Question about headphone wiring

[arsun]

My main usage with the amp will be with headphones. I have a few questions about the wiring of the headphone jack though. Since I won't be using speakers, I will probably install 10 ohm wirewound resistors to load the wiring posts. My questions are about 120 ohm resistors in that configuration. I am using a different kind of jack than the standard one in the kit. Here, the section touching the left channel positive on the headphone plug is one piece and the lug is connected to it at some distance. This is true for the right channel positive and the ground as well. Ok, let me try to find a picture to show what I mean by that.  Yes, I found something This is what I meant by it:

Now my question is as follows: How do you recommend I mount the 120 Ohm resistors to this headphone jack? Thanks...

 

[jrebman]

I can't help you with the headphone jack because I can't see the picture, but you don't have to worry about loading the speaker terminals with a resistor -- just not needed.  You could also just eliminate the speaker wiring altogether if you want.

HTH,

Jim

 

[arsun]

Thank you Jim and Grainger for your comments. I will rather have the option for the speakers as well, so the wiring will be there. Let's just say I will use my Sennheisers most of the time.

Is there a recommendation on how to install the 120 Ohm resistors to that kind of jack?

 

[jrebman]

Arsun,

Yes, that was my thinking too.  Mine also will be used primarily as a headphone amp but I'm also prepared for the fact that this may also make a fine amp with the speakers I have, so why not leave the speaker wiring in.  I'm ready to be pleasantly surprised.

-- Jim

 

[arsun]

Yeah Jim, this is what I was thinking about.

Now back to that problem... I just checked the schematics of the amp and read the instructions again. Here is what I am considering, please bear with me if that sounds weird or wrong, cause apart from basic knowledge, I am not experienced at all. I have an additional terminal strip with 5 posts (same as other strips in the amp). What if I create a similar layout of the jack in that strip? Let me explain this a bit more: From headphone jack to strip we have 3 teflon wires going out: right channel, left channel, ground and thus 3 posts in the strip are allocated. I use 120 ohm resistors from positive posts to other two posts, so that I have right channel positive attenuated by 120 ohm and left channel positive attenuated by 120 ohm at the posts 4 and 5 respectively. After that, the positives from right speaker post goes to the right attenuated positive section, left to the left attenuated section. The grounds will merge. Then from this terminal strip, the outs will be from ground and from right and left posts. Is this wiring ok for that kind of jack? Am I missing something here? Thank you for your help...

 

[Paul Joppa]

If I follow you  :^)  that sounds good. The headphone jack will be attenuated by the 120 ohm resistors, and the speaker output will not.

The only problem is that plugging in the headphones will not automatically disconnect the speakers. As long as you use it for one or the other, no problem. If for both, you can use banana plugs for the speakers which are easily disconnected. (Trust me on this, unscrewing and re-attaching spade lugs all the time is a pain!!!)

There are jacks with independent switches which will do the disconnect function. So far I've only found one cheapo that is readily available; there were some posts on the old forum which I can't find right now - it's late at night! Does anyone have them bookmarked?

I should mention that the 120 ohm resistor is a vexed question. There have been posts on that as well. For high impedance phones (say, 250 ohms or more) it does little; you can leave it out. For low impedance phones, it attenuates them, and if they are sensitive that's important - otherwise you'll get blasted, and the residual noise and hum will be too loud. But some low-impedance phones really need a low impedance drive; they don't work well with the 120 ohms even though it is the official standard - the standard has not kept up with the prevalence of iPods and the like, and too many 'phone makers simply ignore the standard as long as their phones sound OK with iPods and the like. For those you need a low-impedance L-pad or something similar.

 

[arsun]

Thank you Paul for your informative reply. All my speaker cables have banana plugs, so that should be fine. One question I missed: For the three wires from the headphone jack to the strip: Do I need to make them in a twisted fashion? Thanks...

I will be using Sennheisers mostly, however I also happen to have a nice set of iems and I also love some Audio Technicas (I might get them in future as well as a nicely used pair of Grado HP1000s), which all have lower impedance than the Senns, hence I would rather not loose those different options.

 

[arsun]

Since it has been some time, let me ask the question I had in my last post again: There will be wires between headphone jack and the terminal strip for ground, left and right positive. What is the best way to make this connection? Three single teflon wires in a twisted fashion, using shielded twisted pair with drain for ground, the other wires for left and right, or one twisted pair for ground and left positive and another one for again ground and right positive? I am more inclined to the latter, but for no particular reason . I appreciate your comments, thank you.

 

[Paul Joppa]

I'd use teh single STP with shared grounds, with the inside pair for left and right "hot". That way there is no possibility of a ground loop. The headphone jack already has a shared ground, so there's no technical point in splitting that only to return them together at the other end.

 

[arsun]

Thank you for the reply. So, one single STP wire is it then. The drain will be for the ground, red and black ones for the left and right +. I was thinking teflon twisted pair, since all the connections here are with the same wire. STP sounds more convenient though...

 

[arsun]

I think I have a problem with this wiring. I just finished it and soldered everything. There is one single STP wire going from headphone jack to the terminal strip. All the grounds are merging. The output autoformer wiring is to the terminal strips for right and left channel. From these hot posts, there are two 120 ohm resistors going to the posts to which the STP red and black wire are connected. The speaker post connections are again to the posts where I have the output autoformer wiring connected to. I just wanted to check a few things before going on, just to be on the safe side and here is what I found:
First the correct things:
- Speaker posts are not attenuated, they show 0 Ohm from the output autoformer binding posts
- Headphone jack left and right are attenuated by 120 Ohm

Now here is I think I might have a problem:
- The posts of the ground and output autoformer binding posts show continuity somehow, I checked the connections, there isn't anything connecting these, or at least I could not see it
- The left and right channels in the headphone jack show 240 Ohm between them
- The ground-left and ground-right on the headphone shows 120 Ohm
- Speaker ground and hot shows connectivity

Seems like the last three are results of the first issue, but I simply cannot see any connection between these posts in the terminal strip. Am I making a fundamental mistake here or being really silly?

Thank you for your help.

 

[Paul Joppa]

The DC resistance of the transformer output winding is less than 0.5 ohms, i.e. nearly zero. If I understand your post correctly, that is consistent with your readings. How does it sound?

 

[arsun]

Ok, then that makes sense, thank you. I am still in the building phase, but I am checking things as I go along. I thought I made a silly mistake somewhere. I did not listen to it yet, but thanks a lot for the clarification. I really appreciate it...