Ohms and bass heads

Discussion in 'Amps and Cabs [BG]' started by silvermaneZ, Dec 20, 2000.

  1. silvermaneZ


    Oct 10, 2000
    Houston, TX
    Ok, I obviously was never taught this, and did not pay attention to physics class in high school, so this is my stupid question for the week. How does the "X" wattage at "Y" ohms that a head puts out relate to the "Z" power handling at "Y" ohms that the cabinet takes? If the "Y" ohms do not match up, and I going to screw up my equipment? I have seen alot of people talking about the ohms of certain cabinets and heads on these forums, and I honestly have no idea what is going on. If anyone could help me with my ignorance, I would appreciate it.

    By the way, I am using a Hartke 5000 head with a mid 90's Carvin 2-10 and 1-15. Could not tell you the ohms of the stuff if my life depended on it, which if I end up blowing my rig up with me in front of it, my life just might.
  2. You're using a solid state head (transistor), so the rules are as follows:

    Your head has a minimum impedance rating, probably 4 ohms. Don't ever go below this rating, or your amp might smoke itself. The power rating of your head is given in reference to a load impedance, like "400 Watts at 4 ohms". This tells you the output power the head can supply to a 4 ohm load. If you hook something with a higher impedance, like a single 8 ohm cabinet, your amp will only be able to deliver 200 Watts to it cleanly. It won't hurt anything, but you won't be able to get he 400 Watts out of your amp.

    Your speaker cabinets each have an impedance and power rating as well. "8 ohms, 300 Watts" means that the cabinet is an 8 ohm load, and it will not be damaged by anything less than 300 Watts.

    In your setup, you probably have the speakers connected in parallel when you hook them both to your amp. So if both are 8 ohm cabs, then the total when both are hooked up together is 4 ohms. This is what your amp "sees", it may as well be one giant speaker cabinet with 4 ohms impedance. If you'll post the details off your gear (400Watts into 4 ohms, etc.) I can give you specifics for your gear.

  3. silvermaneZ


    Oct 10, 2000
    Houston, TX
    Thanks for the help, chris.

    The Hartke 5000 actually has a solid state and a tube pre amp section, since you said that it was a solid state.

    The head, according to specs, puts out 250 watts per side @ 4 ohms, and 170 watts per side @ 8 ohms.

    The Carvin 210 is listed at 400 watts @ 4 ohms, and the 115 is listed at 500 watts @ 8 ohms.

    I run both of the cabinets out of one side of outputs, and run them biamped.

    Any other info you can give would be GREATLY appreciated.
  4. BassBald


    Dec 20, 2000
    PMFBI (Pardon me for butting in), but...

    My understanding is that you may be tempting fate with your current setup. Combining a 4ohm and and an 8ohm cabinet on one channel can cause you two problems.

    First, unless your head is capable of driving 2ohms per side(it should say on the back or in the manual) you should not use the cabs together. If I remember correctly, a 4ohm and 8ohm cab together make 2.6 ohms. This could conceivably fry your head (amp).

    Second, even if your head can drive 2ohms per side, it's a bad idea to mix ohm-ages. Depending on how your amp works, one cab will draw more power than the other, tending to make things uneven. Of course, it's possible that that is the desired effect... kind of a poor man's "mixer". Usually, however, that is not desirable.

    One last point. There seems to be a common misconception that using higher impedance cabs (lower ohms) allows your amp/head to use all it's power. The logic goes something like this: My amp puts out 170 watts at 8 ohms and 250 watts at 4 ohms, therefore it's better to get a 4 ohm cab so I get to use all my amps watts. The problem with that logic is that, in this example, it would actually take 340 watts to drive a 4 ohm cab the same way 170 watts would drive an 8 ohm cab. A 4 ohm cab has double the resistance of an 8 ohm cab; hence requiring twice the power to drive. Most bass heads/cabs' power does not double when the impedance doubles. I've seen a few very expensive home stereo/theatre amps that do, but very few.

    Bottom line is: I would run your cabs out of separate sides just to be safe.

    Sorry to be so long winded.

    Hope this helps,
  5. MikeyD


    Sep 9, 2000
    It wasn't clear that Silvermane's using it that way, since bi-amping was mentioned. Also, if the 8-ohm cabinet is somewhat more efficient than the 4-ohm unit, conceivably it could sound fine. However, I agree that if the two are in parallel, the resulting 2.67 ohms could create problems if the head is not designed to handle that impedance.

    Higher impedance means higher ohms, since ohms are the units of impedance.

