Impedance Question-8ohm & 4ohm=???

[rodl2005]

Ive just bought the great Fender bassman 300pro=love it!!
But was wondering --- I've got a 4 ohm cab(4x10) & an 8 ohm cab(1x15") what impedance would adding these together come to??? & would the Fender-switchable 8/4/2ohm be able to handle that configuration?

Rod

[ChrisK]

If they're connected in series, 12 Ohms.

Resistances add in series.


If they're connected in parallel, 2.67 Ohms.

Resistances do this in parallel. R1*R2/(R1+R2)

[rodl2005]

How do U mean series parallel? I'm just thinkin of plugging my SVT410HLF cab-(4 ohm) into main spkr out, & the EV 1x15"-(8 ohm) into the other spkr out on the back of the amp! How do I tell re series or parrallel? I mean I know how to do it INSIDE the cab, as I've built cabs B4 & looked into this in detail, BUT don't know re if the cabs are already built & are in the configuration I described above?? HELP please- I don't wanna hurt the brand new Fender bassman300PRO!! which is switchable -8/4/2 ohm! Help PLEASE!!!!
Thanx, Rod.

[ChrisK]

I 'aven't a clue. You'll have to look at the schematic for the amp and see what it does.

I don't see the schematic on the Fender site, so you're on your own.


BTW

You might want to consider reading the user's manual. On page 7, sections K and L indicate that parallel is used.

www.fender.com/support/manuals/pdfs/manuals_elec/basspdf/Bassman_300_PRO.pdf

2.67 Ohms it is. 2 Ohms will probably work since resistances are usually slightly lower that the standard 4 and 8 Ohms.

[rodl2005]

Yep, done that , Ta. Rod.

[JohnH]

Rod - Hi from not so far away (NSW)
Your question is a bit like the issue I had at x2 your Ohms, in this thread:

guitarnuts2.proboards45.com/index.cgi?board=amp&action=display&thread=1150767138

I was working out how to combine my 16Ohm combo speaker and my 8Ohm cab. In fact, see them on the left - (ignoring the little guy)

You are lucky to have those three options for setting the output impedance, and based on advice around here the 2Ohm setting should work in your case. The main thing to watch for is that the 4Ohm cab will produce 2/3 of the output, and the 8Ohm cab 1/3, which might suit you or it might not. I was trying for a balance between the two.

good luck

John

[vonFrenchie]

Heres a handy little impedance calculator. It allows you to use up to 16 speakers in ser-par, 4 in each row and 4 in each column

colomar.com/Shavano/impedance_proc.php

[UnklMickey]

a word of caution:

the calculator seems to work okay.

however, there is a statement in the text that just don't flush.

How Much Power is Going to Each Speaker?
If all of the loads are initially the same, then the power is spread equally across all loads. For example, if you have 2 speakers wired in Series, both rated at 8 ohms, and you apply 100 Watts to the load, then the power is equally distributed among the 2 speakers - in this case, each will absorb 50 watts of power. The same holds true for speakers wired in Parallel - if the loads are the same, then the power will be distributed equally - 2 speakers wired in Parallel with 100 watts of power applied to them will have 50 watts applied to each individual speaker driver.
The same holds true for 4 speakers wired in Series, Parallel or Series/Parallel.

If the different speakers have a different impedance, the lower impedance load will have more power delivered to it. Its a good idea to make certain that any given driver can handle the power you plan to apply to it. I suggest that you build or buy your systems where all of the cabinets have the same impedance (it will greatly simplify dealing with them if you suddenly scale up the number of cabinets that you use).

the statement in red is true ONLY if the speakers are in PARALLEL.

if they are in SERIES, the speaker with the HIGHER impedance will have more power delivered to it.


unk

[ChrisK]

Indeed!




Use delivered load voltage for parallel power calculation,

[VDpar = [Pout*Rparallel]^^0.5]

[Pdriver = VDpar^²/Rdriver]

Which simplifies to:

Pdriver = Pout * Rparallel/Rdriver


and induced load current for series power calculations.

[IDser = [Pout/Rseries]^^0.5

[Pdriver = IDser^²Rdriver]

Which simplifies to:

Pdriver = Pout * Rdriver/Rseries

[sumgai]

Chris,

Indeed!

Use delivered load voltage for parallel power calculation,

[VDpar = [Pout*Rparallel]^^0.5]

[Pdriver = VDpar^²/Rdriver]

Which simplifies to:

Pdriver = Pout * Rparallel/Rdriver


and induced load current for series power calculations.

[IDser = [Pout/Rseries]^^0.5

[Pdriver = IDser^²Rdriver]

Which simplifies to:

Pdriver = Pout * Rdriver/Rseries

Indeed yourself!

The only quibbles I'd have would be that one could conceivably arrive at either formula by using either voltage or current for series or parallel, it doesn't matter which way one starts out. But the math you didn't show is pretty hairy! Thus you made it simple for the non-engineer, and that's good.

But you did refer to impedance as resistance, and that's a Romper Room no-no! If you'll allow me to reconstruct your final formulae, they should read:

Parallel loads, power distribution:
P_speaker = P_total * (Z_total/Z_speaker)

and

Series loads, power distribution:
P_speaker = P_total * (Z_speaker/Z_total)

Of course, all of this is theoretical, it doesn't include core losses, frequency response, back emf, etc. But what's a few semi-minute watts amongst friends, eh? ;D


sumgai

[ChrisK]

FTW

But you did refer to impedance as resistance, and that's a Romper Room no-no! If you'll allow me to reconstruct your final formulae, they should read:

vs

Thus you made it simple for the non-engineer, and that's good.

Well sparky, make up yer mind. ;D

[sumgai]

Chris,

.....
Well sparky, make up yer mind. ;D

Oh? And just because someone in antiquity stupidly decided to assign the moniker "Ohms" to both resistance and impedance, does that mean that we can now say to a newbie, "well, they're really the same thing, so don't worry about it"?

But my real point is, even if some other websites get it wrong (whether for the sake of simplicity, or from sheer stupidity, it doesn't matter), I'd just as soon not perpetuate incorrectness, if that's alright with you.

Enough, I've picked my daily quota of nits. Let's move on, shall we? ;D


sumgai

[ChrisK]

It's not the impedance that I worry about, it's the impudence.

(I'm not sure what this means, but it sounded good.) ;D


Yeah, I know, but I'm the only person that I know that has an LCR meter (personally owned) as well as the ability to actually measure impedance (which I rarely do). 'Course I work for a $30 billion company.

It's impedance for sure, but we ALL just use a DVM to measure it.

[UnklMickey]

sumgai said:

...someone in antiquity stupidly decided to assign the moniker "Ohms" to both resistance and impedance...

not stupid at all!


recognizing the the difference between resistance and impedance :

valuable.

recognizing the similarity:

priceless!




can you say.................vector?

[sumgai]

unk,

.........
can you say.................vector?

Only when I'm on a real bender! ;D


sumgai

[rodl2005]

Well U GOT ME way back too!!!!
All I'll go with- I think - is that it adds up to approx 2.67 & I'm happy for the 4x10" to take more than the 2x10" or 1x15''
Either way- I reckon I'll be able to use 'em!! & with THAT Fender 300pro(all tube) amp-it'll add up to an ALLMIGHTY bass sound Thats for sure- Thanx for the help guys
p.s. sorry I'm not tech. minded enough to join yr debate on impedance/resistance/ VDPAr=rout parallel(sounds like sumthing U catch being a naughty boy!!!)
ROD