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Post new topic 8 Ω vs. 4 Ω Speaker Related to Amp Load
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Author Topic:  8 Ω vs. 4 Ω Speaker Related to Amp Load
Jerry Overstreet


From:
Louisville Ky
Post  Posted 15 May 2021 5:41 am    
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clickhere

I found this interesting concerning 4Ω speakers harder to drive than 8Ω. I always thought it was just the reverse. Not being very familiar with electronics, I'm trying to educate myself more in the field and thought this article was informative. Maybe someone else wil too.
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Georg Sørtun


From:
Mandal, VA, Norway & Weeki Wachee, FL, USA
Post  Posted 15 May 2021 6:52 am    
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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.
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Dan Kelly


From:
Boston, MA
Post  Posted 15 May 2021 8:10 am    
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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 amp’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
Post  Posted 15 May 2021 9:40 am    
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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:
Quote:
Related to my post above, 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 amp’s damping factor is also highest, leading to the flattest frequency response it is capable of, irrespective of speaker impedance characteristics.

as irrelevant for a guitar amp. Using the lowest impedance tap and then a mismatched higher load impedance does increase the damping factor, and I guess if you're an audiophile and want the maximum response flatness, lowest damping, etc., then I suppose if you only push the amp to a fraction of its rated output, maybe you'll get that without causing problems. I tend to doubt that most audiophiles really want to barely push their power amps, but I could be wrong. They do all kinds of crazy stuff to get absolute linearity. But I would never suggest doing that with a guitar amp. Guitarists even looking for a clean sound normally push their amps quite a bit and I think most players are specifically looking for some of the nonlinear aspects of the response, even if it's just a little bit of saturation to get a little extra compression and sustain.

And 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.
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Dennis Detweiler


From:
Solon, Iowa, US
Post  Posted 16 May 2021 4:53 am    
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Does the same apply to connecting two 4 ohm speakers in series (8 ohm)?
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Dave Mudgett


From:
Central Pennsylvania
Post  Posted 16 May 2021 5:22 am    
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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.
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George Biner


From:
Los Angeles
Post  Posted 17 May 2021 1:14 pm    
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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 Indiana, USA
Post  Posted 17 May 2021 3:19 pm    
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Dave Mudgett wrote:

“And 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.”

A 4x10” Bassman or Super Reverb combo is perceived as “louder” because its combined speaker area is greater than that of any other Fender, save for the never-used, boat anchor Super Six.

In short, 4-10’’ combos push more air.
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Dave Mudgett


From:
Central Pennsylvania
Post  Posted 17 May 2021 5:58 pm    
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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.

Quote:
A 4x10” Bassman or Super Reverb combo is perceived as “louder” because its combined speaker area is greater than that of any other Fender, save for the never-used, boat anchor Super Six.

Yeah, at about 1885 square inches. The Quad Reverb is the other Fender boat anchor at about 1810 square inches. Waylon loved those. I've had a couple, too much for me. Basically just a Twin Reverb chassis in a 4x12" cab. It used four 16 Ohm speakers in parallel for the standard 4 Ohm Twin Reverb load. But my Session 500 is definitely heavier than my Quad Reverbs! Talk about a boat anchor! I need to get rid of that thing.
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George Biner


From:
Los Angeles
Post  Posted 17 May 2021 6:35 pm    
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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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Mullen Royal Precision SD10 E9 / Spark Amp / Fender Deluxe Reverb / Sarno Freeloader / Goodrich volume pedal
“Now there is a snappy sounding instrument. That f****r really sings.” - Jerry Garcia
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Dave Mudgett


From:
Central Pennsylvania
Post  Posted 17 May 2021 8:43 pm    
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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.
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Steve Sycamore

 

From:
Sweden
Post  Posted 20 May 2021 12:28 am    
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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.
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Steve Sycamore

 

From:
Sweden
Post  Posted 20 May 2021 12:44 am    
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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.
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Willie Sims

 

From:
PADUCAH, KY, USA
Post  Posted 26 May 2021 1:18 pm    
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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.
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Jerry Overstreet


From:
Louisville Ky
Post  Posted 26 May 2021 3:07 pm    
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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!
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Foster Haney


From:
Oregon, USA
Post  Posted 29 May 2021 7:44 am    
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I have another question that this thread sounds qualified to handle 😁.

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
Post  Posted 29 May 2021 9:29 am    
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Quote:
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?

Output transformers match impedances approximately. Nothing in the real world is perfect.

And none of this says that there is no difference between the sound at various impedances. There are sometimes differences. I think it's usually pretty subtle. But, let's say, a Super Reverb correctly running 'normally' at 2 Ohms into a 2 Ohm speaker set sounds a bit different to me than if the OT was changed to 8 Ohms into a comparable 8 Ohm speaker set (you'd need four 32 Ohm speakers to really compare - I did it as a goof when I got a bunch of old 32 Ohm Jensens at salvage). And even the exact same model speaker is different at different impedances. You'll see this is true if you look at the specs for a typical speaker. E.g., https://www.tubesandmore.com/sites/default/files/associated_files/mod10-50_specification_sheet.pdf
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Foster Haney


From:
Oregon, USA
Post  Posted 29 May 2021 11:03 am    
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I guess I’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 wrote:
Quote:
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?

Output transformers match impedances approximately. Nothing in the real world is perfect.

And none of this says that there is no difference between the sound at various impedances. There are sometimes differences. I think it's usually pretty subtle. But, let's say, a Super Reverb correctly running 'normally' at 2 Ohms into a 2 Ohm speaker set sounds a bit different to me than if the OT was changed to 8 Ohms into a comparable 8 Ohm speaker set (you'd need four 32 Ohm speakers to really compare - I did it as a goof when I got a bunch of old 32 Ohm Jensens at salvage). And even the exact same model speaker is different at different impedances. You'll see this is true if you look at the specs for a typical speaker. E.g., https://www.tubesandmore.com/sites/default/files/associated_files/mod10-50_specification_sheet.pdf

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Dave Mudgett


From:
Central Pennsylvania
Post  Posted 29 May 2021 2:09 pm    
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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.
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Foster Haney


From:
Oregon, USA
Post  Posted 29 May 2021 4:00 pm    
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Thank you for the clarification. That’s very helpful.
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Donny Hinson

 

From:
Glen Burnie, Md. U.S.A.
Post  Posted 30 May 2021 12:31 pm    
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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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