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Thoughts on lightweight amps

Discussion in 'Amps and Cabs [BG]' started by dincz, Jan 29, 2014.

  1. dincz


    Sep 25, 2010
    Czech Republic
    Just some idle thoughts about classD/SMPS. Would appreciate some informed input from those with relevant knowledge and experience.

    I was a believer but after trying various power amps I now have my doubts - not so much about class D - but about the switch-mode power supplies that (usually) feed them. I think it's all about having the headroom (and the power supply is part of this) to deal with the initial attack of a note. Class D/SMPS can handle it provided the amp has available clean power way in excess of what you might think you need - that is, lots of headroom.

    It's significant that most amps are no longer rated for continuous power but for burst power, i.e. power that can be maintained for only a few tens of milliseconds. In fact, it seems from tests I've read that most SMPS powered amps will shut themselves down if you try to get their maximum rated power for several seconds continuously.

    If you want "oomph", which I take to mean the clean power to deal with attack , then stick with iron or choose a class D/SMPS amp with several times the power rating of a conventional amp that would do the job. That's where multiple kilowatts of Powersoft amp comes in handy. On the other hand, there's no reason at all why a SMPS couldn't be engineered for the purpose if some of the size/weight advantages were to be sacrificed.

    I'm not a design engineer so this is more gut feeling than anything else, although I was a maintenance tech in broadcasting for 20 years in a previous life so it's not a totally uninformed gut feeling.
  2. el murdoque

    el murdoque

    Mar 10, 2013
    Well, i'm no engineer myself, but as i understand it, the bass signal is highly dynamic and the peaks are indeed not that long.
    I like Aguilar's approach to that, as far as i know the power section of the Tonehammer 500 is rated at 800 Watts, but within the Tonehammer, limited to 500 Watts - so there is some spare headroom the user can't touch.

    I played and gigged a lot with my Tonehammer. Before that, i played an old Peavey and an old Trace Elliot Head, the Peavey rated at 240W, the TE at 200W and the Tonehammer feels much louder. It competes with the TE where the master is dimed and the gain is set so it just does not clip when i set the Tonehammer's Master around 10-11 O'clock. Mind, there is still a lot happening between 11 and 2 o'clock on the Tonehammer.
    Same with my backup and Reggae head, the Markbass LMII. Rated at 500W as well,
    The output is so high that i can go a lot further than the Peavey or TE could go and still have a lot of headroom left.

    I don't know what would happen if you brought some Lab-Kit and we ran pink noise over longer ammounts of time and measured the output - but frankly, i don't care either.
    These amps are made for amplifying bass guitars and that's what they do, and do well.
  3. dincz,
    I've been looking at the class D technology myself recently with an eye to designing my own amp. And when I realized that the output stage of these digital amps (by their nature) require L/C filters (look it up), I ruled them out completely.
    Why? Because it's hard enough for an analog, direct coupled, mostly resistive power output stage to drive a speaker which is a "complex load". The speaker and it's cabinet is an L/C filter. Any complex load has resonant frequencies which really complicates the linearity of the reproduced bass or guitar waveforms. Now add the L/C output filter in the class D amplifier and you've one set of resonances on top of another set. So much for purity, clarity, definition and neutrality.
    I haven't done any detailed examinations of this issue (so someone please correct me if I am wrong) but I would never buy a class D amp just because they are more efficient and lightweight. Give me a huge power transformer, some big ass capacitors and a direct coupled, class A/B, bipolar transistor power amp any day over the digital units. Especially for bass guitar! Leave all the coloration to external processors and speaker cabinets of your choice.

    Enough said!
  4. Roland GR 88

    Roland GR 88

    Sep 16, 2013
    I find that the resonate frequencies of the complex load running into my L/C filter rather uncomplicated in a linearity sense. The clarity of definition as it relates to neutrality however is best left to the Swiss.
  5. StuartV

    StuartV Finally figuring out what I really like Supporting Member

    Jul 27, 2006
    Manassas, VA
    There you go, listening with your eyes....

    I have 4 all tube amps and 1 Class D 500W. To my ears, the Class D head is very clean and uncolored. With a Sansamp BDDI in front of it, with settings to emulate an SVT, it sounds very much like my SVT - i.e. it sounds really good (to me, of course).

    And, dincz, you might want to check out the just-announced Peavey Mini Mega amp head.

    $500 street price
    1000W continuous, 1500W peak
    8 pounds
    Lots of cool features

    That MIGHT be enough power to give you the clean headroom you want... :D
  6. dincz


    Sep 25, 2010
    Czech Republic
    Maybe so, but I don't have a problem with the tone of class D amps. My doubts are more to do with headroom and dynamics and their relationship to switch mode power supplies - in class D, A/B, G, H or whatever.
  7. dincz


    Sep 25, 2010
    Czech Republic
    It looks interesting and I'm surprised to see a continuous power rating. However, I'm more than happy with my Crown XLS1000 - probably because I run it bridged into an 8 ohm cab and don't really crank it. Plenty of headroom and power supply capacity in that situation.
  8. FWIW: Personally, I really love the new G-K micro bass amps - small compact super lightweight and they kick butt too. The new compact lightweight bass cabs available today are also awesome - this really is a good time in history to be a bass player.

  9. BurningSkies

    BurningSkies CRAZY BALDHEAD

    Feb 20, 2005
    Seweracuse, NY
    I would have pinned transient abilities at least partly to the capacitors an amp is built with.
  10. Passinwind

    Passinwind I Know Nothing Supporting Member Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    Series inductors on the output are a common feature in power amps of pretty much any topology.
  12. This made me laugh out loud :D

    But dad, I was only trying to be completely clear and honest, I swear.
  13. Since when did class D become a "digital" amp? If anyone seriously looks into "class D" typology you will find it is analog.
    From what I have gleaned over-building any power supply for class D would have little benefit.
  14. BurningSkies

    BurningSkies CRAZY BALDHEAD

    Feb 20, 2005
    Seweracuse, NY
    Its the age old myth. Next up, someone tells you because the SMPS switches on and off it is by default digital. ;)
  15. :) In that case you can over-bias a class A/B and make it "digital" as there will be "off" states (devices go into cut-off).
  16. +1, The MB800 has headroom to the sky, just make sure your drivers are up to the task. If you need more DBs than that you will be going FOH anyway.
  17. christw

    christw Get low!

    May 11, 2008
    Dayton OH
    I want to be Tesla (tinkerer at Dayton Amp Co)
    BobbyBld from Peavey posted some good stuff about the demands on class d amps used for bass guitar. I'm on a phone or I'd link it.
  18. I'm not certain but my Markbass CMD121 Jeff Berlin might be a Class D amp. If so it sounds tremendous.
  19. That info is hard to come by. IIRC the LM II is SMPS/class A/B? MarkBass has used class A/B and class D (both with SMPS).
  20. dincz


    Sep 25, 2010
    Czech Republic
    A gentle nudge back to the topic, which wasn't really class D specific. Iron age amps were generally rated for continuous power and could deliver their maximum power (on the brink of clipping) for at least minutes on end - never tried it for more than ten minutes into a dummy load.

    Modern lightweights seem to be rated for burst power - typically around 20 milliseconds - which is less than one cycle at 40Hz. That seems like a serious limitation to me and I'm guessing the weak link is the power supply.