Brad

A transistor amp rated at 8 ohms does not want to see an 4-ohm load; it will produce too much power for the output transistors to handle

A tube amp rated at 4 ohms does not like to see 8 ohms, due to the impedance mismatch there is reflected voltage (fly back voltage), which can cause arcing inside the power transformer

I have been told most 4-ohm speakers are at least 3db less efficient than their 8 ohm counterpart. That makes you think the manufacturers were maybe playing the "Spec Game" to make an amp look more powerful to the buyer.

[This message was edited by Ken Fox on 07 August 2006 at 10:33 AM.]

I wonder what happens to the effeciency on a 2 ohm speaker? The new Fender Jazzmaster 112 speaker (made by Jensen) is 2 ohms.

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Darvin Willhoite
Riva Ridge Recording

[This message was edited by Darvin Willhoite on 07 August 2006 at 11:06 AM.]

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A transistor amp rated at 8 ohms does not want to see an 8-ohm load; it will produce too much power for the output transistors to handle

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Ken, do you mean "does not want to see a 4-ohm load."? {Edit - I was typing while Darvin posted.}

Speakers don't produce any power - a 4 ohm speaker is not "more powerful" than an 8 ohm speaker, per se.

Speakers translate electrical energy from the power amp into mechanical energy. The issue is getting maximum power transfer from the power amp to the speakers. Power tubes inherently have a high output impedance, and tube amps use a output transformer to bring that output impedance down to the range of the speaker - power transfer is greatest when the input impedance of the speaker matches that scaled output impedance. That is a basic principle in Electrical Engineering. Getting maximum power transfer is important not only from the standpoint of efficiency, but also the "flyback" voltage Ken mentioned. With a tube amp, one should use the correct impedance speaker, IMO. I suppose if one uses a very over-engineered output transformer, it may be OK to mismatch some, and some people seem to like that, but I don't think it's a good idea.

Solid state amps are different - power transistors naturally have a low output impedance. If a speaker is directly connected to the power transistors, it will tend to draw more and more current (and power) as the speaker impedance gets lower - that's why such amps generally have a "minimum impedance" rating. If you use a speaker of too low impedance, you can push the power transistors past their current limits and they blow. Still, on a direct-coupled solid state amp, one generally gets the most power using a speaker load of the minimum rated impedance.

On the other hand, some solid state amps (like some of the early Vox solid state amps) had an output transformer. For those, one should use a speaker of the "correct" impedance, IMO.

[This message was edited by Dave Mudgett on 07 August 2006 at 11:13 AM.]

But, the impedence can be more of an issue with using an output transformer and as noted maximum power is transferred when the impedence matches.

As you can see from the impedance curve (the one nearest the bottom of the graph), the impedance runs between 7 and 8 ohms only in the narrow band between 150 and 350 cycles. Outside of that range, the speaker has a strong peak at 50 cycles (where it measures 70 ohms), and it rises pretty steadily from 500 cycles (where it measures 9 ohms) and up to 20 ohms at 4 kilocycles...and it rises even more above that! (Back up to 60 ohms at 20 kilocycles, which is about the top of our hearing range.)

So, as far as a tube amp is concerned, I feel you're pretty safe with anything higher (in speaker impedance) as long as you're not pushing the amp severely for extended periods. Solid state amps are far more fragile, load-wise, and in the case of transistor output stages, it always best not to go lower than the recommended impedance. Going lower causes the transistors to draw more power and generate more heat, and solid-state devices are generally not tolerant to heat! Tubes will run hotter too with lower impedances, but they are much more tolerant to heat. Transformers will fail with heat, too, but it takes fairly long periods of stress, it won't happen in seconds (as it's been known to happen with transistors).

So Gary, a 4-ohm speaker isn't really "more powerful" than an 8-ohm, but it will cause the amp to draw more power, and produce more watts, hence the corelation you mention.

[This message was edited by Donny Hinson on 07 August 2006 at 04:01 PM.]

"I have been told most 4-ohm speakers are at least 3db less efficient than their 8 ohm counterpart."

"The 4 ohm is going to sound better at a lower volume than the 8 ohm."

Oh, boy. All those statements are so far wrong it's astounding.

Dave and Donny have it right.

