Mackie 808S mixer/amp question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by strat68, Sep 11, 2008.

  1. strat68

    strat68 Member

    Messages:
    761
    Joined:
    Jan 25, 2006
    Hi all,

    My band has a gig where the club provides a stereo power amp and main speakers hung from the ceiling with subs on the floor. They provide us two 1/4 cables that would go to the mixer line outs on a combo mixer like the 808s.

    We want to use the mackie's internal amp to drive the monitors. I have the manual where they show some sample scenarios, but I don't recall it having what I described above. (Just checked pdf's- close, but they don't).
    http://www.mackie.com/pdf/ppmseries_hu.pdf
    http://www.mackie.com/pdf/ppm_om.pdf

    So what we did at our first show there (first attempt) was:

    Connect the front end via mixer line out to the power amp. Then one monitor cable to each left and right side speaker. I was hoping the power amp routing in front/stereo mode would help there, but but we could not separate the mix.

    Next and final attempt, we ended up putting the PA routing in main/monitor mode. Plug the two monitor cables Right/Monitor speaker outs only and had the left side unplugged. This worked and we had normal mixing of mains and monitors.

    My concern is... I'm the guitar player, and as us tube amp freaks know, that's a huge no-no in the tube world... a power amp with no impedance load at all. I know it is not so disasterous with solid state amps, but I want to be sure. Does the 808s have a dummy load when the speakers are unplugged? Say for example if you had to use the board as a mixer only with no speakers, to an external amp AND monitor amp.

    Or- is there a better configuration we can use at this club. (Which is a pretty sweet deal, BTW- subs and tops, and it's a restaurant, not a concert venue, with a small dance floor, occupancy maybe 150?)

    Thanks,
    Steve
     
  2. Tony474

    Tony474 Member

    Messages:
    307
    Joined:
    Mar 31, 2008
    Location:
    North Bushey, near London, England
    With the front end mix being fed to an external power amp via the line out sockets, you have two ways of connecting passive monitor speakers to be driven by the internal power amps:

    1. Set the switch to left and right mains and drive one speaker or set/pair of speakers from each channel. The internal amps are supposed to go down to 2 ohms each but I would play safe and keep the load on each side 4 ohms or greater. This way the monitor mix will be exactly the same as the front-of-house mix and the upper (monitor) graphic EQ will have no effect. Neither will any of the other monitor controls such as effects level, etc.

    2. Do what you've already done by switching to left mains/right monitors and using just the right power amp for all the monitor speakers. Again, make sure the speaker load doesn't go too low. This way you can control the monitor mix independently from the FOH mix and all the monitor controls will work. You shouldn't have a problem with nothing plugged into the left output - this is a transistor amp after all - but if you're really worried you could always connect another speaker to it and point it somewhere that won't cause feedback or any other problem.

    Following on from this, I've done exactly what you describe with my 808S - used it as a mixer with no speakers connected - and no ill-effects resulted during a 90-minute set.
     
  3. strat68

    strat68 Member

    Messages:
    761
    Joined:
    Jan 25, 2006
    Thanks Tony, I believe our first attempt was your 1. recommendation. Problem we had was that the fixed mix didn't work well. So we did what you said in 2. and will do so again tonight. I guess we will be ok, but I should probably send this link to Mackie to see if and how the system is protected.

    I've always read that it's "not as bad" to have no load on a solid state amp, but don't recall ever reading that it's a non-issue.

    Thanks,
    Steve
     
  4. Tony474

    Tony474 Member

    Messages:
    307
    Joined:
    Mar 31, 2008
    Location:
    North Bushey, near London, England
    My technical knowledge is a bit limited but it seems to me that operating a tube amp with no load is bad because of the output transformer - the primary winding will induce a current in the secondary and if it has nowhere to go the coil will overheat. On the other hand if a transistor amp's output is open-circuit, theoretically the impedance is infinite and little or no harm should result.

    Of course, I could be talking through my posterior orifice (nothing new there, then) and I'm happy to be corrected by those with superior knowledge. All the same, as I said, I've operated the 808S like that a couple of times and no problems arose either at the time or later when it was back to conventional use. The heat-sink on the back got a bit warm but nowhere near as hot as it sometimes gets when regular speakers are connected in the normal way.
     
  5. zydeco papa

    zydeco papa Member

    Messages:
    50
    Joined:
    Sep 10, 2008
    Location:
    Buffalo-ish
    It won't hurt anything to do that. It's common practice and a non-issue.
     

Share This Page