8 ohms speakers connected to the 4 ohm tap?

Discussion in 'Audio Hardware' started by Radiotron, Aug 27, 2008.

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  1. Radiotron

    Radiotron Tube Designer Thread Starter

    Location:
    Montreal, Canada
    I was checking out the PrimaLuna series on Upscale Audio's website and something caught my attention.

    "Looking at the rear panel, you'll note you have four inputs, and speaker taps for either 4 or 8 ohm speakers, but don't make any assumptions. Just because your speakers say 8 ohms, you can fine-tune the sound by using the 4 ohm tap instead".

    So what happens when you connect your 8 ohm speaker on a 4 ohm tap?
     
  2. DaveN

    DaveN Music Glutton

    Location:
    Apex, NC
    I'm curious about this too. My Onix SP-3 has the same option but I've never tested it. I'd like to know what is happening on an electrical level before I do.
     
  3. bobrex

    bobrex Active Member

    First, please understand that although a speaker claims to be 8 ohms, with few exceptions - Magnepans being the biggest exception, speakers are not a linear load. The impedance varies anywhere from 2 ohms to 20 ohms (sometimes more) depending upon a number of factors (frequency, box tuning, driver selection,...) Using the 4 ohm taps won't harm anything, and you may in fact prefer the sound.

    From an electrical level, you may not get as efficient of a power transfer, so you may lose a few watts (bet you won't notice.) Or you may gain some depending upon the impedance characteristics of your speakers.

    Bottom line - go ahead, no harm done.
     
  4. KT88

    KT88 Senior Member

    It's not a good idea to mismatch impedances. Why do you think tube amps have multiple sets of taps? Obviously, it's important. By keeping the speakers connected to the taps that most closely approximate your speakers load, you will get the most efficient transfer of power at full bandwidth.

    As bobrex has said, it's a moving target because speakers have complex impedance curves, not a simple and static impedance. So speaker makers will rate their speakers nominal impedance which will provide you with the closest impedance for choosing a tap. The real fun part comes when you buy a 6 ohm speaker ;). What happens when you connect an 8 ohm speaker (or any higher impedance) to the 4 ohm tap is that the output transformers do more of the work and they get a bit hotter, so the better the transformer quality, the better transfer and lower distortion you have. The high frequencies will begin to roll off slightly in this example as well. Connecting the same 8 ohm speaker to the 8 ohm tap will result in the most linear frequency response and greatest efficiency of power delivery across the band. Some amplifiers can deliver more power into lower frequencies at lower impedances due to the interface with and the design of their transformers but it is always advisable to keep the load matched with the tap on the transformer.

    The situation can be a bit more costly if you take it further by connecting say a 12 ohm speaker to the 4 ohm tap. Under those conditions, the transformer will recieve a lot more voltage and begin to get very hot as it will not be able to dissipate all of the energy into the load. So you could blow a transformer, which essentially destroys all but the more expensive amplifiers as replacement is very expensive. The surest way to destroy an output transformer is to operate a tube amp without a load connected.

    A bit of a reverse situation is formed inside the amp when you connect a 4 ohm speaker (or any lower impedance) to the 8 ohm tap. The output tubes work harder in this arrangement and may not last as long. Again, you will not get the most efficient power transfer to the speakers but this time, the lower frequencies will be limited in power. So you can use taps as a sort of mild tone control in this way as long as the output transformers and output tubes and surrounding circuitry of the amplifier is of good quality and as long as you don't get too far away from the target impedance. In other words, a 2 ohm speaker on the 8 ohm tap is stretching it too far here as well. A good rule of thumb is to never be off by more than 100% of the value, so nothing below 4 ohms or above 16 ohms should be connected to the 8 ohm taps. look at it as a factor of two and anything more or less than that will be destructive to the amplifier. The speakers are not really effected here other than the limiting of their power bandwidth. If you are going to mismatch impedances, to avoid any amplifier damage, you really need to be careful of input levels as well as that is what drives the voltage swing inside the amplifier. If you crank up the preamp and get a lot of power output, you are going to see higher voltages inside of the amp. Nothing wrong with that as long as the amp is designed to handle it and you are operating it within its specifications but if you are mismatching its expected load and output, you can get into some problem areas.

    There are multiple taps on a tube amp for these reasons. You use each for specific loads and if you use the wrong one, in some situations you could have some problems.
    -Bill
     
  5. Radiotron

    Radiotron Tube Designer Thread Starter

    Location:
    Montreal, Canada
    Bill, you DA man.
     
    George P likes this.
  6. P2CH

    P2CH Well-Known Member

    I always looked at a tube needs a load and a SS amp doesn't need a load. A transistor would be happy all day long if it didn't have to supply power, or current, to a device.

    The output tubes rely on the proper load on them though.

    Maybe this isn't the appropiate way of saying it but either way, I know not to run a tube amp without a load, or the proper load on the proper taps.

    Thank you Bill for a better explaination.
     
    triple likes this.
  7. KT88

    KT88 Senior Member

    Ah, shucks... :nyah:
    -Bill
     
    bajaed and Jawbreaker420 like this.
  8. naru1980

    naru1980 Forum Resident

    Location:
    Los Angeles, CA
    Thread revival:

    My Verity Audio Finn's boast a nominal 8 Ohms in their specs (page 9), yet in the manual it says to connect them to the 4 Ohm taps on a tube amp (page 5): http://www.verityaudio.com/en/pdf/manuals/EN_Manual_Finn.pdf

    Kind of contradictory, no? Does anyone have experience with these or this situation in general?
     
  9. Burt

    Burt Forum Resident

    Location:
    Kirkwood, MO

    A tube amp has a transformer, which is like an electrical gearbox. It has, usually, three "gears" for 4, 8 and 16 ohm loads.

    If you connect a higher load to a given tap it will in turn present a higher load to the output tubes.

    In hi-fi service this is usually not a hazard to anything, because the sustained power is usually low. Note weasel word "usually".

    Speakers are not resistors, they are a wildly dynamic load and the stated impedance of a speaker is a NOMINAL value. Sometimes therefore it works better to use a different tap than the nominally correct one.
     
  10. naru1980

    naru1980 Forum Resident

    Location:
    Los Angeles, CA
    Tried it out on the 4 ohm tap and the bass sounded "off". It would register some bass notes very loudly, but others softly. Not sure if there's a better way to explain it. Anyway, I'll be keeping them on the 8 ohm taps. Thanks
     
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