Stfoth, as Mapman indicated the net result of the myriad factors and tradeoffs that are involved in a middle ground situation such as you've described will come down to a combination of the specific designs and listener preference. And chances are that a multitude of different approaches will work well for many listeners, as we often see here.
A good understanding of those factors and tradeoffs can reduce the randomness of the selection process, and the risks of wasted time and expensive mistakes, but in the end, as the saying goes, the proof of the pudding is in the eating. Or in this case, the listening.
Best regards,
-- Al
P.S: Ghosthouse, thanks for the nice words :-)
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Is there some speaker performance or design spec that signals when a voltage vs a current source is preferable? If the impedance of the speaker is relatively high, and does not vary greatly in magnitude over the frequency range, and does not have phase angles that are severely capacitive (i.e., extremely negative) at any frequency (especially at frequencies at which the impedance magnitude is low), and has medium to high sensitivity, chances are it would be suitable for use with either type of amplification. To the extent that those criteria are not met, as Ralph (Atmasphere) has said in the past it can often be helpful to try to determine the intentions of the designer. For example, it can be inferred that a small two-way speaker having an impedance in the vicinity of 4 ohms in the bass region and 8 ohms at higher frequencies is probably intended for use with solid state amplification, which will augment the bass by delivering more current and power at bass frequencies than at higher frequencies, for a given output voltage. While the classic Quad ESL-57 electrostatic speaker was designed before solid state amplification existed, so it can be presumed to be suitable for use with tube amplification even though its impedance varies widely over the frequency range and is highly capacitive. Best regards, -- Al |
Hi Ghosthouse,
A theoretically ideal voltage source has an output impedance of zero, and a theoretically ideal current source as an output impedance that is infinite.
Neither exists in practice, of course. However most (but not all) solid state amps have an effective output impedance that is low enough in relation to speaker impedance to enable them to be considered as voltage sources for practical purposes. Provided, of course, that they are operated within the limits of their maximum voltage, current, power, and thermal capabilities.
On the other hand, tube amps have output impedances that are usually significant in relation to speaker impedances, and also tend to differ widely among different designs. Consequently tube amps fall at various points on a continuum between ideal voltage source and ideal current source.
An ideal voltage source will maintain an output voltage that is constant as a function of load impedance (again, as long as it is operated within its maximum voltage, current, power, and thermal capabilities). Per Ohm’s Law, if the load impedance is purely resistive the current that is supplied will equal that output voltage divided by the load resistance. It gets more complicated when the load has a significant inductive or capacitive component.
An ideal current source will maintain an output current that is constant as a function of load impedance, and per Ohm’s Law the voltage it will supply into a purely resistive load will equal that current multiplied by the resistance, as long as it is operated within its maximum capabilities. And again, it gets more complicated when the load has a significant inductive or capacitive component.
Best regards,
-- Al
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Hi Jim, As for that -3db item regarding 4 ohm impedance speakers, is this attribute a constant with respect to power across the bandwidth?
IN other words, with a 4 ohm IMP whatever speaker will it always be 3db down in SPL at what ever rated amplifier output that it would be if an 8 ohm load was present?
I got the impression making a speaker IMP 4 ohms incidentally obtained automatically about a 2 – 3db gain in response or sensitivity.
If a 4 ohm speaker is rated to produce the same SPL at 1 meter as an 8 ohm speaker, and both ratings are specified on the basis of an input of 2.83 volts (rather than 1 watt), and the SPL and impedance ratings are accurate, the 4 ohm speaker will require twice as many watts (i.e., 3 db more power) to produce the same volume as the 8 ohm speaker, at a given distance and at all frequencies for which those ratings are accurate. But also keep in mind that most solid state amplifiers can deliver significantly more power into 4 ohms than into 8 ohms, and in some cases twice as much. Regularly we see here and elsewhere, output to input IMP should have 1 to 10 ratios. 1K out IMP needs a 10K input or better. Although that guideline is commonly stated, as you indicated, it’s not that simple. See my post dated 10-3-2016 in the following thread: https://forum.audiogon.com/discussions/audio-research-ls-25-mkii-compared-to-actual-ar-production Is there as well a general rule of thumb for preamp gain and amplifier gain one should keep in mind irrespective of the load when choosing a preamp & power amplifier match? Power amp gains generally tend to be in the area of 25 to 30 db or so, although some are significantly higher and some are significantly lower. The gains of active line stage preamps in recent decades generally tend to be in the area of 6 to 15 db or so, although some are considerably higher, and at least a few are somewhat lower. Differences in gain will affect what settings of the volume control will be used, but of course won’t affect the maximum amount of power that can be delivered to the speakers. Volume control positions will also be affected by speaker sensitivity and the output level of the source components. If any of these four factors is considerably higher or lower than usual attention should be given to the possibility that the volume control may have to be used too close to the bottom of its range, or at the other extreme that it might even run out of range at the top with some recordings. Best regards, -- Al |
Hi Jim,
Strictly speaking, speaker efficiency would be the ratio of acoustic power out to electrical power in.
However, it is common practice for the term "efficiency" to be used to refer to the sound pressure level (SPL) that would be produced at a distance of 1 meter in response to an input of 1 watt.
It is also common practice for the term "sensitivity" to be used to refer to the SPL that would be produced at a distance of 1 meter in response to an input of 2.83 volts.
2.83 volts into 8 ohms corresponds to 1 watt. (2.83 squared/8 = 1). So for an 8 ohm speaker, that is truly 8 ohms, sensitivity and efficiency are equal.
2.83 volts into 4 ohms, though, corresponds to 2 watts (2.83 squared/4 = 2), which is 3 db more than 1 watt. So the efficiency of a 4 ohm speaker would be 3 db less than its sensitivity, assuming those terms are used in their most usual sense.
