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I need help with a very simple problem please!

post #1 of 14
1/27/10 at 9:24pm
Thread Starter 
i am making a box to connect my amps speaker output to my headphones however, there are some issues i can't figure out...

here's a horrible schematic of what i think the circuit looks like (sorry for the size)



i'm pretty sure a resistor goes at A and B, perhaps a 220ohm? but i'm not sure what to do with the two - lines @C or if they also need a resistor on, D?

I also don't know what wattage resistors i need, i am guessing 2watt?

all resistors will be socketed (am using gold through-hole IC pins for this) to make this box as flexible as it can be, and to even play about with different kinds/qualities of resistor for some fun

any help will be really appreciated!

kind regards,

andrew
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post #2 of 14
1/27/10 at 10:57pm
Quote:
Originally Posted by googleborg View Post
i am making a box to connect my amps speaker output to my headphones however, there are some issues i can't figure out...

here's a horrible schematic of what i think the circuit looks like (sorry for the size)

i'm pretty sure a resistor goes at A and B, perhaps a 220ohm? but i'm not sure what to do with the two - lines @C or if they also need a resistor on, D?

I also don't know what wattage resistors i need, i am guessing 2watt?

all resistors will be socketed (am using gold through-hole IC pins for this) to make this box as flexible as it can be, and to even play about with different kinds/qualities of resistor for some fun

any help will be really appreciated!

kind regards,

andrew
I think you don't want to do that. Unless your headphones are designed to handle the kind of power that comes out of a speaker output (e.g. AKG 1000) you'll fry them.

Take for example the AKG K701. The maximum power rating for those is 220 mW (that's mili watts). If you feed them even 1 watt, you'll literaly blow them.

So, say that your amp has a power output of 10 watts, if you put 2 watt resistors there, those will blow up too.

While I'm sure it's technically possible to hook up your 'phones to your amp´s speaker output, it's not as simple as your original plan, and I'm not sure if it's worth the hassle.

cheers !
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post #3 of 14
1/27/10 at 11:19pm
the_equalizer: The amp willl not put out the maximum power output unless you max the volume knob.

googlebord: I'm looking at a diagram by Joseph Grado, and he only put the resistors on the signal lines, none on the ground channel. I'm sure someone can post a digital version the diagram. However, you might want to get 10W resistors.
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post #4 of 14
1/27/10 at 11:36pm
Thread Starter 
thanks both for the replies indeed it's definately possible, and i just noticed on that grado thingy he uses 10watt resistors but only of 2 and 20 ohms , i was thinking of using 1 watt at 220 :P

and your right he doesn't seem to put anything directly on the ground ones but the ground in connected to the other wire with a 2ohm 10watt resistor, but i'm not sure he's actually connecting them together like i'll have to to connect them to the ground of my headphone plug

i basically just want to move my amplifiers headphone socket outisde of the case, and i know it uses resistors inside to supply it's headphone jack...but they are not small...

here they are
http://i865.photobucket.com/albums/a...e/DSCF0586.jpg

according to an online resistor calc they are in order of physical size (IF i am reading the colour correct, only have that photo as a guide atm, will open my amp to confirm later), bid to smallest

Resistance 220 Ohm
Tolerance +/- 5 %
Resistance range @ +25 C 209 Ohm .. 231 Ohm
Temperature coefficient +/- 50 ppm
Resistance range @ 0 .. +40 C 208.7 Ohm .. 231.2 Ohm
Resistance range @ -40 .. +70 C 208.3 Ohm .. 231.5 Ohm
Resistance range @ -55 .. +125 C 208.1 Ohm .. 232.1 Ohm
IEA "E" series 6, 12, 24

Resistance 10 Ohm
Tolerance +/- 5 %
Resistance range @ +25 C 9.5 Ohm .. 10.5 Ohm
IEA "E" series 6, 12, 24, 48, 96, 192

Resistance 1 Ohm
Tolerance +/- 5 %
Resistance range @ +25 C 0.95 Ohm .. 1.05 Ohm
IEA "E" series 6, 12, 24, 48, 96, 192

am sure the solution is really simple, but i dare not mess it up! :P
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post #5 of 14
1/27/10 at 11:44pm
Thread Starter 
this is the joe grado image for reference, judging by the speaker plugs it isn't recent hehe

http://i865.photobucket.com/albums/a...headphones.png
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post #6 of 14
1/27/10 at 11:49pm
You definitely need to be careful about connecting the two negative speaker poles. If teh amp is single ended and the negative poles are grounded, you're fine but if the amp has a differential output you could ruin the amp. Notice that in the Grado diagram, each negative speaker output went to its respective negative headphone wire.
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post #7 of 14
1/28/10 at 12:06am
Thread Starter 
thanks for the info regarding single ended and differential i've no idea, but my amplifier is a NAD c320bee if that helps...

i think i can see that the negative poles are fed from the same pcb trace....i will try to confirm this
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post #8 of 14
1/28/10 at 12:12am
mudgecoffee has a point. The safer way to do it is to get rid of the 1/4 TRS jack. Either recable your headphone to XLRs (3 or 4 pin), or reterminate your headphones to banana plugs.

I made a special cable for connecting my HD650 to an Adcom power amp, and I didn't use a TRS plug. It was bare wire straight to the speaker terminal.
I used 1/2 watts resistors and it was fine, though it was just an experiment and I didn't really run it for weeks. 1-2 watts should be more than enough. Maybe the 10 watts Grado used is just for safety, cause I'm sure with the impedance rating of the headphone, not even the 300W Adcom was outputting anywhere near 10 watts to the HD650.
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post #9 of 14
1/28/10 at 12:23am
Thread Starter 
it's comforting to know that headphones and a 300w power amp worked together

just examined the amp and the negative traces are indeed joined together on the PCB, where they also meet a wire coming direct from the power supply board, this has be the ground i take it edit: yes it's ground.

at least i know connecting them together will be fine, as they originate from the exact same point anyway. i've got some more integrated amps i don't use and i'll examine them too later on, see how they compare. will examine the resistor traces now and see how it's all connected up O_o
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post #10 of 14
1/28/10 at 12:40am
Quote:
Originally Posted by ting.mike View Post
the_equalizer: The amp willl not put out the maximum power output unless you max the volume knob.

<snip>
Agreed, still, if it's too powerful...

Quote:
Originally Posted by googleborg View Post
this is the joe grado image for reference, judging by the speaker plugs it isn't recent hehe

http://i865.photobucket.com/albums/a...headphones.png
Cool! One learns a new thing everyday in these forums.

cheers!
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post #11 of 14
1/28/10 at 12:41am
Quote:
Originally Posted by googleborg View Post
it's comforting to know that headphones and a 300w power amp worked together
I know, it was a daring move..
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post #12 of 14
1/28/10 at 12:43am
Quote:
Originally Posted by the_equalizer View Post
Cool! One learns a new thing everyday in these forums.

cheers!
Head-Fi has taught me much as well.
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post #13 of 14
1/28/10 at 12:53am
Thread Starter 
well from what i can tell it appears the nad c320bee just uses a single 220ohm resistor per channel, all those other resistors and caps seem to be part of a connected circuit of some kind, probably the soft clipping...i'm sleepy now though, thanks for the help! :P just looking at how a plain old amp is put together breaks my mind hahaha
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post #14 of 14
1/28/10 at 1:03am
Oh, the higher resistor you use, the bassier your cans will get.

Later.
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