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Ohmage and stuff!

Discussion in 'Amps and Cabs [BG]' started by DCman, Oct 24, 2002.

  1. DCman


    Apr 7, 2002
    Hi guys,

    Just wondering if you could answer a couple of questions. I have the Ashdown MAG250 head, with the MAG410 cab and a Peavey 115 BXBW. I noticed on the back of the Ashdown cab that it said that the two inputs are wired in parallel, therefore, one can be used as an output to another cab.

    What will give me more power:

    1) Using both outputs from the head, and going into each cab separately,

    2) Going into the Ashdown cab, and then out of that, into the Peavey 15"?

    Or will it not make any difference at all?

    The Ashdown cab is 8ohms, and the Peavey is 4ohms (i think)


    DC :)
  2. ingmar


    Oct 14, 2002
    Normandy, France
    Actually the total resistance of an 8 Ohm and a 4 Ohm cab is 8/3= only 2.67 Ohms, so be careful and check whether your amp is able to handle such low impedance and make sure it gets enough fresh air as it might get warm at high levels.

    If your head is a mono amp, plugging both cabs into the amp or chain one through the other cab should not make a difference. If your cables are identical length, there might be the tinyest advantage in connecting both cabs to the amp as they will get the amps signal exactly the same time... but we're talking nano-seconds I believe, probably not audible allthough some high-end hifi-freaks will tell you different!
  3. jobu3

    jobu3 Artist formerly known as Big Joe

    Feb 17, 2002
    Mountain Top, PA
    i'm not sure but i don't think that ashdown stuff goes as low as 2. more likely 4. this means that hooking up the 2 cabs is a no-no. an 8 ohm 4x10 and an 8 ohm 1x15 would be perfect and then you wouldn't run into the problem of the 4x10 being so much louder than the 15 because of less impedance... equal imepdance usually means equal power to both cabs not necessarily equal volume either...
  4. Matthias


    May 30, 2000
    Vienna, Austria
    Sorry, but this is pure nonsense.

    There's a nice example why it takes the current virtually no time to 'run through' a cable:
    Imagine a tube (sort of water pipe, not the ones in the amp...) filled with balls of the same diameter from one end to the other. If you push in one more ball at one end, one ball will fall out immediately at the other end. This how information is passed on in a cable as well.
    Sorry if my English is weird, I hope you know what I want to say...

  5. JMX

    JMX Vorsprung durch Technik

    Sep 4, 2000
    Cologne, Germany
    There's no such thing as ohmage, it's either resistance or impedance (for AC).

  6. Maximum power is delivered when the output impedance is matched by the load (ie the imoedance of the cabs). Your head will produce 250 watts at 4 ohms. You will be able to get the 250 watts into the Peavey 1 x 15 cab. If you are wanting to use both, then I'd suggest you consider replacing the 4 ohm cab with an 8 ohm one.

    2 8 Ohm cabs gives a load of 4 ohms, a 4 ohm and an 8 ohm cab give a load of 2.67 ohms - which is not recommended for an amp that produces maximum power at 4 ohms.

    Hope this helps

  7. Thank You JMX, that word drives me insane!
  8. 1st of all, how do you figure ohmes into parallel? I've alwayes added up all the cabs 8+4=12 then devided 12 by the lowest ohms ( 4 / 12 ) = 3. Then I devide highest ohm cab by the result 8/3 = 2.6666666666. OK say you've got an 8 a 4 and a 2? That comes out to 1.14!!!!! (can you say OVER HEATED) Check your manual and see if your can is stable at loads below 4 ohms.

    As for your second question "Using both outputs from the head, and going into each cab separately,
    Going into the Ashdown cab, and then out of that, into the Peavey 15"? Or will it not make any difference at all" Yes it does make a difference but not in the ohms of the speakers. The difference is in line loss do to wire length and effective wire ga.. Say you decide to go from the amp into the 4X10 and from the 4X10 into the 1X15. You use 2x3ft. of 16 Ga. speaker wire. The length of wire will be the total of the 2x3=6ft. and the Ga. would remain at 16. If you plug both into the amp and run them to each speaker cab the length would only be 3ft and the effective ga. would be 8 ga. This is preferable as to the fact that the longer a cable run the more power and damping factor loss especially at loads below 8 ohms. You can counter act this to a point by keeping cable runs as short as possible and by using the biggest speaker ga. By running both into the amp you are doubling your speaker diameter and shortening your cable run. Can you hear this? It all depends on what you are listening for.;)
  9. For parallelling resistances (or impedances):

    1/Rtotal = 1/R1 + 1/R2 + 1/R3.......+ 1/Rn

    And parallelling (2) runs of 16 Gauge wire does not give you the same effect of (1) 8 Gauge wire. It doesn't work like that.

