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Can a head with stereo outputs be summed to mono?

dB

Silver Supporting Member
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4,214
Suppose you had a head with 4 stereo outs, 2 left and 2 right. Most of the time you just ran 2 extension cabs, 1 left and 1 right. Occasionally you would like to run an additional cab, say a 1x15, in mono...almost as a center channel type speaker. Is it possible to somehow use the extra two stereo outs to feed one cab a mono signal?
 

Kyle B

Silver Supporting Member
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5,109
How easy this is depends on the nature of the amp. Does it have four discrete power amplifiers, or is it two amps with two parallel outs on each? Is the word "bridge" anywhere on the amp or the manual?
 

donnyjaguar

Member
Messages
4,194
Once again, KyleB is on the right track. If your amplifier doesn't have provisions for bridging, I wouldn't parallel the two outputs into one speaker if I were you. If you do, it'll create more heat than light, or in this case more heat than sound.
 

drbob1

Gold Supporting Member
Messages
29,058
Most guitar amps aren't advanced enough to have a Bridge mode, and if they did, it would probably bridge the whole thing not just the paralleled outputs, so you'd lose stereo imaging. The best solution would be to built a line out for each side (or use the line out if it has one), sum the line levels together and run that into a separate amp with the 15" speaker. You could use something as simple as the EHX Magnum44 and a 15" speaker cab. If the idea is to have the 15" do duty as a subwoofer, the line out idea is even better: line out to crossover with low end to power amp to 15".
 

Bruce Gerard

Member
Messages
136
Is it a tube or SS amp? I'm guessing SS. Above advice is good, and unless it is known that the amp is bridgeable - why take the chance? Also doing this can cause your amp to oscillate & it will pop very quickly - doing severe damage. I fixed an amp once where the guy bridged the L&R channels at the speaker outs. Amp wasn't designed for that & it cast him a big repair bill.
 

walterw

Platinum Supporting Member
Messages
39,238
Suppose you had a head with 4 stereo outs, 2 left and 2 right. Most of the time you just ran 2 extension cabs, 1 left and 1 right. Occasionally you would like to run an additional cab, say a 1x15, in mono...almost as a center channel type speaker. Is it possible to somehow use the extra two stereo outs to feed one cab a mono signal?
not a chance.

speaker outputs can't be "summed", period. (bridging is something very different.)
 

donnyjaguar

Member
Messages
4,194
Actually, you can sum L & R channels in a tube stereo amp to bridge it. Check out a Dynaco ST70 for the schematic. However, the OP is talking solid state and that's a different story.
 

drbob1

Gold Supporting Member
Messages
29,058
At the risk of blowing things up… Some amps actually use jumping to accomplish bridging, sort of. The Stewart PA50/100/200B series you push a button and use the hot from one side and the ground from the other in bridge mode…
 

slybird

Member
Messages
6,463
I have a Alesis RA-300 that has a bridge switch. My Mesa Boogie 2:Fifty will not bridge. But I have to ask why? A 100 watt amp really is not much louder than a 50 watt amp.
 

walterw

Platinum Supporting Member
Messages
39,238
Actually, you can sum L & R channels in a tube stereo amp to bridge it.
which is it? my lay definition of "summing" is the two outputs paralleled right together, like with a Y-cable.

bridging inverts one output and uses the two outs as the + and -, with the speaker load always between them.
 

donnyjaguar

Member
Messages
4,194
To confuse matters further, some higher powered amplifier are in reality two amplifiers bridged all the time. I think most every car stereo deck falls into this category. I know from my car-audio adventures many guys assume that the - lead of each speaker is common with ground. They ain't! :)
 

walterw

Platinum Supporting Member
Messages
39,238
oh yeah, many modern high-powered class D micro bass amps are like that; if it's the size of a dictionary, cranks out 900w, and won't do 2Ω, it's likely two power amps permanently bridged.

+1 also to the output ground thing, i've read where folks use speaker-level DIs on these and Bad Things Happen between pin 1 of the DI and the mixer channel it's connected to.
 






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