Newbie question about wiring (series vs. parallel)

[queequeg99]

I have a very basic wiring question about a Tuba18 I’m building.

The speaker has two MCM 55-2421 drivers (4 ohm speakers). I’m using a BASH 300 plate amp. Right now, I’ve just built this one cab but I fully expect to build a twin cabinet in the next month or so. Based on my limited understanding of speaker wiring (there is a reason I decided not to become an engineer during my freshman year), I think I should be wiring the speakers within each cab in series. Then I could run the two cabs off my amp in parallel (I would split each channel within the little box I’m building for the amp and have two sets of binding posts in the box - one for each cab). Does this sound like the appropriate approach?

[sine143]

honestly, you can do either or. Read page 19 of your plans. It states 2 parallel drivers as 4 ohms, 2 series as 10 ohms. compare this to the power handling of the MCM and the output of the bash 300 at 4 and 10 ohms.

One thing about series wiring and MCM. if you wire and series, and have your amp turned up "where you like it" and happen to suffer a driver failure in one of your cabs, its likely you'll blow up the other one as well, because impedance will drop for the system. The same is not necessarily true for parallel wired drivers.

[Tom Smit]

If you wire the drivers in series (inside the cab), they might be able to handle the power. But, you would have to be careful. Do voltage measurements with sound tracks.

Two cabs can be hooked up to the existing two amp terminal. That will save you some work.

[sine143]

tom, bash 300 is 300 into 4 ohms, not into 8 ohms, just saying.

[queequeg99]

honestly, you can do either or. Read page 19 of your plans. It states 2 parallel drivers as 4 ohms, 2 series as 10 ohms. compare this to the power handling of the MCM and the output of the bash 300 at 4 and 10 ohms.

One thing about series wiring and MCM. if you wire and series, and have your amp turned up "where you like it" and happen to suffer a driver failure in one of your cabs, its likely you'll blow up the other one as well, because impedance will drop for the system. The same is not necessarily true for parallel wired drivers.

Thanks for pointing out page 19. That helps. I understand the logic behind your advice to wire the speakers within the cabs in parallel. However, if I connect the speakers inside the cabs in parallel, I'm concerned about then running the two cabs off the amp in parallel as well. If I'm understanding things correctly, if I connect the cabs to the amp in parallel, won't I be starting with a system that has dangerously low impedance? The minimum impedance for the bash is 4 ohms. Should I connect the two cabs to the amp in series?

[sine143]

Thats up to you to decide. a couple hours of music playback in each configuration should highlight a clear winner based on faceplate temp alone. IMO the bash should run 4 ohms fine at music levels. Esp considering your going to adjusting the input attenuator up if you series wire them (run the amp at a higher output level) to match a pair of parrellel cabs. 4 ohms is within the operating range of the bash.

[queequeg99]

I think I was concerned that if I take two cabs that each present 4 ohm loads and connect them to the amp in parallel, the total impedance will be well under 4 ohms (i.e. less than the bash's minimum impedance). On the other hand, if I connect the cabs to the amp in series, total impedance will be 8 ohms, which the bash can handle (albeit at reduced output power). FWIW, I won't be cranking these things up to reference volumes.

There is also the distinct possibility that I have no idea what I'm talking about. This entire project has been a tremendous learning experience.

[sine143]

read my post. the one you quoted. Page 19 of the plans, 1 cab does not present a 4 ohm load. a horn adds impedance to the load. the values are listed in both my post and page 19 of the plans

[queequeg99]

Thanks for bearing with me sine. You've been very helpful and I am honestly not trying to waste your time. Yes, I see that I mis-stated the total impedance for series wired drivers. My apologies. When I made my immediately prior post, I was assuming that I would parallel wire the drivers in each cab (an assumption I'll stick with for now).

