8 Î© vs. 4 Î© Speaker Related to Amp Load

Jerry Overstreet · From: Louisville Ky

Georg Sørtun · From: Mandal, Agder, Norway

Right. On paper it takes about twice the energy to drive a 4ohm speaker compared to driving an 8ohm. In reality it varies with spread in ohm with frequency, as "impedance" in a voice-coil in a magnetic field, isn't flat with freq.

Dan Kelly · From: Boston, MA

Very interesting, Jerry!

It seems the discussion is centered around Audiophile quality sound systems. However, I can imagine that most of the discussion is applicable to instrument amps / speakers.

This statement really caught my attention - and, I will have to try it and think about it for awhile. Quote:

"using the lowest impedance tap on a tube power amp not only usually results in the lowest distortion and best sound the amp is capable of, but the ampâ€™s damping factor is also highest, leading to the flattest frequency response it is capable of, irrespective of speaker impedance characteristics."
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Dave Mudgett · From: Central Pennsylvania and Gallatin, Tennessee

To say that lower impedance loads are 'harder to drive' than higher impedance loads grossly oversimplifies the situation, IMO. It depends on the situation. Yes, given a fixed driving voltage, the amount of current required to achieve the same power level in a speaker is inversely proportional to the load impedance. As Georg points out, this is not DC and impedances of practical loads vary by frequency, but that's not the only issue here.

IMO, the real issue is power transfer, which is governed by the power transfer equations, and it can be readily shown that maximum power transfer occurs when the load impedance matches a power amp's output impedance. The output impedance of a typical solid-state amp is very low, so you typically get more power available with a lower impedance load by drawing more current. The problem is that power amplifiers are current-limited devices and if you draw too much current, you can tax or even fry the amp. But yes, lower impedance loads definitely draw more current from the amp, which I assume this is what they mean by saying it "works the amp harder."

But for amps like tube amps with an output transformer, the situation is quite different. Tubes are high-impedance and the output transformer is there specifically to approximately match the amp's output impedance to the load impedance. Tube power amps typically generate a relatively high voltage swing which is transformed into the higher current swing in the load.

The other thing is that it is generally a bad idea to run a tube amp with transformer into a higher-than-rated load impedance owing to increased flyback voltage to the output transformer which can shorten its life and in extreme cases, fry it. I've seen it. So I view statements like this:

Dennis Detweiler · From: Solon, Iowa, US

Does the same apply to connecting two 4 ohm speakers in series (8 ohm)?
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Dave Mudgett · From: Central Pennsylvania and Gallatin, Tennessee

Two 4 Ohm speakers in series 'works' the amp as hard as a single 8 Ohm speaker, using their terminology. Audio frequencies are low enough that the wavelength of the electromagnetic signal is long enough to very accurately treat the load as a lumped impedance, not a distributed one. So the amp just sees a lumped 8 Ohm load.

George Biner · From: Los Angeles, CA

I'm guessing that the original issue is thinking intuitively that a load impedance of 4 ohms sounds like it should "impede" less and be easier to drive than an 8 ohm load.

Impedance is similar to resistance, which is the ratio of (Ohm's law) voltage to current inside the load element. High resistance means more resistance to current - amplifier output voltage should track input voltage therefore Vout is predetermined and should not be influenced by the load -- the power output of an amp is output voltage times current -- if the load resistance is higher, the output current is lower, and the power output is lower, and the amp is not working as hard.

It may also help to think of the extreme case of attaching a shorting wire to an amp -- zero resistance load, amp working very hard to put out infinite current.
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Eric Philippsen · From: Central Florida USA

Dave Mudgett wrote:

â€œAnd will somebody please tell me why the (arguably) best sounding Fender amps ever made for guitar are the 4x10" tweed Bassman and Super Reverb, with their nominal 2 Ohm loads? Yes, this is about preferences, but for guitar, that's a pretty widely held view. I know lots of guitar players who don't use Supers and tweed Bassmans any more, but it's not because they don't sound good but that they are frequently too loud for the gig.â€

A 4x10â€ Bassman or Super Reverb combo is perceived as â€œlouderâ€ because its combined speaker area is greater than that of any other Fender, save for the never-used, boat anchor Super Six.

In short, 4-10â€™â€™ combos push more air.

Dave Mudgett · From: Central Pennsylvania and Gallatin, Tennessee

I don't see this as 'harder to drive'. Simply that, given a fixed driving voltage, more current flows through a lower impedance load. One says that a lower impedance presents a higher load to the amp, I guess because power amps are generally current-limited. But current is not work. It doesn't mean it's 'harder to drive'.

The instantaneous power delivered to the load, which is the first time derivative of work, is the product of instantaneous voltage and current. And thus the work over a period of time is the time integral of power. Current is just one aspect of this. So I don't agree that a lower impedance load 'works the amp harder'. That depends on several things.

But my issue was some of the extrapolations of this idea made on that thread, at least about tube amps. A typical tube amp can handle a short (infinite load) better than an open circuit load (zero load). Not that either are recommended.

George Biner · From: Los Angeles, CA

The output voltage is predetermined by the input voltage and the gain of the amp if the amp is operating linearly which it should. A low impedance load will create more output current, hence more output power, hence the amp is working harder with a low impedance load.
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Dave Mudgett · From: Central Pennsylvania and Gallatin, Tennessee

If you're talking about a solid-state amp with no output transformer, then if you attach a lower impedance load with impedance no smaller than the matching load, then more power will be produced because you're closer to the max power load. I would call that "easier to drive" - same driving voltage produces more power, or you could say less driving voltage (by lowering preamp gain) gives the same power. I will preemptively agree that this 'harder' or 'easier' is just semantics. That's why I don't really care for such descriptions.

