Simple Headphone Amplifier won't work

[AndrewCE]

all polarized caps are electrolytic. all resistors are metal film. all nonpolarized caps are metalized film. "m" is the virtual ground. My headphones measure 37 ohms DC.

HERES THE PROBLEM:
the signal is clear, but not loud enough. i've tried short circuiting the inverting input to "m", and that forces it into clipping (square wave buzziness). I've tried short circuiting the inverting input to ground, and I get no audible signal. I've tried putting a 1K resistor from inverting to ground and I get no audible signal. I've tried putting the 1K resistor from inverting to "m" and the signal is audible, but just as weak as it is when there's no 1K resistor there(how the circuit is in the schematic).

any ideas on how to up the gain cleanly?

[Gus]

2uf and 37ohms?   
work out the math

[sean k]

That 1uf from output to non inverting, maybe 1nf at the most, more likely 100-330pf, and the 1uf on the inverting to "m" should have a resistor in front of or behind it make a highpass or low pass. 200k is maybe a little big as you'd want less voltage amp and more current amp so maybe 10k with a 2.2k over or under the cap to "m"

The 10k at the in doesn't need to be there either and I think Gus is talking about your high pass at the output, try 100uf.

[AndrewCE]

So the feedback capacitor needs to be a smaller value? (to tell you the truth, I have no idea why it's there. I just put it there because someone else told me to, and the signal got cleaner)

but a 10K and a 2.2K would give me a gain of 5. My current gain is 200 and I need the output to be louder. Does this have something to do with impedance?

What is gus saying about 37ohms?

[Paul Marossy]

What is gus saying about 37ohms?

Headphones are usually 16 ohms or 32 ohms. It sounds like you have 32 ohm headphones.

So the feedback capacitor needs to be a smaller value? (to tell you the truth, I have no idea why it's there. I just put it there because someone else told me to, and the signal got cleaner

My understanding is that it is for limiting the bandwidth of the opamp. The excess bandwidth that is. On various schematics, I usually see values of 100pF to 560pF. A 330pF is probably a good place to start.

[Gus]

the 2uf and 37ohm headphone driver forms a high pass.

1/(2PIRC)= -3db point = 1/(2 x 3.14 x 37 x 2 ^ -6) about 2.1KHz

100uf is still too small IMO about 43Hz

470uf gives about 9Hz,  330uf gives about 13Hz, 220uf gives about 19Hz

Fix this and you will need less gain .  Whats the output level of the source?  What is the sensitivity of the headphones?

google look in books etc for opamp non inverting amp you need to fix your schematic.  google headphone schematic etc there is a god site with headphone amp circuits.

[AndrewCE]

Thanks guys, here's the current schem:

I'm about ready to call this thing "final". The gain is good, the only problem is, it's noisy. There's a constant hissing. The only way I've figured to make it go away is to increase the value of the feedback cap, but then the tone gets all dull. Unless someone has a few quick tips on how to reduce noise, I'll move this thing from breadboard to PCB.

btw, I believe the MXR Dist+ equivalent of my 100K to "m" is MXR's extra 1M that goes into the network of 1M resistors. This answers both the question of where to connect the other end of the 100K and why my design doesnt include that extra resistor (it does!).

and i'm assuming that a ceramic cap reading "10" means 10pF. And I'm also assuming that the black mark on the tips of my round orange caps doesnt mean anything. correct me if i'm wrong.

[sean k]

It's better to have the volume changed in the feedback loop by making the 100k a pot and changing the gain of the circuit instead of having your volume control on the input. The way you have it the gain is still all out even at lower volumes so the noise floor is higher at lower volumes as its always being amped at full even though the signal isn't. Variable gain means variable noise floor.

Put a 100 nanofarad cap accross the power supply rails. Only the 100k (+ input) and the 1k should go to "m", the other points go to earth.

[AndrewCE]

grrrrr

i got it set up, but i get no signal through the headphones. just pure hum. i'm gonna check all my connections, but here's my schematic as of now. does anybody see where i went wrong? I added a DC jack for a 9v input. there are 2 connectors that are "normally closed", and when the DC plug is in, one of them is the shell and the other is not connected. the node labeled "dc jack center pin" is not involved in the switching.

the reason i added this is so that when my 9v adapter is plugged into it, the adapter powers it. then when it is removed, it goes to battery power. right now i get no signal on either power source. any help? ive done plenty of continuity checks

i wonder, was i supposed to use a heatsink when soldering my chip? cause i didnt. could i have burned out the chip? and how do i check for that?

ps. when i touch the pot shaft (or even put my finger near it) the hum gets louder. im using a plastic enclosure(all radio shack has, and my altoids tin won't fit a 1/4 in jack).

EDIT: ok, i didnt change anything, and now with the battery supply it sounds like an extremely low-fi fuzz box. and the pot taper has problems, which, btw, it has never done while in the gain-control configuration. there is no sound in the power supply operation (but i'm in the process of correcting that) can anyone spot the reason for my fuzz ?

[Nasse]

http://www.analog.com/library/analogDialogue/archives/41-08/amplifier_circuits.html

Is that ok value for that .047 uf cap....

[AndrewCE]

http://www.analog.com/library/analogDialogue/archives/41-08/amplifier_circuits.html

Is that ok value for that .047 uf cap....

um, i think so... why? do you think it should be bigger or smaller? this is the cap that limits the bandwidth on the DC-side. i dont hear any bass loss or bassy hum with this value...

[Nasse]

I always thought it does affect audio bw too, cap in that position. Perhaps there is some other problem. But I think your opamp choice is not best for low impedance loads. I think they use 5532 for headphones

[R.G.]

LM386

[AndrewCE]

guys, i'm starting to try to calculate the frequency rolloffs of the filters on this design, and i dont know how to handle the opamp feedback bandwidth. I saw on one website that I would take the 1Kohm and 0.047uF together to see what is the frequency of the highpass filter.I assume this would also mean to take the 100K pot and 220pF cap together as a filter to calculate the lowpass filter. But my common sense tells me the opposite of this system; I think the 100K pot and 0.047uF should be taken together as the highpass and then the 1K and 220pF should be taken together as the lowpass.

Am I right or is the website right? and could you please explain the reasoning behind it?