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Author Topic: JC 120 speaker question  (Read 276 times)

Psc

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JC 120 speaker question
« on: March 11, 2020, 10:15:50 AM »

Hi all.  Quick question, hopefully someone can show me what I'm missing.  I understand that the speakers used are 8 ohm.  The schematic shows that when you connect an external speaker you should also use 8 ohm, but it shows them connected in parallel, which would make it 4 ohms.  I would have thought 4 ohms was too low.

https://www.ssguitar.com/index.php?action=dlattach;topic=4786.0;attach=8708

Thanks!

joecool85

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Re: JC 120 speaker question
« Reply #1 on: March 11, 2020, 10:53:32 AM »

Hi all.  Quick question, hopefully someone can show me what I'm missing.  I understand that the speakers used are 8 ohm.  The schematic shows that when you connect an external speaker you should also use 8 ohm, but it shows them connected in parallel, which would make it 4 ohms.  I would have thought 4 ohms was too low.

https://www.ssguitar.com/index.php?action=dlattach;topic=4786.0;attach=8708

Thanks!

I believe it is stable for 4 ohms.  The reason they use 8 ohm speakers internally is so that you can then connect the external speakers without dipping below 4 ohm total.  Also, circuits output more wattage at lower impedance.  So when running external speakers along with the internal, when set up as parallel and the amp sees 4 ohms total, it is a boosted output.  If you had internal and external 4 ohm speakers and hooked in series, it would work, but you would have the amp seeing 8 ohm and have a lower total wattage.
« Last Edit: March 11, 2020, 10:55:01 AM by joecool85 »
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Psc

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Re: JC 120 speaker question
« Reply #2 on: March 11, 2020, 01:15:03 PM »

Ok, that makes sense.  I have another stereo amp that needs a 4 ohm load.  So I could build one 2-12 speaker cabinet with 4 ohm speakers and turn both amps into heads?  That might simplify things for me.

joecool85

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Re: JC 120 speaker question
« Reply #3 on: March 11, 2020, 02:07:06 PM »

Ok, that makes sense.  I have another stereo amp that needs a 4 ohm load.  So I could build one 2-12 speaker cabinet with 4 ohm speakers and turn both amps into heads?  That might simplify things for me.

Sounds like that would work for you.
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g1

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Re: JC 120 speaker question
« Reply #4 on: March 11, 2020, 02:22:30 PM »
The schematic linked shows parallel arrangement, but you must be sure that is the correct schematic for your unit.
If I remember correctly, there are versions of the JC120 that put the ext. spkr. in series.  Those units would not support 4 ohm operation.

Psc

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Re: JC 120 speaker question
« Reply #5 on: March 11, 2020, 11:43:34 PM »
I'll definitely double check.  It looks correct so far.  The amp was modified, so I'm going to have to go through it pretty extensively.  I'm not sure what the guy was up to.  He added two inputs to the back, and three relays.  He also put a small circuit board in there, and a transformer.  The end result is that none of the effects work, so I'm going to try and put it back to stock.  I don't think he changed much on the boards, mostly the connections between the boards.  The schematic I posted matches the serial number of my amp.

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Re: JC 120 speaker question
« Reply #6 on: March 12, 2020, 01:38:59 PM »
  The schematic I posted matches the serial number of my amp.
You should be good then.  Just wanted to verify that.