Hopefully simple impedance question

Discussion in 'Amps and Cabs' started by iMatt, Aug 19, 2008.

  1. iMatt

    iMatt Member

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    I have the line out from from my Univalve going into a TC effects unit and then to a Samson 120a solid state power amp. This has stereo out-puts with each output rated at 4ohm minimum each. I believe that with a solid state amp it is perfectly safe to connect higher impedance speakers.

    I have an 8ohm THD cab and would like to connect this to both of the Samson outputs at the same time - the outputs are binding posts so it should (I believe) just be a matter of conecting the cab speaker cable to one pair of posts and jump a short patch of speaker cable to the other pair of posts ( I use George L cable).

    Is this perfectly OK? I am assuming that each of the Samson outputs will "see" an 8ohm load and that will be fine but just wanted to check there isn't somthing else going on as a result of using the same 8ohm cab for both speaker channel outputs rather than a separate 8ohm cab connected to each channel.



    The reason I want to do this is so that I can run the output from a looper (Boss RC50) into the second input on the Samson (the Univalve via TC going into the other) so that the Samson effectively mixes the looper signal with the "direct" signal from the Univalve. I will have a Lehle A/B switcher between the TC and Samson/looper so I can send the effected signal either to he Samson via the looper or straight to the Samson bypassing the looper. In this way I will be able to record a loop but can then bypass the looper when not recording so it doesn't mess with the main guitar signal played over the loop.

    Gave me a headache dreaming this up so I hope it will work OK.
     
  2. Blue Strat

    Blue Strat Member

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    I don't think this is safe unless the Samson has a "bridged" mode available. What you're proposing is connecting one output to another which, in general, is a bad idea.

    "Perfectly OK" would be the furthest description from the truth as far as I can tell.
     
  3. iMatt

    iMatt Member

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    I kind of hoped you wouldn't say that.

    The Samson does have a bridge mono mode which is how I have been running it up till now. The line out from the Univalve goes to the TC and from there to the Samson and on to the THD cab. In bridge mono mode only one of the Samson inputs works so I can't use it to "mix" in the signal from the looper (the Samson has separate output volume controls for each channel which would be useful if I could use it in the way I was hoping).

    This doesn't mean game over. I think what I will have to do if your advice is correct (which it no doubt is) is mix the looper and direct signals before going into the one "bridged" channel of the Samson. I want to avoid using a mixer or the like to try and keep things simple so guess a "Y" cable would do the job - although those I have seen seem to be low end.

    So I'm kinda hoping you are being over cautious and I can still do what I want - but I don't hold out much hope because I know very well that you know exactly what you are talking about (incidentally I currently have a nice Brimar 6V6 in the Univalve that I bought from you a while back - haven't been back to you simply because I really got nailed for import duty when it came into the UK).
     
  4. Blue Strat

    Blue Strat Member

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    Hate that import duty. Gotta pay for the Matriarchal overhead though;)

    It might be ok, but I can't give it my "blessing" in good conscience.

    Since there are obviously inputs for each channel, can't you use both inputs as your "Y" cable?
     
  5. iMatt

    iMatt Member

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    Using both inputs as the Y cable was plan A but it turns out only one input is active when in bridge mono mode. This was a big dissapointment which sent me back to the drawing board and my hair brained scheme of connecting the cab to both outputs.

    If I were to try it and it all goes horrible wrong I assume it is the Samson that would go "pop"? The Samson is relatively inexpensive and whilst I really wouldn't want to willfully fry the thing, if the potential upside is my ideal (for now) set-up I might decide to take the gamble as you have given me some glimmer of hope that it might pay (and you get to say you told me so if it blows).
     
  6. rickenbackerkid

    rickenbackerkid Member

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    Is it a stereo cab? Why don't you plug the univalve into the left speaker. (with your TC in the effects loop)

    And then go -->

    Unvalve line out --> Looper --> Sampson amp --> Right speaker.

    That would be much safer and do the same thing.

    Ben
     
  7. Plague Dog

    Plague Dog Member

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    To bridge the amp you don't connect both outputs in parallel as your suggesting in your post above.

    The speaker cabinet should be connected on the hot leads of both channels and the negative leads would be jumpered out.
    Sometimes they will place the binding posts equal distance from the red to the black and from red to red and from black to black so you can plug you banana jacks in bridged. Normally there will be a bridge switch that connects the two black posts and then you plug your banana jack accross the two reds.
    If you're unsure on how to bridge a stereo amplifier I wouldn't do it.
     
  8. Blue Strat

    Blue Strat Member

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    I don't think this addresses his need to mix two signals.
     
  9. iMatt

    iMatt Member

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    Just picked this up again this morning - different time zone to some you guys. Whilst PlagueDog is totally correct on the "how to bridge" issue - as Mike says that is not my problem in this particular instance. Cutting through all the extraneous detail in my original post, it comes down to having a solid state stereo power amp which has separate input level controls for each channel and wanting to use it to mix two signals. Folllowing the manufactures instructions there is no problem at all doing this if I connect separate speakers to each channel (4ohm minimum in each case) - the question is can I safely connect the same 8ohm cab to both sets of speaker outputs and as you can see Mike has cautioned against trying that without accepting some risk of damaging the power amp. (I only need to consider this because unfortunately with the amp switched to bridge mode only on of the two inputs remains active.)

    Unfortunately Bens perfectly sensible suggestion is not open to me without rewiring the cab because the cab is not stereo - and to be honest I am not sure I would want to rewire it that way as it has two different speakers which add to the flavour and I would miss that (i imagine).

    JUST EDITING THIS AS I CAN NOW STOP WASTING EVERBODY'S TIME - I HAVE JUST GOT THIS FROM SAMSON SERVICE:

    "That will absolutely NOT work. You will destroy your amp in that fashion."


    So thank you Mike for stopping me in my tracks and to others for chipping in with suggestions.


    I may possibly give some consideration to rewireing the cab in stereo - but most likey I will just get the Y cable (or pehaps a decent switcher with A+B option).


    Matt
     
    Last edited: Aug 20, 2008

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