But the simple version -

Speakers are rated at a certain impedance roughly measurable by DC resistance( which will not be exactly the same). Amplifiers are designed, mostly, to be wired up to a particular impedance.

Some amps have several impedance hookups for more flexibility.

Ohms have NOTHING to do with power.

Ohms have NOTHING to do with efficiency.

Ohms have NOTHING to do with sound at certain volume levels.

Ohms have nothing to do with volume levels AT ALL.

Ohms just "are". The most important general rule is to "match" the impedance of your speaker(s) (in total - and if you do not know how to calculate it, have it done for you)to the amplifier's output. You will get the best sound, the most volume and the least strain on an amplfier by hooking up 4 ohms to 4 ohms; 8 ohms to 8 ohms...etc.

If an amp wants to "see" 8 ohms and you hook up a 4 ohm speaker you get LESS power and lousier tone (in most cases. Not always true with certain solid state systems of specific design. If you don't know if you have one - assume you don't).

Speaker efficiency is what generates volume, more so than power, and certainly more than the irrelevant "ohms". and ohms have nothing at all to do with the efficiency of certain speaker designs. They are unrelated.

This is a REALLY simplified version. There are quite a few holes in what I said as far as acoustical engineering. But for the AVERAGE player, the rule is this - match your speaker(s) impedance to the impedance your amp is designed for. That will get you the best volume and tone. There are exceptions...and if you don't know what they are, pretend they don't exist, period. It will save you money and keep your equipment running well.

OK, now go to the WeberVST website...and do a lot of reading....if you don't understand or agree. Ted Weber puts a lot of it into good, basic terms.

[This message was edited by Jim Sliff on 07 August 2006 at 10:04 PM.]

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I know the 4OHM is suppose to be more powerful BUTTT ?

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If the amp is 4 ohms it will sound best with a 4 ohm speaker. An unmatched 8 ohm speaker will not sound as loud, and in the long run it puts a strain on the amp.

If the amp is 8 ohms, a 4 ohm speaker will draw more power and sound louder [edited: wrong - see posts below], but in the long run that puts a strain on the amp.

So a speaker with lower ohms will always sound louder than a speaker with higher ohms with the same amp [edited: wrong - see posts below]. But, it is not good to use a speaker with lower ohms than the amp is made for.

A 4 ohm speaker with a 4 ohm amp rated at 100 watts will not sound louder than an 8 ohm speaker with an 8 ohm amp rated at 100 watts (if that is wrong, somebody please correct me) [edited: seems to be right]. It is the mismatching of a speaker with an amp that results in different volumes. So it is not good to try to get more volume by mismatching with a speaker of lower impedance than the amp. And if you mismatch with a speaker with higher impedance than the amp, you will loose volume.

As a practical matter, it seems that most amps (tube or solid state) can temporarily tolerate a speaker with half the impedance, or twice the impedance. So if you are in a pinch you can play a gig that way, especially if you don't run the amp on 10 all night. But it doesn't make sense to permanently use a mismatched speaker with an amp. I don't pretend to understand the electonics, but this is the practical information I have gotten from many previous discussions of this on the Forum, and from personal experience. If I've got anything wrong, somebody who knows better, please correct me.

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Student of the Steel: Zum uni, Fender tube amps, squareneck and roundneck resos, tenor sax, keyboards

[This message was edited by David Doggett on 08 August 2006 at 10:56 AM.]

That's the big misconception. A mismatch either way *decreases* power.

The exception is SS amps that have multiple taps. There, a 4-ohm tap on an output transformer may provide more power.

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I think what Gary is saying is that a 4 ohm speaker will sound louder than an 8 ohm speaker. But what everyone is saying above is that isn't necessarily a good thing for the amp.

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No, that's not what I said. That is true for a direct-coupled solid-state amp with a very low output impedance. Max power transfer always occurs when amp output impedance and speaker input impedance are matched. If the amp output impedance is lower than any possible speaker impedance, then the speaker will draw more and more power as its impedance decreases. The safety issue is that if it's lowered below a certain point, it may draw more current than the power transistors can handle, and blam, they overheat and blow. With a direct coupled amp like this, the amp tends to run hotter as the speaker impedance gets lower, but puts out more power, all else equal. But never run it below the stated minimum impedance.

