4ohm v.s. 8ohm

Harold Dye · From: Cullman, Alabama, USA

How will replacing a 4ohm speaker(BW)with an 8ohm speaker effect the amp. I am considering doing that with my NV 400 because of weight. These amps are set up for 4ohms and I don't want to damage the amp with this change. What would be the effect other than loss of power(and how much would that be)? Would the amp run hotter? What would be a good speaker choice? I am considering a Kappalite.

Jack Stoner · From: Kansas City, MO

In general terms it will just about halve the max power output. The electronic formula is P (watts)= E(voltage) squared/ohms. e.g. if the output of a power amp is 27 volts and you have a 4 ohm speaker load the power would be 27X27/4 or 182.25 watts. The same 27 volts with an 8 ohm speaker would be 27X27/8 or 91.125 watts.

The output voltage is listed on the back of the Nashville 400 so you can make exact calculations.

ADDED: if you plan to replace the BW with some other speaker, you need to consider the mounting screws. I had thought about replacing the BW speaker with one of the Eminence "Delta Lite" speakers (same as was used in some of the Evans SE-200 amps) but the speaker mounting screws are too short and would have had to be replaced with longer ones as the mounting lip on the Delta Lite was much thicker.

Terry McCumbee · From: West Virginia, USA

yes you are right Jack.I replaced mine with the NEW DELTA LITE 11 Speaker that EVANS is useing in there new AMPS.They have the Screws to for the Speaker.

Jim Sliff · From: Lawndale California, USA

Roger Light · From: Sheldon, IL

Greg Cutshaw · From: Corry, PA, USA

Actually the max 27 volts coming out of the amp is not a constant. It varies based on the load and the internal resistance of the amp. In reality, going to an 8 ohm speaker will result in a fairly small drop in power output and an even smaller drop in volume level based on the logarithmic relationship of power level to volume level. Even if the amp lost 1/2 it's power, the user would only hear a 3 db drop in volume, really no big deal.

There would also be changes in the damping factor and tone of the amp. These are usually optimum when the amp's load is equal to its internal resisitance but an individual user may like the tone better with a mismatched load impedance.

With a 4 ohm resistor load, my Nashville 400 puts out 28.8 volts RMS at 1 Khz (measured). The current is 7.225 amps and the voltage across the load is 28.9 volts (measured). The output power is 208.8 watts. This agrees well with published specs and the 4 ohm resistor is a decent approximation of the average complex speaker impedance.

With an 8 ohm load, the current is 4.817 amps and the load voltage is 38.53 volts giving a power of 185 watts.

With a 2 ohm load the current is 9.63 amps, the output voltage is 19.26 volts and the output power is 185 watts.

With a 16 ohm load the current is 2.89 amps, the output voltage is 46.2 volts and the output power is 133 watts.

As the speaker impedance is reduced below the amp's internal resistance, the current increases but the load voltage decreases. At some point the amp can overheat as the current gets high enough. Many amps have protection and compression circuits which will limit the max current and could affect these results.

As the speaker impedance is increased above the amp's internal resistance, the current decreases but the load voltage increases. Again the amp is limited by the power supply rails and any compression or protection that is built in could modifiy these results.

The results obtained by my scope readings indicate that the Nash 400's internal resistance is close to 4 ohms.

There are newer servo amp designs that can actually have totally different behavior than the above results including the new Class D amps.

Greg

Mike Wheeler · From: Delaware, Ohio, USA

Greg, the numbers you gave for a 16 ohm load are actually for a 2 ohm load...correct?
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Greg Cutshaw · From: Corry, PA, USA

Mike,

I corrected the info above and included both 2 and 16 ohm results.

Not only is a speaker's impedance highly variable as a function of frequency (see graph below), it can also be affected by the enclosure it is mounted into. An example of this would be a sealed enclosure restricting air movement, changing the mechanical impedance dynamically on transient music passages. Nevertheless, in my experience, bench measurements with resistive loads come awfully close to an amp's real performance specs.

Greg

Mike Wheeler · From: Delaware, Ohio, USA

Greg, nice presentation. Smile

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Harold Dye · From: Cullman, Alabama, USA

Thanks guys this is very helpful. I know nothing about electronics but I had heard that mismatching speakers with an amp might cause some problems. I would like to decrease the weight of the NV 400 but not at the expense of the amp. Hopefully someone will enter the market with a good sounding 4ohm speaker with lots of power and lighter weight. I have other amps in head cabs so I will just leave the 400 as is for now.

Eric Jaeger · From: Oakland, California, USA

OK, I admit to knowing just enough about this topic to be dangerous...

Are guitar amplifiers fundamentally different than audiophile-grade stereo amplifiers? Because solid state stereo amplifiers don't seem to care what load they see, with the exception of some unusual ribbon speakers (Apogees) whose reactive load could drop below 2 ohms. In fact, most stereo amplifiers specify power at 2, 4, 8, and 16 ohms. And if fidelity suffers as a result of an impedance mismatch I haven't heard it and the rags haven't written about it.

What am I missing?

