The ole less ohms than the amp can handle question ...

[uwe]

Ok, I've got a transistor Markbass combo, delivering 500 watts at 4 ohm, no 2 ohm setting, it's not an Ampeg after all. It has an 8 ohm 15" onboard speaker and I add - parallel of course - an 8 ohm 2x10" external cab, so I draw speaker power with 4 (8:2=4) ohms, so far so good. I have another Markbass 8 ohm 12" speaker and always wondered how it would sound if I added that - again in parallel - to the onboard 15" and the external 2x10". I understand that I will then be drawing with roughly 2.7 (8:3=2.67) ohm from a 4 ohm amp, is that an issue if I ALWAYS play the combo at less than half (more like 1/4 or 1/3) of its potential volume?  I'm no expert at all, but is the difference of only 1.3 ohm life-threatening?

Of course, mixing a 15", a 12" and 2x10" speakers might sound real crap (or make no real difference at all), but that is another issue. I've always had a penchant for combining different speaker sizes, in my heyday I ran 2x18", 1x15" and 4x10" together (and never had issues with not being heard well even if I didn't play loud).

[Aussie Mark]

Most modern amps have a lot of inbuilt protection circuits that kick in under different circumstances.  I don't know for sure, but it is quite possible that the Markbass will shut down if it detects a load like that.  If not, it will run a lot hotter, which will likely kick in the protection trigger at a certain temperature and shut it down anyway.  Either way, I don't think you will kill the amp during a short test.

[amptech]

Never tinkered with transistor designs, but in an old book on (early) transistor amps I have different output stage designs is described. Some can handle near short loads, and fry with too high loads - other will fry with a near short load but can tolerate higher impedance loads.

Any way, even though you might have a protection circuit, proper load should still be applied.

As a general rule, it is not advisable to mix speakers so that the impedance does not add up.
But fortunately, since you have three 8 ohm speakers, can´t you just hook up two of them in series to get 16 ohms,
then hook up the remaining one in parallel ( = total 8 ohms ) - and plug into your 8 ohm output?   

[Psycho Bass Guy]

The Markbass has a switching supply AND a switching output section; that's a whole boatload of regulation you're going to be taxing in ways it may or may not be able to handle, even for short periods. (Out of curiosity, what is its mains fuse rating?) Keep in mind that these amps are are designed to be disposable, period, so the ways you could "cheat" with older conventional designs don't apply. Their "expected" lifespan is roughly a decade at best whereas, older class A/B amps were good for 30-40 years, and with maintenance, could last indefinitely.  Using the speakers the way you suggest may work just fine, or it might let out the magic smoke. If you want the acoustic (spl) benefit of using all four cabinets and don't mind the phase smear and loss of damping, (the mids will be blurry and the amp will be decidedly less punchy) amp tech's recommendation to put some of the speakers in series is the safe option.

[uwe]

Thanks, good advice from all.

[slinkp]

I'm no amp tech -- nor am I  amptech ;-)   -- but, an 8 ohm load in parallel with a 16 ohm load will not give you a total of 8.
You'll get  N where  1 / N =  1 / 8 + 1 / 16.    So N = 5.33 ohms... which I'm guessing would be fine for your amp.

Also, maybe not super important but it might be interesting to be aware that the 8-ohm load will get more than half the power, and the other two will split whatever's left.  I cannot seem to remember how to apply Ohm's Law to figure out exactly how much power goes to each in this mismatched scenario ....  But, is it a bad thing? I don't think so, but it might translate into one cab sounding noticeably louder than either of the others.

It sounds like it wouldn't hurt anything to try,  though, so if you like the result, more power to you ;-)

[slinkp]

I remember now.  The one cab will get 2/3 of the total power,  and each of the others will get 1/6th.

So if your amp was delivering 300 watts into the total load of 5.33 Ohms, you'd be putting 200 watts into one cab and 50 watts into each of the others.  I think. Can anybody confirm?

[Psycho Bass Guy]

but, an 8 ohm load in parallel with a 16 ohm load will not give you a total of 8.

Where are you getting this number? The only way the amp "sees" a 16 ohm load is with a series connection somewhere.

[slinkp]

Maybe I misunderstood the suggestion, but Uwe was originally trying to figure out how to hook up THREE 8-ohm cabs, and Amptech said:

"But fortunately, since you have three 8 ohm speakers, can´t you just hook up two of them in series to get 16 ohms,
then hook up the remaining one in parallel ( = total 8 ohms ) - and plug into your 8 ohm output? "

So that's where the 16 comes from ... two 8-ohm cabs in series.

I can't figure out any way to combine three 8-ohm cabs and end up with 8 ohms.

The options I can think of are:

A. All 3 in parallel = 2.66 ohms

B. All 3 in series = 24 ohms

C.  2 in series, the 3rd in parallel with the pair = 5.33 ohms total if I got my previous post right ... this was what I thought Amptech was suggesting...




or

D. 2 in parallel, the 3rd in series with that pair = 12 ohms total  ... or maybe this is what Amptech meant?

[amptech]

As I said, I would not recommend mixing speakers that will not add up - but by hooking up two cabs in series
and one in parallel across them will be the close enough to try out (though not exactly 8 ohms - sorry for being inaccurate here).

[slinkp]

Just want to be clear I wasn't trying to bust your chops nor show off my crappy diagram-making skills... to the contrary I'm  trying to learn something here....  I was hoping to either confirm I understood the three-cab wiring options correctly or that someone tell me where I went wrong.  I really appreciate having guys here who understand amplifiers a damn sight better than I do.

[amptech]

No problem, I really should have used a near equal sign here (= 8 ohm total) but don't have one on my keyboard