    16 AWG has 4.73 ohms / 1000 feet, and 8 AWG has 0.739 ohms / 1000 feet.

  10. Right 16 AWG is 4.73 ohms / 1000 ft. However when you take 2 16 GA. wires and run them from the same source arn't you in fact doubling their GA?:confused:
  11. geshel


    Oct 2, 2001
    Time for a little wrap-up, perhaps?

    1. yes 8+4 ohms = 2.67 ohms

    2. You're thinking of tube amps. SS amps have a very low (<.1 ohm) output impedance

    3. Actually, neither you nor I have any idea what the amp's maximum power at 2.67 ohms would be. It might very well be higher than 250w. It also might overheat the amp. Again, I think you're thinking of tube amps.

    4. He did say "nanoseconds", which in my book is currently "virtually no time". Granted this means "no time" in regards to a bass amp, but no signal travels instantaneously.

    Just be thankful they're not all writing "pw3mg".

    Or, maybe it's "homage"? It's an homage to all those earlier posts that asked the exact same questions, and are nicely archived for just such a purpose? :D
  12. Naw man. You are doubling your current capacity, but not the Gauge. Gauge doesn't correspond linearly with ampacity.

    16 AWG is good for 10 Amps or less. 8 AWG is rated for at least 40 Amps or less (depending on the insulation type, it can go as high as 55 Amp rated).

  13. Matthias


    May 30, 2000
    Vienna, Austria
    Yes, not instantaneously, but nearly at light speed. This would be 3 meters in 1 nanosecond. You're right. But one can't hear this. milliseconds (think of delay lines) yes, but not nanoseconds.
  14. Just a note; Humans don't perceive anything less than about 30 milliseconds as being delayed. This goes for audiophiles too!
  15. Matthias


    May 30, 2000
    Vienna, Austria
    Sounds good. Compared to speed of sound 2/3 light speed is still 'nearly light speed' to me...

  16. In terms of max power I was referring of course to the statement that appears on the back panel of amps- The statement that says Max Power 250 watts@4 ohms !!! This is the manufacturers recommendation nothing more - I tend to go with them!!!!

    I also believe that this will be the same for Tube or SS amps!!!

    Also, as power is given by the expression

    P=V I

    We can substitute in for the known values and see what comes out!!! So we probably could work out what the output would be at 2.67 ohms!

    One other thing, maximum power in a circuit is reached through a process called impedance matching! I got my Phyisics degree about 13 years ago, so can't go through all the particular details anymore - but I remember the basic idea!
  17. geshel


    Oct 2, 2001
    Given that the Ashdown is rated at 4 ohms minimum, I'll support your point that it wouldn't be good to run it with 2.67 ohms.

    As for the rest of what you say, I'll be concise: you don't know what you're talking about (I understand you've been educated, but you've forgotten too much). If anyone wants me to go into further detail, you know where to find me. :)

  18. OK - I'll admit it has been a long timeand I know I've forgotten a lot, but I'm still interested in this stuff!!! Gonna PM me and show me where I've gone wrong!!:)


  19. geshel


    Oct 2, 2001
    Ahh good, I was just editing my post to make it more diplomatic, but I see that's not necessary. :D

    I'll post it here for everyone's "benefit" :)

    - impedance matching is something that applies in certain circumstances and doesn't really have much to do with a solid-state amp driving a speaker. (the most obvious one that comes to mind is RF circuits, where impedance mismatches cause reflections that make the circuit less efficient)

    - A SS amp is essentially a (variable) voltage source. The current, and therefore power, at maximum voltage are determined by the load impedance. Reduce the impedance, increase the power. At some point, the amount of current the amp can supply is maximized, and it clips/melts/whatever. In the case of the Ashdown, what I meant was (and I think you agree, sort of, since you mentioned P=IV) that at 2.67 ohms the max power output would possibly be more than 250w. It's also possible that it couldn't supply the necessary current to put >250w into 2.67 ohms, so the max power would be less. Either way, not recommended as the amp wasn't designed for it (and here we're definitely in agreement).

    - tube amps use an output transformer to buffer between the (quite high-voltage and low current) tubes and the speakers. Different taps on the transformer are available for different load impedances, so that the load the tube sees is more or less the same. Now, I'm not certain about this part, but: because the tube circuit is designed fairly specifically for this load impedance, if you mismatch the driver with the transformer setting (for example, using a 4-ohm load with the 8-ohm tap), the circuit clips (voltage peak for too-high loads, current peak for too-low loads) before max power is reached. -oOOo- If somebody who knows more about tube amps could tell me if (and how) I'm wrong, I'd appreciate it.

  20. Thanks for the info, and no, from my perspective no increased levels of diplomacy were required. Yes I was educated once - but as it hasn't been a daily requirement from my job, I have forgotten the precise facts!!! I guess my statement about education was more - 'you can tell me in reasonably technical terms and I'll understand it' And wasn't 'I've got a degree, I'm right your wrong'.

    Once again - thanks:)

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