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Old 01-31-2013, 08:18 PM   #1
andyhatescrass
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circuit board on marshall cab

Alright, so all I know about the pcb board in marshall cabs is that they make it possible to wire speakers a certain way to get around series/parallel wiring. I've been doing some work on one of my speakers that has been out of my cab for a while. Today I screwed it into the baffle and switched on my amp, but forgot to connect that speaker to the crimp connectors that connect everything else. It took me about 20 minutes to realize that only 3 of my speakers were working and immediately powered off and connected that speaker. My amp seems fine, but it made me raise the question if I was running an odd impedance? I normally run my head at 16 ohms into the 16 ohms input on the back. I know that the pcb board makes it possible to connect speakers of any impedance, and be running the correct load. But did it correct the load for me or was the cab running a load other than 16 ohms?
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Old 01-31-2013, 08:40 PM   #2
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No it doesn't. All it does it change the way they are wired up. You still have to use the correct impedance speakers. It doesn't magically alter the impedance, it just allows you to switch between parallel or series/parallel connections.
And yes, you were running at the wrong impedance while the speaker was disconnected. It is unlikely to cause any damage but it is certainly sub-optimal.
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Old 01-31-2013, 09:31 PM   #3
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Originally Posted by Cathbard
No it doesn't. All it does it change the way they are wired up. You still have to use the correct impedance speakers. It doesn't magically alter the impedance, it just allows you to switch between parallel or series/parallel connections.
And yes, you were running at the wrong impedance while the speaker was disconnected. It is unlikely to cause any damage but it is certainly sub-optimal.


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Old 01-31-2013, 09:35 PM   #4
andyhatescrass
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Just out of curiosity, what would the impedance actually be with only 3 16 ohm speakers connected instead of 4 wired exactly like this? I don't know how to figure out impedance in the first place without looking at diagrams.



It was one of the bottom speakers that was not attached.

Cathbard, you posted it in a thread of mine a while ago, but here it is in case anyone else chimes in.

Last edited by andyhatescrass : 01-31-2013 at 09:37 PM.
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Old 01-31-2013, 09:47 PM   #5
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Quote:
Originally Posted by andyhatescrass
Just out of curiosity, what would the impedance actually be with only 3 16 ohm speakers connected instead of 4 wired exactly like this? I don't know how to figure out impedance in the first place without looking at diagrams.

It was one of the bottom speakers that was not attached.

Cathbard, you posted it in a thread of mine a while ago, but here it is in case anyone else chimes in.


Rtot = 16 + 8 = 24 ohms
if it is in series-parallel

1/Rtot = 1/16 + 1/16 + 1/16 = 3/16
Rtot = 16/3 = 5.3 ohms
if it is in parallel
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Old 01-31-2013, 10:07 PM   #6
andyhatescrass
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Originally Posted by gumbilicious
Rtot = 16 + 8 = 24 ohms
if it is in series-parallel

1/Rtot = 1/16 + 1/16 + 1/16 = 3/16
Rtot = 16/3 = 5.3 ohms
if it is in parallel


Thanks! Pretty sure what I have wired is a series-parallel wiring, so 16 into 24 ohms isnt horrible for a few minutes..... Glad I found that out, my mind is at peace.
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