impedence for DIY project help

[jaygeorge1979 10+ year member]

I am trying to figure out how home made passive crossovers affect impedence. I know components of a crossover have impedence, i just dont know if it affects the overall impedence of the system...for example, i have a two way loudspeaker...both the tweeter and the midwoofer are 8 ohm...does the final load need to also be 8 ohm or do most home audio amps do 4?...

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[PV Audio 5,000+ posts]

most receivers are four ohm stable //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif. the impedance of the crossover will not likely affect the longevity of the amp unless you choose some ridiculous 2 ohm components (this is assuming you are using a parallel circuit...)

 

[thylantyr 10+ year member]

Dictionary; Impedance

: something that impedes : HINDRANCE: as a : the apparent opposition in an electrical circuit to the flow of an alternating current that is analogous to the actual electrical resistance to a direct current and that is the ratio of effective electromotive force to the effective current

The speaker has a DC coil resistance as can be measured with a meter. ie, a

8 Ohm impedance drivers may have a 5.6 Ohm DC coil resistance. But when

you send different frequencies to the driver the impedance of the driver changes,

that's what the impedance chart tells you, at 'x' frequency impedance is 'y'.

So.. the 8 Ohm nominal impedance is really variable and this changing impedance

affects how the crossover works because the crossover is designed for a certain

fixed impedance and when impedance is all over the map depending on what

frequency is playing by the driver, then the crossover will perform differently.

The zobel is suppose to stabilize the impedance so the crossover 'see's a

more stablized impedance.

When you split the frequency range, high pass for tweeters, band pass for midrange, low pass for woofers, if each driver is 8 Ohm nominal, the amplifier

effectively 'sees' 8 Ohm as the load.

Also, amplifiers don't freak out as much driving low impedance tweeters vs.

midranges and woofers.

If you have a 'dumb' amplifier you can drive a much lower load if the load is

tweeters. ie, if your amp is only 4 ohm stable, you can drive 1 ohm tweeter loads

usually, but don't expect to drive 1 ohm woofers and they will be a bigger burden.

I drove a 1 ohm tweeter load on my Adcom home amp in bridged mode. It's

only stable in bridged mode at 8 Ohms, but I took it down to 1 Ohm.

Right now, that same amp is driving 0.8 Ohms on the tweeter array per channel.

Piece of cake.

 

[jaygeorge1979 10+ year member]

are you saying that you have tweeters running in their own seperate amplifier channel? this is because you have an active crossover right? each frequency gets its own amplifier channel...so...if i have two 8 ohm drivers, a passive crossover WITH zobel, wired in parallel then that channel of the amp would see 4 ohm?

 

[PV Audio 5,000+ posts]

bi-amp? what? //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

 

[thylantyr 10+ year member]

are you saying that you have tweeters running in their own seperate amplifier channel?

The line array I'm running is fully active, the tweeters are driven by a 2ch home

amp. The midranges are driven by a 2ch pro audio amp.

this is because you have an active crossover right?

I'm using the DCX

each frequency gets its own amplifier channel...so...if i have two 8 ohm drivers, a passive crossover WITH zobel, wired in parallel then that channel of the amp would see 4 ohm?

You are confusing an active setup with passive.

A passive setup is;

Amplifier output -> passive crossover -> tweeter

Same amplifier output -> passive crossover -> midrange

Same amplifier output -> passive crossover -> woofer

If the drivers are 8 Ohm nominal, amplifier 'sees' 8 Ohm nominal.

An active setup is;

Active crossover [high pass output] -> Amplifier #1 -> tweeter

Active crossover [bandpass output] -> Amplifier #2 -> midrange

Active crossover [low pass output] -> Amplifier #3 -> woofer

The drivers are directly connected to the amplifier. The active crossover

feeds each amp the proper frequency range set by you. The amplifier 'sees'

the driver impedance.

You can also do that with a passive setup, but it's dumb;

Ampifier #1 -> passive crossover -> tweeter

Amplifier #2 -> passive crossover -> midrange

Amplifier #3 -> passive crossover -> woofer

Or

Amplifier #1 -> passive crossover -> tweeter

Amplifier #1 -> passive crossover -> midrange

Amplifier #2 -> passive crossover -> woofer

Amplifier #1 can be whimpy and still perform good while amplifier #2 needs

more balls to drive the woofers, perhaps it can be a higher powered amp.

Or

Amplifier #1 -> passive crossover -> tweeter

Amplifier #1 -> passive crossover -> midrange

Active crossover -> Amplifier #2 -> woofer

 

[jaygeorge1979 10+ year member]

each frequency gets its own amplifier channel...so...if i have two 8 ohm drivers, a passive crossover WITH zobel, wired in parallel then that channel of the amp would see 4 ohm?

You are confusing an active setup with passive.

i am bad with words...first of all, i understand how your active setup works and why each amplifier channel would hypothetically see an 8 ohm load...that being said.....in car audio, when we would wire two 4 ohm subs in parallel, it would give a two ohm load taht the amplifier would see....unless im doing taht wrong, its been so darn long...i am trying to figure out if home audio when using passive crossovers for my loudspeakers, and ONE channel of my amp for a speaker cabinet which contains BOTH a mid and a tweet......8 ohm tweeter in parallel with 8 ohm midrange lets the amp see a 4 ohm load, assuming i am using a zobel network to stabalize impedence?

 

[thylantyr 10+ year member]

i am bad with words...i understand how active/passive setups differ...in car audio, when we would wire two 4 ohm subs in parallel, it would give a two ohm load taht the amplifier would see....unless im doing taht wrong, its been so darn long...i am trying to figure out if home audio is the same...8 ohm tweeter in parallel with 8 ohm midrange lets the amp see a 4 ohm load? but you said it would see 8? how is this possible?

