4 OHM vs. 8 OHM - AVS Forum
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post #1 of 19 Old 12-01-2003, 06:16 AM - Thread Starter
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Why do some speaker makers use a 4 ohm design and others use a 8 ohm design?If 8 ohm is more efficient and easier to amplify what are the benefits of 4 ohm speakers ?
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post #2 of 19 Old 12-01-2003, 07:37 AM
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Quote:
Originally posted by RONM
Why do some speaker makers use a 4 ohm design and others use a 8 ohm design?If 8 ohm is more efficient and easier to amplify what are the benefits of 4 ohm speakers ?
RONM,

Like anything else - it's a design choice.

Why do some speakers use a box that is 4 cubic feet in volume, and others use a box
that is 5 cubic feet in volume? Why are some speakers 2-way, others are 3-way, and
some are 4-way?

There is no "right" speaker design - the designer is always trading off performance
characteristics with cost and other factors. The speaker impedance is just another
design parameter - although most designers keep the impedance within the 4 - 8 ohm
range for compatibility with most amps and receivers.

An 8 ohm speaker is easier to drive - since for a given voltage it takes less current to
achieve that voltage than for a 4 ohm design.

Dr. Gregory Greenman
Physicist
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post #3 of 19 Old 12-01-2003, 07:51 AM
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But it's all an approximation - impedance varies hugely relative to frequency - I dont think designers have much control over it and are not that concerned as long as impedance doesnt drop too low. an 8 ohm pseaker may have an impedance range of 5 to 50 ohms, it's all a weighted average.

dunno if this forum accepts images....heres a Spendor BC1 impedance graph

http://www.john-a-harper.com/spendor-bc1-imp.jpg
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post #4 of 19 Old 12-01-2003, 08:55 AM
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Why would 8 ohm speakers be easier to drive than 4 ohms?!

Ohms is a unit for resistance, the greater the number, the more resistance.

An amp able to feed 100 watts at 8 own would be able to feed something around 140 watts at 4 ohms.

So an amp can feed more power to 4 ohms speakers than 8 ohms.. So its the other way around, 8 ohms are less efficient than 4 ohms.

No?
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post #5 of 19 Old 12-01-2003, 09:11 AM - Thread Starter
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Thats what I thought but speakers that are a 4 OHM all recommend seperate high power amps to make them sound best. In particular the POLK LSI 4 OHM speakers are recommended by POLK not to be used with recievers in general and with seperate amps with at least 100w per channel.
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post #6 of 19 Old 12-01-2003, 09:54 AM
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Quote:
Originally posted by Grandarf
Why would 8 ohm speakers be easier to drive than 4 ohms?!

Ohms is a unit for resistance, the greater the number, the more resistance.

An amp able to feed 100 watts at 8 own would be able to feed something around 140 watts at 4 ohms.

So an amp can feed more power to 4 ohms speakers than 8 ohms.. So its the other way around, 8 ohms are less efficient than 4 ohms.

No?
4 Ohm is not "easier" or "more difficult" to drive than 8 Ohm (and vice-versa). It's all relative to what the amplifier was designed to drive. For an amp to deliver, say, 100 Watt into 8 Ohm a rail voltage of about 30-35 Volt is needed. To deliver 100 Watt into 4 Ohm would take only around 20-25 Volt is needed. That means when that first amp drives a 4 Ohm load at a 100 Watt it will have to burn that extra 10 Volt into heat, at 5 Ampere effective that means the amplifier has an extra 50 Watt per channel in heat to deal with.

So, there's more heat to deal with for an 8 Ohm amp, and it'll have to deliver more current, when dealing with a 4 Ohm load vs. an 8 Ohm load.

For an amplifier that's designed to drive 4 Ohm it'll use a lower rail voltage, so less heat, and output devices that are better at dealing with the higher currents required. I've seen amplifiers designed to drive very-low impedance loads (under 1 Ohm). They'd do very poorly with 8 Ohm speakers, there's no voltage there to drive them, but they do great at delivering loads of current, far more than any regular amplifier could deliver.

Many regular amplifiers (ie. designed for 8 Ohm loads) have a setting/switch that lowers their rail voltage so they don't produce as much heat driving 4 Ohm loads. It's probably a good idea to use that, especially if you like to crank it up.

