LM386....?

[pwdrive]

Per 2nd schematic in the link below.

1) Expected DC voltages on pin 5 and pin 7, no AC signal input, Vs=12 volts?

2) Would rotating the volume control alter the said?

http://www.hobby-hour.com/electronics/lm...-audio.php

Excuse my ignorance on this matter.

Lawrence.

[Old Sparky]

I'd expect the voltage on pin 8 to be approx. half the supply voltge, Lawrence, but I'm not sure whether rotating the volume control would affect the DC conditions. In theory it would as the resistance between pin 3 The + or non-inverting i/p and ground will vary between zero and 10k. Others feel free to correct me if I'm wrong!

[ppppenguin]

Looks odd as you'd expect the inputs to be at the half rail point. But it's taken straight from the maker's app note: http://www.ti.com/lit/ds/symlink/lm386.pdf

[Terry]

As Jeffrey says, the output (pin 5) should be at half the supply voltage ± any offset.

Regarding the DC coupled input control, the datasheet says:

Input Biasing

If the dc source resistance is less than 10 kΩ, then shorting the unused input to ground will keep the
offset low (about 2.5 mV at the input, 50 mV at the output). For dc source resistances between these values we
can eliminate excess offset by putting a resistor from the unused input to ground, equal in value to the dc source
resistance. Of course all offset problems are eliminated if the input is capacitively coupled.

So if Vs=12V the output will be 6V ± 50mV.

As for pin 7, this isn't specified but as it at the junction of two 15k resistors between, essentially, the output and Vs, I would expect it to be 9V.

   

For more information, see the manufacturer's data sheet as linked to by Jeffrey and, for a laugh, have a look at the revision history for Issue B which must have been a bit confusing to follow!

[reset]

Per 2nd schematic in the link below.

1) Expected DC voltages on pin 5 and pin 7, no AC signal input, Vs=12 volts?

2) Would rotating the volume control alter the said?

http://www.hobby-hour.com/electronics/lm...-audio.php

Excuse my ignorance on this matter.

Lawrence.[url=http://golbornevintageradio.co.uk/forum/newreply.php?tid=6474&replyto=70248][/url]

Someone here will tell you get the DC biasing right first and then look at the AC signal path. This is good advice but we sometimes  see circuit presented without the conditions being defined. I think the circuit is being presented for learning about the LM386 and not as a functional circuit. With no external circuitry pins 2 and 3 see ground through the 50K resistors. The output balances and pin  5 would be 1/2 Vs. Adding the volume control across pin 3 does not change the potential applied to pin 3 the circuit remains in balance.  If you apply a DC voltage to the volume control you will throw the circuit out of balance and cause the output to swing as you vary the pot.

   

The complete circuit might look like this ( from datasheet).

   

Note Cc, the coupling capacitor prevents DC from reaching the volume control while allowing the input to see AC excursion.  This preserves the circuit balance as far as DC bias is concerned while allowing the AC signal to be amplified. In a classroom we would spend time building the circuit possibly starting with a bridge circuit and onward. The site may have done that but with the wonder of web browser you can land in the middle of a thread.

[reset]

I want to try and do a better job of expressing myself.  Some times diagrams are presented as part of the theory of operation and others are presented as suggested construction circuits. Both of the schematics in my previous post were from the datasheet. One was representing the basic circuit operation without regard to biasing the other was a construction drawing. Here are similar diagrams for my present project.

   

this is from the first page of the datasheet. It just presents the idea that you could build an amp using the device.

   

The evaluation board looks a little more complicated.

   

This is the circuit as built.

The second and third pictures are from the AN that comes with the evaluation board.
They leave a lot out of the datasheet expecting us to know about the devices. Twenty years ago when the devices we new the datasheets were more detailed.
Quite often we see what is sometimes called "book circuits" they are representative of the circuit but not functional. (or not fully detailed)

[pwdrive]

Thanks for the replies, I take it then that the DC voltage on pin 5 should be approx. half of the supply rail voltage....

Lawrence.

[ppppenguin]

Yes. That also gives maximum possible output voltage swing and hence maximum possible output power.

[Nick]

Thanks for the replies, I take it then that the DC voltage on pin 5 should be approx. half of the supply rail voltage....

Indeed. From the first page of the datasheet http://www.ti.com/lit/ds/symlink/lm386.pdf, see below:

   

[pwdrive]

Cheers all.

Lawrence.