Almost all tube amps and some solid state amps have output transformers that are specifically there to match impedance with the speaker. In such cases, it is my opinion that the speaker should be matched as closely as possible to avoid stressing the output transformer. Some tube amps - for example Marshalls, Laneys, and some later SF Fenders - do have multiple output transformer taps. That's very useful, and allows different speaker impedances to be used without any fear whatever. In fact, it's not hard to find retrofit multi-tap output transformers for some old Fenders in the aftermarket.

I agree with Donny that if a tube amp is not pushed hard and the output transformer is very robust, it may be reasonable to run more mismatched. But my view is that most people like to push tube amps a bit to get that "tube sound". In that situation, I think that mismatching places unneeded stress on both the output transformer and output tubes. Stress on the transformer is from flyback voltage. Stress on tubes is from running them harder due to reduced efficiency because impedances aren't matched. At the price of the NOS tubes I use, I don't want to stress them any more than I have to. And I definitely don't want to blow the old output transformers on my old amps.

What Ohm speakers were used on all the greatest steel recordings of all time???

Can anyone match the famous steel recording with the speaker Ohm-age???

Also, On the big steel hits, what was the steel Amps rated output, and what was the actuall speaker rating?

[This message was edited by Pete Burak on 08 August 2006 at 10:08 AM.]

Obviously, the mismatching didn't break my amps either. But I will take the word of people who seem to know that running mismatched speakers at high volume for extended periods of time is inviting amp damage.

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"a 4 ohm speaker will draw more power and sound louder"

That's the big misconception. A mismatch either way *decreases* power.

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Hmmm...maybe I should tell the folks at Fender; my Stage 112SE amp is rated at 118 watts into 8-ohms, and 160 watts into 4-ohms (when using an extension speaker). That's a pretty significant difference! Maybe it is a "numbers" or rating thing, and they de-rate amp with a mis-matched speaker for improved amplifier reliability? Still, it makes little sense to design an amp for 160 watts output when you know 98% of the customers will use it in a 118 watt mode.

Anyhow, other than the "power thing", the sound of different speakers is very subjective. Some players use an 8-ohm speaker in their amp (designed for a 4-ohm speaker) simply because the sound is different, "better", to them). Sure, they probably lose a little power, but they're going after an elusive sound, and not a few extra watts. If you're not maxing out the amp, you can always just play a little louder, and maybe get a different sound, one more to your liking.

Interestingly enough, Dave said that the range we're usually using a speaker in (in my previous graph) is one where the impedance is pretty nominal. But wait! You'll also notice that the efficiency of the speaker (the "SPL", or upper line in the graph) is also lowest near the nominal impedance. When you move away from that nominal impedance, the speaker just sort of comes alive, and indeed imparts it's own response, or "sound" to the amp (and your playing). This is why a lot of people select speakers based on their sound, and not strictly their output, efficiency, or impedance. (The engineers do that! )

You see, if it's not giving you the sound you want, the best numbers match in the world is pretty meaningless. Some players even EXPECT their speakers to fail eventually (just like tires on a car), while others hope to retain them stock and functional for a lifetime. In the end, whichever is right depends only on the player. There are few hard and fast rules when it comes to a musician and the sound he's after.

[This message was edited by Donny Hinson on 08 August 2006 at 01:40 PM.]

But if you talk about a "fixed" impedance OT, any mismatch - up or down - will *decrease* power.

And yes, the *sound* is really key....but for the average Joe who knows nothing about servicing amps, it's a lot safer to stay with matched impedances. Heck, most tube amp players really have no idea how (or tools to) bias an amp!

Oh - FWIW the "experiments" with Fender amps mentioned a couple posts earlier ARE easy to analyze:

1) Fender tube amps are overengineered, and can handle 100% mismatch either way for the most part, and

But when these amps are pushed, the speaker impedance and reflected voltage change the way the amp feels and sounds significantly, in my experience. I've done this many times, and when pushed hard, virtually every tube amp I've tried that with lost noticable volume when I used, for example, an 8 ohm speaker where a 4 ohm was expected. Further, the amp felt different - the interaction between me, the guitar, and the amp changed. When everything is just right in a tube amp, when I push the amp, it pushes back in a very nice way, giving a great touch-response. I first noticed how much difference there was when I bought an old Tweed Champ (a very long time ago) which had an 8 ohm replacement speaker, for guitar. I could not believe how much better the amp sounded and felt when I put a good 3.2 ohm speaker in there. Still, I know guitar players that swear by mismatching impedances, especially on the big amps. They argue that they can push the amp harder and get more distortion at lower volume. Fine, but to me, they'd be better off using a smaller amp more appropriate to the situation.

