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Ampxperts: What about miniature power amps?

Discussion in 'Amps and Cabs [BG]' started by JohnP, Jun 15, 2001.

  1. JohnP


    Jun 9, 2001
    Endorsing Artist: Rick Turner Guitars, DTAR, Seymour Duncan,
    While looking at the various amps available to power subs and bass rigs , I kept seeing these class d miniature amplifiers..the 300watt@2 ohm Acoustic Image Clarus comes to mind..almost fits in the palm of your hand, 5.5 lbs with a preamp. Who makes these ultra small featherweight things? The lightest smallest commercial amp i saw was the qsc plx, but that was big @ 17 lbs/2 rack spaces compared to this.

    What's the story with these miniature amps? (just the amp, not a pre) Are they commercially available? is there a limitation on watts at this size..could you get a 500 or 800 watt one that would only be margainally bigger?
  2. MikeyD


    Sep 9, 2000
    Beware of strangers bearing gifts in small packages! ;)

    Seriously, I'm not familiar with that Clarus, but I suspect the power ratings are not RMS - or they are exaggerated, or both. It is difficult to get honest, RMS, continuous power in a package much smaller or lighter (in watts per cubic inch or watts per pound) than QSC's PLX series, IMO. Sometimes you see small stuff for car audio applications, but that's partially because they already have a DC source to draw on. Bass amps need to be powered from AC, so their power supplies occupy substantial space, and in many cases do weigh a lot. Also - a lot of this has to do with heat dissipation. It is not good to cram lots of power transistors into a small space without adequate heat sinks and/or forced cooling.

    Again, I'm not familiar with particular mini amps of the sort you describe, but I recommend being very skeptical until you find solid information on all aspects of their performance and reliability.
  3. http://www.qscaudio.com/support/library/papers/amptalk.pdf

    The above will teach you about switching amplifiers, and switching power supply amps.

    The QSC PLX series is a switching power supply that drivers a class AB or class H output, depending on the model of the amp. Class H is more efficient and found in the higher-end models.

    Switching amps and supplies are sophisticated pieces of engineering. IMO, you get what you pay for. I had a long conversation with QSC engineering, and they say the PLX line failure rate is the same as that of the MX/RMX series, which are non-switching power supplies. The result is a dramatic reduction in weight. My MX-3000a weighs 69 pounds compared to 21 pounds for the PLX series. To be fair, my MX still puts out more power than the largest PLX (to date), but this is negligible at the higher ratings.

    As I understand it, class D amps are a switching output class, most likely combined with a switching power supply. I have a friend with a tiny one (Clarus?) and he loves it for the small size and light weight. I can understand this, as my situation is very heavy. However, after being inside the QSC amps, I am firmly committed to their quality and construction, and will buy them again cuz they're built like Harley-Davidsons. Think of the PLX as a small block hawg.. still a hawg, just a smaller one. Teenie amps like the Clarus scare me. YMMV.

    For me personally, I will stay with separates because I bi-amp. Also, I believe a separate preamp is far superior to one bundled into these tiny amps. Eventually I will replace my heavy MX/RMX with a pair of PLX.
  4. JohnP


    Jun 9, 2001
    Endorsing Artist: Rick Turner Guitars, DTAR, Seymour Duncan,

    Mike-that's the Clarus site..where the claim is 300 watts rms @ 2 ohms. AC power. 5.5 lbs with an integrated preamp.

    Thanks for the qsc site Bruce. I learned a lot.

    I still don't get it. I have just done a search on Class D amps, and there are a lot of them for cars, as you said Mike.. they also all claim to actually run cooler, not hotter.

    Chuck brought up the 1400 watt stewart @11 lbs.

    but I am more interested in the real miniatures, like the Clarus or Walter Woods. Who makes these amps..not the pre's, just the amps.
  5. MikeyD


    Sep 9, 2000
    As bgavin suggests, the Class D switching output stages do enable the amp to be more efficient, which means it will generate less standby and other losses (i.e., heat). With good engineering and attention to heat transfer, I think it is possible to have a fairly high-power switching amp in a small package. The only other thing I can add is - I remember seeing some discussion (I believe it was on the pro sound newsgroups in usenet) about switching-type output stages and how a number of people didn't like them. These stages "build" an analog audio signal out of discrete voltage steps (in essence, a high-power D/A converter), and there may be some switching distortion and other anomalies introduced (such as high-frequency artifacts that have to get filtered somehow before they reach your speakers). I don't know enough about them to comment further. If I were considering them, I'd wait to get as much real-world appraisals from knowledgeable people as possible. And I have no idea of this company's reputation - something I would want to determine. So - after my obligatory "cautious" approach - I say keep looking at them.
    - Mike
  6. I was wondering about this recently, as I noticed two integrated bass heads made by the same Company in the latest M/F catalog. Both are rated the same power, but one is 36 lbs heavier than the other. They are: The Ampeg B2R Rack - 350watts, weighing 15lbs. The other is: The Ampeg SVT-350H
    Classic(solid state)- 350watts, weighing 51lbs.