    This is incorrect. Watts are units of real power delivered to a speaker, so a 4-ohm cabinet will receive more wattage from a given amplifier than an 8-ohm cabinet. If the speaker design is equivalent, this wattage usually translates to more volume. All else being equal, a 4-ohm cabinet will be louder than an 8-ohm one.

    No, a 4-ohm cabinet has *half* the impedance of an 8-ohm cabinet (nominally speaking, of course, since impedance varies with frequency and temperature, etc.), so it will typically require less voltage to drive it at the same wattage input. A lower impedance (or resistance - the DC component) means a higher *load* for the amplifier.

    I think you meant to say: power output usually does not double when impedance is halved.

    - Mike
  6. Mo' bass

    Mo' bass

    May 4, 2000
    Right on, Mike!!!

    The PRE-amp of the Hartke 5000 has both a tube and a transitor section. The POWER-amp is all transistor.

    I assume that you mean that each cab is connected to a separate output since otherwise no bi-amping is possible. This means that you are not going to damage anything with this setup. Are U sure that the 115 is 500W/8Ohms?? I just checked the Carvin website and it sais 400W/4Ohms for the RL115. Can you check again??
  7. MikeyD correctly pointed out the misconceptions. One thing I will add is that the "more power to a 4 ohm than an 8 ohm" is only true for solid state. Tube power amps use transformers which allow them to do full power into any load that they have a tap for, my Ampeg V4B has 2, 4, and 8 ohm switch settings. (MikeyD, I know you know this, just pointing it out to others.)

    Solid state amps are basically a voltage source, and since V=IR and P=IV, then Power = V^2 / R. Maximum V is fixed for a solid state amp, so reducing R to the minimum results in max output power.

  8. BassBald


    Dec 20, 2000
    Thanks for the clarification. My bad. I guess I should have paid more attention in class, too.

    This is something that's still a bit grey to me. Perhaps you could help.

    In the case of speaker cabinet sensitivity, it's (usually) measured at 2.83 volts, which equates to 1 watt into 8 ohms. Wouldn't it take 2 watts into a 4 ohm cabinet to net the same 2.83 volts? (and hence the same "volume level"?) If not, where am I going wrong?

    (Apologies to SilvermaneZ, if this is getting off topic)

  9. You're thinking in terms of keeping a constant voltage, when you probably should be thinking in terms of constant power. You're right about the 2.83VRMS across 8 ohms = 1 watt. For the same amount of volume on a 4 ohm speaker, you'd need 1 Watt again, so you'd only need 2VRMS. I think speakers are sensitivity rated at 1W/1m, so the impedance should be factored out by the fact that the power is kept the same. If it was a 16 ohm speaker, it would take 4VRMS to get 1 Watt into it. They would have to adjust the voltage for different impedances to keep the power the same.

    Think of the voltage as the supply, and the power as the result of a certain voltage across a certain load. Your amp supplies voltage, and when an impedance is connected to this voltage supply, current flows through the load as a result. When voltage is across a load, and current is flowing through it, then power is being dissipated, Power= Current * Voltage.

    It helps me to think of it this way.

    Voltage=Pressure, Current=Gallons per minute, Resistance=restriction of flow.

    Think of a garden hose with a spray nozzle on the end. When the nozzle is closed and no water is flowing, there is still pressure (voltage), this is like your amp with no speaker connected, no power is put out. When you open the nozzle a little bit, a little spray of water comes out (high impedance resulting in low current for a given voltage), like hooking a 16 ohm speaker to your amp. When you open the nozzle all the way, a lot of water comes out (low impedance, high current for the same voltage), like hooking a 4 ohm speaker to your amp.

  10. BassBald


    Dec 20, 2000
    Wow, great explanation! I think I finally understand it all. You ought to write an "Amplification for Dummies" book. I know I'd buy the first copy :)

    Thanks again.
  11. MikeyD


    Sep 9, 2000
    Yeah - I try to be careful when giving technical advice. If I'm not sure of something, either I will refrain from giving an answer or else I will qualify it somehow (e.g., "I think", "as I recall", "this is a guess on my part", or "in my opinion" - that kind of thing). I make mistakes, too: I made a very recent little faux pas on a usenet group, but at least I said something like I was guessing the answer! :)

    A couple of relations we should always keep handy:
    _ (1) Power = Voltage x Amperage
    _ (2) Voltage = Amperage x Impedance (resistance, if DC)

    Now, substituting eq. (2) into (1):
    _ (3) Power = (Amperage squared) x Impedance
    Inverting (2),
    _ (4) Amperage = Voltage/Impedance
    Substituting eq.(4) into (1):
    _ (5) Power = (Voltage squared)/Impedance

    You can see that, if voltage is held constant, power goes to infinity when impedance goes to zero. This is not physically possible, but the math shows the important relations between quantities. When impedance is *lower*, the amplifier has to deliver more current at a particular voltage, hence its *load* has increased.

