"summing" the output of a combo amp to run a cab

[amra]

I am wondering if you can sum the output of a combo amp, in sort of an external "bridged" mode with some kind of external device to run a single cab?

Can it be done?

[Zaphod B]

Would this be for the purpose of a power increase, or blending the characteristics of two amps?

[Zaphod B]

But for the record I'll say that I've never heard of anyone doing it. I don't know if it can be done or not.

If you really want to run two amps into one cab, you could just rewire the cab so that it has two inputs, and dedicate different speakers within the cab to different amps.

[ggunn]

Originally Posted by amra

I am wondering if you can sum the output of a combo amp, in sort of an external "bridged" mode with some kind of external device to run a single cab?

Can it be done?

Nope, sorry. You never want the outputs of two amplifiers to merge. As Zaphod sez, you could wire up a cab so that the separate amps drive separate speakers, but driving the same speaker with two amps will damage both amps and possibly the speaker as well.

[amra]

Sorry guys, I should have been more clear....

The combo amp is 2x35 watts, and I want to combine it's two channels (like a bridge switch would do) to run one cab, instead of the two speakers in it's own cabinet....

[Zaphod B]

Not unless it has a bridging function, or a channel blend.

Your combo amp has two discrete 35W power sections?

[timthetortoise]

Originally Posted by Zaphod B

Not unless it has a bridging function, or a channel blend.

Your combo amp has two discrete 35W power sections?

Not too odd. If it has modelling capabilities or an effects loop it's probably stereo.

Also, I'm not saying this would work, but why wouldn't it? I mean, as long as polarities are matched and impedances are paralleled, the output transformer really wouldn't know the difference, would it?

[Zaphod B]

Originally Posted by timthetortoise

Not too odd. If it has modelling capabilities or an effects loop it's probably stereo.

Also, I'm not saying this would work, but why wouldn't it? I mean, as long as polarities are matched and impedances are paralleled, the output transformer really wouldn't know the difference, would it?

You go first and let us know how it goes.

[amra]

Thanks for the replies.
Yea, I don't know about the 2 discrete power sections, it does have an effects loop, though. It is an '86 Randall RC-235 with stereo chorus. Pretty nice sounding for a solid state. I am just wishing it was a 70 watt mono, instead of 2x35 watts...

[BrentDomann]

If you can get a stereo cab, that would probably be ideal. Same overall output and number of speakers, and you have both a cab that was designed for your stereo output and one that won't fry your amp.

[timthetortoise]

Originally Posted by Zaphod B

You go first and let us know how it goes.

If I had a combo try it with, I probably would. But the question remains, why wouldn't this work?

[Zaphod B]

Originally Posted by timthetortoise

If I had a combo try it with, I probably would. But the question remains, why wouldn't this work?

The main reason I can think of is that the output of the amp is looking for an impedance of 4, 8, or 16 ohms. I would imagine that the impedance of the output section itself, viewed as a load of another output section, would not be anywhere in that range, and so you would be paralleling the impedance of two output stages with a speaker. What's the resulting impedance as seen by either output section? Whatever it is, it isn't going to look anything like a speaker to the output section and if it's really low it's going to fry them.

[mixsit]

Sounds like a) We know it's possible to design for 'brodged vs dual and b) you'd be guessing untill you know how.

[Zaphod B]

OK, I had to do a little research to re-familiarize myself with amp output impedances, etc.

The output impedance of amplifiers has to be very low in order to reduce the voltage induced by speaker cone motion. One rule of thumb I'm reading is that the amp's output impedance should be about 1/10 of the speaker load. So for an amp optimized for 8-ohm loads, its output impedance would be less than 1 ohm.

If you wire the outputs of two of those amps together they are going to see a < 1-ohm load and they will likely fry from overcurrent unless they are built like a fucking tank.

That's the way it looks to me, anyway. Maybe someone who works with this stuff more frequently can shed some light.

[dave.....]

years ago i wired 2 amps to one speaker .... bad idea .......... both amps up in smoke the instant i turned on the second amp .......

