Speaker Question: 16 Ohm Speaker with an 8 Ohm Output Transformer

[Simon Barden]

It depends on the amp design and the standard of component. It was suggested in the manual for my old 100W Mesa Nomad, that in the 50W/half power mode, you plugged the 8ohm speaker into the 4 ohm output for the correct impedance match (with only two valves working). But it also said that it was fine to leave it in the 8 ohm jack and that it wouldn't cause any damage, you'd just get a fatter, darker sound. But note that it was an output transformer rated for at least 100W, which would be running at 55W, so the mismatch wasn't going to cause the transformer any issues. (at least at one speaker ohmage step difference).

[fender3x]

If the question is "is it safe" to run a 16 ohm speaker out of a tube amps 8 ohm jack, I think the answer is yes. Will an "all things being equal" 16 ohm speaker sound the same as an 8 ohm speaker? No, but that is not necessarily bad, particularly if you have high enough power. There's a good article on it here.

http://blog.hughes-and-kettner.com/o...and-impedance/

What Simon describes is what I understand to be generally true. It's why a lower (16 Ohm) resistance speaker sounds quieter than a 8 Ohm.

I think what JimC is referring to is the problem of running with no load. If the amp is "on" a valve amp in particular should have a speaker connected. I don't think it makes a difference whether it's 8 or 16, but their should be some resistance. The other thing that is confusing for a dyslexic person like myself is that "lower" resistance is a higher number.

[Simon Barden]

What Simon describes is what I understand to be generally true. It's why a lower (16 Ohm) resistance speaker sounds quieter than a 8 Ohm.

No, no and no for valves. Solid state amps are basically voltage amplifiers, so as the speaker resistance increases, the power output of the amp decreases. Roughly a doubling in speaker impedance results in 50% drop in the output power produced. in So the same model of speaker in a 16 ohm guise will be quieter than the 8 ohm version.

But valve amps are basically current amplifiers, and they need an output transformer to match the speaker impedance to the valve power amp circuitry. Because of this, a 50W valve amp with, 4 8 and 16 ohm output taps on its transformer will put out 50W into a 4 ohm load (using the 4 ohm tap), 50W into an 8 ohm load (using the 8 ohm tap) and 50W into a 16 ohm load (using the 16 ohm tap). But it also means that if you set the tap wrong (or the amp doesn't have a suitable tap for the speaker connected), the amp will see the wrong impedance.

In the case of most valve amps, the output transformer primary also forms part of the output stage biasing circuit arrangements. The speaker impedance is reflected across the transformer and becomes part of the biasing circuit. If that impedance is wrong, the output stage is biased incorrectly and that can result in very unhappy output valves.

At best, the amp will 'sound different' at worst, it will go pop.

Modern amps will normally tolerate a small impedance mismatch e.g. 8 ohm tap into a 16 or 4 ohm speaker,as long as you don't run the amp flat out. But a 4 ohm tap into a 16 ohm speaker is normally pushing the limits. But it really isn't a great idea, especially with vintage amps or faithful copies of them, where the manufacturers normally skimped on equipment ratings to fit the cheapest components they could.

What H&K say should really only be applied to H&K's own amps, where they know the designs and equipment ratings. But don't quote H&K saying it was OK to do a 4 ohm/16 ohm mismatch if you are using a different manufacturer's valve amp and it goes up in smoke!

And never, ever, run a valve amp without a speaker connected.

[fender3x]

And never, ever, run a valve amp without a speaker connected.

Well at least I got that part right... I stand corrected! I do not want to be responsible for an amp going "pop"!

[Andy40]

I'm down to two now... a Fender Deluxe Reverb Tonemaster (released last year) and a Roland Blues Cube Artist.
Both around the 1200 dollar mark. I sold my last two tube amps last year.

SNAP!

I love my Roland BC Artist! Got all the juicy Tone Capsules too.

I tried the Tonemaster at Guitar Bros but couldn't really get into it due to time and noise restrictions (of course).

[Andy40]

New speaker for the 5E3 now, then later another 16 ohm speaker to make an 8 ohm 2x12" cab.

Simon and I are on the same page here

[DrNomis_44]

No, no and no for valves. Solid state amps are basically voltage amplifiers, so as the speaker resistance increases, the power output of the amp decreases. Roughly a doubling in speaker impedance results in 50% drop in the output power produced. in So the same model of speaker in a 16 ohm guise will be quieter than the 8 ohm version.

But valve amps are basically current amplifiers, and they need an output transformer to match the speaker impedance to the valve power amp circuitry. Because of this, a 50W valve amp with, 4 8 and 16 ohm output taps on its transformer will put out 50W into a 4 ohm load (using the 4 ohm tap), 50W into an 8 ohm load (using the 8 ohm tap) and 50W into a 16 ohm load (using the 16 ohm tap). But it also means that if you set the tap wrong (or the amp doesn't have a suitable tap for the speaker connected), the amp will see the wrong impedance.

In the case of most valve amps, the output transformer primary also forms part of the output stage biasing circuit arrangements. The speaker impedance is reflected across the transformer and becomes part of the biasing circuit. If that impedance is wrong, the output stage is biased incorrectly and that can result in very unhappy output valves.

