LM386 Amplifier Project

[bountyhunter]

On his website, **broken link removed**
Rod Elliot measured 3% distortion across a ceramic capacitor that was attenuating its own distortion so its distortion was worse.

I'll study it more thoroughly when I get a chance, but this is likely where the disconnect is:

What is often missed completely, is that capacitors used for signal coupling must have a very low impedance for all frequencies that one expects to pass through the system

That is simply flat wrong for the design I used with FET input amps which have very high impedance. The capacitor's varying impedance doesn't matter if it is varying from "very negligible" to "extremely negligible" compared to the impedance it is driving into. In some designs, the coupling may be critical if the input impedance is low. Mine is very high. I learned that from an old designer (always use a good high impedance buffer stage first).

EDIT: perused the article, and I do have to give him points for honesty: from the article:

Perform all the blind tests you can with capacitors used in real circuits. Having done this, if you still think there is a difference (and can demonstrate it to others in a blind test), then you will probably be the first to do so.

And:

These effects are examined by a combination of simulation and actual testing. Simulation features heavily here, simply because most of the effects are extremely difficult (some are almost impossible) to measure. The resolution of the simulator is far greater than any known test instrument, but one has to be careful to ensure the models used act in the same way as real components.

Yep, that's where the train always jumps the tracks.....

 

[TheNewGuy]

Since you are building two amps anyway (for stereo) you could put ceramic in one and film in the other then rig it to play a single channel through one or the other with a selector switch and see if you can hear the difference.

Wait, does my amp have to have two LM386 IC's because it is stereo? Or do I create two seperate circuits?

 

[bountyhunter]

Wait, does my amp have to have two LM386 IC's because it is stereo? Or do I create two seperate circuits?

The LM386 is a single amplifier. You need two complete (identical) circuits for stereo. One for each channel.

 

[TheNewGuy]

Would a circuit with two integrated LM386's work?

It being a single amplifier means that it only amplifies one channel (or one side of the headphone only)? Just so I understand what I'm doing.

 

[audioguru]

Hi TheNewGuy,
A stereo amplifier has an amplifier for each channel. So it has two amplifiers.
A TDA2822M is a stereo amplifier with two amplifiers similar to two LM386 amplifiers.
It has only 8 pins. Its gain is 98 which might cause hiss to be heard when the volume control is turned down. Its datasheet shows a headphones amplifier and has a pcb design.

Hi Bountyhunter,
Your headphones amplifier has only one coupling capacitor. Its value is so high that it attenuates frequencies below 0.03Hz. Instead of the huge 4.7uF ceramic capacitor I would use a more reasonable 0.022uf (22nf) film capacitor for response down to 7Hz and attenuation at lower frequencies.

 

[bountyhunter]

Would a circuit with two integrated LM386's work?

yes

It being a single amplifier means that it only amplifies one channel (or one side of the headphone only)? .

correct

 

[bountyhunter]

Hi Bountyhunter,
Your headphones amplifier has only one coupling capacitor. Its value is so high that it attenuates frequencies below 0.03Hz. Instead of the huge 4.7uF ceramic capacitor I would use a more reasonable 0.022uf (22nf) film capacitor for response down to 7Hz and attenuation at lower frequencies.

At the time I think I was going for as close to DC bandwidth as possible on the low end so I used a big cap. I don't think it would matter to go smaller on it.

 

[TheNewGuy]

Hi TheNewGuy,
A stereo amplifier has an amplifier for each channel. So it has two amplifiers.
A TDA2822M is a stereo amplifier with two amplifiers similar to two LM386 amplifiers.
It has only 8 pins. Its gain is 98 which might cause hiss to be heard when the volume control is turned down. Its datasheet shows a headphones amplifier and has a pcb design.

Hello everyone,
Hmm...if I were to use two LM386's, I would not get this hiss? Because it's gain is lower? But it would probably be easier to use the TDA2822M...I wonder if it is possible to lower the gain on the TDA2822M to remove/lessen this hiss?

EDIT:
Found a data sheet for the (click here) TDA2822M.

Found one also for the LM386 while I was at it (click here).

 

[audioguru]

The gain of the TDA2822M is set to 98 by two internal resistors (for each amplifier) that you cannot change.

 

[TheNewGuy]

I think I'll stay with the LM386's. Because a hiss can be anoying. I could also add a switch to each chip to switch from 20X to 200X.

It seems like the LM386 has a bit more features, including mute?

 

[audioguru]

The LM386 is a simple ordinary little power amplifier that also has hiss but the hiss is reduced because its gain is only 20. It does not have a mute function.

 

[TheNewGuy]

Oh, I thought I read it had mute somewhere.

 

[TheNewGuy]

If you replace the low level mic with a high level AUX input then the gain of the LM386 must be set to only 20 and a volume control must be used to reduce the input level. The resistor that powers the electret mic must be removed. a resistor attenuator must be used to feed headphones so that a loud signal does not deafen you.

How would a make a resistor attentuator though? Would it help with the hiss?

 

[audioguru]

If your headphones are only 8 ohms per ear then a series resistor will attenuate the loud sounds and hiss. Try 22 ohms to 100 ohms.
If they are 32 ohms then increase the value of the series resistors.

 

[TheNewGuy]

I'm trying to find a schematic that has two IC's. Maby I can do one of my own?

A series resistor is a a couple resistors connected in "series"?

 

[TheNewGuy]

Dang, this is harder than I thought. I had the idea you just slap some components on a board and ta-da!

I realize how much I don't know!

 

[audioguru]

An LM386 amplifier has only 7 parts. A stereo amplifier has only 12 parts.
It is easy to make on stripboard. Simply slap the parts onto a board and ta-da!
Make a pcb if you want.

The series resistor to attenuate the output so that headphones do not blow up your ears is just one resistor from the output of one amplifier circuit to one ear of the earphones.

 

[TheNewGuy]

I'm going to try to draw all this on a schematic.

How would you integrate both onto one circuit (what connections between the two IC's have to be made) ? Or is it better to have two seperate ones? A 2 in 1 deal would be better and compact right?

 

[TheNewGuy]

They don't connect do they?

 

[audioguru]

They don't connect do they?

Of course all the grounds are connected together and the +9V points are connected together.

This circuit does not have a resistor attenuator at its outputs so it might deafen you. Read the text of its article to see the impedance of their headphones or how they prevent the sound to be too loud.