Question for our experts - why aren't more amps designed for 2 ohm loads?

[agedhorse]

Yeah I'm not encouraging people to try it just saying it's the reason why that particular amp is not 2 ohm stable

No, that's not the reason. It's internally bridged which is an essential part of the topology but not the cause. As an example, the Subway 800 watt models are internally bridged yet easily capable of driving a 2 ohm load reliably.

It's a design choice, nothing more than that.

 

[SunByrne]

Someday I'm going to buy a Mesa amp just as a way of honoring @agedhorse's contributions to TB.

 

[Pop C]

I checked the OM and it states 2 ohms minimum.

It's possible there is some problematic interaction between the amp and these particular cabs. The impedance rating of a cab is a nominal value, and actual impedance varies by frequency. The actual impedance at some frequencies drops below the nominal rating. You could try the amp with different cabs.

It's also possible the amp has a problem. Contact DarkGlass.

I have no idea how common it is, but you are not the first to have problems running a DG amp at 2 ohms.

I appreciate the feedback and your expertise. I sold 2 of those cabs some time ago. As I'm sure you are aware of, those particular speakers have a dual voice coil switchable between 4 & 8ohms. I have no idea how close to 2ohms I really ended up with, running 3 of them as I did. I have used the Darkglass with the one that I kept, as a single cab setup in a small venue with it set at 4ohms with no issues. Obviously, I didn't have it turned up much at all because it's a 900watt head and a 300 watt cab combination. I recently got ahold of an Ampeg Portabass 250, which is a much better match!

These cabs, right, with the selectable 4/8 Ohm impedance?

Regardless of the reliability of your amp at 2 Ohms (which, it seems, might be questionable), don't run two of those 112s at 8 Ohms apiece and the other at 4 Ohms. Doing so, you're stressing the 4 Ohm cab way more than the others (since it's getting twice the juice) and significantly limiting how loud your rig can go (since the 4 Ohm cab will hit its limit when the other two are halfway there).

Set all three cabs to 8 Ohms and your amp to 2 Ohms. You'll get more usable power from the rig (even if the amp will make a little less in theory, it'll make more - and more usefully distributed - before toasting a cab), present an easier load to the amp (2.7 vs. 2 Ohms), and your rig should sound better (at least fuller and more even) since all of the 12s will see equal power.

Now, a 212 + two 112s, the 212 should be 4 Ohms, but that's still equal power to each woofer.

Easy mistake to make, but trying to run at 2 Ohms vs. 2.7 isn't helping you in this case.

EDIT: With a solid state amp like this, setting it to "2 Ohms" means "2 Ohms minimum." It'll be fine (and have an easier time) running a 2.7 Ohm load.



Having recently bought a fridge with my mom, I appreciate hearing this. We asked about how often the models we were looking at came in for service (and for what) and in the course of the conversation got a glimpse into some of what you're saying re. parts used (among other things).

Figured, with something like a fridge, it wouldn't hurt to know what the guy repairing them would get for his mom.

EDIT: No, not an Ampeg fridge - the kind with a compressor (one of the parts we asked about).

EDIT EDIT: No, not that kind of compressor...

Thanks! Never thought of that 8-8-4ohm thing. Makes sense though. It's all musical history now, as I sold 2 of those cabs off a couple years ago. I kept 1 to run at 4ohms for small venues, but recently bought 2 PJB C-2 cabs for that purpose running them with an Ampeg Portabass 250. For outdoor shows I have a PJB M-500 and 2 C-9 cabs. Still have the Darkglass, but pretty much bring it along as a backup these days. Thanks for sharing your knowledge!!

 

[MarkA]

I appreciate the feedback and your expertise. I sold 2 of those cabs some time ago. As I'm sure you are aware of, those particular speakers have a dual voice coil switchable between 4 & 8ohms. I have no idea how close to 2ohms I really ended up with, running 3 of them as I did. I have used the Darkglass with the one that I kept, as a single cab setup in a small venue with it set at 4ohms with no issues. Obviously, I didn't have it turned up much at all because it's a 900watt head and a 300 watt cab combination. I recently got ahold of an Ampeg Portabass 250, which is a much better match!

Thanks! Never thought of that 8-8-4ohm thing. Makes sense though. It's all musical history now, as I sold 2 of those cabs off a couple years ago. I kept 1 to run at 4ohms for small venues, but recently bought 2 PJB C-2 cabs for that purpose running them with an Ampeg Portabass 250. For outdoor shows I have a PJB M-500 and 2 C-9 cabs. Still have the Darkglass, but pretty much bring it along as a backup these days. Thanks for sharing your knowledge!!

Cool - glad you've got something that works better for you (sounds like the Ampeg might be more to your taste in general, too). Sounds like you've got the big rig sorted, as well.

I generally like the feeling of a big (read "powerful, responsive") amp behind me, working well within its powerband, but try the cab at 8 Ohms with the Darkglass. As big as the power/power handling gap is between amp and cab, you should still have plenty of dynamic headroom and you might feel that the pair feels subtly tighter and more responsive at 8 Ohms than 4... or you might not.

If you dig the feel and sound better at 4 Ohms (and I can see why you set it this with the Ampeg Portabass), you can put it back - you're already aware of the power/power handling differential and know to use your ears and an appropriate degree of caution.