Current amp vs Voltage amp

I think that you'll find this thread and this one to be informative.

Also, if you haven't already, see Atmasphere's paper on Paradigms in Amplifier Design.

Regards,
-- Al

Hi Bruce (Bifwynne),

I've looked at the references you provided, and read your post, and I think that everything you've said is essentially correct.

Basically, your VS-115 has a lower output impedance than many or most other tube amps, on a given tap. The significant amount of negative feedback it uses is presumably a major reason for that. And it can be presumed that the output impedance of its 4 ohm tap is approximately half of the output impedance of its 8 ohm tap.

The lower the output impedance of a tube amp, the more closely the effects of the interaction of that output impedance with the impedance vs. frequency characteristics of the speaker will approach those of a solid state amplifier (which will have near zero output impedance, in most cases).

The one slight qualification I would make to your post (which you probably already realize) is to emphasize that JA's reference to obtaining a flatter frequency response with the 4 ohm tap than with the 8 ohm tap applies to his "standard simulated loudspeaker load," which has impedance characteristics that are quite different than those of your (and many other) speakers. As you probably already realize, for that and other reasons a generalization should not be drawn that 4 ohm taps will inevitably provide flatter frequency response than 8 ohm taps. It depends on the speaker.

In this particular case, though, given that your speakers were voiced for solid state amps, it can be expected that lower output impedance will result in a flatter response, everything else being equal.

Best regards,
-- Al

A note on terminology: Unless there are some extremely unusual exceptions out there, all audio power amplifiers amplify both voltage and current, so the terminology in the subject line of the thread is a bit misleading.

Solid state amplifiers having near zero output impedance are sometimes referred to as being voltage sources, meaning that for a given input voltage their output voltage will remain constant regardless of load impedance (within the limits of their capability). Referring to them as being voltage sources is accurate terminology, to a very close approximation. (The reason it is a very close approximation, rather than being exact, is that no real world amplifier can have an output impedance that is exactly zero).

Tube amplifiers, having significant output impedances, are sometimes referred to as being current sources, which strictly speaking would mean that for a given input voltage the current they output would remain constant regardless of load impedance (within the limits of their capability). That is NOT accurate terminology, except in the extremely loose sense that their output current will vary LESS as a function of load impedance than in the case of an amplifier having near zero output impedance.

It may be helpful to read this Wikipedia writeup on the voltage divider effect. In the first figure, consider Z1 as representing the output impedance of the amplifier, and Z2 as representing the impedance of the speaker, which will be different at different frequencies.

Regards,
-- Al

Hi Unsound,

I suspect it to be true that more often than not speakers having widely varying impedances will provide flatter frequency responses when driven by solid state amps. But if so, that would simply be because more often than not speakers are designed and voiced with the expectation they they are most likely to be used with solid state amplifiers.

But there are certainly a great many speakers having widely varying impedances that will do better with tube amps than with solid state amps. Many electrostatics are notable examples, along with a goodly number of dynamic (box-type) speakers.

The effects of amplifier/speaker impedance interactions are, of course, just one of a great many factors affecting the frequency response and tonal balance of a speaker. Which kind of amplifier stands the best chance of being optimal after all those factors net out depends, as Atmasphere has said in similar threads in the past, mainly on the intentions of the designer.

Best regards,
-- Al

05-05-13: Unsound
Al, the wouldn't sensitivity of the speaker vary along with the impedance?

Sensitivity, defined as the sound pressure level (SPL) generated by the speaker at a given distance, in response to a given voltage at its input terminals, at a given frequency, is what it is for a given speaker.

If the impedance of the speaker varies widely as a function of frequency, and if the speaker is driven by a tube amplifier having significant output impedance, then the voltage that appears at the input terminals of the speaker, which will be essentially the same as the amplifier's OUTPUT voltage, will vary significantly as a function of frequency, for a given INPUT voltage to the amplifier.

