impedance vs sound quality

illusha · #1 02-04-2011, 12:22 PM

curious as to what you guys think on the topic of impedance vs sound quality... a vey comon technique with 8-ohm cabs is to pair them up in parallel, which presents a 4-ohm load to the amp... at a first glance seems like lots of free additional power, right? but I know that everything comes with a price, so... what's the catch?

I realize that impedance varies with frequency, and that lower frequencies generally yield higher impedances, with 2 major peaks due to box resonances... so an 8-ohm speaker's impedance will vary from 4 to 50 ohms... I also realize that lower impedance puts more strain on the amp's power supply and output transistors due to the increased current flow... also, lower impedances compromise the damping factor, but how overrated is it anyway? how is the sound dynamics affected?

so, I realize that sound quality will be marginally better with an 8-ohm load vs a 4-ohm load... but is it worth the loss of free power??? what are the advantages and disadvantages? I think this may be difernent for subs, mids, and highs... any opinions or recomendations???

Vinny D · #2 02-04-2011, 01:40 PM

People try to debate this all the time, I highly doubt anyone listening to any cabinet (and not knowing what it is) could tell the difference if they are listening to a 8ohm cab or a 4ohm cab.

okcrum · #3 02-04-2011, 01:43 PM

Amplifier damping factors are generally so low that load impedance is a nonissue as far as sound quality.

T-Bird · #4 02-04-2011, 01:44 PM

Hi.

The only thing to worry about is to match the load with an amp that's known to take it. Safely.

The myth about one nominal impedance being better than another must originate from OT windings of a tube amp, highest tap using all the winding etc.etc.

On a SS, if the amp can handle the load, it can. That's regardless of what the amp manufacturer may tell, not all the "4Ohm" amps can deliver what the manufacturer may promise.

On a well engineered combination, it makes no difference to the sound whether the nominal load is 8 or 4 Ohms, but as the "free power" you speak of gives only 3dB, why bother. Unless the available cab(s) and amp(s) combination is easier to set up as 4 than 8 ohms.

I personally always aim for 8Ohms, as 4Ohm capable bridgeable amps are rare in my budget. (and my current amps are all 8Ohm only)

Regards
Sam

walterw · #5 02-04-2011, 02:01 PM

the norm seems to be "8Ω per large driver";

a single 18" sub will typically be 8Ω, a 2x18" 4Ω. a two-way top box with one 12" or 15" + horn will be 8Ω, while a 2x15" + horn box will usually be 4Ω.

i think the idea is to distribute enough power into each big speaker without taking too much or limiting options for which amps push how many boxes.

crapshack · #6 02-04-2011, 09:44 PM

I'd really like to A/B two of the exact same cab but 8 or 4ohm - with one head at 3 precise volume and tone levels. Who has done this? Results? Which can you push harder with the same head?

walterw · #7 02-04-2011, 10:22 PM

by definition it won't be the "exact same cab"; the speakers will have different amounts of wire wound onto them, to get the different impedance.

in reality this doesn't matter, except that amps create more power running at 4Ω/side than they do at 8Ω/side.

with modern PA stuff, there is no "impedance vs. sound quality" question. it's just a matter of how much power the boxes get.

TimmyP · #8 02-04-2011, 10:34 PM

I'd say that if there is a difference, there would be a 50/50 split as to which one folks preferred. Most bass cabinets are so bad as to be considered sound producers instead of reproducers, and folks can't even agree on the best reproducers.

illusha · #9 02-04-2011, 10:48 PM

ok, maybe I wasn't quite clear... I was not talking about building a cab with either a 4-ohm or an 8-ohm driver in it, I realize that most single-driver pro cabs are 8-ohm, and most dual-driver pro cabs are 4-ohms...

what I was talking about was hooking up TWO identical 8-ohm subwoofers to power amplification... an amp that I have in mind (IPR) can handle both of those subwoofers if I run them parallel'ed to present a 4-ohm load on the amp... if I run them separately at 8-ohms, then the amp's power gets reduced in half and it can only handle one subwofer - therefore I need 2 amps... so it's not a matter of gettng +3dB...