If the load impedance is smaller than the matching load, then more voltage will be dropped over the output impedance than the load, and output power delivered will drop.

Some of my Peavey solid-state amps exhibit this type of behavior. For example the Studio Pro 40 delivers (at clipping) 25 Watts RMS into 16 Ohms, 45 Watts into 8 Ohms, and 40 Watts into 4 Ohms, 2 Ohms not recommended, no doubt the current would be too much. With others, the output impedance must be lower, because power increases monotonically as load impedance is reduced to the lowest-rated value.

With a tube amp, the picture is changed somewhat by the output transformer, which is set to give output impedance in the range of the load impedance. As always, theoretical max power is achieved with load equal to the complex conjugate of the amp's output impedance. Of course, this is not achieved in practice. A load not matched in either direction gives less power. I'd call that 'harder to drive' with that particular amp. Again, I dunno why that description matters, but it's not the same as "lower impedance speakers are (implied always) harder to drive".

I guess, in a sense, yone can argue either line of what is 'harder' or 'easier', depending on your point of view. My point of view is that if a lower driving voltage produces the same power, that load is easier to drive.

As far as linearity goes - lots of my amps are routinely operated in a nonlinear region to some extent. Sometimes to a large extent. I'm not talking about a hi-fi amp.

Steve Sycamore · From: Sweden

Yes, definitely the situation changes and becomes more complex with tube amplifiers. The reactions to changing voltages, speaker impedance and current demands create a type of reflection phenomenon back from the output transformer towards the power tubes that can build up voltage much higher than the tube itself generates.

If the power amp is well designed the bias will be chosen to support a particular output impedance. That is especially important for true class A power sections. Using a speaker double or half that impedance will give the effects described but may degrade the power amp's class a bit. That can also shorten or lengthen tube life.

Steve Sycamore · From: Sweden

P.S. The Physics definition of work is Force times Displacement or Energy transferred to an object. I think in either case both the different speakers end up producing the same output or baffle displacement as long as the power amp handles either impedance with equal efficiency. Hence neither works any harder than the other. But as Dave mentioned, with tube power sections, deviating from the designed or optimal output impedance lessens the tube's ability to efficiently transfer power.

Willie Sims · From: PADUCAH, KY, USA

jerry this has nothing to do with your question,ken fox said a 8 ohm speaker. would make the power tranistor's operate cooler,than a 4 ohm speaker.with some loss of output.but not much
my evans fet 500 still had more power than I could use.with a 8 ohm speaker.

Jerry Overstreet · From: Louisville Ky

Yes, Willie. That is mentioned in the conversation in my link there. It does make sense and I think I understand it better now that I've read through it again.

Good to see you posting, hope you're well and looking forward to celebrating our birthdays in another month!

Foster Haney · From: Ojai, CA

I have another question that this thread sounds qualified to handle ðŸ˜.

I have a MojoTone Princeton clone that has an upgraded output transformer with multi ohm tap options. I currently have it wired at 4 ohms with the EPS-15c (4 ohms). Is anything upstream from the OT effected by the change in Ohm rating? Or does the OT regulate that perfectly?

Good posts up above, thank you!
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Dave Mudgett · From: Central Pennsylvania and Gallatin, Tennessee

Foster Haney · From: Ojai, CA

I guess Iâ€™m asking more from a technical standpoint than a tonal.

Would switching an OT have an effect on how the circuit behind the OT works?
Does the circuit have an impedance of its own, or is the OT the only determining factor?

Thanks for the reply Dave!

Dave Mudgett · From: Central Pennsylvania and Gallatin, Tennessee

The circuit in front of the OT definitely has its own output impedance. Tubes circuits are generally pretty high impedance, which is why the OT is needed. There would be a helluva mismatch going directly into a speaker.

So yes - the output tube section is going to see a different load input impedance if the OT is wound differently. I think the tonal impact of the OT can be pretty large.

Foster Haney · From: Ojai, CA

Thank you for the clarification. Thatâ€™s very helpful.
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Donny Hinson · From: Glen Burnie, Md. U.S.A.

Having been around back in the '50s and early '60s, the era of "high-fi" amplifiers like McIntosh, Kenwood, Fisher, and Sherwood all had multiple taps on the output so they could work with any speakers. And likely as not, most of the cognoscenti (and some of the manufacturers) said to just hook up the speakers up to the terminals that "sounded best". Because the equipment was over-engineered to start with, and because the stress on "super loudness" (which came about on the mid-to-late '60s) had not yet arrived, that's what we did...and we never had a problem. The tone, response, and output differences were fairly obvious, caused by the the different loads. The speaker affects the amplifier just as the amplifier affects the speaker. But I think it's important to keep in mind that what much of we generally read about speakers and amplifiers on the internet should be taken with a grain of salt, since most of these articles aren't written by anyone with a knowledge of pedal steel. We play a rather unique instrument, one whose demands are very different from those sought by most guitar players or audiophiles. We're not interested in in overdrive, distortion, or "crunch", and we're also not interested in true flat frequency response and ultra-fidelity.

In like manner, people who drive taxis don't listen to what Ferrari or Rolls Royce drivers say when they're looking to buy a new taxi. Winking

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