Chas' example is different. I don't know much about Evans amps, but the power sections are solid-state, and I'd be surprised if they're not direct-coupled. I don't have any schematics for them, so I can't say for sure. But their website suggests running an 8 ohm extension speaker is OK, so assuming the internal speaker is also 8 ohms, they're obviously OK for 4 ohm operation. As far as saying "why" the 4 ohm BW sounds better - well, it's possible that has more to do with the inherent sound of the BW, as compared to the original speaker, than anything else - I think that the kind of thing alluded to in Donny's last post. The only way to test that hypothesis would be to run other types of 4 ohm speakers and see if they also improve the tone similarly. On the other hand, if the Evans is transformer coupled, it's a different analysis altogether.

Donny - I never suggested that speakers shouldn't be chosen for tone (or efficiency, for that matter). Even though I'm trained as an electrical engineer, as a musician, I of course agree that they should. I also never said that the peaks don't affect the tone - of course they do. All I said was that the frequency band that typically contains most of the signal energy has a pretty flat impedance which is closely related to the nominal impedance - this affects the electrical interaction between the output section and the speaker, and that can affect both the tone and the way the amp feels to me, as a player. IMO.

So with that said, if I have a choice of getting a particular speaker type (one that has a tone and efficiency that I like) at the rated impedance or at a different impedance, I'm gonna get the matching impedance version. YMMV, and I know plenty of people who disagree. Let me say that I've also watched smoke coming out of some of those amps as they pushed them to the point where the OT burned up. Of course, pedal steel players never play loud enough to allow any distortion, so we don't have to worry about this, right?

If you don't believe that tube amps aren't sensitive to load impedance, and you have an amp you'd like to destroy (so don't do this unless you want to kill the amp) - try removing the speaker load entirely and play through the turned up amp. For most amps, it won't be long before you see smoke coming from the output transformer. I have, unfortunately, done that accidentally when a speaker wire came loose during a gig - one of those clip-on speaker connectors shook loose from the vibrations. Another time I just had to run an 8 ohm JBL D-130 extension cab out of my old Deluxe Reverb, for a 4-ohm total load. It was a laid back, fairly quiet gig, and sounded fantastic - right up the the second that I turned around in horror to see smoke come out of the input jacks as the OT fried. Yes, OTs are not all created equal - some can take it and others not. I'm not into playing Russian Roulette with my old Fenders.

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Still, it makes little sense to design an amp for 160 watts output when you know 98% of the customers will use it in a 118 watt mode.

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You use what the amplifier was designed for for. And with the exception of amplifiers (a very few tube amps suitable for steel, and a few more solid-state) with multiple impedance outputs, you have ONE "correct" choice as far as impedance, unless you like playing dangerously with your electronics.

PA amps and bass amps commonly have multiple taps. so do some Marshall heads, but those aren't usually steel amps either.

Simple, basic rule - find out what impedance your amp is designed for, and use the best speaker you can find (for the sound you want) rated for both that impedance and the amplifier's power output.

If you're refering to the distortion characteristics of such a mismatch, then, yes there will be a very noticable change. The amp will approach it's max available output (which will be lower than a correct match would provide) and sound "driven hard", but, at a lower volume level. Here, the amp is overdriven and not the speaker. This could account for the "It sounds better" comment. At full, matched impedance, power the distortion character will be quite different again, since the speakers themselves will be contributing to the distortion.

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Point out a famous steel song, or ride, that was cut with a 4-ohm, and one that was cut on an 8-ohm, and let us decide for ourselves what to buy/use.
What else could possibly matter?

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No, there is no dramatic difference in sound. There might be a dramatic difference in amp life. I would venture to say that the vast majority of the great steel playing you have heard and admired was done on an amp with a matched speaker - pros tend to have professional equipment and use it in a professional way. If the speaker didn't match, you wouldn't have been able to hear it. But maybe the guy's amp went South sooner. Maybe that's not dramatic either. Maybe he got 80 or 90% of its life.

