Audio change/boost

[msknight]

OK - here's the situation - Benq monitor..... volume is low.... I believe I know the source of the problem.... the actual input level from HDMI is low. As the monitor is being used on the end of a number of different devices, not all of which have separate volume control on board... there's nothing I can do at the source. - This has been checked as I have a second Benq monitor and swapped them for testing.

The monitor has a headphone output... but it's not putting out enough signal, coming through listenable but not by much, on a pair of standard impedance headphones (roughly 50 ohms I'd say) and even if I hook up to my old Logitec PC speakers, turning up to max volume on both those and the monitor, are just about comfortable listening, but still noticeably low, and no headroom for more power.

Hooking the headphone output to a decent amplifier works, but the volume has to be turned up high and there's hum present. Couldn't make out much on the oscilloscope.

There is a 5v USB power supply nearby, but I'm running out of mains sockets.

I did have a headphone amp unit, (Line5 headphone amplifier) but now that seems to not be working at all... with anything. So I might take that apart sometime down the line.

I'm presuming I want to boost it enough by a set amount so that the Logitec speakers can grab hold of something to work with. Possibly double, maybe tripple the signal.

[msknight]

OK - so this is how it's going to go down.

Firstly, I managed to get a small headphone amp to work. That told me roughly what I needed to know... but it's not pretty. It's currently feeding an old pair of Logitec speakers.

In the interests of getting all this working off 5v and without needing mains sockets, however, I have a second pair of Logitec speakers which I've turned to passive... ie I've simply cut out the transformer and wired the speakers directly to the input jack. - Plugged to the small headphone amp, they work, but the amp just isn't powerful enough to drive them. Fair enough. At least I know I'm in the right ball park. Also turned out that there's only one speaker in the things. The second is just a flexible plate to give it the appearance it has a second speaker in there.

So now I just need an amp and I think I can use two LM386, for which 5v will be close to the lower end that they'll run, and hook up to a 10K pot with two wipes... https://www.instructables.com/Simple-Audio-Amplifier-With-LM386/ ... so... yeah, all I need now is the LM386 chips... which are currently in bound from China. ... and the Line5 box will be the perfect housing, complete with input and output sockets, a nice little on-off switch and maybe I can use the on-board pot as well.

Not a bad solution for a beginner, and I think I can pull this off.

[Buriedcode]

I would get a test signal going, I'm sure youtube has some 1kHz or 440Hz video.  Then scope the amplitude out of the headphone socket.  Scoping the signal with the headphones plugged in will give you a rough amplitude on the scope and what volume that sounds like to you.  You can then do the same with a different source thats "loud enough" for you ears to see what the difference in amplitude is.  It could be you need a fair bit of gain before your headphone/power amp.

The LM386 is ancient but will work.  I think its gain is fixed at...20? So you might need a preamp first.  I knocked up a decent sounding headphone amp (note: standard 32 ohm over the ear cheap headphones) with a TS922 opamp.  It has enough oomph to drive headphones without noticable distortion and its rail to rail IO, so it'll happily run off 5V (your USB) without any clipping.  Then there are of course pretty good deidcated headphone amp IC's, some without output caps so to pops/clicks when turning them on/off.

It can be quite hard to get a handle on what kind of levels devices output and what "sounds" reasonable volume to you, because whilst there are standards they vary wildly between devices, and of course - our ears also vary a fair bit.