Your original post said "2 parallel drivers as 4 ohms", which is totally consistent with page 19 ("A pair of 4 ohm drivers parallel wired give 4 ohms nominal impedance."). Am I correct in assuming that the end result of 4 ohms in both statements already takes the effects of the horn into account? Otherwise, I would expect the total impedance of a pair of parallel-wired 4 ohm drivers to be just 2 ohms. If my assumption is correct, wouldn't a cab that contains two parallel-wired 4 ohm drivers present a 4 ohm load to the amp? If not, I must be missing something very fundamental. But if my assumption is correct, I get back to my concern about connecting two cabs (each presenting 4 ohm loads) to the bash in parallel.

Or does the increase in impedance due to the horn need to be taken into account at every level of connection (i.e. both when wiring the individual drivers together as well as when you're considering how the wire the cabs to the amp)? For some reason that I cannot quite articulate, this does not make intuitive sense to me. It seems like double dipping. However, there isn't much about electromagnetism that makes intuitive sense to me.

[Grant Bunter]

Each individual cab (in this case) adds 2 ohms impedance.
2 x 4 ohm drivers in one cab parallel wired is 2 ohms, + 2 ohms for the cab, = 4 ohms. Correct.

Normal direct radiator cabs do not add 2 ohms of impedance, ie 2 x 4 ohm drivers wired parallel in them would have a cab nominal impedance of 2 ohms.

You're also correct when you say that 2 x T18's with parallel wired MCM 4 ohm drivers, with both cabs run in parallel also, will have a nominal impedance of 2 ohms.

Given your bash 300 can only deliver a 4 ohm load, your plan to run 2 x T18 cabs (as described above) in parallel is not achievable.

Despite the caveat earlier from Tom, that if one driver blows in serial wired cabs, then that cab stops working, or perhaps blows the other driver as well, you can achieve a (greater than) 4 ohm overall load by serial wiring each cab and parallel wiring both cabs together.

In that scenario, your values are:
4 + 4 + 2 = 10 for one cab
10/2 = 5 ohms for two cabs wired in parallel

When drivers are wired in series in the cab, the voltage (we speak volts here, and voltage is one of the products, and measurable, used to calculate watts) is doubled. I don't know what the specified voltage for a single MCM 4 ohm driver would be (the T18 isn't a pro sound cab so the plans don't give maximum voltages).
Suffice to say, you wouldn't want to run your bash to maximum volume.

Hope this helps...

[sine143]

Bahaha im so dumb, missed the dual loaded cab caveat. Series wire the drivers in the box parallel the boxes together!

[queequeg99]

There we go! Excellent. Confusion has been eliminated. I won't be pushing this system to reference volumes (or even anywhere close) so the suggestion to wire the drivers in series and the cabs in parallel is the approach I'll take.

Thanks all for helping me along.

[Tom Smit]

[tomlang]

Excellent explanation. In my mind I'll think of this as "electric impedance" and "acoustic impedance". The electric impedance being the speaker impedance (eg 4 ohms) and acoustic impedance (eg 2 ohms).

OK, so now I have plans for the David speaker. It, like the Tuba 18 referened earlier, has in the plans stated a 6 ohm woofer impedance using the 4 ohm MCM speaker.

I'm thinking of getting a dvc speaker with two 4 ohm coils wired in parallel. OK, then 2 ohm "electrical impedance" and another 2 ohm "acoustical impedance" for a total of 4 ohms load now not 6 ohms.

The David speaker uses a crossover that would need to be modified for use with a 2 ohm impedance speaker.

The question is: Do the inductor and capacitor values in the plans reflect the 4 ohms "electrical impedance" or 6 ohms "electrical + acoustical impedance"? I need to know so I can scale the inductor and capacitor values. For example, if the woofer crossover was originally designed for 6 ohms, I need to scale the inductors by 2/3. If the woofer crossover was originally designed for 4 ohms I need to scale the inductors by 1/2.

Thanks!

[Bill Fitzmaurice]

There's nothing to be gained by using a 2 ohm load. Stick with a four ohm driver.