Line audio amplifier help?

[Amiga1992]

Hey guys,

For convenience's sake I want to add a headphone plug to my Amiga. I'd need to build a little line amplifier and I need some advice.

I guess I will use an LM386 based design since they are simple and easy to do. But I am stumped as to what capacitors to use.
I've seen a particular schematic I like which uses two 220 microfarrad capacitors (doesn't specify the voltage), but this circuit is for making a headphone amp for an ipod, meaning it will already receive headphone signal input.

What kind of capacitor would I need to lift up a line signal to headphone levels?

Also: how would one go at making a stereo separation mixer? It would be great if I can add circuitry to be able to, with a pot, regulate the amount of stereo separation I get, from no separation to fully separated, since composing on the Amiga with hard panning is painful at best if using headphones.

Thanks for your input

[mfilos]

Check out this article Akira mate.
http://tangentsoft.net/audio/cmoy-tutorial/

It has really useful info

[Amiga1992]

Thank you mfilos!
So a "headphone amplifier" works well for upping a line level signal to hearable levels?

This one seems a bit more complicated than what I was looking at, I guess because it doesn't use the LM386. USing the LM386 is very easy:
http://www.instructables.com/files/d...6P8.MEDIUM.jpg

But none of this explain anything about stereo sep[aration. I was wondering oif some one here had done something similar to what I want to do. At first I will keep it as an external box to the Amiga but I am planning to build it inside eventually.

[mfilos]

This is a guitar amplifier as I see it mate!

An awesome alternative although a bit more pro is this one:
http://gilmore2.chem.northwestern.ed.../meier_prj.htm

...unless you wanna be imba pro with tube headphone amp as Illuwatar did :P http://www.illuwatar.se/project_page.../headphone.htm

[Amiga1992]

Well I thought about a tube amp BUT I need a headphone output only, and I dont want to change the warmth of the Amiga's AD/DA. Maybe at some point
For now I want something simple, based off the LM386, that amplifies teh stereo signal to headphones, with a volume pot. That's all.

[8bitbubsy]

It's really simple: You power the op-amp with two 9-12v sources, then you feed the Amiga's output lines into the two IN pins on the op-amp, then you feed the OUT pins to IN on the volume pot, then the OUT from the volume pot goes to the headphone jack.
For the caps needed, just make sure you get a steady ripple-free vcc for the op-amp.. For other caps, check the op-amp's schematics for an example use.

I recommend something like NE5532P for the op-amp.

[Stedy]

Hi,

My original Audio mixer with an LM386 op-amp is here:
http://www.ianstedman.co.uk/Amiga/de...dio_mixer.html
Ideally it should have AC coupled inputs but it has been working fine for 8 years now installed inside a PC.

I found some nice headphone amplifier circuits using transistors, if you want something different. I have used transistors for audio and video amplifier circuits successfully.

@8bitbubsy

Your design recommendation will create a wonderful high power, possibly high frequency oscillator that will be destroyed if you turn the volume right down. An op-amp with no negative feedback has a gain of 100,000 and the same signal applied to both inputs will cause the device to amplify the smallest difference between the inputs, with a gain of 100,000 the output will transition between the supply rails.

There is an old joke, to design an oscillator, design an amplifier, likewise for an amplifier, design an oscillator. I have come to understand this saying after an interesting experiment once.

Ian

[Amiga1992]

So I ended up buying a dirt cheap headphone amp from China. It seems it has a chip similar to this:
http://www.ti.com/lit/ds/symlink/lm4917.pdf

The problem with this little device is that it has no volume pot. Where would one add the volume pot? According to 8biitbubsy's suggestion, I'd have to pinch the output of audio from teh chip and put the voolume knob between this and the output. Is that correct? And what kind of pot do I need that would take the two signals? Can I add a separation mixer to this?

Teh PCB is real small so I like it. I'd just have to add a voltage regulator and it would work inside the AMiga

[Stedy]

Hi,

Buy one of these from your local Maplin store:
http://www.maplin.co.uk/standard-dua...tiometers-2207
You want a 10K log potentiometer, FX09K.

The human ear's response to changes in audio level is logarithmic, so you want a volume control with a logarithmic response. If you use a linear part, the perceived volume will go from too quiet to too loud very quickly.

Connect the potentiometer to the input of the amplifier. You keep the amplifier the same, just adjust the input audio level.

If you goto Maplin, also purchase a TS2950CT-3.3, Order Code: N68CA. This is a simple 3.3V regulator. You can connect it to the +5V supply within the Amiga without a massive power loss in the regulator. You will need a 1uF capacitor for the voltage output for stability and of course stripboard and wire.

Ian

[Amiga1992]

Awesome, thanks Stedy!!

[Amiga1992]

Alright, I got the 10K logarithmic pot, and it works weird. The volume is jumpy and it goes to a point and before it ends it lowers, so when it's at max, it's actually not at the loudest volume. I triple checked to see if I was given a linear one, but it is indeed a log pot.

I put the pot between the source and the input of the amplifier.
Did I actually need a different pot?

[Stedy]

Hi,

Akira, have you wired up the potentiometer correctly?

With the pins of the potentiometer facing you you should wire them up like this:
Left most pins, connect to ground on the amplifier board or the Amiga.
Middle pins, connect to the audio amplifier input.
Right most pins, connect to the audio output of the Amiga.

Turning the volume control clockwise will now increase the volume. I suspect you ahve not connected the potentiometer to ground.

Ian

[Amiga1992]

I connected it wrong, you are right. I will fix this ASAP! Thank you Stedy

[Arnie]

I have been thinking about adding small internal speakers to my Amiga with a headphone jack and volume control but, I have no experience to make it happen. I have sourced a couple of small speakers that fit into the Amiga so I just need the amp, volume and headphone jack...lol.

@Ian...any chance you could make me one...pretty please

[Amiga1992]

You could use the one I am using, it's a FIIO E3, or you could get the better E6.

[Arnie]

Originally Posted by Akira

You could use the one I am using, it's a FIIO E3, or you could get the better E6.

I have no idea what you just said

[Amiga1992]

Originally Posted by Arnie

I have no idea what you just said

If you read through the thread, you will realize I bought a pre-made line audio amplifier. You could try to use that, it's cheap and readily available.

[Arnie]

Originally Posted by Akira

If you read through the thread, you will realize I bought a pre-made line audio amplifier. You could try to use that, it's cheap and readily available.

No offence intended Akira but, I can't find any reference to you buying a FII0-E3 or E6.

As I have said, I have no experience of these things, so Schematics, diagrams etc. go straight over my head.

[Stedy]

@Arnie,

A one off system designed by me is too time consuming especially when there are some good off the shelf modules available. Found two possible modules here:
http://www.esr.co.uk/cebek/index.htm (note add VAT to the prices).
Look at the ES3 and ES5 modules. In the technical information it shows how to connect up the unit.

@Akira

Have you re-wired your amplifier yet?

[Amiga1992]

Stedy, I have not found time, I am finalizing my entries for Datastorm 2012. Perhaps I don't get to do it before next week! But I sure know the wiring is wrong so I gotta change it (I put ground in the middle)