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In Reply to: RE: Efficiency & Resistance posted by saki70 on February 14, 2015 at 14:39:02
My take on it is this.
The higher impedance speaker is more sensitive to voltage making current less of a requirement.
This makes them more suitable for smaller amplifiers like SETs etc, that semantically are voltage amplifiers.
Audio Illuminati
Follow Ups:
The efficiency does not change with the impedance. If you have an amp with selectable taps on the output transformer (4-8-16 ohms is typical) then when the amp is matched to teh speaker the system will have the same maximum output, independent of the speaker impedance.
If you have no taps, as is typical with solid-state amps, then you have a limited voltage, in which case the voltage sensitivity (dB per volt) is the relevant measure and a higher impedance speaker will have less maximum output.
To change form one impedance to another, replace all the speaker drivers and the crossover with otherwise-identical parts rated for the new impedance. You can keep the boxes. :^)
I don't mean to be disagreeable, but there are exceptions to the statement, "If you have no taps, as is typical with solid-state amps, then you have a limited voltage, in which case the voltage sensitivity (dB per volt) is the relevant measure and a higher impedance speaker will have less maximum output." An example of this is the pair of Nelson Pass First Watt F3 solid state stereo amps I use for the mids and highs of my triamped DIY horn speakers. They do not have ouput transformers or taps, but their rated output is 15 watts into 8 ohms but only 10 watts into 4 ohm or 16 ohm loads.
Edits: 02/15/15
True enough. Most high-end solid state amps at least claim to have no significant current limit (the "arc-welder" school of amp design) but of course any class-A amp will have such a limit. I did indeed generalize.
Lets say that we have two speakers , A & B . Both speakers are driven by a SET tube amp and theoretically identical except that ...
Speaker A is 98db efficient with a nominal impedance of 8 ohms .
Speaker B is also 98db efficient but with a nominal impedance of 14 ohms .
How will speaker B respond differently than speaker A ?
Thank You
If speaker B is in fact 98dB/watt at 1 meter, same as A, then they will sound the same on the 14 and 8 ohm taps respectively. Assuming you have those taps, and assuming the transformer is wound so that there is no practical difference in its performance on those taps.
But it is very likely that the speakers are rated dB/2.83v at 1 meter. This is a sensitivity rating, not an efficiency rating - even though it is far too often claimed to be an efficiency rating. In that case, the 14 ohm speaker is nearly 3dB more efficient and will be louder.
This confusion has been around for decades, so I have no expectation that it will go away anytime soon - there are still flat-earth believers, after all.
In my experience/opinion, all things being equal (Which they rarely are) the higher impedance speaker will sound more tactile.
The higher resistance is easier for the amp to grip.
The lower impedance speakers will need more current, all things being equal, the amp wont be able to swing the voltage as high. Yes there are impedance taps on the amp, but you are leaning more towards a current amp at that point.
It's more challenging/complex to build an amplifier to deliver the current needed to energize a lower impedance coil and have the fidelity comparable to a high impedance / higher voltage speaker.
The lower impedance speaker would be more suitable with modern type gear which are generally solid state devices etc that can't swing the required voltage to get the watts into a 16 ohm speaker, but can deliver the amperage to excite the speaker at a lower voltage.
Audio Illuminati
Good answer Garg0yle , very understandable !
So here is an aspect that puzzles me... there are two 'system' manufacturers that are very well respected , Audio Note and Coincident Technology . Both put out medium to high efficiency speakers but Audio Note stays in the 8 ohm realm while Coincident goes up the ladder into 14 ohm territory . Yet both camps have well received SET driven sound .
Just how tactile is the higher impedance speaker in actual usage ?
Can the average listener hear the difference ?
Or is it that Audio Note SET amps delve into a higher amperage output ?
Again thank you for your time .
Whether or not you could hear a difference would depend on how well the Audio Note is able to supply the extra time varying current with enough fidelity to match the same wattage/SPL level of the Coincident. (Assuming equally efficient speakers)
Of course there are many, many variables involved with this comparison, so I can't say which one sounds better from sitting here, but the Coincident does have a head start from a design aspect. (In the context of high voltage, low current tube amps)
"Or is it that Audio Note SET amps delve into a higher amperage output ?"
Yes for a given SPL on otherwise comparable speakers, there will be more current going through the 8ohm speaker.
Audio Illuminati
McIntosh MC2125/2120
DAMPING FACTOR in STEREO
16 at 2 ohms output, 50 at 4 ohms output, 20 at 8 ohms
output, 14 at 16 ohms output.
It would seem that this amplifier has its best performance at 4Ω.
The autoformer Mc developed and uses takesnthe speaker impedance and adjusts it so the amp sees a fairly uniform impedance regardless of the actual speaker impedance. Think of it as converting the speaker to amp relationship of the ss amp to look and act similar to a tube amp and speaker system. One advantage is the power is uniform regardless of the speakers impedance. It has an advantage when using speakers with complex and difficult crossover's that challenge direct coupled amps such as speakers with very high capacitance. An example is e'stats such as the old KLH Nines and JansZens.
A benefit, of course is when speakers have largest impedance swings as most do thereby having to deal with varying power and current levels that may be heard by sounding bass shy or coloring the sound, etc. the Mc amps gender to be more uniform and a speaker may significantly sound different on them but closer to what the speaker will sound like on a tube amp. This is why some persons dislike the Mc sound immensely not realizing the Mc is coming closer to presenting the speaker closer to an ampless bias sound, so to speak.
I am amazed more companies never followed Mc though in the earliest ss era a few did use output transformers to try to keep the germanium outputs more stable. The autoformer can be eliminated in the Mc and used as a dc amp. Some years ago this was tried as an experiment to see how if it could be done and the result was the Mc was stable, the voicing very close but the speakers' sound changed quite a bit, sounding as if on the Mc power amp that did not use autoformer at the time. I can not remember the model number; it was the first offered without them back in the '70s. The reviewers panel pretty much represented the public's opinion on which amp "sounded" better.
I have used Mc amps from just before their first ss until just a couple of years ago and while I have had some very fine ss as flow through the house found the Mc SS amps to be generally be more satisfying as I grew up with and used tube amps in my formative years as sell as through out my hobby years. As for ss that were not Mc, I tended to find those with capacitor coupled outputs to be more satisfying but, not sure why.
Don Brian Levy, J.D.
Toronto ON Canada
"It would seem that this amplifier has its best performance at 4Ω."
I disagree.
Damping may quell oscillations in some speakers with oscillation problems or inappropriate cabinet design, but does it not describe how well the amplifier will work or sound with all speakers to merit such generalities.
If the speaker/cabinet combination is sufficiently dampened mechanically, then it means very little with typical high impedance speakers.
It means more to sloppy high excursion-low impedance woofers that need the negative feedback to help control the woofer. These have looser suspension and soft surrounds. The voice coil has less turns so it makes a weaker electromagnet then a comparable high impedance coil.
This aligns with what I said about the challenges to building a current type amplifier. The negative feedback is an attempt to correct the inherent flaws of such topologies, the lack of grip on the voice coil.
If you short the terminals on an 8 ohm and a 16 ohm speaker that are otherwise the same, you will find it harder to push the cone in on the 16ohm speaker. That is because it has a better motor, these motors will be more responsive to changes in voltage.
Audio Illuminati
Yes impedance ! I do get it right sometimers !
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