For what it's worth, I'm with KeithD that impedance as stated by manufacturers is only an average. Impedance of most speakers varies widely over frequency. Driving a low impedance at high frequency would not be much of a problem as there's little power at that part of the spectrum. Driving a low impedance at low frequencies is the opposite though.

-Rob-

Lies, damn lies, and interconnects...
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post #7 of 19 Old 12-01-2003, 11:44 AM
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Ronm,

I used to have LSI's and yes they definetely sound better with a separate amp. They seemed to really strain my old receiver.
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post #8 of 19 Old 12-01-2003, 12:05 PM
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Accepting lower impedances give you more flexibility in choosing drive units and configurations. For example, accepting a 4 Ohm impedance allows me to use a pair of 8 Ohm woofers in parallel to get a 6dB sensitivity increase.
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post #9 of 19 Old 12-01-2003, 03:42 PM
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Lower output impedance acts as a shunting resistor between the speaker terminals and increases control over the speaker cone. Simplified, this generally means a higher damping factor for a lower impedance amp. (Damping factor is a ratio between load impedance and the amp's output impedance at a specific frequency)

Of course, the inductor in passive crossovers introduces additional resistance and makes the damping factor mostly pointless.

Az
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post #10 of 19 Old 12-02-2003, 04:38 AM
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Quote:
Originally posted by Az Barber
Lower output impedance acts as a shunting resistor between the speaker terminals and increases control over the speaker cone. Simplified, this generally means a higher damping factor for a lower impedance amp. (Damping factor is a ratio between load impedance and the amp's output impedance at a specific frequency)

Of course, the inductor in passive crossovers introduces additional resistance and makes the damping factor mostly pointless.
A 4 Ohm amplifier does not have 4 Ohm as its output impedance. Neither does an amplifier for 8 Ohm speakers have 8 Ohm output impedance. For audio there is no impedance matching (where load, source, and transmission line would have the same impedance, common at high frequencies). All amplifiers, pretty much regardless of type or power rating, have a very low output impedance, generally a small fraction of an Ohm.

-Rob-

Lies, damn lies, and interconnects...
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post #11 of 19 Old 12-02-2003, 05:43 AM
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yes, the impedance does vary with frequency, but a (good) speaker designer can precisely control the impedance of a system at any frequency. Large changes in phase go hand-in-hand with impedance changes.

Many designers make speakers that have relatively flat impedance across the audio range. (this isn't as hard as it sounds)

-matt
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post #12 of 19 Old 12-02-2003, 06:33 AM - Thread Starter
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eulogytool,
what amp did you use for your polk lsi's.My ADCOM GFA 7605 is IMO not cutting it with my system.The sound is thin with dull bass,poor midrange .
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post #13 of 19 Old 12-02-2003, 08:34 AM
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Quote:
Originally posted by Grandarf
Why would 8 ohm speakers be easier to drive than 4 ohms?!

Ohms is a unit for resistance, the greater the number, the more resistance.

An amp able to feed 100 watts at 8 own would be able to feed something around 140 watts at 4 ohms.

So an amp can feed more power to 4 ohms speakers than 8 ohms.. So its the other way around, 8 ohms are less efficient than 4 ohms.

No?
Grandarf,

Afraid not.

Ohm's Law is V = I x R or Voltage = Current x Resistance.

Divide both sides by Resistance and you get an equation for the current -

I = V/ R or Current = Voltage / Resistance

Your amp or receiver has no trouble developing the needed voltage or "pressure" if
you think of an analogy with water flowing in a hose.

However - your amp or receiver may not have enough current - which is analogous
to the amount of water flowing. As you can see from above - I = V/R - if you make
the load resistance R smaller for a given voltage - then the current has to go up.

If you halve the Resistance - you must double the current to support the same voltage.
Your amp / receiver may not be able to deliver that much current - which is why a
4 ohm load is harder to drive than an 8 ohm load.

One of the early Apogee ribbon speakers, the "Scintilla", had a nominal resistance of
just 1 ohm. It took a LOT of current to drive one of those - and, in fact, the Scintilla was
infamous at destroying amps that tried to drive them.

Additionally, the amplifier designer designs the amp with a certain load in mind - usually
on the order of 4 to 8 ohms. If the load impedance goes to low - lower than what the
designer was shooting for - such as if you took two 4 ohm speakers and wired them in
parallel for an effective resistance of 2 ohms - then the amp can go unstable. That is
the circuit including the load changes from an amplifier circuit to an oscillator circuit and
the amp destroys itself that way.