At best, the amp will 'sound different' at worst, it will go pop.

Modern amps will normally tolerate a small impedance mismatch e.g. 8 ohm tap into a 16 or 4 ohm speaker,as long as you don't run the amp flat out. But a 4 ohm tap into a 16 ohm speaker is normally pushing the limits. But it really isn't a great idea, especially with vintage amps or faithful copies of them, where the manufacturers normally skimped on equipment ratings to fit the cheapest components they could.

What H&K say should really only be applied to H&K's own amps, where they know the designs and equipment ratings. But don't quote H&K saying it was OK to do a 4 ohm/16 ohm mismatch if you are using a different manufacturer's valve amp and it goes up in smoke!

And never, ever, run a valve amp without a speaker connected.

I thought Valve amps were voltage-amplifiers and Transistor amps are current-amplifiers, there are a few reasons why Valve amps use output transformers, firstly it isolates the high anode voltages on the anodes of the power Valves from the speaker, secondly it transforms the impedance of the speaker up to an impedance that the power Valves are happy working with so they can deliver their maximum power into a speaker load, thirdly the output transformer allows push-pull operation, although some Valve amps use a single power valve.


A typical Transistor amplifier doesn't really need an output transformer because the output is already at a low impedance, however, there have been Transistor amp designs in the past that do use an output transformer, most modern Mosfet amplifiers usually run off a +/- DC supply, usually something like +/- 70V or so, at a few amps, normally the output voltage at the speaker, when the amp is running idle, is usually something close to zero as ideally possible.


Interestingly enough, my Peavey Renown, which is a 100% Transistor/Solid-State 160 Watt amplifier that can handle a minimum speaker load of 4 Ohms (it uses some ICs in the pre-amp circuitry, and two of those metal TO-3 packaged power Transistors in the power amp), uses an output transformer, my guess is that Peavey did that to try and get the amp to sound more Valve-like, they were doing a lot of experimenting with Solid-State amps back in the 80's when Valves were starting to become scarce, personally, I think the Renown has kind of a hybrid Valve/Solid-State sound to it, you can get some good cleans and overdrives out of it, if you set the tone controls, etc right.


On a side-note, here's an interesting Wikipedia page on something called the Maximum Power Transfer Theorem:

https://en.wikipedia.org/wiki/Maximu...ansfer_theorem


It does have some complex maths equations on it, but there are some simple to understand diagrams and explanations on it, this is the main reason why a Valve amp needs an output transformer to match the impedance of the speaker load to the amplifier, although there are Valve amps in existence that don't use an output transformer, these are known as OTL, or Output Transformerless amplifiers, which you'll more likely see used in Audiophile HiFi applications, for example driving a pair of HiFi headphones, I find them quite fascinating, would love to build myself one.


Here's another interesting article I found, it basically shows how a speaker's impedance varies with frequency, I thought it was relevant enough to post a link to it:

http://education.lenardaudio.com/en/05_speakers_3.html


So, what that says is that the 8 or 16 Ohms impedance rating of a speaker is really just a nominal rating.

[Simon Barden]

Nominal, but not that nominal. It's still an indicator of the overall impedance, so a 16 ohm speaker will have a higher overall impedance across the frequency range than an 8 ohm speaker.

Impedance matching (apart from being necessary in this instance to get a lot of power from output valves) is done to get maximum current (and therefore power) transfer from the power section to the speaker. Valve output impedance is well up in the kilo ohms, maybe 8k ohms for a push-pull pair, so you need a high ratio transformer to translate 4, 8 or 16 ohms into the kilo ohms range. So you are using the transformer to turn 4, 8 or 16 ohms into something approaching 8k ohms. So the different impedance the power valve outputs see if the wrong speaker tapping is used isn't in the 4 to 12 ohm range, but the 8k to 32k ohm range. The output transformer is used to keep the impedance the amp sees around that 8k value (or whatever the value is for that particular amp), which is why the amp's power output remains constant despite different impedance speakers being used with it, as long as the correct transformer tapping is used.

Before the advent of transistors, impedance matching used to be the way signal levels were transferred between items of equipment to maximise current transfer, but these days, low impedance signal outputs feeding high impedance inputs are the norm, where it's all about signal voltage levels, not current.

Solid state power amps have a very low output impedance, and can quite happily drive 4 ohm speakers directly, (some can do 2 ohms). But without a matching transformer, the output voltage level remains almost constant but the output current drops as the speaker impedance increases. So as the speaker impedance increases, the amp's output power decreases. Too low a speaker impedance load and a solid state amp can burn out its power transistors as it tries to provide more current than the devices are rated for. But most tranny amps will run all day into an open circuit (though some designs can become imbalanced and don't like it for too long). You can't do that at all for a valve amp.

In that instance, the valve power amp sees an infinite resistance but its still trying to drive a lot of current through the transformer. None of that power is being transferred to the speaker, and it has to go somewhere, namely into heating up the output transformer and the output valves. It's a matter of seconds at any reasonable power output before the transformer winding insulation burns off and shorts out, and/or the valves burn themselves out.

[Andy40]

I'm learning things by the minute here

[DrNomis_44]

I'm learning things by the minute here

I think we are all learning things here, including myself.