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Watts and Ohms?

Discussion in 'Amps and Cabs [BG]' started by xgrantgx, Mar 28, 2013.

  1. xgrantgx


    Apr 1, 2011
    Hey guys, I'm pretty much clueless when it comes to watt / ohm compatibility between the amp and the head. I'm looking into buying a Sunn 2x15 and have an offer for one for $225. I'm told the amp has a capability of 300 watts and 8 ohms while the head I have is a 480/50 watt bi-amp (Gallien-Krueger 700RB-II). Will they be compatible? What are the do's and don't of watts and ohms? Thanks.
  2. figuredbass

    figuredbass Supporting Member

    Jul 11, 2007
    NYC vicinity
    Please read the TalkBass "sticky" for a primer on this:

    BTW, the Sunn 2X15 is a speaker (or "speaker cabinet", or simply "cab"). It's your GK 700RB-II head that's an "amp".

    "amp" = amplifier = head
    speaker = speaker cabinet = "cab"
  3. Don't blow up the cab is the really hard rule.

    Firm rule: don't under load a solid state amp.

    Semi firm rule: don't over load a tube amp

    Load being the impedance presented to the amp. More impedance is more load on the amp.
  4. dincz


    Sep 25, 2010
    Czech Republic
    The opposite is true really - for solid state amps. Lower impedance means heavier load and the amp can deliver more power.
  5. figuredbass

    figuredbass Supporting Member

    Jul 11, 2007
    NYC vicinity
    +1 Although it's a good general rule to avoid excessively loading any type of amplifier.
  6. Troph


    Apr 14, 2011
    Kirkland, WA
    The OP asked about a GK 700RBII, which is a solid-state amp rated to safely handle cabinet impedances as low as 4 ohms, so the amp would be at no risk here. The cabinet on the other hand has an impedance of 8 ohms and a power handling of 300W, then your GK 700RB-II could theoretically blow it, since it can produce 320W at 8 ohms. But as long as you don't turn the volume all the way up, you'll be fine.

    Here's more info than you asked for, because I'm bored waiting for a build to finish at work: :)

    Think of it as "more ohms == more resistance to current flow". So more ohms means less current flow. Less current flow means less power is drawn from the amplifier.

    Solid state (i.e., transistor-based) amps can only supply so much current/power before the transistors start to exceed their designed Safe Operating Area and literally overheat. Your GK has been designed such that it can safely power a 4-ohm or higher speaker combination without overheating. Some bass amplifiers can safely run at 2-ohm impedance, but 4-ohms is most common.

    The other factor to consider is "effective impedance" when you hook up multiple cabinets to a single-channel amplifier (most bass amps are single-channel). The formula for parallel resistance can be found online, but by far the most common scenario is two 8-ohm cabinets, which yields a 4 ohm effective impedance. As you could probably guess, two 4-ohm cabinets would yield a 2-ohm effective impedance. Because your GK is only rated to safely drive a 4-ohm load without overheating, you would therefore not want to hook up two 4-ohms speakers to it! But two 8-ohm speaker cabinets would be OK (in fact, this is how I run my GK 700RB-II).

    Is your amplifier so powerful that it could blow out your speaker cabinet at max volume? Just compare the amp's power output to the cabinet's max power handling at the given impedance. If the amp is more powerful than the speaker can handle, then don't turn the volume all the way up!

    Some people are also confused by power handling when two cabinets are in use. Let's say you have two 8-ohm cabinets, each rated for 200W. These will have a 4-ohm effective impedance when hooked up to the amp. Let's say the amp produces up to 500W of power at 4-ohms. The power gets "split" evenly between the two cabinets, such that each one only "sees" 250W maximum. So theoretically, you could blow both speakers at max volume in this made-up example.

    If you have two cabinets with different impedances hooked up, the calculations get messier. It's not a good idea anyway.
  7. seamonkey


    Aug 6, 2004
    Can somebody point out any Ss amplifier that doesn't have built in safety features like "Short Circuit Protection", "Over Current", "Thermal protection"?

    There are just not any since the early days of SS
  8. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    Plenty of us are using valve amps, so probably best not assuming that SS ones are going to have effective protection. Plenty of them still get blown up.
  9. I'll stick with more cabs in parallel reduces the impedance load on the amp. If you want to talk colloquially of overloading an amp that's why there is the confusion.
  10. Troph


    Apr 14, 2011
    Kirkland, WA
    Point taken. Of course, no protection system works perfectly. And they are designed for failsafe protection in common scenarios, not to allow you to operate the amplifier outside of design specs on a regular basis. The protection systems simply cannot protect against many failure cases, and they do not work 100% reliably.
  11. dincz


    Sep 25, 2010
    Czech Republic
    There's the problem. If you reduce the impedance, you increase the load. There's no such thing as "impedance load". "Load" generally refers to the amount of current being delivered - low impedance = high current = heavy load.

    EDIT - I guess you meant load impedance
  12. ThisBass


    Aug 29, 2012
    My SS Laney amp is built in the late 80s. It is a 4 Ohm amp
    The data sheet for the output stage transistors tells a SOA of appro >1 Ohm.

    I believe that the rectifiers and the transformers of the supplies are a more weak part whenever a load <4 Ohm sucks the same power by decreased voltage but increased current

    If the load impedance is 2 Ohm the max power of the amp is not increased. The max output power of the amp can't be enlarged.
    The equation Pmax=Urms*Irms remains constant, but Urms and Irms are variables depending on the amount of load.
  13. ChuckTrucks


    Jul 28, 2012
    When I was young all the cool kids understood 2, 4, 8, and 16 ohms and how to get there. Car stereos were very popular in the 90s. lol

    I actually learned a lot from that stuff.
  14. ThisBass


    Aug 29, 2012
    Sometimes 2 x 8 Ohm = 4 Ohm is very close to 3 x 8 Ohm = 2.7 Ohm.

    Very often the total impedance, the average of impedance curve, is significant below the nominal rating, whereas sometimes the total is significant above the nominal.
    That means that sometimes the equation of 2 x 8 Ohm of speaker type A equals the equation of 3 x 8 Ohm of speaker type B.

    By the other hand it is not uncommon that a 4 Ohm amp has got some headroom at 4 Ohm. That means some 4 Ohm amp are rather a 3.5 Ohm amp or something comparably.

    Ohms is only a constant number for the nominal rating.
    Measuring voltage and current with a pink noise signal the equation of Urms/Irms calculates to a value that is close to the nominal.
    Giving +10dB of bass boost to the pink noise signal impacts the frequency spectrum, which impacts the equation of Urms/Irms and therefore the load. In general the load is increased. Lets say appro 5..6 Ohm in this case.

    Bass guitar signals are very powerful at low frequencies. A boost of +10dB at low band frequencies impacts the total impedance very significant.
    In practice that means a total impedance of appro 6..8 Ohm at note B0/E1 but a total impedance of appro 4..5 Ohm at note G2/C3.

    4 Ohm Valve amp outputs match to a 4 Ohm nominal. But very often they don't match to the total impedance in real live situations.

    In a daily use it is right to suppose the impedance of any given cab as a constant number. But the trueness is very often a different animal. Not knowing the boundary conditions means not knowing the impedance!

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