Dr. Gregory Greenman
Physicist
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post #14 of 19 Old 12-02-2003, 08:50 AM
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Quote:
Originally posted by Rop
4 Ohm is not "easier" or "more difficult" to drive than 8 Ohm (and vice-versa). It's all relative to what the amplifier was designed to drive. For an amp to deliver, say, 100 Watt into 8 Ohm a rail voltage of about 30-35 Volt is needed. To deliver 100 Watt into 4 Ohm would take only around 20-25 Volt is needed. That means when that first amp drives a 4 Ohm load at a 100 Watt it will have to burn that extra 10 Volt into heat, at 5 Ampere effective that means the amplifier has an extra 50 Watt per channel in heat to deal with.
Rob,

I'm afraid the above is somewhat misleading.

Although amps are rated at a given power - 100 W into 8 ohms, say - what that REALLY
means is that they have an output stage supply voltage of about 28 volts (RMS).

It's really this output stage supply voltage that the amp modulates to provide the output
current that drives the speakers. An amp doesn't really have a fixed power that can be
apportioned to voltage and current - it's more like it has a fixed voltage. [ Not strictly true -
but a better approximation than saying it has a fixed power ].

Think of a pump. A pump is rated to supply a given pressure. It has a very easy time
developing that pressure when you put a nozzle on the hose. But you leave the end
of the hose open - and you don't get much of a spray.

Your house has about 40 - 60 psi in the water lines. If you connect a garden hose to the
outside spigot - and put your thumb over the end - i.e. you make a high resistance nozzle -
you can get a pressure drop across that "nozzle" that is a substantial fraction of the
40 - 60 psi house supply.

But if you tried to get a 40 - 60 psi drop at the end of an open hose without a nozzle -
your house couldn't supply enough water flow to do it.

Unquestionably, a 4 ohm load IS more difficult to drive than an 8 ohm load - it just
plain takes MORE CURRENT to do so relative to an 8 ohm load.

Dr. Gregory Greenman
Physicist
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post #15 of 19 Old 12-02-2003, 08:54 AM
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Quote:
Originally posted by Rop
A 4 Ohm amplifier does not have 4 Ohm as its output impedance. Neither does an amplifier for 8 Ohm speakers have 8 Ohm output impedance. For audio there is no impedance matching (where load, source, and transmission line would have the same impedance, common at high frequencies). All amplifiers, pretty much regardless of type or power rating, have a very low output impedance, generally a small fraction of an Ohm.

-Rob-
Rob is correct here.

The ratio of the load impedance divided by the amp's output impedance is called the
"damping factor".

One of my amps has a rated damping factor of 400, if memory serves; into an 8 ohm load.
Therefore, the output impedance of this amp is 0.02 ohm.

Dr. Gregory Greenman
Physicist
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post #16 of 19 Old 12-02-2003, 10:31 AM
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Quote:
Originally posted by Morbius
Rob,

I'm afraid the above is somewhat misleading.

<<SNIP>>

Unquestionably, a 4 ohm load IS more difficult to drive than an 8 ohm load - it just
plain takes MORE CURRENT to do so relative to an 8 ohm load.
Morbius,

In the paragraph after the one you quoted I stated it takes more current for the same power at 4 Ohm vs. 8 Ohm.

Not to keep whipping a dead horse, but there is not just one limitation (constant power, constant current) when it comes to amplifiers. Yes, their power supply has a finite capacity when it comes to delivering power. So with voltage fixed that means there's a limit to the current it can deliver. However, all amplifiers I'm aware of will deliver quite a bit more current than what they are rated for at an 8 Ohm load. They generally can drive a 4 Ohm load, but total dissipated power starts to be a problem. With, these days, 7 channels in a receiver of fixed size (and a need not to have noisy fans) power dissipation is a big problem.

I stand by my statement that it's all about what the amplifier was designed to drive. Driving one vs. another load is not inherently more difficult. It is more difficule for any amplifier to drive a load (higher or lower) it wasn't designed for. As mentioned, I know of one design that was meant for driving a ribbon (less than 1 Ohm impedance). It could do that comfortably. Driving an 8 Ohm load would be a big problem for that amplifier though, it doesn't have the voltage to generate much power in that load.

