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#1
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Isn't paralleling the outputs dangerous?
The amplifier has output transformers, so it shouldn't be. The Dynaco ST-70 was intended to be operated in parallel, and my RCA theatre amps have instructions to be operated in parallel. I can't really see what the problem would be so long as they're in phase. |
#2
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Bridging Guitar Amp
"Max Holubitsky" wrote in message ... Peter Mulvey wrote: Hi all, Anybody have any thoughts on the feasibility of bridging a 2 x 50 W tube power (guitar) amp? It's a Marshall 9005. Schematic of one side of the amp is available he http://www.drtube.com/schematics/marshall/9005.gif I would like to use this amp to drive a large cab, and 100W would be nice. I could strap the inputs together using the link terminals, but what to do with the OT outputs. Is it common practice to bridge tube amps, or is this just a plain bad idea. Many thanks for your thoughts....... If you link the inputs together, you can parallel the outputs. This will not be ideal, but it will work. the output impedance will be half of whatever taps you tie together - i.e. 8 ohm taps tied together gives a 4 ohm output impedance. If you want to bridge it, you'll have to invert one of the input signals. Really though, if it's a large cab, why not just connect 2 speakers in the cab to one 50W amp, and the other 2 speakers to the other amp, and then tie the inputs on the amp together? Pete. Isn't paralleling the outputs dangerous? Why not just run 2 or more speakers? If you have the inputs bridged then the speakers will be in phase and you will have effectivelly 100W. Too damn loud IMO, but then again I like 5W champs... |
#3
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Max Holubitsky wrote:
Isn't paralleling the outputs dangerous? The amplifier has output transformers, so it shouldn't be. The Dynaco ST-70 was intended to be operated in parallel, and my RCA theatre amps have instructions to be operated in parallel. I can't really see what the problem would be so long as they're in phase. Do not put amplifier outputs in parallel. This has nothing to do with O/P transformers. Any decent amplifier has a very low Thevenin equivalent generator output impedance, well below one ohm, likely around 0.1 ohm. BTW, this has nothing to do with the nominal 8 ohm speaker rating which is a measure of the ability of the amplifier to deliver current. Thus, if the individual outputs are in any way different the difference signal would see this load , i.e. the other amplifier, and a huge current would flow that could cause damage. Just being "in phase" is no guarantee of no difference signal, e.g. the gains could be different. The correct way to get double the power is to bridge them (see elsewhere) or much better, IMO, use two speakers - they can even be in the same box. Cheers, Roger -- Roger Jones, P.Eng. Thornhill, Ontario, Canada. "Friends don't let friends vote Liberal" |
#4
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Do not put amplifier outputs in parallel. This has nothing
to do with O/P transformers. Any decent amplifier has a very low Thevenin equivalent generator output impedance, well below one ohm, likely around 0.1 ohm. BTW, this has nothing to do with the nominal 8 ohm speaker rating which is a measure of the ability of the amplifier to deliver current. Thus, if the individual outputs are in any way different the difference signal would see this load , i.e. the other amplifier, and a huge current would flow that could cause damage. Just being "in phase" is no guarantee of no difference signal, e.g. the gains could be different. I find it hard to believe that any tube amplifier designed for guitar use will have such a low output impedance. I know what you are saying, and it is 100% true for a solid state amplifier, especially one with a large dampting factor, however, the amplifier in question will likely be just fine. I doubt that a tube guitar amplifier will have a better damping factor than, say, a Dynaco ST-70, and the Dynaco is intended to be paralleled this way. The correct way to get double the power is to bridge them (see elsewhere) or much better, IMO, use two speakers - they can even be in the same box. Agreed 100% Cheers, Roger -- Roger Jones, P.Eng. Thornhill, Ontario, Canada. "Friends don't let friends vote Liberal" |
#5
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Jimmy wrote: YEs, you can paralell them, I done it and it worked OK. However driving two separate speakers worked even better. Obviously you are wanting some serious volume. Just paralelling the amps will give you 3db more. At the levels we are talking about I doubt if you will here the difference between one amp and two. You Will hear a substanial difference if you feed each amp into their own speakers. Again, however, the increase is only 3 dB. You can't get sumthin' fer nuthin'. Cheers, Fred -- +--------------------------------------------+ | Music: http://www3.telus.net/dogstarmusic/ | | Projects: http://dogstar.dantimax.dk | +--------------------------------------------+ |
#6
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Well you can, sort of, The two amps with two speaker systems sounds
noticably louder,fuller than either the 1 amp and 1 speaker system or the 2 amps and one speaker sytem. I chalk the difference up to efficecy of the speakers. Guitar pluckers having been doing this forever. "Fred Nachbaur" wrote in message ... Jimmy wrote: YEs, you can paralell them, I done it and it worked OK. However driving two separate speakers worked even better. Obviously you are wanting some serious volume. Just paralelling the amps will give you 3db more. At the levels we are talking about I doubt if you will here the difference between one amp and two. You Will hear a substanial difference if you feed each amp into their own speakers. Again, however, the increase is only 3 dB. You can't get sumthin' fer nuthin'. Cheers, Fred -- +--------------------------------------------+ | Music: http://www3.telus.net/dogstarmusic/ | | Projects: http://dogstar.dantimax.dk | +--------------------------------------------+ |
#7
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Hi Fred,
