Need assistance outlining earphone speaker circuit

[Garhurd Jung]

I need to outline an earphone speaker circuit. We are told the impedance of the earphones and the base voltage they require and furthermore the voltage source. In the wake of inquiring about I have discovered that the LM386N-4 is a prevalent decision. (an example from datasheet LM386N-4 - https://www.icrfq.com/part/2377557-LM386N_4.html ). I've seen many circuit graphs yet what I don't comprehend is the means by which to pick the voltage increase required for the speaker.

I initially believed that you would need to know the info sound flag voltage and when you increased that by the pick up that would need to give the base voltage required by the earphones, yet it gives the idea that what is important is the present which I don't get.

Also, would you say you are ready to plan such a circuit without thinking about the sound information flag? In the event that you do need to think about it what is it you have to think about it and why?

I would greatly appreciate any help as I am very lost.
Thanks in advance!

[Charlie Schultz]

Hi Garhurd and welcome to the forum.

Can we clarify a couple things? First, the LM384 is a power (voltage) amplifier and it will provide the power increase needed for the earphones (typical application circuit from the datasheet below).

And by "sound information flag", I think you mean the input (pickup) signal?

[Garhurd Jung]

Hello Charlie, thanks for the welcome and clear me the speaker circuit.

Yeah, I mean input signal and now I got my point.

Thanks, thanks a lot.

[Chad Schmidt]

Hello Garhard,

Here's a link a thread that shows a circuit schematic I used recently and have been happy with.

viewtopic.php?f=4&t=3976&start=20#p38858

If you search for Jenkins Headphone Amplifier, you should find a report (I think this was a student thesis) that discusses the design tradeoffs and process.

That same linked thread also shows a schematic for a stereo circuit by Vellman. There is also some documentation on this circuit you may be able to find with a web search.

Hope this helps!

[David King]

I hate the LM386, it's specs were garbage when it came out in the 1970s. We've gone through dozens if not hundreds of evolutions of "pocket sound" since then and there's no reason not to look for a better alternative or simply buy something off the shelf, or as a kit to get this job done. I'd look through some of ST's offerings here: http://www.st.com/en/audio-ics/headphon ... fiers.html
I'm sure TI also has some great options. There are some good legacy chips from the 1980s and 1990s that would serve well albeit use way more current than the latest generation. Most of the modern packages are designed as SMT only and will undoubtedly pose an issue for a one-off implementation but I'm sure there are ways around that.

[Beate Ritzert]

Before i would talk on circuits i would do a "rule of thumb" design.

which impedance does the speaker have (the variance may be huge)? What is the maximum current (or power) needed to deliver? From that follows the necessary voltage output swing, and that - of course with some margin - will define the operation voltage of the device. Power should be less a problem than output impedance.

And then design from the last stage backward voltage source means small output impedance which might mean Op-Amp (well check if it can deliver the power), or a source resp. emitter follower stage. Maybe even a valve. The rest follows: which amplification is needed for Your signal? Maybe You need some NFB to improve linearity and to reduce the output impedance? Do You have enough open loop gain for this?

And then it is time to chose the technology for the implementation.

[David King]

Beate,
Agreed! In this case choosing a headphone that matches up well with the instrument. Headphones/earphones suitable for bass might be a very different choice from ones for guitar or uke etc.

[Rodger Knox]

Most of the modern packages are designed as SMT only and will undoubtedly pose an issue for a one-off implementation but I'm sure there are ways around that.

You can always design and etch your own board and solder on all the SMT components. It's not easy, but it can be done.

This is one of the MIMF preamps designed by Vlad Arlean, the board is 2" x 1 1/2".

[rymon sandy]

Hello,

I am wanting to design, prototype, and build a very simple circuit that will take a 3.5mm headphone jack as an input, amplify the signal (hi gain, very low distortion), and output the signal via another 3.5mm jack where I could connect my headphones or speakers.

[David King]

Rymon,
You might look at kit offerings on eBay. Unless you really want to reinvent the wheel there's not much reason to look past what's currently available for a few dollars. Keep in mind that headphones and speakers have different power and impedance requirements so you might need to build two devices. For a few dollars more you could include a Bluetooth transmitter and have the sound appear wirelessly at your wireless headphones, smart phone or home stereo. The input also depends on what instrument you are playing into it, be it an electric guitar, an acoustic instrument with a piezo pickup or something else that requires a microphone.

[Halgeir Wold]

If you google "Headphone amplifier kit" you will find plenty of kits- some with suitable boxes included or as options, but first you need to set some specifications for your needs. What is you intended use, and what will be your input sources? Headphones come in a variety of impedances, from 8 ohm to typically 600. Any small 1-2W amplifier will usually deliver enough signal to drive a set of headphones. When using headphones with larger power amplifier, one usually includes a voltage divider to drop the signal to the headphones. Feeding a headphone amplifier from an electric guitar may need a small preamp to set the correct load impedance for the guitar pickups. Piezo pickups usually have their own preamps, but a small preamp may still be a good idea.