# Quick yes or no Question Regarding impendance?

Discussion in 'Amps and Cabs [BG]' started by mz91, Oct 9, 2003.

1. ### mz91

Apr 19, 2002
Zug, Switzerland
Sorry but i need to check if i got this straight...If you run a Poweramp in bridge mono mode into a speaker does it half the impedance. So if i run a poweramp bridge mono into one 4 Ohm cab, ill be running at 2 Ohms????
And second question is if its alright to run a Poweramp on stereo mode but just use one cab so just using the left side of the poweramp. Is that okay for the amp?
Thanks for answering the retarded Questions.
Michael

2. ### throbbinnut

For bridge operation, add both channels minimum impedance. If each channel is rated at 4 ohms, then you can only go down to 8 ohms in bridge mode.

You can run just one side of a stereo amp by itself.

Chris

3. ### Petebass

Dec 22, 2002
QLD Australia
Not always. I was just talking to someone about Camco amps - 2 ohms each side bit it bridges at 1 ohm apparently......

You'd better check the specs for your particular amp. What model is it?

4. ### joshsmog

Aug 25, 2003
call up the company of your amp tell them model number and ask away.

5. ### BillyB_from_LZSupporting Member

Sep 7, 2000
Chicago
I agree with Throbbinut as long as the stereo amp is solid state...

Pete, those numbers seem completely backward. If you look at it from an Ohm's (and Watt's) law perspective, bridging a stereo amp, doubles its potential voltage output since one channel handles positive voltage swing and the other channel handles negative voltage swing. Assuming that the maximum output current stays the same in the stereo and bridge modes (same output transistors), the minimum load impedance that would relate to double the voltage but the same current is double.

Example: 800 watt amp into 2 ohms causes 20 amps to flow (P = I*2 x Z) and the voltage is P/I or 40 volts. If you double the voltage but keep the current the same, the load impedance would double (80 volts/20 amps = 4 ohms).

Thus at 2 ohms, the amp puts out 800 watts per channel and bridged, it puts out 1600 watts (20*2 x 4) bridged into 4 ohms.

Clear as mud?

6. ### Petebass

Dec 22, 2002
QLD Australia
Shows to go ya! Don'y believe everything you're told. He's a reliable source usually. Ah well......... thanks Bill.

7. ### mz91

Apr 19, 2002
Zug, Switzerland
Okay my post wasnt clear to begin with. I want to buy a poweramp that i can use at home. And i have a Aguilar DB659 pre and a Epifani 2*12 cab. So one of the cheap alternatives to a poweramp would be the Crown XLS 602 (http://www.crownaudio.com/amp_htm/xls.htm). So lets say i plug the epifani into it at bridge mono mode. Since its a 4 Ohm cab the poweramp would be running at 2 Ohms? See i was going to buy this poweramp as a backup one since its cheap and the tech told me that he wouldnt cause it would be running at 2 Ohms with a 4 Ohms box . I was confused cause i thought a 4 Ohm box onto a poweramp would mean it would be running at 4 Ohms. The only way i thought you could change impedance was with running multiple cabs and either running them parallel or series. I am pretty confused as you can see. What i guess im going to have to do is save up the money for a QSC PLX, but those puppies are expensive here in europe! If i bought the QSC, and ran bridge mono, it would be running at 2 Ohms? or 4? Okay i need to stop this is starting to make my head spin!!
Thanks for all the replies guys!!!
Greetings.
Michael

8. ### Petebass

Dec 22, 2002
QLD Australia
So you've got a single cab with 2x12's in it that's rated at 4 ohms.

Which ever amp you use will see a 4 ohm load when connected to this cab.

All you have to worry about is wether or not the amp can handle 4 ohms in bridge mode.

Ignor all that stuff about 2 ohms. The tech probably misunderstood you. He probably thought you had 2 seperate cabs and they were 4 ohms each (2 ohm total).

9. ### nonsqtrThe emperor has no clothes!

Aug 29, 2003
Burbank CA USA
Hi mz91, I think you were right in the first place. Ohms is ohms and that's all there is to it. If you have a 4 ohm cab it will always be 4 ohms and the only way to change that is to put more cabs in series or parallel.

Now as far as what your amp "sees", that's a different story. It depends on whether it's a tube or solid state amp, whether there are transformers involved, etc etc.

It sounds like maybe the concern your tech was trying to express is that 4 ohms is below the minimum impedance in bridged mode. But the spec sheet in your link clearly indicates that it is acceptable for the amp to run bridged at 4 ohms, so this shouldn't be a concern. I wouldn't run it at full power for too long (it'll tend to get hot), but with that kind of power it shouldn't be necessary to turn it more than halfway up.

As far as a bridged amp changing the impedance of a speaker, that's not possible. Always trust the manufacturer's spec. If they say it's okay, it probably is.

10. ### mz91

Apr 19, 2002
Zug, Switzerland
thanks pete, see thats what i thought...Did you check the specs to the XLS 602? Should be fine no? Or you think i should save up serious cash (more than double!!) and go for the QSC PLX?
Michael

11. ### mz91

Apr 19, 2002
Zug, Switzerland
maybe i should just forget the bridge mono mode and run it on one side on stereo mode???? should be okay right?
Michael

12. ### Petebass

Dec 22, 2002
QLD Australia

Holy Moley. Bridged, that amp pushed 1,680W at 4 ohms. That's...um.... a lot! And it's only 12.3 Kg.

Looks like a good buy to me, on specs anyway. I can't comment on the sound or reliability though. Maybe someone out there can give a first hand account? Anyone? Anyone?

Lots of people on TB use PLX amps and they've proven to be very good, reliable amps. Bob Lee from QSC hangs around here quite a bit which is a bonus to all QSC users.

13. ### mz91

Apr 19, 2002
Zug, Switzerland
Thanks alot for the reply. Yea Bob is really a blessing to QSC costumers..I have a RMX. I want a PLX but its hard to afford since im a student!
Michael