impedance

[RxJxVx]

checking out some new bass amps at a local music stoer last week, i noticed a brand new amplifier (ampeg svt) with an impedance selector switch with an option to run the amplifier @ 2 ohms.

it has been my understanding that the 2 ohm load is unstable and not necessarily very good for older amplifiers.

so my question is, if i actually run the 2 ohm load with vintage gear, is there a greater risk for the speaker or the amplifier?

also, is it likely that the new amps are specifically designed to run safely @ 2 ohms, or is that just an option for the craziest of the crazy that ampeg thought they would still equip the svt head with? 

i know NOTHING about svt heads(not that i pretend to know anything about any other amplifiers), but i know that my old ampeg tube head also has a 3 way impedance selector and i presume the svt heads have had the same feature since their inception.

[basiklybass]

From what I know (possibly not much) as long as the amp is designed for the load, there is no problem. The tubes are coupled to the output transformer which is coupled to the speakers. If the transformer is wound for 2 ohms, the tubes should work normal. One assumes the curents and voltages have been thought of by the designers and so the power supply and power transformer are capable of supplying the needed power as well.

I belive it is a situation where most older amps were not designed for the 2 amp load like many amps today are. I know some older Sunns have an 8 ohm or 16 ohm output...and some, like mine have 8 and 4 ohms....the transformers for the 60 watt Sunns usually have 3 taps for 4, 8 and 16. they only used two of the taps but you could use the other and have a wider choice.

I run my 200S at 4 ohms all day long and have not had any problems.

[JoeArthur]

What basiklybass says about tube amps and the output transformer is correct. If there is a two ohm tap, don't worry about it.

There is nothing about OLD TUBE amps that have problems with a 2 ohm load, assuming there is an impedance match for it. Old tube amps do not work any differently than new tube amps.

Consider that the 1959 Bassman had a normal 2 ohm load with it's 4-10" speakers, as did the Super Reverb. The output transformer was wound with a 2 ohm secondary. Not a problem.

2 ohm loads can present a problem for solid state amps.

Remember that the 2 ohms, or whatever ohms you care to mention is a "nominal load", not a constant one, nor a real one in real life - it varies with frequency.

Real speakers wired to create a nominal load of 2 ohms can dip below a one ohm "real load" at specific frequencies. Tube amps, with a 2 ohm output tap, can easily handle it, but certain solid states can have a problem with 1 ohm real loads - resulting in overheating (concert series), or having the "short circuit protection circuitry" activate (coliseum series) which causes some really terrible noises to happen to your sound.

Both of these cases with solid state amps depend on the actual amp design, what the impedance actually is (real time, not nominal speaker ratings)... and what amount of power you are really trying to get out of the amp (i.e. is it on "10" or "2").

[basiklybass]

I love the knowledge that is out there...here.

So being an electrician by trade, I know that if the current increases the voltage drops and vice versa. I know that the load is independent from the current and voltage...work is work is work. I know that when speaking audio, speakers and amps, the impedance varies with the frequency, the parameters of the speaker, cabinet and several other things. I know that there are trade offs in designing a system. You lean towards one aspect sacrificing another.

Unfortunately, I do not know the intricacies of amplifier design. Can anyone explain what the advantage of having such a low impedance load is? One would think that if as Joe said, the speaker can in real life approach or go below 1 ohm.....1 ohm is not much when dealing with 100 watts….if I am correct and a watt is a watt is a watt...... It is a virtual short. So why would anyone design or want a system that places such a load on the amplifier? In my world, if it’s a short…it’s broke. While a motor may have only 2 or 3 ohms at rest, once they get moving and the EMF kicks in, the impedance rises significantly. I seems that speakers may be the opposite? Does the impedance drop as the travel increases and the frequency goes down?

Along this line, I know that Db drag racers have enormous capacitors in their cars to supply the huge current needed for the amazing levels they reach. I know the cables are big also, I have seen speaker cables that I could run a house on. They are only using 12 volts of course, still, those systems run incredible watts (or so they say) and the loads are almost always 2 ohms at the top levels. Aren’t the watts of a 12 volt amp the same as a watt of a 120 volt amp? It just takes 10 times the current to get the same results. What does a Coliseum amp on 8 put out as far as voltage? Anyone know?

Thanks for making me smarter guys…and gals if there are….