-Rob-

Lies, damn lies, and interconnects...
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post #17 of 19 Old 12-02-2003, 10:48 AM
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Quote:
Originally posted by Rop
A 4 Ohm amplifier does not have 4 Ohm as its output impedance. Neither does an amplifier for 8 Ohm speakers have 8 Ohm output impedance. For audio there is no impedance matching (where load, source, and transmission line would have the same impedance, common at high frequencies). All amplifiers, pretty much regardless of type or power rating, have a very low output impedance, generally a small fraction of an Ohm.

-Rob-
I never said otherwise.

Az
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post #18 of 19 Old 12-02-2003, 01:06 PM
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Quote:
Originally posted by Rop
Morbius,

In the paragraph after the one you quoted I stated it takes more current for the same power at 4 Ohm vs. 8 Ohm.

Not to keep whipping a dead horse, but there is not just one limitation (constant power, constant current) when it comes to amplifiers. Yes, their power supply has a finite capacity when it comes to delivering power. So with voltage fixed that means there's a limit to the current it can deliver. However, all amplifiers I'm aware of will deliver quite a bit more current than what they are rated for at an 8 Ohm load. They generally can drive a 4 Ohm load, but total dissipated power starts to be a problem. With, these days, 7 channels in a receiver of fixed size (and a need not to have noisy fans) power dissipation is a big problem.

I stand by my statement that it's all about what the amplifier was designed to drive. Driving one vs. another load is not inherently more difficult. It is more difficult for any amplifier to drive a load (higher or lower) it wasn't designed for. As mentioned, I know of one design that was meant for driving a ribbon (less than 1 Ohm impedance). It could do that comfortably. Driving an 8 Ohm load would be a big problem for that amplifier though, it doesn't have the voltage to generate much power in that load.

-Rob-
Rob,

We'll have to agree to disagree.

Given that in practically all amplifiers that I'm aware of - it's the VOLTAGE - not power
that is regulated.

The power supply is regulating the voltage available to the output stage. For a given
output stage voltage - it will always be more difficult to drive the lower impedance than
the higher impedance.

Now in the case of the amp meant for driving the 1 ohm load - it's an apples and oranges
comparison - because it doesn't have the voltage that the 8 ohm amp has.

Saying that the 4 ohm load is more difficult to drive, is like saying it's more difficult for
a car to go down the highway towing a trailer than it is when it doesn't have a trailer.

Now farm tractors are meant for towing trailer - but they don't do well on the highway
because they may have only a 60 HP engine vs. a 200 HP engine for a car - apples
and oranges.

For a given output supply voltage [ which is what the regulation circuitry is trying to
achieve ] , it will always be more difficult to drive the lower impedance. You have to
put out the same voltage - but at more current for the lower impedance.

Dr. Gregory Greenman
Physicist
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post #19 of 19 Old 12-02-2003, 04:38 PM
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Lemme see if I can throw in an easy-to-grasp explanation here:

As the audio signal voltage fluctuates, as audio signals are known to do, the current fluctuates right along with it (with the exception that the current's waveform sometimes leads (for a capacitive impedance) or lags (with an inductive impedance) the voltage wave).

What determines the magnitude of the current relative to that of the voltage at any given moment in time is the impedance (A.C. resistance, simply put) of the load (and the rest of the circuit: wire, crappy terminals, etc.) The lower the impedance, the greater the current.

Some people call the lower-impedance load "harder to drive", because more current is required to acheive a given voltage level. However, keep in mind that a specific power is reached at a lower voltage level because of the greater current. Plus, a lower impedance generally plays louder at this same lower voltage, for them same reasons.

Before the advent of step-up DC-DC power supplies, car audio amps were limited to somewhere around 13.5 watts/channel, due to the 12-volt system. That's why low-impedance speakers became popular. In the home, especially with tube amps, higher-voltages worked more easily (there's that word again) with lower current and higher voltage driving 16-ohm speakers.

Some people might consider a high-impedance load "harder to drive" because a higher voltage is required to induce a given current, but again, due to Ohm's law, the voltage helps develope a certain power level. The real bottom line is proper source/load matching. An amp can be designed for best power transfer into any impedance.

The amp maker can't know what the characteristics of your speaker will be, and vice versa. The best they can do is give impedance (and other parameter) recommendations. Impedance fluctuations do occur, and they are of both capacitance and inductance (collectively known as 'reactance'). The best we can ask for is an average rating, or range, with which to choose an amp.

So, which is easier to drive, a speaker that requires a high voltage level, or a high current capacity? (I guess it depends on your definition of "is".)

How's that?

Larry

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