Amplification conundrum

Discussion in 'Amps, Mics & Pickups [DB]' started by Daniel Baskin, Feb 21, 2014.


  1. Daniel Baskin

    Daniel Baskin

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    I tried doing a search on this, but if you know of a past thread that covered a similar issue, let me know...

    I use an Aguilar GS 112 and an Eden Nemesis 320 watt head. I've used this amp setup for about eight years and it has generally worked really well for me if I only use it for an hour or so at a time, and is only loud enough if I crank the volume and set the gain at half capacity--but it is excellent-sounding under those constraints.

    I played a 4.5-hour background jazz gig at a downtown restaurant/bar last night. I'd played this gig before, and usually my setup does alright, though my sound tends to lack definition and clarity. Well, last night I decided to crank the treble and and mids AND put the gain at 2/3 to 3/4 capacity (with volume cranked). Being fed the signal from mixing my magnetic and piezo pickups, I had a blast with the sound I was getting.

    However, the amp completely pooped out about 40 minutes from the end of the gig. In the past, I've had to crank the amp more and more throughout the gig to maintain the same volume, but I always attributed it to fatigue / failing to pull the strings as strongly as at the beginning of each gig.

    A final note: Sometime during my first year of owning the amp, it fell off of a tall cabinet from being shaken by sound vibration during a loud rock gig. I got it repaired soon after, and it seemed to work just fine, but I don't know if this is what has been causing it's issues of pooping out and needing to be continually cranked higher.

    My question is, do I need a new amp? A new speaker? Both?

    If I *do* need a new amp, does it need to be a stronger wattage than my current one, or do I just need one that hasn't been beaten up (or both)? Is my speaker--which uses a 12" cone--paired with me cranking my mids and highs--which are better produced using smaller diameter cones--causing my amp to overwork?

    Thanks!
     
  2. Champagne

    Champagne

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    Is that a tube amp? Failing power tubes or a bad power supply will exhibit that problem.
     
  3. Daniel Baskin

    Daniel Baskin

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    No, it's not a tube amp; but I do think it's a power supply problem as you say.
     
  4. DukeLeJeune

    DukeLeJeune Only immortal for a limited time Gold Supporting Member

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    One of the dirty little secrets that we speaker manufacturers don't like to admit is, our cabs are subject to thermal compression.

    Basically as the voice coil heat up its resistance rises, reducing its sensitivity, and in turn over time the voice coil heats up the magnet, reducing its strength (flux). The net result is, your cab may actually lose 3 dB or more over the course of the evening. So you keep turning up the amp to compensate, which in turn heats up the voice coil and magnet even hotter, not to mention making your amp work that much harder (you'd need to double the wattage output to make up for a 3 dB loss in sensitivity). It's a viscous cycle, and eventually something fails. In this case, it sounds like your amp overheated before your voice coil was fried.

    In general, large diameter voice coils and heavy magnets resist heating better than small voice coils and lightweight magnets, but imo what you really need on the speaker side is, more speakers.

    You see, most of the overheating occurs in those last few decibels. So suppose your cab was never asked to do those last few decibels. Instead, suppose you added a second identical cab, so that now each cab will be seeing only half as much power to achieve the same SPL. Thermal compression is greatly reduced, and your equipment stands a much better chance of making it through the gig. You might have 1 dB of thermal compression at the end of a long evening instead of 3 dB.

    On the other hand if you double up on amplifier power, your cab will exhibit just as much thermal compression as before at the end of the night, except that now you'll be able to deliver more power to offset it. At some point the voice coil will get hot enough to melt the glue that binds it to the voice coil former. You might make it through the gig and you might not, but now you have a "blown" woofer because the voice coil will be scratchy.

    If you don't want to haul two cabs to your next 4.5 hour gig, find a single cab solution that has a significantly better combination of efficiency and power handling than the cab you're using now. What you have now is rated at 95 dB and 300 watts, and two such cabs would give you 98 dB efficiency and 600 watts power handling.
     
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  6. Daniel Baskin

    Daniel Baskin

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    Wow! That was *incredibly* helpful! 6/5 stars.

    I am willing to haul a 2nd cab to the gig--and this may be a stupid question--does the cab have to be identical, or just have an ohm rating that makes it work out in the way it's wired to not overwork the amp?

