Headphone practice amp

[marcelomd]

Hi,

This is my first post. I just wanted to share something I did.



This is a simple headphone practice amp with instrument + aux inputs. The instrument input is loosely based on a MXR Microamp using whatever parts I had. The aux input and headphone driver uses a NJM4556 (aka JRC4556) which is low distortion, cheap, and can drive headphones.

Works with both bass and guitar. I tested with a G&L L2500 and a vintage Telecaster clone. Both sounded awesome.

Feel free to use. I'm open to critiques =)

Thanks.

[ElectricDruid]

Welcome!

Very nice work. About my only nitpick would be that it's inverting, but I guess for a headphone amp where you're never going to be feeding the signal into something else, that doesn't really matter. Anyway, I don't see a simple way to avoid it, so it's best how it is.

Tom

[antonis]

R1 & R2 values could be down to 10 times lower and C3 could be up to 100 times biger and an electro of 47 to 470 μF added to PS but if it works OK, it works OK...

And Wellcome, of course..!!

[sajy_ho]

A cab simulator after the input stage would be nice in case of using distortion pedals, maybe this one from tonepad: http://www.tonepad.com/getFile.asp?id=150

[samhay]

You can't turn the gain down to less than 1 and I would consider it is bad practice to build a headphone amplifier that you can't turn all the way down/mute. I guess you could argue that your guitar volume control can do this duty, but for the next revision, I would consider adding a volume control as per your auxiliary volume control.

An easy tweak - I would use an NE5532 instead of the 4558.

[rutabaga bob]

Welcome!  I just built an MXR headphone amp for a friend, not too long ago...

[marcelomd]

Thanks for the feedback, guys =)

I mostly forgot about supply filtering because I was using a battery. Will fix that. And I will put a volume control for the instrument. After playing a bit more with the amp, it makes more sense. This thing is kind of loud anyway.

I picked the 4556 because it has higher output current. According to this guy, it was designed to drive headphones =)

I'll check the cab simulator... maybe make it switchable, one for bass, one for guitar.

I'll post the updated schematic later.

Thanks!

[marcelomd]

Latest schematic:



Added a volume to aux input, electro cap to power supply. Cab simulator will come later.

Thanks!

[Fancy Lime]

Hi Marcelo,

nice design! I was wondering: How much difference does the buffer on the reference voltage make? Did you just put it in because you had half an op-amp to spare anyway, or is there a serious benefit for this application?

Cheers,
Andy

[marcelomd]

Hi,

A lot from column A, a little from column B hehehe.

A buffered ground supposedly provides a more stable, lower impedance ground than resistors. It also sources/sinks more current, which is nice for driving bigger headphones.

I had a spare opamp, so why not?

How much difference does it make: For this application? None at all. Or none that I can hear, anyway. It's a matter of context: I use it with a crappy headphone, playing distorted guitar along rock music at a reasonable volume. Any real improvement over a simple divider gets lost in the noise I'm making =)

Other people with better phones, better ears, playing other songs, other styles, might notice something.

This is for a headphone driver. For a reference voltage in a pedal, where currents are so low, I wouldn't bother.

[dschwartz]

If you're not scared of smd soldering, try the maxim max9722a. 130mW per channel, no decoupling caps..stereo
I just built one, its great!!!

[marcelomd]

This MAX9722A looks impressive. Except I can't find it anywhere near me (not without paying an arm and a leg in shipping) =\

[PRR]

As said elsewhere, "It would be much quicker to help ,if you list all the amplifier chips that you know for 100% sure you can get some items of."

But let's sniff for TDA2003. It is going out of production, but zillions were made, and back-stock is not hard to find.

[marcelomd]

Hmmm I can find the TDA2003 (and TDA2005) easily around here. Wow... 10W is a lot. Do we need that much for headphones?

I'm not very familiar with this kind of IC, so I wouldn't even know what to look for.

[PRR]

> 10W is a lot. Do we need that much for headphones?

That's a Show-Off extreme case: high voltage, low load impedance, and hefty heatsink.

Headphones are higher impedance so we get less power for the same voltage. We may use lower voltage. 9V supply will be about 2.5V RMS output which is 0.2 Watts in 32 Ohms. This is still LOUD, don't go there. The benefit is that the 2003/2005 are real easy to find, usually low price, sound pretty good (when not cranked to 11 on cheap car speakers), and will be loafing in this job.

TDA2003 is the classic Single, you need two. TDA2005 is a dual TDA2003. Whichever falls to hand easier.

[marcelomd]

Oooh got it. I'll try these ICs next time I go shopping =)

Thanks a lot!

[Bill Mountain]

> 10W is a lot. Do we need that much for headphones?

That's a Show-Off extreme case: high voltage, low load impedance, and hefty heatsink.

Headphones are higher impedance so we get less power for the same voltage. We may use lower voltage. 9V supply will be about 2.5V RMS output which is 0.2 Watts in 32 Ohms. This is still LOUD, don't go there. The benefit is that the 2003/2005 are real easy to find, usually low price, sound pretty good (when not cranked to 11 on cheap car speakers), and will be loafing in this job.

TDA2003 is the classic Single, you need two. TDA2005 is a dual TDA2003. Whichever falls to hand easier.

I have a small DIY 8W amp kit with a TDA2003.  I was planning on building a small bench amp with it (its been sitting around for years so I'm obviously in no rush).

Seeing you mention it here has me thinking of trying it as a headphone amp instead.  I have a set of 47ohm cans.

Do I really need two separate amps could I hook the headphones in parallel at the output maybe with some series resistance to each side?

Here's a link to the kit instructions which includes the schematic
  I tried to attach a photo of the schematic but the file was too large.

http://www.kitsrus.com/pdf/k47.pdf

[marcelomd]

@Bill

If I understood everything, a single TDA2003 can drive both sides comfortably. A 10-100 ohm resistance to each side should be enough. It will be mono, though.