1
\$\begingroup\$

Ultimately I would like to assemble this simple audio amplifier:

enter image description here

Given my lack of any significant knowledge in the topic I have several problems and questions.

Grounding:

In the thread What is ground and what does it do? it has been said that grounding is basically about creating a reference point and:

Remember, a complete circuit is required for current to flow. You would need connections to earth ground in two places for current to flow in and out of your circuit from earth ground.

Let's take a lightning. If I am struck by it, how come the circuit is closed? The current certainly flows through me. And also if there was no current flow through grounding wire in any case, what would be a physical reason for attaching it?

In the schematic I showed there are signs of earth ground in multiple places. I saw some video of a man creating this amplifier exactly and it seemed to me (the footage is not that precise) that he created a stand alone device, therefore there could be no physical attachment to actual ground by any external wires. How then should I understand the grounding symbols?

In the LM386 data sheet I found a statement that it features "Ground referenced input". How am I supposed to understand that? Also about pin number 4... Should I attach the pin to the ground (as possibly shown in the schematic) or I could attach all the wires ending with the ground symbol to this pin? If it's the latter how is the pin constructed inside the chip? How come it serves as a ground?

Amplifiers:

I have already assembled some amplifier that looked more or less (rather less) like the one shown. The problem is that it worked with an input from a regular PC jack but it did not work at all with an input from 2.5 mm telephone jack. Someone said that it's because PC audio already has boost and the telephone's port does not. Did he mean a power booster by it? Like "push–pull output"? But what is the general difference between push–pull output and this amplifier? I mean, in this case I am trying to increase voltage of the signal and, as I understand, also a power output (P=UI=U^2/R). So I need a booster to boost my signal so I can boost it again? I don't understand. Someone also said that it could work if I assembled my amplifier correctly. If I assemble this very circuit correctly without any additional parts, will it work with the telephone jack I mentioned (Panasonic KX-DT333)?

Edit:

Also what is the function of the 0.05 mOhm capacitor? What happens if I use 1 mOhm? Also there should be no problems if I use 220 instead of that 250 one, yes?

\$\endgroup\$
3
  • \$\begingroup\$ In that circuit all the ground symbols must be connected together, and to the negative terminal of the power supply. There is no need to connect anything to Earth Ground (even if some sources say that that ground symbol means "earth ground" - the different ground symbols are not used consistently despite various descriptions you may find). \$\endgroup\$ Oct 7 '14 at 19:30
  • \$\begingroup\$ So if I use USB as my supply I should simply connect all the wires ending with the earth symbol to the negative wire, yes? \$\endgroup\$
    – user51704
    Oct 7 '14 at 19:41
  • \$\begingroup\$ yes - connect your circuit ground to the USB ground. \$\endgroup\$ Oct 7 '14 at 19:43
0
\$\begingroup\$

Lots of questions.

Grounding

  1. If you're struck by lightning, that's because there was such a strong potential difference in charge between you and the clouds that the air itself broke down, forming a plasma, and equalizing that potential difference in a very short amount of time. Since it's a lot of energy in such a short amount of time, that means a lot of power. I hope you brought a change of underwear.

Schematics

  1. In schematics, symbols like ground and Vcc (power supply) are special in that multiple instances of them typically mean "connect these all together".
  2. The LM386 is somewhat unique among ICs because its inputs have a resistor-to-ground built in. Imagine a 50k resistor from pin 3 to pin 4, connected inside the chip. There's one from pin 2 to ground also, but in this circuit that's moot because pin 2 is grounded. You are supposed to understand that by looking up and reading the datasheet, noting especially the parts about "input resistance" or if you're lucky, the "simplified schematic" which shows the inner workings of the chip.

Amplifiers

  1. It's hard to guess why one amp didn't work and another did with so little information to go on. A concept that is probably going to help you is "impedance matching" which means basically that some amplifiers are designed to efficiently drive certain types of loads, and are less efficient with different types of loads. PC audio outputs are generically classified as "line level" which means they're optimized to deliver a couple volts into a 600 ohm load. BUT they will still likely "work" for headphones that are 30 ohms. Why? The impedance mismatch is not very noticeable. Instead of delivering a couple volts it delivers a couple millivolts. But headphones are efficient and so the resulting signal is good enough for many purposes.

  2. The phone you linked to has a 2.5mm "headset jack", which probably means you can treat it like a headphone jack on a typical audio player, or even like your PC's audio output.

  3. "boosted" output on the phone - don't worry about this right now. The phone has enough output to drive headphones, which means it will work with the circuit in your image. You don't need a "booster" and the person you're talking to either doesn't understand this or isn't framing it in a helpful way for you.

  4. Push-Pull - way off topic. This is a kind of amplifier arrangement which is often used inside integrated circuit chips. In fact, there's at least one inside the LM386. But learning about this is tangential to today's goal of "make the phone louder".

  5. I think this circuit will work with the Panasonic phone.

Capacitors

  1. Capacity is measured in Farads, or often (because a Farad is huge) micro-Farads, abbreviated µF. Ohms are a measure of resistance. Very different things.

  2. The function of the 0.05 µF capacitor is a complex subject. For now, just take for granted that it forms a "filter", together with the 10 ohm resistor, that prevents this particular circuit's amplifier from wasting power. If you omit these components or deviate from the suggested values too much, the amplifier will oscillate at a high frequency (too high for you to hear) which will waste power and may harm the speaker.

  3. Yes, you can use a 220µF instead of the 250µF capacitor. The only difference will be slightly less bass response from the speaker. But if you're using this with voice it probably won't be missed.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy