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Topic: How to wire an external speaker jack? |
Tommy Boswell
From:
Virginia, USA
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Posted 9 Apr 2011 7:03 am |
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I want to add an external speaker jack to my combo amp that will cut the signal to the internal speaker when the jack is in use. I'd like to use a Switchcraft shorting jack. Can someone verify this wiring scheme for me?
amp pos+ wire --> jack tip --> jack short --> internal speaker pos+
amp neg- wire --> jack ring --> internal speaker neg-
When a plug is inserted, the connection to the internal speaker positive terminal is broken. My main question is about the negative connection, which is not broken. Is that OK? For the internal speaker to share a negative connection with the external speaker? |
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Roger Kelly
From:
Bristol,Tennessee
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Posted 9 Apr 2011 8:52 am
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Yes it is O.K. as you see here....
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Tommy Boswell
From:
Virginia, USA
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Posted 10 Apr 2011 9:07 am
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Thanks for the diagram.
Is there a place on the forum where stuff like this is available? I see "SteelGuitarForum.com" at the bottom of the diagram. |
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Roger Kelly
From:
Bristol,Tennessee
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Posted 10 Apr 2011 9:26 am
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| Tommy, I'm sure that there are topics that has covered this subject somewhere in the archives of the Forum. You might do a search on this subject and see. Anything that is uploaded to the Forum like pictures or diagrams will have Steel Guitar Forum.com on it. |
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Dave Grafe
From:
Hudson River Valley NY
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Posted 10 Apr 2011 10:32 am
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| FYI, you will want to have your amp turned off when you plug and unplug this jack... |
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Glenn Uhler
From:
Trenton, New Jersey, USA
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Posted 10 Apr 2011 10:55 am External speaker jack protection
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Dave,
You are correct and that's what should be done. But, what happens if someone unplugs the speaker without turning off the amp. We will have to ask Ken Fox for sure, but I think that with no load, the voltage on the output transistors will spike high enough to possibly blow the transistors.
You might be able to prevent this by installing a power resistor (say 200 ohm and 20 watt) across the pos and neg terminals to prevent the voltage spike. With no speaker, the output sees the big resistor. When a speaker is plugged in, the transistors hardly see the big resistor and nearly all the power goes to the speeaker.
_________________
1974 Marlen S-12 1968 Tele 1969 Martin D-35H |
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Ken Fox
From:
Nashville GA USA
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Posted 11 Apr 2011 1:08 pm
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A direct coupled transistor amplifier output can tolerate an open circuit but will fail if it see a shorted circuit.
A tube amp will not tolerate an open circuit on the secondary of the output transformer, this can result in damage to the primary windings when a signal is applied. That is why the classic tube amps used a short to ground via the contact on the jack when the speaker was unplugged. That dead short would protect the output transformer. |
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Glenn Uhler
From:
Trenton, New Jersey, USA
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Posted 11 Apr 2011 2:52 pm Protect you amp!
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Advice from the expert!
Thanks, Ken!
_________________
1974 Marlen S-12 1968 Tele 1969 Martin D-35H |
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Ken Fox
From:
Nashville GA USA
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Posted 11 Apr 2011 3:04 pm
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"Is there a place on the forum where stuff like this is available? I see "SteelGuitarForum.com" at the bottom of the diagram."
No, actually that is inserted by the Forum on any photo or drawing uploaded. |
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