-eric

David Doggett · Posted 9 Mar 2007 9:29 pm

Eric, I probably know less than you about this, but I suspect it may have something to do with the fact that audiophiles don't tend to play their amps wide open, but stick near the middle of the volume range where the amps are most accurate. If that's not loud enough, they get a bigger amp and stick to the middle of its range. But guitar players, especially tube amp users, like to run their amps near top volume. If that is too loud, they will get a smaller amp and run it wide open. I imagine a missmatch is more dangerous for an amp run near maximum volume all night; whereas, running an audiophile amp mismatched but far below its maximum is much safer.

Eric Jaeger · From: Oakland, California, USA

DD, with tube amps I think you're absolutely right. An impedance mismatch or open circuit on the output can toast the OT (I've done that to several Dynaco Stereo 70s before I learned).

But with solid state, I honestly admit I don't know. With those sub-2-ohm Apogees some amps, even solid state, would go unstable, and the only power amps that could reliably handle them were huge Krells -- at over $10K a side (not mine!). But I've used the same solid state stereo amps on different speakers whose nominal impedance runs from under 4 to over 16 ohms (big electrostatics) with no problem.

As I said, I don't get it. Oh well.

-eric

Mike Wheeler · From: Delaware, Ohio, USA

A discussion about audiophile amps is a journey into the Twilight Zone. There is so much baloney mixed with facts that it's almost impossible to get to the truth. So, let's not even go there, please.

Eric, with solid state amps, a good rule to follow is, stay at, or above, the lowest rated impedance. Going higher than the highest rated impedance usually won't damage it, but the power and distortion specs will go to heck. In that case, your ears will tell you what works.

As far as guitar amps are concerned, context is everything. A given amp's ability to survive a mismatched load depends on the amp's design, how it's being used and the environment it's in. One size does not fit all cases.
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Jim Sliff · From: Lawndale California, USA

Greg did a superb job of posting all the technical backup relating to what I was summarizing. That's stuff anyone interested should probably save a copy of.

Also, one of the biggest misconceptions is that it's safer to use a higher-impedance speaker in a tube amp than one that's too low.

While either one can cause problems, using (as an example) a 16-ohm Celestion in an amp that wants to see 4 or 8 ohms can cause a phenomenon called "flyback". Without getting too technical, you can compare it to an ocean wave's backwash at the beach - signal gets bounced back, right to the output transformer. And that transformer (which is the heart of the amp's tone) can get cooked.

As far as the variable output of some SS amps, it's part of the design whereby the output power is adjusted by the circuit depending on what impedance it "sees". Tube amp circuitry does not work the same way - usually if different impedance speakers can be used with a tube amp there's an impedance switch with fixed settings. FWIW some amps are more forgiving than others; most Fender tube amps can handle a 100% mismatch (though power and tone will be affected), while most Marshalls can't tolerate much variation from the rated impedance.
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1921 Weissenborn Style 2; Hilo&Schireson hollownecks
Appalachian, Regal & Dobro squarenecks
1959 Fender 400 9+2 B6;1960's Fender 800 3+3+2; 1948 Fender Dual-8 Professional

Ken Fox · From: Nashville GA USA

From the Old Session 400 manual:

210 watts at 4 ohms 1% T.H.D.
120 watts at 8 ohms 1% T.H.D.
80 watts at 2 ohms 1% T.H.D.

Not at all in line with the results above. Oh well!!

Jonathan Cullifer · From: Gallatin, TN

Different amps can have drastically different responses to different speaker ohms, depending on a lot of factors. I suspect the Session 400 power amp is different than the Nashville 400 one.

Ken Fox · From: Nashville GA USA

Other than DDT compression on the front end, not a lot different.

Greg Cutshaw · From: Corry, PA, USA

Ken,

The results you specify are quite well in line with what I posted. The 8 ohm results at 210 watts are right on the money!

At 2 ohms current limiting kicks in limiting the output power and at 8 ohm the voltage limiting kicks in either from amp protection, voltage compression or the amp's supply voltage rails being reached. I wish I still had a Nash 400 to bench . It wouldn't take long to scope at 8 and 2 ohms. Most amps without compression or limiting will post results close to what I posted.

Greg

Donny Hinson · From: Glen Burnie, Md. U.S.A.

Mike Wheeler · From: Delaware, Ohio, USA

Having a bad day, Donny?
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Dyke Corson · From: Fairmount, IL USA

I can tell you my experience: I put a 8 ohm Kappalite in my Peavey LTD. It was lighter weight, but I had to crank the amp considerably higher than with the 4 ohm BW, and to my ear it was not as clean and the tone was not as good except at low volume. I am hoping to see a NEO 4 ohm 15" BW from Peavey someday.

T. C. Furlong · From: Lake County, Illinois, USA

Dyke, the reason that the Kappalite was not nearly as loud was probably due to the lower efficiency of the driver rather than impedance. The higher impedance would decrease the output approx. 3dB.

Interesting fact: I just learned that all of the old JBL D series speakers had the same voice coil regardless of how they were marked for impedance. So feature that. A 16 ohm JBL had the same voice coil as the 4 ohm or an 8 ohm. Nominal impedance means "named impedance" not minimum impedance like you would think.
TC

Jim Sliff · From: Lawndale California, USA

Dyke Corson · From: Fairmount, IL USA

Very interesting TC! Have you found a NEO style 15" that sounds good and has good efficiency and light weight? We know you found a good 12", Your cabs are GREAT!

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