    Good posts, throbbinnut!

    - Mike
  12. What every one wants to know on this board is loudness and we always talk about power which isn't loudness. some companys are start telling you just how loud that their speakers are. So 120db is a little less than an air hammer, 130db is the threshold of pain and 140db is an explosion. I have seen the add for a subwofer at 136 db. When we talk watts and ohms it tells us nothing but how to avoid hurting our amplifiers. So I'm in favor of a rating system that means somthing to musicians, tell us how loud the thing is.
  13. MikeyD


    Sep 9, 2000
    Bassdude - you have the right idea. Unfortunately, even "loudness" has to be taken with a grain of salt. Our ears are most sensitive in the upper midrange (around 3 kHz), and least sensitive in the extremes of the audible spectrum. For example, a 30 Hz. note at 110 dB sounds only as loud as a 1 kHz note played at 90 dB. So what might seem deafening at normal frequencies might be only moderately loud in the low bass (another reason why we bassists usually haul bigger amps around than our guitarist counterparts).

    I agree with your point, though. Ampeg's catalog lists the 1-watt sensitivity as well as the maximum power loudness of their cabinets. However, it's not clear to me how either of these measurements were made, in terms of the frequency content used or whether anechoic (full-space) or half-space, etc. Of course, one also has to be mindful of how far from the speaker the sound level is being measured. I think that both the sensitivity rating of the speaker and its maximum power loudness are useful. For example, one might say the SWR Megoliath is capable of huge volume output, but it might require far more power to do so than, say, a big folded-horn cabinet, because the latter is likely to have much better sensitivity. Then there's the matter of "throw": the ability of the speaker to project far from its location - a subject which I'm studying presently for another post in talkbass. Stay tuned.

    - Mike
  14. the iceman

    the iceman

    Nov 27, 2000
    MikeyD-- I also was wondering about the sensitivity ratings Ampeg has for their cabinets. I don't know why they are the only manufacturer that I've seen that shows both the regular and "maximum" sensitivity ratings. Could it be just to make their cabs sound like they're better than other by adding this stat (a SPL of 125db is pretty impressive to me!) or what?

    I think I understand how this maximum sensitivity can be reached (just by a very high frequency note?), but I still wonder how applicable it is to normal bass ranges. I would think it would be better to just look at the standard SPL rating taken at 1watt/1meter like most other companies have (and which I assume Ampeg uses for their base measurements), but what do you think?
  15. MikeyD


    Sep 9, 2000
    Ooops - I gotta clear this up a little! It's not "maximum sensitivity", it's maximum output level (at the maximum power input). The sensitivity is only the typical sound pressure level (SPL) at 1 watt @ 1 meter thing. Ampeg's maximum loudness is what the cabinet will yield under full power input. For example, their SVT-215E cabinet is rated for a 103 dB SPL sensitivity (1w/1m). The RMS power handling is 400 watts, and they claim a maximum SPL of 129 dB. This makes sense to me, assuming the cabinet does not "go non-linear" and compress its output at high wattage levels. Using SPL = 10log (P/Pref) + SPLref, where P = 400 watts, Pref = 1 watt, and SPLref = 103 dB, then SPL would be 129 dB. Some of the specs on their other speakers don't quite fit this formula, and I don't know why. (They may be typo's, since I've seen a lot of discrepancies between the catalog and their web site.)

    So - to finish my answer, if the cabinet can produce 103 dB in the middle range of your bass with a 1 watt input at a distance of 1 meter, then if you pump 400 watts of the same notes, you can expect to be exposed to 129 dB at 1 meter directly in front of the cabinet. That is, under the conditions under which Ampeg developed those figures (which may be anechoic or semi-anechoic, and which may not have used typical bass frequencies - I have read that sensitivity measurements are often made at 1 kHz).

    - Mike
  16. the iceman

    the iceman

    Nov 27, 2000
    thanks for clearing that up--I understand it a lot better now
    I guess the only reason Ampeg puts those stats up there is to confuse people like me! (heh heh, just kidding, i know this information can be useful) Oh well.