specs: subs - Peavey SP-FH at 800 wrms, 1600 wprgm, 3200 wmax, 8-ohm
amps: Peavey IPR-3000 bridged 1680 wrms @ 8-ohm, 3000 wrms @ 4-ohm

walterw · #10 02-05-2011, 12:01 AM

Quote:

Originally Posted by illusha

what I was talking about was hooking up TWO identical 8-ohm subwoofers to power amplification... an amp that I have in mind (IPR) can handle both of those subwoofers if I run them parallel'ed to present a 4-ohm load on the amp...

you mean both cabs on one side of the amp? that'll work just fine.

Quote:

Originally Posted by illusha

if I run them separately at 8-ohms, then the amp's power gets reduced in half and it can only handle one subwofer - therefore I need 2 amps... so it's not a matter of gettng +3dB...

usually it's not exactly half, so one box per side will get marginally more power than both boxes on one side. don't know what you mean by "only handling one subwoofer", or "needing 2 amps".

Quote:

Originally Posted by illusha

amps: Peavey IPR-3000 bridged 1680 wrms @ 8-ohm, 3000 wrms @ 4-ohm

uh, i don't think that amp will bridge...

in general, whatever configuration of whatever subs and amps delivers the most clean watts into the most boxes will sound best. "there's no replacement for displacement."

illusha · #11 02-05-2011, 12:08 AM

that IPR-3000 amp WILL bridge fine, as will 1600, which has been confirmed by Peavey engineers, and many users on their forum... and yes I am thalking about bridging an amp, any amp, or taking a mono amp, and either using it at 8-ohm output, or at 4-ohm output... what I'm trying to learn is how that affects sound quality, power demands, amp's lowest impedance abilities, etc... and I am planning on having more than plenty of power available to those subs from amps, which will be digitally limited anyway, so having more then necessary is not going to do me any good...

walterw · #12 02-05-2011, 12:22 AM

ah.

so if there is a way of safely bridging that amp, then yeah, bridged into the 4Ω load of the two boxes in parallel will get you the most power, which will therefore sound the best for that combination of stuff.

there is an argument that bigger amps in parallel sound better than smaller amps bridged just because they're not getting "slammed to the wall"; most amp specs seem to indicate slightly more distortion in bridge mode, and they definitely run hotter and draw more power from the wall that way.

it's like a big V8 cruising along vs. a little 4cyl. screaming in 3rd gear.

illusha · #13 02-05-2011, 12:49 PM

well, sorry to disagree again =) but, actually, 2 amps each running bridged into 8Ω will provide slightly more power (1680w each) compared to 1 amp running bridged into 4Ω (3000w)... either configuration can drive 2 subs... but 2 amps cost more than 1 - about twice as much =) that's the dilemma...

again, power is not an issue - either at 4Ω or 8Ω I get plenty of power for my application, with headroom left over... the question is: should I buy twice as many amps and run them at 8Ω, or half of those amps running at 4Ω - to power the same amount of hornloaded subwoofers (8)...

seems like a simple answer, but there's gotta be a catch... any loss in sound quality? any loss of control of the driver? anything? anyone? anyone? Bueler?

steveksux · #14 02-05-2011, 01:19 PM

Don't worry about a little impedence. It happens to all guys at one point or another. Maybe you were just tired?

Worrying about it just makes it more likely to recur the next time.

Randy

walterw · #15 02-06-2011, 10:48 PM

Quote:

Originally Posted by illusha

well, sorry to disagree again =) but, actually, 2 amps each running bridged into 8Ω will provide slightly more power (1680w each) compared to 1 amp running bridged into 4Ω (3000w)... either configuration can drive 2 subs... but 2 amps cost more than 1 - about twice as much =) that's the dilemma...

not sure who you're disagreeing with...

i was referring to bigger amps not bridged vs. smaller amps bridged. bridging pushes the amp harder and has a little less fidelity (most specs show a higher distortion figure in bridge mode). a little distortion is not a big deal with subs (you don't hear it), but i have to think the non-bridged amps would be a little more reliable over the long haul.