audio gets you up close and personal with the electrons rocketi…

www.instructables.com

A scope is key to make sure your layout is not osillating, that could have burned out one of the LM386's,
and its oscillation above audio freq so you never heard anything.

Noisy LM386 based audio amplifier

remember to keep the remember to keep the capacitor’s legs as short as possible, that also helps.

www.robotshop.com

LM386 Instability Issue

Hi all. A while ago I started looking into designing a heterodyne bat detector circuit. I posted a thread on here to ask for advice. So for reference you can find this thread here: http://www.electro-tech-online.com/threads/ultrasound-transducer-for-bat-detector-project.126293/page-2 Since...

www.electro-tech-online.com

Also not all bypass caps, for the same C, are as effective in their ESR performance -

Regards, Dana.

 

[betwixt]

In the original post you have to consider what signal passes through the middle amp.

In a bridge configuration, the same signal is fed to both amps but one wired to invert it, the result is twice the voltage between the outputs because unlike a single ended configuration, the polarity across the loudspeaker can be reversed instead of just reaching near ground and near supply levels.
Power = voltage squared divided by resistance "W =(V^2)/R" so doubling V can theoretically give four times the power. In practice, it doesn't because of losses in both output stages.

In post #1, the middle amplifier carries the difference between left and right channels. So the loudspeaker is fed from one channel and the inverse of the other, in theory it could produce zero output if fed from a mono source and something weird when fed from a normal stereo source. You need the same signal but inverted to drive the second output.

Brian.

 

[Audioguru]

Dana's bridged LM386 amplifiers in post #2 almost overheats the low power LM386 amplifiers.
1) A graph in the datasheet shows with a 6V supply and an 8 ohms speaker, the output with low distortion is 0.2W which is 3.6V p-p.
Another graph shows the max p-p output is 4V. 3.6V/4V= 0.9 times.

With a 9V supply, the max p-p output is 6V so with low distortion it will be 6V x 0.9= 5.4V p-p which is 0.46W into 8 ohms.
Another graph shows that the dissipation (heating) is 0.55W when its output is 0.46W.

Bridging doubles the p-p voltage, current and losses. The 8 ohm speaker appears to be 4 ohms.
A graph shows that with a 12V supply and a 4 ohm load, the max p-p output is 3.5V so with low distortion will be 3.5V x 0.9= 3.15V p-p which is 0.31W into 4 ohms. The heating is 1.2W. Then the bridged low distortion output onto 8 ohms is 1.24W and the total heating is 2.4W(close to its max allowed heating).
With a 9V supply the bridged low distortion output into 8 ohms is also 1.24W and the heating is only 1.6W.

 

[KlausST]

Hi,

can you please give me any pointers to find a "more modern circuit"?

Every IC manufacturer provides selection tools, applicatiin notes ... for free. So do the distributors, too.

Klaus

 

[Alistair Ballantyne]

Did a LM386 overheat ?

In your schematic above you are directly loading output with 1000 uF,
curious why ?

Datasheet typical application shows -

View attachment 167871

If you are seeking higher power there are many solutions out there -

https://www.ti.com/audio-ic/amplifiers/speaker-amplifiers/overview.html

Tales From the Chip: LM386 Audio Amplifier

Tales From the Chip: LM386 Audio Amplifier: Audio is one of the most entertaining, time-consuming and (eventually) wallet-draining ways of learning about and falling in love with electronics. Reproducing, recording and amplifying audio gets you up close and personal with the electrons rocketi…

www.instructables.com

A scope is key to make sure your layout is not osillating, that could have burned out one of the LM386's,
and its oscillation above audio freq so you never heard anything.

Noisy LM386 based audio amplifier

remember to keep the remember to keep the capacitor’s legs as short as possible, that also helps.

www.robotshop.com

LM386 Instability Issue

Hi all. A while ago I started looking into designing a heterodyne bat detector circuit. I posted a thread on here to ask for advice. So for reference you can find this thread here: http://www.electro-tech-online.com/threads/ultrasound-transducer-for-bat-detector-project.126293/page-2 Since...

www.electro-tech-online.com

Also not all bypass caps, for the same C, are as effective in their ESR performance -

View attachment 167873



Regards, Dana.

Thanks Dana - good material to investigate.
The 1000uf option I understand gives superior smoothing, filtering and stability.
Certainly performs better than the usually prescribed lower uf options.
Regards,
Alistair

--- Updated Mar 5, 2021 ---

Dana's bridged LM386 amplifiers in post #2 almost overheats the low power LM386 amplifiers.
1) A graph in the datasheet shows with a 6V supply and an 8 ohms speaker, the output with low distortion is 0.2W which is 3.6V p-p.
Another graph shows the max p-p output is 4V. 3.6V/4V= 0.9 times.

With a 9V supply, the max p-p output is 6V so with low distortion it will be 6V x 0.9= 5.4V p-p which is 0.46W into 8 ohms.
Another graph shows that the dissipation (heating) is 0.55W when its output is 0.46W.

Bridging doubles the p-p voltage, current and losses. The 8 ohm speaker appears to be 4 ohms.
A graph shows that with a 12V supply and a 4 ohm load, the max p-p output is 3.5V so with low distortion will be 3.5V x 0.9= 3.15V p-p which is 0.31W into 4 ohms. The heating is 1.2W. Then the bridged low distortion output onto 8 ohms is 1.24W and the total heating is 2.4W(close to its max allowed heating).
With a 9V supply the bridged low distortion output into 8 ohms is also 1.24W and the heating is only 1.6W.

Thanks for that useful insight.
Therefore the bridged circuit above should not overheat?
If it did overheat does it mean the LM386 should be discarded?

 

[danadakk]

Just plug into a protoboard suspect part and do a simple test to see if
it still can drive a speaker. Use typical example in datasheet for circuit.

Regards, Dana.

 

[Audioguru]

An amplifier with a 1.24W (low) output power that produces 2.4W heating power (twice as much) as its output power is useless.
You should never operate an IC near its maximum 'allowed" temperature because then it might fail on a warm day or if it is enclosed.

 

[danadakk]

A sim with low distortion, eg. not overdriven, producing 270 mW into a 8
ohm resistive load. Not exactly a good model for a speaker. There is
a lot more inertia component (inductance) in a speaker.

Regards, Dana.

 

[Audioguru]

Hi Dana.

Your low level of output swing is far from the max output where clipping will occur, but your negative output swing is noticeably more than your positive output swing causing even harmonics distortion, probably because the 12V supply is too high. The fairly low distortion is shown only with a 6V supply.

The output swing and power will be almost the same when the 12V is reduced to 9V, but the distortion will be much less.
The overheating of the IC also might be causing some of the distortion.

 

[danadakk]

Here is 200 mV of drive, for 1 W out, die at ~ +50C over room.

Regards, Dana.

--- Updated Mar 6, 2021 ---

Almost 3W out in bridge, but starting to distort -

Regards, Dana.

 

[Audioguru]

Without bridging, with a 9V supply and an 8 ohm load the distortion should be fairly low.

 

[danadakk]

Of course we are frying eggs on these parts doing 1.4W RMS each at thermals > 50 C/ W.

Regards, Dana.

 

[danadakk]

Without bridging, with a 9V supply and an 8 ohm load the distortion should be fairly low.

The distortion, bridge circuit, 9.3% for 9V supply, 7.3% for 12V supply, from sim.

Cannot speak to the completeness of the model or the fact load is not a real
speaker model.


Regards, Dana.