headphone amp - for click track

[carrejans]

We want to use clicktracks with our band. I use one channel of the mp3 for the samples sent to the monitor; the other for a click sent to the headphones of the drummer.
He complained that the clicks weren't loud enough.

I tried to put a 2N5088 circuit, with a pot in between; but this isn't amplifying hard.

Any suggestions? It doesn't need to be anything fancy. Just a simple volume boost with one pot. (and no rare parts)

thanks...

[carrejans]

Would this LM386 circuit do the trick?

http://www.techlib.com/electronics/audioamps.html#LM386

[PRR]

A 2N5088's output POWER  is quite limited, and you 'd have to know how to work it to get any real LOUDness.

With 32 ohm phones, and no "high fidelity" pretensions, a LM386 power amp is hard to beat.

600 ohm phones will need a higher voltage _power_ amp.

[Mark Hammer]

If it's merely a click you wish to hear, then a 386 will be hard to beat.  Just pay attention to the value of the output cap and don't be afraid to experiment with different values of gain-bandwidth setting caps in the pin-1-to-pin-8 loop

[carrejans]

Thank you all!

[carrejans]

Ok, I finished this one.

I soldered it on a board, tested it with a battery, and it worked perfect.

Now I added a led, used a DC jack in stead of battery; added a switch; and built it inside a hammond enclosure.
Result: doesn't work anymore. A lot of buzzing!!

I think something is wrong with either my grounding; or my switch wiring.

I have drawn a schematic of my wiring. Maybe you guys see something really stupid, I did.

Thanks...

[carrejans]

Some remark: the input mini-jack is made of metal, and thus makes ground contact with the enclosure.
But when i remove it's connection with the chassis; the problems remain the same.


- SWITCH OFF: really loud hum coming out OUTPUT 1; OUTPUT 2 is dead quiet
- SWITCH ON: loud hum coming out OUTPUT 1 (along with the click); OUTPUT 2 is normal

[carrejans]

I made a drawing of the headphone amp, that I made on perfboard.




Here is the schematic, where I based my layout on:

[Manny]

I noticed that you have a separate ground to DC, are the two grounds (DC, output) connected?

Judging by the drawing below, even when your switch is off, current is still being drawn as the dc is never disconnected so the circuit is still amplifying something (albeit not what's coming from the stereo jack), sounds like a grounding or shorting problem on the board to me, where the 386 is recursively amplifying low level noise or ground buzzing.

Did you ever test the circuit directly, i.e. just connect two mono jacks, dc and ground and running a signal through to make sure it amplifies correctly?

Manny

[carrejans]

Thank you for your answer, Manny!

I noticed that you have a separate ground to DC, are the two grounds (DC, output) connected?

Yes, they are connected with that orange wire.

Judging by the drawing below, even when your switch is off, current is still being drawn as the dc is never disconnected so the circuit is still amplifying something (albeit not what's coming from the stereo jack), sounds like a grounding or shorting problem on the board to me, where the 386 is recursively amplifying low level noise or ground buzzing.

Yes, that's probably gonna be one of the problems. I'm gonna change this, right away.
That's probably explaining the loud buzz, when the switch is off.
But when the switch is on, there is a buzz mixed with the signal too. That's still another problem, I think. (I assume some grounding issue??)

Did you ever test the circuit directly, i.e. just connect two mono jacks, dc and ground and running a signal through to make sure it amplifies correctly?

Yes, before I put it into an enclosure I tested it; and it worked perfect.
So most probably a problem with the offboard wiring.

[Manny]

If all the audio jacks are metal and the chassis is metal. You shouldn't need to connect their grounds with wires.
I'd use a multimeter to test that the rings on the jack sockets are electrically common and if so, remove their connecting wires.

Perhaps there's a ground loop in there somewhere that's causing this buzz. 

[carrejans]

Ok, I changed my wiring a bit. And as expected the hum is gone when the switch is OFF.

But still A LOT of buzzing when switch ON; on output of lm386 amp.

The ouput jacks and DC jack are isolated from the enclosure (plastic); the input jack not (metal).
When I remove the connection of the input jack and the enclosure, there still is the buzzing.


I made a better drawing of my offboard wiring.

[carrejans]

When I touch the chassis with my hands, I hear a different buzz.
So this means that there is still some voltage over the enclosure, no?

- I removed the input jack from enclosure
- the ouput jacks are isolated
- dc jack is isolated
- I removed the potentiometer from enclosure
- I thought maybe the led was giving contact to the enclosure; so I cut it off

Still no result...


I also swapped the LM386 for a new one.

[Manny]

Looking at your updated offboard wiring, everything looks ok.

Perhaps it is the board after all, maybe the 386 is fried (although they usually handle 12v so it's improbable) or theres a ground issue on there somewhere.
Personally I'd use an audio probe (you can build one if you don't have one, this one http://www.diyguitarist.com/PDF_Files/DIY-AudioTester.pdf works great).
Use one end on the input of the board and trace the signal path on the circuit to see where the buzz starts.

I hope this helps 

[carrejans]

Ok, I just made an audio probe.

At the input there is no problem.

I marked some points on my layout.
At point 3 the sound is better than at point 1 and 2.

What can be wrong?

[Manny]

Sounds like a ground problem somewhere after point 3. Were you applying voltage during the test?

[Rob Strand]

You should try adding the 10uF across the power supply, place right at the board.

[smallbearelec]

Just a thought:

Are you using and un-modified wall-wart for the power source? Those often have only a bare minimum of filtering. Maybe try a battery temporarily and see if that cures the hum. If it does, you either need to add filtering to the wall-wart, or build a Small Wart:

http://www.smallbearelec.com/Projects/SmWart/SmWart.htm

Regards
SD

[carrejans]

Steve, it was the last thing that I would expect. But it was indeed a bad wall-wart. Thank you for your tip.

I thought about this; but didn't investigate, because I was using the wall-wart of my EHX Holy Grail Plus. It's their official wall-wart, that came by the pedal.
That pedal doesn't have any hum. But probably will have circuitry for that on it's pcb, I guess.

Pffff, it took me so long, for such a small project. It can sometimes be frustrating.
Thank you ALL for your input!



---

I still hear some minor hum, that changes when I touch the enclosure with my fingers. That's still not normal, I think???

[Manny]

I'm glad you finally found a solution.
Happy clicking!