You Will hear a substanial difference if you feed each amp into their own speakers. Again, however, the increase is only 3 dB. No it ain't :-) and the explanation for us tech oriented types is quite simple and has nothing to do with the electrical power, but with speaker efficiency: When you stack speakers (notably woofers) close enough together, fed with the same signal, power, and in-phase, they act (because of overlapping wave compression effects) as a single cone with a somewhat larger cone area and efficiency than two cones separated far enough. The drawback is higher directivity, but this might even add to the audible effect when you are in the "beam". You can ask any old fashioned PA systems guy why in those olde days of under-kilowatt-PAs the speaker cabinets were stacked like that the cones in the cabinets are located as near as possible to each other. Or, try to find some info about how "Schallzeilen" work; many comparatively little diameter speakers positioned nearly together in a vertical line - used e.g in churches, parliament halls, and so on. And D'Appolito design speakers get added efficiency this way, although the primary intention was another goal (single point wave source). You can't get sumthin' fer nuthin'. That's right. In the case under discussion the resulting higher speaker efficiency is gained by more distortion and more directivity. But the priciple works just fine for geetah and bass-ment stacks, and for stacking PA cabinets, and for "Schallzeilen". Back to the topic of this thread, I wholeheartily agree that two amps with two cabinets (or one multispeaker cabinet, if you can split the speakers in it to be fed by two amps of the same type) are _the_ way to go. An added plus is that no modding whatsoever of the gear itself is required. If you still don't believe, just take a classic geetah stack (consisting of bottom cabinet, upper cabinet and piggy-back amp) separate the cabinets at least 1 metre horizontally apart, plug in your favourite axe and compare the result to the verticaly oriented setup with cabinets stacked vertically. If you don't believe your ears, just use a sound preasure meter. You won't be disappointed about the added dBs of sound pressure :-) Tom -- Anyone who considers protocol unimportant has never dealt with a cat. - R. Heinlein |
#8
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Tom Schlangen wrote: Hi Fred, You Will hear a substanial difference if you feed each amp into their own speakers. Again, however, the increase is only 3 dB. No it ain't :-) and the explanation for us tech oriented types is quite simple and has nothing to do with the electrical power, but with speaker efficiency: When you stack speakers (notably woofers) close enough together, fed with the same signal, power, and in-phase, they act (because of overlapping wave compression effects) as a single cone with a somewhat larger cone area and efficiency than two cones separated far enough. The drawback is higher directivity, but this might even add to the audible effect when you are in the "beam". [...snip for brevity] Good info here! Thanks, Tom! Cheers, Fred -- +--------------------------------------------+ | Music: http://www3.telus.net/dogstarmusic/ | | Projects, Vacuum Tubes & other stuff: | | http://www.dogstar.dantimax.dk | +--------------------------------------------+ |
#9
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(snip) Roger wrote: Do not put amplifier outputs in parallel. This has nothing to do with O/P transformers. Any decent amplifier has a very low Thevenin equivalent generator output impedance, well below one ohm, likely around 0.1 ohm. BTW, this has nothing to do with the nominal 8 ohm speaker rating which is a measure of the ability of the amplifier to deliver current. Thus, if the individual outputs are in any way different the difference signal would see this load , i.e. the other amplifier, and a huge current would flow that could cause damage. Just being "in phase" is no guarantee of no difference signal, e.g. the gains could be different. Max Holubitsky wrote: I find it hard to believe that any tube amplifier designed for guitar use will have such a low output impedance. Roger said: It likely won't. It will be a rather crude PP design with no NFB and lots of distortion by Hi-Fi standards. The output impedance will be quite high. I was going on speculate on the value - how does over 50 ohms sound? Likely, you could put two in parallel with no damage - but why? Max: I know what you are saying, and it is 100% true for a solid state amplifier, especially one with a large damping factor, Roger: The same goes for good tube amplifiers. IIRC, damping factors of 20 are common so the Thevenin generator is a less than 1/2 ohm source. Max: however, the amplifier in question will likely be just fine. I doubt that a tube guitar amplifier will have a better damping factor than, say, a Dynaco ST-70, and the Dynaco is intended to be paralleled this way. Roger: I can't comment on the Dynaco. Does it have any NFB? Roger (before): The correct way to get double the power is to bridge them (see elsewhere) or much better, IMO, use two speakers - they can even be in the same box. Max (before): Agreed 100% Cheers, Roger -- Roger Jones, P.Eng. Thornhill, Ontario, Canada. "Friends don't let friends vote Liberal" |
#10
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I forgot to add. I you paralell the out puts you must put a 1 ohm several
watt resistor in series with each amp output or risk frying the amp. You also need some way of closely matching the gain setting s of each amp. In general it is a PITA and not suitable for any practical use. "Jimmy" wrote in message ... YEs, you can paralell them, I done it and it worked OK. However driving two separate speakers worked even better. Obviously you are wanting some serious volume. Just paralelling the amps will give you 3db more. At the levels we are talking about I doubt if you will here the difference between one amp and two. You Will hear a substanial difference if you feed each amp into their own speakers. "Peter Mulvey" wrote in message ... Hi all, Anybody have any thoughts on the feasibility of bridging a 2 x 50 W tube power (guitar) amp? It's a Marshall 9005. Schematic of one side of the amp is available he http://www.drtube.com/schematics/marshall/9005.gif I would like to use this amp to drive a large cab, and 100W would be nice. I could strap the inputs together using the link terminals, but what to do with the OT outputs. Is it common practice to bridge tube amps, or is this just a plain bad idea. Many thanks for your thoughts....... Pete. |
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