Little Headphone Amp

mictester · Post by **mictester** » 12 Apr 2011, 18:11

I was asked today for a circuit for a simple headphone amp. The LM386 does most of the work for you and is very cheap. It's not the lowest noise amplifier IC, but we're only asking for low gain so it's acceptable. It has a lowpass filter after the volume control (prevents radio interference and excessive shrillness), a highpass filter made of the input capacitor and the volume pot resistance. It's not far from the datasheet circuit!

Here's the circuit - Vero and PCB to follow:

: Based on the datasheet!; Headphone Practice Amp.png (2.18 KiB) Viewed 3491 times

mictester · Post by **mictester** » 12 Apr 2011, 18:48

Following on - here's the Vero and the PCB:

: HeadphoneAmpTracks.gif (2.64 KiB) Viewed 3484 times

Grendahl · Post by **Grendahl** » 12 Apr 2011, 19:09

Would it be alright to use a stereo output jack and just bridge the L/R channels together? I'd hate to have sound in only one ear.

CRBMoA · Post by **CRBMoA** » 12 Apr 2011, 19:19

Grendahl wrote:Would it be alright to use a stereo output jack and just bridge the L/R channels together? I'd hate to have sound in only one ear.

Or build two and run them side by side? Becomes a mixer in a hurry, but one or two dual ganged pots would make it a winner.

Not that I don't appreciate what is there. Just thinking out loud.

I learn alot by posting dumb things and getting corrected.

Steven_M · Post by **Steven_M** » 12 Apr 2011, 20:08

I thought with 386 amps you were supposed to put a zobel network on the output jack if you're going to be using headphones. Does anyone know what the benefits would be?

Hides-His-Eyes · Post by **Hides-His-Eyes** » 12 Apr 2011, 20:21

Any reason for 22k specifically? Just to get 74Hz highpass rolloff?

100k there would give you no bass roll-off I guess, but would 10k not be just as appropriate (I'm guessing you don't want too much below 150Hz in headphones anyway...) Or does that combined with the lowpass on the output give too little bass?

mictester · Post by **mictester** » 13 Apr 2011, 06:54

Grendahl wrote:Would it be alright to use a stereo output jack and just bridge the L/R channels together? I'd hate to have sound in only one ear.

If you look at the schematic, you'll see that it has a stereo output jack with the tip and the ring joined - you'll get sound in the middle of your head!

mictester · Post by **mictester** » 13 Apr 2011, 07:01

Hides-His-Eyes wrote:Any reason for 22k specifically? Just to get 74Hz highpass rolloff?

100k there would give you no bass roll-off I guess, but would 10k not be just as appropriate (I'm guessing you don't want too much below 150Hz in headphones anyway...) Or does that combined with the lowpass on the output give too little bass?

The output network (10R in series with 47n) is a Zobel network for stability as recommended in the datasheet.

The 22k pot was a compromise - the roll off at 74 Hz (-dB down there) was to help reduce low frequency "handling noise" and any mains hum. I'm going to put up the "deluxe" version later, which includes an op-amp input buffer for high input impedance to allow better connection directly to a guitar, and an option to switch in a little distortion too.

Grendahl · Post by **Grendahl** » 18 Apr 2011, 12:05

mictester wrote:
Grendahl wrote:Would it be alright to use a stereo output jack and just bridge the L/R channels together? I'd hate to have sound in only one ear.
If you look at the schematic, you'll see that it has a stereo output jack with the tip and the ring joined - you'll get sound in the middle of your head!

Yes, I see it now. Guess I should actually take time to read the schem instead of just glancing at it!

dennism · Post by **dennism** » 18 Apr 2011, 19:33

So, I whipped this up this morning and it sounds great! Thanks for posting it! It's surprisingly clean for a 386 based circuit and naturally I'd like to distort it up a bit. I know Mictester said that he will be coming back with a deluxe version, but I'm impatient and looking for a sneak preview.

How would I add some dirt to it? Would it be as simple as putting a pot or a cap across pin 1 and 8 like in the Ruby? Or is it more complex than that?

Thanks again, very cool little circuit, just got to box it up now.

mictester · Post by **mictester** » 18 Apr 2011, 23:01

dennism wrote:So, I whipped this up this morning and it sounds great! Thanks for posting it! It's surprisingly clean for a 386 based circuit and naturally I'd like to distort it up a bit. I know Mictester said that he will be coming back with a deluxe version, but I'm impatient and looking for a sneak preview.

How would I add some dirt to it? Would it be as simple as putting a pot or a cap across pin 1 and 8 like in the Ruby? Or is it more complex than that?

Thanks again, very cool little circuit, just got to box it up now.

I'm glad you like it! I built one the other day for a neighbour who has a 15-year-old son who is just starting to play. I built him a "deluxe" with a rechargeable battery pack to keep running costs down. I laid the front panel of the diecast box out to look like an AC30, but with just three controls (Volume, Overdrive and Tone). I tried it out with three different pairs of headphones, and it sounded great!

mictester · Post by **mictester** » 19 Apr 2011, 05:13

I'm drawing up the deluxe version and both Vero and PCB layouts. They should be up here later today.

jaguarow · Post by **jaguarow** » 20 Sep 2012, 15:34

Here's a vero layout for this. It sounds really great, with a little noise but nothing to complain about in a simple practice amp

rocklander · Post by **rocklander** » 22 Sep 2014, 22:00

I'm looking at building a headphone amp for our drummer (personal monitoring).. would this be suitable or is it more 'guitar oriented'?.. I was just gonna take an aux feed from the 'thru' of a powered monitor speaker...

grrrunge · Post by **grrrunge** » 23 Sep 2014, 11:00

rocklander wrote:I'm looking at building a headphone amp for our drummer (personal monitoring).. would this be suitable or is it more 'guitar oriented'?.. I was just gonna take an aux feed from the 'thru' of a powered monitor speaker...

Take a look at the TDA2822M. It's a dual 1W amp chip in a DIP8 package, and doesn't require more than 10-15 components around it to operate. Runs on 3-15VDC. Dirt cheap at tayda...
Your drummer will love you for the stereo

rocklander · Post by **rocklander** » 23 Sep 2014, 11:46

grrrunge wrote:
rocklander wrote:I'm looking at building a headphone amp for our drummer (personal monitoring).. would this be suitable or is it more 'guitar oriented'?.. I was just gonna take an aux feed from the 'thru' of a powered monitor speaker...
Take a look at the TDA2822M. It's a dual 1W amp chip in a DIP8 package, and doesn't require more than 10-15 components around it to operate. Runs on 3-15VDC. Dirt cheap at tayda...
Your drummer will love you for the stereo

thanks. there's no stereo signal for him, and I'm 90% sure he'll only have one plug in anyway.. this is just foldback for vocal reference.. we just can't seem to eliminate feedback with the current speaker setup. the stage we play most often has glass behind it and it's just all reflecty and crap.

rocklander · Post by **rocklander** » 24 Sep 2014, 10:34

tried building one of the 'less component' ones off the web and found it noisy and even picked up a radio station

I built this one and it's silent.. works a charm...

simple, fast build.. did it dead bug style..

thanks Mictester.. much appreciation!