Also, keep in mind that as can be seen in measurements provided by John Atkinson in Stereophile, and measurements that are provided in reviews at SoundStage.com and elsewhere, manufacturer sensitivity and efficiency specs are often overstated by a few db. In some cases, I believe, because the manufacturer’s rating may be based on a single frequency, rather than an average across a wide range of frequencies.
Best regards,
-- Al
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Kosst_Amojan 12-14-2017
It's called impedance matching. The closer a load matches the impedance
of the source, the more efficient the energy transfer. Generally you
want the load to be at least twice the impedance of the source to avoid
odd coupling effects. The ratio of impedance difference also defines the
damping factor which can significantly effect speaker behavior.
Kosst & Miketuason, The following statement appears in the manual for the McIntosh MC601 that is listed in Mike's system description. I believe similar statements appear in the documentation of other McIntosh solid state amplifiers which use autoformers. And this is what I was alluding to in my previous post, although I referred to performance benefits rather than benefits to long-term reliability. (Kudos to Mattmiller for citing both benefits): All solid state power amplifier output circuits work best into what is called an optimum load. This optimum load may vary considerably from what a loudspeaker requires. In the case of more than one loudspeaker connected in parallel, the load to the power amplifier may drop to two ohms or even less. A power amplifier connected to a load that is lower than optimum, causes more output current to flow, which results in extra heat being generated in the power output stage. This increase in temperature will result in a reduced life expectancy for the amplifier.
The special Balanced Winding Autoformer creates an ideal match between the power amplifier output stage and the loudspeaker. Also, regarding the comment about damping factor, the MC601 has a specified damping factor of "greater than 40," which presumably means effective output impedances of the 8, 4, and 2 ohm taps of approximately 0.2 ohms, 0.1 ohms, and 0.05 ohms respectively, all of which of course are very small fractions of the impedance of most speakers. Which in turn reinforces the notion that the multiple taps are provided mainly to benefit the amplifier's output stage, as opposed to optimizing interactions with the speaker, such as impedance interactions, bass damping, and energy transfer. Regards, -- Al |
Mike, as I understand it the rationale for the use of autoformers in McIntosh solid state amps is to enable the output stage of the amp to "see" a higher load impedance when driving low impedance speakers. Specifically, to "see" the same load impedance when a 2 ohm load is connected to the 2 ohm tap as when a 4 ohm load is connected to the 4 ohm tap, and as when an 8 ohm load is connected to the 8 ohm tap. Thereby making life easier for the output stage (i.e., improving its performance) when the amp is required to drive low impedances. Of course, that benefit will trade off against whatever sonic downsides may be introduced by the autoformer itself.
Regards,
-- Al
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Mucho gras Al.
Would you mind clearing up this thought of your’s?
Any amplifier having low output impedance can be rated to deliver twice as much power into 4 ohms as into 8 ohms, if the 8 ohm rating provides a large enough margin relative to actual capability
Margin? Section? Area? Range? Who picks them or it? The amp maker, Yes? No>? Hi Jim, Yes, I’m referring to ratings that are claimed by the designer/manufacturer. I’ll explain further with a hypothetical example: During the design process the designer targets a capability of 120 watts into 8 ohms and 180 watts into 4 ohms, based in part on a belief that he can accomplish that with a design that will fall into a price range he wants to compete in. When he tests his initial prototype of the design he finds that it can meet but not significantly exceed those numbers, on a continuous basis (as opposed to just being able to meet them for say a few minutes without self-protection mechanisms being triggered), with reasonably low distortion, when supplied with an AC input of 120.0 volts. He will then presumably want to allow some margin in his published numbers, relative to those numbers, to account for sample-to-sample differences that may occur in production, and to take into account reasonable differences that can be expected in line voltage, ambient temperature, and other variables. So let’s say that he decides to introduce a margin of approximately 16% (about one sixth) into each of the published specs, resulting in published ratings of 100 watts into 8 ohms and 150 watts into 4 ohms. However **some** designers in that situation may instead choose to publish ratings of 75 watts into 8 ohms and 150 watts into 4 ohms, which as George has pointed out may help sell amplifiers because it can lead potential customers to consider the amp as being able to "double down." While from a marketing standpoint the downside of that kind of "specmanship" would be mitigated in the minds of many potential customers by the fact that 75 watts is only 1.25 db less than 100 watts! The only sensible thing to do, If both are the same or quite similar topology, is to weigh the two final amp choices, and buy the heavier one. Weight is actually a criterion that can indeed be helpful to take into account in choosing an amplifier, IMO, as there does tend to be a correlation between weight and quality (albeit a loose correlation of course), among amplifiers having similar topologies and that provide roughly similar power capabilities. Unfortunately, though, weight also tends to be significantly correlated with price, given those similarities. Best regards, -- Al |
Regarding the disagreement about whether or not some solid state amps can "double down": As I see it both sides are correct, but are focusing on different things. One side appears to be focusing on **measurable** maximum power capability, and the other side appears to be focusing on **rated** maximum power capability.
Any amplifier having low output impedance can be rated to deliver twice as much power into 4 ohms as into 8 ohms, if the 8 ohm rating provides a large enough margin relative to actual capability. A reputable manufacturer will, or at least should, choose a margin that is large enough to be comfortable relative to anticipated sample-to-sample variations, reasonable variations in line voltage, etc., but at the same time is not a margin that is so large as to be misleading. And likewise when it comes to choosing the distortion percentages upon which the power ratings are based, and in defining whether the power that is being referred to can be delivered continuously or just on a short-term basis.
Regards,
-- Al
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