Headphone amp for 55Ohm Akg headphones ??

[Cap]

Hi all guys!!

..yeah, I know there are TONS of info about headphone amp and I'm really sorry if there is already another topic about what I'm asking for, but...I tried the ruby, mxr headphone amp from GGG...etc with standard headphones.Not very satisfied even if they all worked!

Now what i need is a simple and clean headphone amp for my Akg-k141 headphones (rated impedence 55ohm and 200mW max input power).

The idea is to use it exactly instead of my Marshall after processing the guitar signal with stompboxes (maybe a cab sim at the end as preamp to give the "amp feeling" and EQ??), a volume knob and my ears

I've found this topic but I'm not good in electronics ..only soldering and reading schematics 
http://www.diystompboxes.com/smfforum/index.php?topic=88880.0

Any suggestions???

Thank you!

[Johan]

google "LM386 datasheet"...I'm sure there is an application note there. that chip was made for headphone amps. most people here think it's a tiny guitaramp chip but thats just our interpritation of what it can do
J

[Cap]

Thanks for reply..I've already read the datasheet but all the applications are for 8 ohm load...

ok I try to change the question..what wuold happen if I connect the 55 ohm headphones to a ruby for example? Only low volume? Dead headphones?  my house blown up??

Cheers

[Top Top]

Generally, for solid state amps, and especially small chip amps, higher impedance is not a problem for the amp or the speaker (headphones in this case).

If the impedance of the speaker is too LOW, it can burn up the amplifier because the current moves through more quickly than it is designed for.

It will work as a headphone amp for almost any headphones. The 386 is very forgiving when it comes to impedance, just don't plug in a 2Ω speaker and crank it and you will be fine.

[Cap]

Thank you for the explanation Top Top!

I'll work on it

[PRR]

> ...I tried the ruby, mxr headphone amp from GGG...etc with standard headphones. Not very satisfied

"Not satified" HOW?? Too loud? Soft? Distorts easy? Hard? Dull? Smells bad? If you are looking for something else, an explanation of "not satified" would be a good place to start.

> give the "amp feeling"

You will, of course, *NEVER* get the "feeling" of a high power speaker-amp with headphones. Guitar amps often run at levels which impact skin and chest/guts. Rooms are sometimes big enough to have "a sound". Headphones are a VERY different experience.

> ..what wuold happen if I connect the 55 ohm headphones to a ruby for example? Only low volume? Dead headphones?   my house blown up??

Birds fall from the sky, polar ice stacks up in your driveway, Mubarak falls out of power.....

Oh, no.

With 9V supply and round numbers, a good amp (LM386 is pretty good) will output 3 Volts RMS. Power is volts squared divided by impedance.

3V in 8 ohms is 3V*3V/8= 1 Watt

3V in 16 ohms is 3V*3V/16= 0.5 Watt (yes there is 16 ohm data on the LM386 datasheet)

3V in 55 ohms is 3V*3V/55= 0.16 Watt = 160mW = essentially the headphone rating. If 160mW is not enough, 200mW won't be real different.

The LM386 will run cooler, the battery will last longer, than when driving 8 ohms.

With some amps, you need to check for a "maximum load impedance". Some tube amps can be hurt by high levels into very high impedance loads. As Top Top says, the LM386 does not care, it will happily drive nothing (infinite impedance).

...... I just realized..... I was using LM386 to drive 2*100 ohm loads 35 years ago. When Mubarak was still just VP, and we thought Anwar would rule forever.

[km-r]

http://gilmore2.chem.northwestern.edu/projects/cmoy2_prj.htm

heres one that i currently have in my mobile hifi...

[Cap]

@PRR
thanks for reply, not satisfied means that i had some distortion and not enough volume...I tried the mxr headphones amp from ggg with my breadboard.
The ruby with zobel network is on a shelf now, but I had similar problems like distortion...I tested it with three 386 chips but was the same.

when i finish the exams next week I'll try all again and I hope to give a better explanation!!

@km-r
..never found the OPA here in my country...bought one from ebay and lost...somewhere I've seen someone building it with the NE5532 with some mods..is it possible?

[km-r]

yes actually the first design i did was made with an NE5532 and it will work with little or no modifications!
the mod i did was to reduce the input impedance because the NE5532 is a BJT input opamp and its noise performance is appreciated with impedances below 20k.
I reduced R2 to 10k and removed R1 and C1 or increase C1 it to about 1 - 2uf

I wanted to have a minimal parts count because i have to stuff it inside a small mint box with a 9v battery for mobile purposes.

the voltage divider is okay as is with your headphones but in my case because i use low impedance headphones i have to make an active divider.

[PRR]

> its noise performance is appreciated with impedances below 20k.

The relevant noise impedance is the SOURCE impedance. When driving with an iPod/etc, the 5532's noise current sees 32 ohms in parallel with whatever bias resistor. Doesn't matter if the  bias resistor is 5K or 50K: 31.8 versus 31.98 ohms.

[km-r]

you are right. but i think the resistor mod i did helps keeping the impedance below 20k. some ipod's use line-outs where the impedances are rather high.