    Not considering tone, which is subjective, couldn't one assume that the heavier one would be much more reliable? Also, the SVT is listed as having MOSFET output, I think FET stands for Field Effect Transformer, but what does the MOS stand for?

    No need for disclaimers, I'm just curious.

    Mike J.
  7. Not anymore.

    If you can get a good conversation with a support engineer at Ampeg (and not a salesman), you might get the unvarnished truth about reliability. I've had the good fortune of having these conversations with both Shure and QSC so far.

    The weight is the difference between a switching power supply vs a transformer based supply. My QSC MX weighs 69 pounds, and damn near all of it is the (2) transformer power supplies. The QSC engineer assured me the failure rates for the light weight PLX (switching) supplies are the same as the transformer based rigs.. heavier is not always an indicator of more reliability.

    There is a chunk of sophisticated engineering in a circuit that accurately switches 230,000 times every second. I think a cheap transformer rig might be more reliable than a cheap switching rig. When you get into the QSC - Crown - Crest (Big Three) arena, this is no longer an issue.
  8. alembicbones


    Nov 10, 2000
    Seattle, WA
    Check out a Stewart World 1.2. One rack space, weighs ~12 lbs. and delivers 1200 watts in bridged mono at 4 ohms, 750 watts at 8 ohms.

    Great amplifier. I've had mine for four years and it's been fantastic.

  9. MOSFET= Metal Oxide Semiconductor Field Effect Transistor. Just another type of transistor than your regular (bipolar) transistor. They have distinct advantages. Some amplifiers with certain circuit topologies seem to sound a bit like tube amps. So I've heard. My old amp is MOSFET. Very fortunately it doesn't sound anything like a tube amp.
  10. There is a caveat with Bridged Mono mode for all amps: this is the mode where the amp works the HARDEST.

    B/M mode operates in a Push/Pull mode using both channels, and doubles the voltage applied to the speaker terminals. You can realize LETHAL voltages as high as 170 volts on the higher powered amps. This requires fully insulated Class One wiring and touch-proof connections to keep yourself from getting killed or wounded in action.

    The other part of B/M mode is the two channels divide the speaker cabinet impedance between them. For example, your 8-ohm cabinet is actually seen by the amplifier channels as a pair of 4-ohm loads. NEVER run an cabinet impedance below 4 ohms, because this presents itself as a pair of less-than 2-ohm loads to the amp. Always check the manufacturer recommendations first. If unclear, get clarification before running 4 ohms or less in B/M mode. You don't want to let the Magic Smoke escape from the amp. Once the Magic Smoke is let out, the amp no longer works.
  11. Some of the above is over my head, but I think a switching amp AKA digital power supply type probably are OK with a true 15 or 20 amp service. But my experience with the QSC 2402 is that if you are in a club with less than OK voltage, your digital amp will sound weak and hard to hear. I love tube amps, but they are too heavy. I use Eden amps as they are a compromise. Yes, they have a SS power amp, but they sound very good. I think some MOSFET amps are OK too. But all in all, I believe you need some iron in your amp for headroom and solid sound.
    I think the Acoustic Image and Walter Woods stuff is used more for stand up bass amplification.
    It has a reputation for being very accurate but sterile.

    Some of this is my opinion, some experience, and some hear say.
  12. I've heard this also, but cannot verify it.

    The tradeoff is weight and complexity. My MX-3000a weighs 69 pounds compared to the 21 pounds of the QSC PLX models. Mine uses a pair of very heavy toroid transformers and capacitor banks, and is built like tank. The PLX uses a highly sophisticated circuit that switches 230,000 each second, then smoothes and reintegrates this series of steps into DC voltage.

    The damping factor in the PLX series is higher than the MX. This implies tighter bass control, but is assumed at nominal input voltage.
  13. Thanks for the input. I don't know how to reprint your paragraph, but you mentioned damping factor.
    I have a practice amp, a Fender Bassman 135 ( 4x6L6GC's), and lots of Iron for 100 watts. I play it through a 1-15 Musicman cab with a Black Widow in it.
    I.m sure the damping factor is not great, but this amp is Tight!.
    Big Iron, Big Bass!
  14. brianrost

    brianrost Gold Supporting Member

    Apr 26, 2000
    Boston, Taxachusetts
    Guess what, they are the same amp (go check them out in a store)!!!

    The difference is the "SVT" version comes in a honking big wooden case so it looks like a tube SVT.

    There ya go, triple the weight so it looks cool :)

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