    Thanks soooo much!
     
  7. robobass

    robobass

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    The amp might also be overheating. Open up the amplifier and clean out eight years of dust and grunge. Also, play it at home with the cover off and see if anything in there is heating up excessively. An example would be a power transistor which no longer makes good contact to a heat sink.

    Moronic safety warning, which some moron will chime in with if I don't: Unplug of course before you stick your fingers in there.
     
  8. DukeLeJeune

    DukeLeJeune Only immortal for a limited time Gold Supporting Member

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    Thank you very much. 6/5 stars is a first for me.

    Doesn't have to be an identical cab, but imo you don't want one whose power handling is much less than the one you already have. Assuming you like the tone you're getting, the standard recommendation would of course be to get an identical cab because then you largely preserve that tone (though it will get a bit warmer), and neither cab is the limiting factor relative to the other one.
     
  9. robobass

    robobass

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    Yeah Duke! That was an excellent rundown on thermodynamics relating to speaker drivers. Never gave it much thought before.
     
  10. Mike Arnopol

    Mike Arnopol Supporting Member

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    Until I figured that out I was wondering why all of my amps were sounding crappy on the third set!

    We tend to base our speakers and amps around the max volume we need. Now I use speakers and amps that give me around 3db more than my max perceived volume.
     
  11. robobass

    robobass

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    In our quest for ever smaller and lighter rigs it seems we are increasingly bumping up against physics, just like the CPU designers. Best to try out any amp or cab in a long, loud rehearsal before laying down hard cash for it.
     
  12. DukeLeJeune

    DukeLeJeune Only immortal for a limited time Gold Supporting Member

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    Thank you.

    Actually I think that we have both short-term and long-term thermal effects. The latter are well documented, and result in the loss of sensitivity over a fairly long term with magnetic flux loss playing a major role as I've described above.

    There may well be a significant short-term "thermal modulation" going on, which audibly affects only the voice coil.

    The heating up of the voice coil from a loud peak is virtually instantaneous, like touching a several-hundred-watt soldering iron to solder. The cooling off of the voice coil takes longer. So we have this asymmetric cycle, with a significant part of the cooling-off taking place as the heat is "sinked" into the magnet.

    The effect of this short-term thermal modulation is, a softening of the peaks on loud transients as the voice coil's resistance rises. The solution is, large-diameter voice coils and/or multiple voice coils (multiple woofers), and high enough efficiency that high power levels aren't needed.

    Imo in the high-end home audio world (where my background is), thermal modulation is an unrecognized epidemic. Speaker efficiency is generally low because good bass extension in a small cabinet is the priority, but thermal power handling is usually only adequate to provide a margin of safety against failure, while thermal modulation sets in long before the voice coil gets hot enough to soften the glue. As a result, many high-end home audio speakers sound smooth but emotionally dead because the dynamic contrast is reduced (mainly because the peaks are softened).

    As a general rule of thumb, I like for the drivers in my home audio speakers to be able to deliver the peaks at only 10% of their rated RMS (not "peak", not even "music program") power handling. On paper it looks like overkill, but imo this helps restore liveliness.

    So back to bass cabs... unfortunately for guys like me who build lightweight, high power handling, modest efficiency "ubercabs", each of those choices work against us thermally. Those big heavy 410 cabs probably lose less output to thermal compression over the course of the evening... well, until you get that second lightweight uber cab!

    One more thing: Tube amps do a better job of maintaining the same wattage output into a higher impedance load, while solid state amps (which behave more like a true constant-voltage source) decrease their wattage output as the voice coil impedance is thermally raised. So there's a valid basis for claims that tubes sound more lively, whether we're talking home audio or prosound.
     
  13. fnordlyone

    fnordlyone Supporting Member

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    Duke kicks facts like a millipede :bassist:
     
  14. tappingtrance

    tappingtrance Cooke Harvey Gold Supporting Member

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    I always thought there was something like that going on throughout the night in addition to the "ears giving up or getting used to the loudness (?Fletcher-Munson?)-

    To the power point I've used the GS 112's for years - love them, you need more power (rule of thumb - please you geektards don't jump on me) 2x the power handling capacity of the cabinet for a clean, powerful, peak handling sound) my thought is when you said 320 watts, that is too little IMO.
     

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