Q:Will it hurt my amp/output transformer/tubes to use a mismatched speaker load?
Simple A: Within reason, no.
Say for example you have two eight ohm speakers, and you want to hook them up to an amp with 4, 8, and 16 ohm taps. How do you hook them up?

For most power out, put them in series and tie them to the 16 ohm tap, or parallel them and tie the pair to the 4 ohm load.

For tone? Try it several different ways and see which you like best. "Tone" is not a single valued quantity, either, and in fact depends hugely on the person listening. That variation in impedance versus frequency and the variation in output power versus impedance and the variation in impedance with loading conspire to make the audio response curves a broad hump with ragged, humped ends, and those humps and dips are what makes for the "tone" you hear and interpret. Will you hurt the transformer if you parallel them to four ohms and hook them to the 8 ohm tap? Almost certainly not. If you parallel them and hook them to the 16 ohm tap? Extremely unlikely. In fact, you probably won't hurt the transformer if you short the outputs. If you series them and hook them to the 8 ohm or 4 ohm tap? Unlikely - however... the thing you CAN do to hurt a tube output transformer is to put too high an ohmage load on it. If you open the outputs, the energy that gets stored in the magnetic core has nowhere to go if there is a sudden discontinuity in the drive, and acts like a discharging inductor. This can generate voltage spikes that can punch through the insulation inside the transformer and short the windings. I would not go above double the rated load on any tap. And NEVER open circuit the output of a tube amp - it can fry the transformer in a couple of ways.

Extended A: It's almost never low impedance that kills an OT, it's too high an impedance.

The power tubes simply refuse to put out all that much more current with a lower-impedance load, so death by overheating with a too-low load is all but impossible - not totally out of the question but extremely unlikely. The power tubes simply get into a loading range where their output power goes down from the mismatched load. At 2:1 lower-than-matched load is not unreasonable at all.

If you do too high a load, the power tubes still limit what they put out, but a second order effect becomes important.

There is magnetic leakage from primary to secondary and between both half-primaries to each other. When the current in the primary is driven to be discontinuous, you get inductive kickback from the leakage inductances in the form of a voltage spike.

This voltage spike can punch through insulation or flash over sockets, and the spike is sitting on top of B+, so it's got a head start for a flashover to ground. If the punchthrough was one time, it wouldn't be a problem, but the burning residues inside the transformer make punchthrough easier at the same point on the next cycle, and eventually erode the insulation to make a conductive path between layers. The sound goes south, and with an intermittent short you can get a permanent short, or the wire can burn though to give you an open there, and now you have a dead transformer.

So how much loading is too high? For a well designed (equals interleaved, tightly coupled, low leakage inductances, like a fine, high quality hifi) OT, you can easily withstand a 2:1 mismatch high.

For a poorly designed (high leakage, poor coupling, not well insulated or potted) transformer, 2:1 may well be marginal. Worse, if you have an intermittent contact in the path to the speaker, you will introduce transients that are sharper and hence cause higher voltages. In that light, the speaker impedance selector switch could kill OT's if two ways - if it's a break befor make, the transients cause punch through; if it's a make before break, the OT is intermittently shorted and the higher currents cause burns on the switch that eventually make it into a break before make. Turning the speaker impedance selector with an amp running is something I would not chance, not once.

Several solid state amp manufacturers, built / build there amps with mis-matched output / speaker ohm ratings.

For example:

Webbs show 250 watts output with a minimum ohm load rating of 2 ohms which is close to nothing. They originally came with a 4 ohm JBL K 130 speaker, then a 4 ohm E 130 and then finally went to an 8 ohm E 130 JBL when the K 130 and 4 ohm speakers were no longer manufactured by JBL.

The same with the Evans. I beleive that the old FET 500's were actually rated at 2 ohms minimum load, or what good is the second speaker outlet as they all came with a 4 ohm Eminence Eclipse as standard equipment.

The newer Evans SE 200's all were using 8 ohm speakers and I think that the amp is actually rated at 4 ohms output. Scott at Evans always claims that an 8 ohm speaker or load will help an amp rated at 4 ohms output run cooler and "breath easier."

Again, just some more to think about or analyze.