In car audio, you did this;

Amplifier -> 4 Ohm sub in parallel with another 4 Ohm sub = 2 Ohm load.

-- no passive crossover.

If you did this;

Amplifier -> passive crossover -> 4 Ohm sub in parallel with another 4 Ohm

sub = 2 Ohm load.

Both woofers are connected together in parallel.

You arn't doing this with tweeters. You are not paralleling tweeters with

midranges and woofers.

The amplifier is driving three seperate crossover sections that in turn drives

each speaker {tweeter, midrange, woofer}.

You have to remember that this is impedance, impedance changes with

frequency. Each driver can have a different impedance depending on

what frequency is playing. Effectively, you can assume the impedance to

be 8 Ohm when you make a loudspeaker with passive crossovers if the

tweeter, midrange and woofer is 8 Ohm nominal, but this 8 Ohm 'total'

impedance isn't written in stone either, it changes.

Even when you paralleled those two 4 Ohm woofers for 2 Ohm, that 2 Ohm

rating is still nominal, it will change also, could be higher or lower but can't

drop below the DC coil resistance of the woofer. So, if your 4 Ohm woofer is

3.2 ohms DC coil, two in parallel would be 1.6 Ohm load worse case when

you think it's really 2 Ohms. /hehe Could be higher, the box affects impedance

too. What if it's really 2.8 Ohm playing some frequency? You thought it's suppose

to be 2 Ohm.. /hehe

Suppose you made a 3 way loudspeaker using 16 Ohm tweeters, 8 Ohm

midranges and 1 Ohm woofers using a passive crossover. The amplifier will

get punished by that 1 Ohm woofer because impedance is very low. The other

drivers won't drain the amp by comparison.

What if you use a 1 Ohm tweeter, 8 Ohm midrange, and 16 Ohm woofer in a design? The amplifier wouldn't freak out as much as the 1 Ohm tweeter won't

consume alot of power like a 1 Ohm woofer can. Also, that 1 Ohm may not

be 1 Ohm as it is also dependent upon which frequency plays.

When you build loudspeakers you can use lower impedance tweeters as long

as you have a 'dumb' amplifier. Even lower impedance midranges are ok as it

won't thrash the amplifier. Woofers are hogs, you really punish an amplifier.

A dumb amplifier would be one with no protection circuit that may detect

a low impedance and turn off to protect itself... Even though the tweeter

won't cause the amp to overheat, the amplifier may not cooperate with you.

I prefer amps with no impedance detection circuits. Heat sensor is fine, if it gets

hot then turn off. //content.invisioncic.com/y282845/emoticons/laugh.gif.48439b2acf2cfca21620f01e7f77d1e4.gif

 

[jaygeorge1979 10+ year member]

ahhhh i still dont get it...you say we are not paralleling the tweets,mids, and woofers....but arent we wiring their crossovers in parallel...doesnt each crossover read 8 ohms and doesnt this mean that we have 3 8 ohm drivers wired in parallel? maybe i dont understand the wiring....i would have one pos and one neg wire feeding from the amp into the loudspeaker, then i would connect three different xovers to it and then connect those to their respective drivers...how is this not wiring in parallel?

 

[thylantyr 10+ year member]

The key thing to remember is the loudspeaker impedance as a whole is only a nominal value.

Tweeter, midrange, woofer all 8 Ohms using a passive crossover would imply a 2.6 Ohm

nominal load because of the three parallel circuits. How many 2.6 Ohm store bought

speakers have you seen that use a single tweeter, midrange and woofer in the design

using 8 Ohm nominal drivers? none, they don't rate them for 2.6 Ohm, rather 8 Ohm

nominal because the frequencies are being split up and effectively the amplifiers sees a

a nominal 8 Ohm load. You can design a passive crossover with more 'stuff' that can

place more loading on the amplifier and the crossover itself will be placing more burden

on the amplfiier and total impedance can drop. If your AV receiver rated for 8 Ohms drives

your 3 way loudspeaker using three 8 Ohm drivers using passive crossovers, it works

fine it's done everyday without issue and those AV receivers aren't very robust in design,

if the impedance was 2.6 Ohms the revceiver would barf. Passive crossover design can

be tricky to make an excellent one.

 

[jaygeorge1979 10+ year member]

well put...well put indeed...finally cleared some stuff up for me...for this, i thank you

 

[thylantyr 10+ year member]

I stopped playing with passive crossovers, oh.. 20 years ago.... because I'm a 'power user'

who wants more from the sound system and only an active crossover system lets

me do this easy without worrying about all the passive crossover gremlin issues. An active

system will require more amplifier channels and a good active crossover but it more like

'plug and play' than a passive 'fixed' system. If you get your test bench running with a DCX,

you'll see how easy it is, you'll be spanking yourself for thinking about passive crossover design when

you realize why primitive it is. Seriously, a DIY'er who has passion for audio is wasting their time with passive.

But to be fair to the other camp, if you have product to sell the passive system is better. One speaker than

the consumer hooks up and it's all done, simple. You don't care if the consumer likes it, let them shop around

for 1000 loudspeakers to find the cool sounding one. I can make my loudspeaker give me different

sounds by tweaking the crossover. Why settle for one sound when you can create many?

/evil

 

[jaygeorge1979 10+ year member]

hey man...i jumped onto the DCX bandwagon a long time ago...im just asking these questions cuz i think knowing the answers could potentiall benefit me....say, if i decide to build myself some computer speakers or my brother some new surrounds for his HT

 

[thylantyr 10+ year member]

Charge labor fees when building for others. /evil

 

[jaygeorge1979 10+ year member]

i plan on it...*insert evil laugh here*

actually, for family i probably wouldnt charge too much...i look at it like this...they pay for parts, i play with em, everybody wins