[amra]

Ok, so you can't just hook them up together. I pretty much figured that would be a bad idea.

I am wondering though, if you could replicate how a mono bridge switch works, but with external components. I mean, it seems to me based on what Zaphod says, that the bridge switch must somehow handle the low impedances somehow when combining two seperate channels. Does anyone know how that works? If you could build an external box that did the same thing, it would be pretty sweet.....

[Zaphod B]

amra, I wouldn't think so. A bridging swich has got to effect the signal path in several places within the amp, and I just can't see how an external box could do that.

[ggunn]

Originally Posted by amra

Ok, so you can't just hook them up together. I pretty much figured that would be a bad idea.

I am wondering though, if you could replicate how a mono bridge switch works, but with external components. I mean, it seems to me based on what Zaphod says, that the bridge switch must somehow handle the low impedances somehow when combining two seperate channels. Does anyone know how that works? If you could build an external box that did the same thing, it would be pretty sweet.....

That's a different animal, What bridging does is send a signal to one power amp and its exact inverse to another identical power amp, and then runs the speakers on the positive terminals of the two (reverse polarity) amps, letting the negative terminals float (or ties them together, I'm not sure). It generates a waveform with twice the voltage swing of either power amp alone. It depends on the amps being identical and the input signal being identical but polarity flipped.

It's typically done on solid state PA amps whose outputs are not transformer coupled; I don't know if a transformer coupled output could even do this at all, even if the other conditions could be met, and it's certainly not done to somehow blend the tonal characteristics of two different amplifiers. The upper half of the waveform would be different from the lower, at the very least, and I don't think you'd find that pleasing.

[Zaphod B]

Good stuff, ggunn.

[timthetortoise]

Yeah, I understand impedance limitations. But let's say you've got 16 ohm outputs with an OT that can handle 4, 8, or 16 ohms, why wouldn't you be able to wire parallel for an 8 ohm load? Does the impedance on the OT depend solely on which set of taps you're using? If so, then I understand why you wouldn't be able to do it.

[Zaphod B]

No, the output impedance of the amplifier is much lower than the speaker load - it's going to be 1/10 or less of the speaker load. So if you just hook up the outputs of two amps, rather than the amps "seeing" a 4, or, 8, or 16-ohm load, they're going to be trying to drive a load that is < 1 ohm.

[ggunn]

Originally Posted by Zaphod B

No, the output impedance of the amplifier is much lower than the speaker load - it's going to be 1/10 or less of the speaker load. So if you just hook up the outputs of two amps, rather than the amps "seeing" a 4, or, 8, or 16-ohm load, they're going to be trying to drive a load that is < 1 ohm.

And that's not to mention the question of what the external voltage swings applied to the secondaries of the output transformers are going to look like at the terminals of the primaries, but hey, if you're determined to find out for yourself why it won't work, go ahead on and try it. I wouldn't try it with any amps that you care about, though. Have a fire extinguisher handy, too.

Experience holds a dear school... ;^)

[amra]

You guys are obviously alot smarter than me when it comes to this, so I will gladly take your word for it. I'll file it under "pipe dream" and not waste anymore thought on it.

[NRS]

I too can attest....absolutely dont do it! I tried that on my first guitar amps which was a small Crate 40 watt stereo combo and immediately heard a loud pop followed by smoke. Fortuneatly it only managed to kill one channel before I shut it down so I was still left with a 2x8 mono 20 watter but am at least glad the lesson got learned early on a cheap piece gear than later on a more expensive one.

Just rewire your cab for stereo so each speaker gets its own juice. Even if you could combine the channels I dont think 100watts mono into 2 12" speakers is going to give you any better results than seperately powering them stereo at 50 watts each. The only reason I can see you'd even want to do that is if you had only one single speaker to power between the two amp sides, but even in that case your better off just getting an extension cab instead because 2 12" speakers at 50 watts a piece will definately be louder than 100watts pushing a single speaker.