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Old 10th January 2011, 08:15 PM   #1
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Default Help with headphone jack

Hi all.
I am in the process of building my first gainclone, using chipamp.com's LM3886 stereo kit. I would like to have a headphone jack in addition to speaker outs. Any advice on how to do that?

I would love to avoid adding another switch/knob to the front of the chassis. I'm picturing power switch on the left, volume knob and LED in the middle, and a headphone jack on the right.

If it matters, my current headphones are Grado SR80, and Shure SRH440.

Thanks in advance for any advice,
Josh
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Old 11th January 2011, 04:20 AM   #2
djoffe is offline djoffe  United States
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headphone jack has three terminals...gnd, right hot, and left hot...
connect amp gnd to jack gnd.
amp right==>100 Ohm 1 Watt==> right hot
amp left==> 100 Ohm 1 Watt ==> left hot

This leaves the speaker outputs live all the time...as well as the headphones...
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Old 11th January 2011, 04:45 AM   #3
singa is offline singa  Singapore
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Quote:
Originally Posted by djoffe View Post
headphone jack has three terminals...gnd, right hot, and left hot...
connect amp gnd to jack gnd.
amp right==>100 Ohm 1 Watt==> right hot
amp left==> 100 Ohm 1 Watt ==> left hot

This leaves the speaker outputs live all the time...as well as the headphones...
There is a way to connect them like commercial units
when you plug in headpone the speaker is disengaged. I think the
ground connection is common to both so either one is on or off.I'm not too
sure but only plug in/out headphone when power is off or else may damage speak or amp and volume at zero.Hope this helps.

edit: I think 100 ohm alone is too dangerous better use a voltage divider to make sure heaphone
amp only gets a certain amount of signal voltage to not over drive it.There is a way to calculate
a voltage divider just do a search.That is the tap to headphone is between the 2 resistors and
the other end to ground if that makes any sense to you,that's the proper way I think.
Still the safest way is the switch method.This is a high power amp.
Regards.

Last edited by singa; 11th January 2011 at 04:56 AM.
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Old 11th January 2011, 05:28 AM   #4
djoffe is offline djoffe  United States
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Here's a typical part with the built in switch.
Attached Files
File Type: pdf KC-301065.pdf (127.5 KB, 65 views)
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Old 11th January 2011, 06:46 AM   #5
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Thanks all. I'm intrigued by the jack/switch combo you suggested. Since I have a baby in the house, I'd like to plug in phones and have the speakers switch off. I cannot tell from the diagram of the part how I would go about hooking it up. Sorry to be ignorant, but I'm new at this... Can you help with what wires go to which terminal? Do you still think 100 Ohm 1 Watt resistors for the phone jack is the ticket?
Josh
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Old 11th January 2011, 06:54 AM   #6
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Maybe I would hook the grounds from the speaker jacks to terminal 3, and a wire from terminal 7 to the star ground? If I'm thinking about it right, when you plug in a phone plug, the ground to the 2 speakers would be broken. Yes? Thanks.
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Old 12th January 2011, 12:32 AM   #7
djoffe is offline djoffe  United States
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Default Here's the wiring

I'm guessing a bit about left versus right....the supplied pinout with the earlier pdf was kind of vague.

Still, you won't go wrong if you remember that Right is Ring and Left is Tip, at least according to my Grado Headphones.

With no headphones plugged in, 2 and 3 are connected.
With no headphones plugged in, 7 and 6 are connected.
So, with no headphones, the right amp connects to right speaker, and the left amp connects to the left speaker.

Connecting the headphones pushes apart 2 and 3, and pushes apart 6&7, disconnecting the speaks from the amps.

Note also with this arrangement, the headphones are always energized, even though the speakers get cut on and off.

Hope this helps.
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File Type: png switching headphone jack.PNG (15.0 KB, 115 views)
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Old 12th January 2011, 01:18 AM   #8
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Ah, I see. Thank you.

One more question, slightly unrelated, but since I'll be shopping at Mouser... Can you recommend good RCA inputs and banana jacks for my project?

Thanks again,
Josh
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Old 12th January 2011, 02:19 AM   #9
djoffe is offline djoffe  United States
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starting here...too many options...panel mount, pc, right angle...etc.

you'll have to narrow it down...

Phono (RCA) Connectors
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Old 12th January 2011, 03:29 PM   #10
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by jbernst5 View Post
Can you recommend good RCA inputs and banana jacks for my project?
if you want speaker turn off when TRS plug is inserted you will have to re-route the speaker wiring through the TRS socket first and then to the speaker output terminals.
When you do this, all the other connections become irrelevant to sound quality options.

I agree with Singa.
We both advise that you create a voltage divider. You need the two 100r as already suggested and two 10r.
Wire the 100r as said in series from speaker to TRS socket.
wire the 10r from Headphone Hot to Headphone Ground on the back of the socket.

I'm still thinking how the TRS socket allows speaker switching and resistor ladder for headphone attenuation.
A normal TRS socket has 6 connections, one pair on each of T, R & S, not as shown in post7
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regards Andrew T.

Last edited by AndrewT; 12th January 2011 at 03:33 PM.
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