2 Ohm 410 design/ Acoustic 470 question

Discussion in 'Amps [BG]' started by wighat, Dec 25, 2013.


  1. wighat

    wighat

    Joined:
    Apr 27, 2005
    Location:
    Port Aransas, Texas
    On the heels of my recent success building a 215 for my V4B, I am interested in building a 2 Ohm 410 for my Acoustic 470. The amp is rated at 170 Watts "independent on load" - how is this possible? Am I really only going to get 170 Watts out of this, no matter the cab's impedance?

    Anyone have any driver suggestions? As far as frequency response, I like 'em low.
  2. Steve Dallman

    Steve Dallman Supporting Member

    Joined:
    Mar 5, 2006
    Location:
    East Central Wisconsin
    I just read through the service manual. The specs are indeed 170 watts into a 4 or 2 ohm load. I don't get it, I don't have a reason to doubt it.

    Four 8 ohm speakers in parallel will yield 2 ohms.
  3. bgavin

    bgavin Supporting Member

    Joined:
    May 3, 2001
    Location:
    Orangevale, CA 95662
    Looking at the 470 Catalog Page this confirms 170w regardless of load.
    Power = Voltage² / Impedance
    This tells me the voltage has to drop as the impedance goes lower.

    Voltage is what makes the cones move.
    Any gains from all-parallel wiring appear to be negated by the decreasing voltage.
  4. Rob22315

    Rob22315

    Joined:
    Oct 23, 2010
    Location:
    Alexandria Virginia
    IIRC, speakers are inherently magnetic, magnetic force is proportional to the change in current, not voltage.

    It's possible that there's a built in power limiter that caps at 170w because of some other constraint in the design. It's also possible that this amp is a current source over its operating range, not a voltage source. That would make the load irrelevant. I don't think that design is common in power amps but I can't say I've actually surveyed power amp designs to be authoritative on which designs are prevalent and which were one-offs or unusual.
  5. wighat

    wighat

    Joined:
    Apr 27, 2005
    Location:
    Port Aransas, Texas
    How/why would the voltage drop when lowering the impedance of the load? The amp would put out the same amount of power into a 32 ohm load as it would a 2 ohm load? I don't understand.
  6. Rob22315

    Rob22315

    Joined:
    Oct 23, 2010
    Location:
    Alexandria Virginia
    Amps can be designed so either the output voltage or the output current is proportional to the input voltage. A current controlling amp would keep the current proportional to the input and allow the voltage to vary to match the load. A Google of 'current source amplifier' should get you some basic circuit designs. Get into the diyaudio forums will get you into both the designs and the history of current source power amps. There's also transconductance designs as well but those details are lost to me in the sands of time.
  7. bgavin

    bgavin Supporting Member

    Joined:
    May 3, 2001
    Location:
    Orangevale, CA 95662
    I=E/R
    When the voltage goes down, so does the current.
  8. wighat

    wighat

    Joined:
    Apr 27, 2005
    Location:
    Port Aransas, Texas
    Soooo, back to the cabinet. What I would really like is an original Acoustic cab that would have come with this head. That's unlikely to happen, at least not anytime soon. What would be a really cool/appropriate speaker to build for this thing?

    Attached Files:

  9. Steve Dallman

    Steve Dallman Supporting Member

    Joined:
    Mar 5, 2006
    Location:
    East Central Wisconsin
    I have been having tremendous luck with MCM's line of speakers. From the series I like (I use 15's and lately 12's) their 10" might be a great match to your Acoustic.

    http://www.mcmelectronics.com/product/MCM-AUDIO-SELECT-55-2981-/55-2981

    While not as efficient as some drivers, the low extension is impressive. Xmass is good. I use a pair of 12's in my rig (along with a 6.5" mid and tweeter, crossed at 800Hz and 5k) and I get sub like lows from those 12's.

    The high end of the 15's and 12's are limited. I don't know about the 10's. I have maybe 20 of the last series MCM sold (Eminence Beta 10 knockoffs that are far better than the Beta 10's I have) and they are fantastic. I got them on closeout for $11 each.

    MCM runs sales on these speakers often, and recently had them for $15 each (plus shipping). Their sales fliers are on their website.

    May I suggest wiring your 4X10 in stereo, with two 4 ohm pairs. Or wire two 4 ohm pairs and put in a series/parallel switch, which would give the cabinet a 2 ohm or 8 ohm option, making the cabinet a bit more versatile.
  10. Steve Dallman

    Steve Dallman Supporting Member

    Joined:
    Mar 5, 2006
    Location:
    East Central Wisconsin
    Someone gave me an old 4X12, 2 ohm Acoustic cabinet. It is insanely heavy...the heaviest cabinet I own, and I have a lot of cabinets. I use it in my practice room with my 370 head (also given to me.) If you were in the area, I'd give it to you, but shipping from northern WI to Texas would be crazy expensive.
  11. Rob22315

    Rob22315

    Joined:
    Oct 23, 2010
    Location:
    Alexandria Virginia
    Yes, that's true for passive circuits but an amp is designed to control either voltage or current in proportion to an input signal. Within the design range (min/max load, upper/lower bounds on the other parameters), the other will vary as needed to keep that ratio.

    Here's a link to a simple voltage-to-current amp design for use in instrumentation.

    http://www.allaboutcircuits.com/vol_3/chpt_8/7.html
  12. bgavin

    bgavin Supporting Member

    Joined:
    May 3, 2001
    Location:
    Orangevale, CA 95662
  13. wighat

    wighat

    Joined:
    Apr 27, 2005
    Location:
    Port Aransas, Texas
    Let's check on that shipping, Steve!
  14. axelrod

    axelrod

    Joined:
    Jun 11, 2013
    Location:
    ohio
    I am also building a cab and currently looking for an amp to power 1 2x10 now and another 2x 10 later both 4 ohms. Looking at peavey nitrobass or other old school amps that will do the 4ohm now and 2 ohm later, and ran across this acoustic 470 on ebay? Should I bid on it?

    ebay acoustic 470
    My cab build

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