If the same speaker is driven by a solid state amplifier having near zero output impedance, then the voltage that appears at the input terminals of the speaker will NOT vary significantly as a function of frequency, for a given INPUT voltage to the amplifier.

What matters is minimizing the variation of SPL, as a function of frequency, for a given INPUT voltage to the amplifier. Which of those two scenarios will produce the best results in that respect depends on the design of the particular speaker, and what kind of amplifier the designer envisioned it being used with.

Deadlyvj, thanks for providing the interesting link. A bottom line summary seems to me to be contained in this quote:

Due to voltage driving, the sonic performance of virtually all loudspeaker systems is severely impaired by the diverse electromotive forces (see above) induced in the voice coil motor corrupting the ideal behaviour of the amplifier interface....

The effect is not unknown and can be traced down, but Meriläinen strongly feels that the noise mechanisms related to traditional voltage driving have not been adequately addressed, not in the literature, nor in practice.

No amplifier can remove the internal EMF voltage components at the driver motor, but their diverse effects on the speaker/amplifier interface and hence, on the sound quality, can be diminished with a sufficiently high source impedance.

I think that commenting on that intelligently would require both study of the details that are contained in his book, and more knowledge of speaker design than I possess. But his thesis, as I see it, does not change the fact that if a speaker has wide variations of impedance as a function of frequency, it is likely to perform best with an amplifier having whatever kind of output impedance the designer envisioned it being used with.

Best regards,
-- Al

Hi Unsound,

Yes, "such a ss amplifier would almost always be able to provide more linear output regardless of whether they are ESL's or dynamic speakers, and that would hold true whether or not the impedance swings were wide or not." (Assuming that by "linear" you are referring to flat frequency response).

However, as I indicated that is not what matters. What matters, with respect to this issue, is the frequency response flatness of the output of the SPEAKER, as judged against the INPUT to the AMPLIFIER. Which in turn depends on how suitable the match is between amplifier output impedance and the design of the particular speaker.

Frequency response flatness at the amplifier output/speaker input has no direct relevance. Speakers whose impedance varies significantly as a function of frequency, and that match most optimally with tube amps, can be presumed to require a non-flat frequency response at their input terminals to produce an acoustic output whose frequency response is flat. If such a speaker is mated with a solid state amplifier having near zero output impedance, frequency response flatness will have been optimized at the output of the amplifier, but it will be wrong at the output of the speaker.

Best regards,
-- Al

I think that Bombaywalla may, somewhat understandably, have misconstrued Bruce's situation. AFAIK he hasn't had nearly as much interaction with Bruce and his speaker/amplifier concerns as I (and Ralph) have had. Bruce is certainly not in "speaker hell." He's just trying to develop as good an understanding as possible of the speaker/amplifier interface, which is certainly to his credit, and something that is likely to be helpful in the future.

Bombaywalla, I would add just one point to your IMO excellent list of reasons for the proliferation of speakers having problematical impedance curves. And that is that there seems to be a tendency among many audiophiles to equate the ability of a speaker or other component to resolve hardware differences with its ability to resolve musical information. Thus, if on the basis of reviews, user comments, etc., a speaker acquires the reputation of making amplifier selection particularly critical, it will in the minds of many audiophiles create an expectation that it will resolve musical information and detail better than a speaker for which amplifier selection is less critical. While of course, as this thread makes clear, that is by no means necessarily the case.

Best regards,
-- Al

Unsound,

If the output impedance of the amplifier is a good match for the particular speaker, the output of the amplifier WILL adapt appropriately.

Think of the amplifier as an amplifier having zero output impedance in series with a resistor whose value corresponds to the actual output impedance. If the impedance of the speaker varies significantly as a function of frequency, and if the amplifier's output impedance is significantly greater than zero (e.g., 1 or 2 or 3 ohms or so, as in the case of most tube amps), the voltage divider effect resulting from the interaction of that output impedance (corresponding to Z1 in the first figure of the reference) and the impedance of the speaker (corresponding to Z2 in the first figure of the reference) will result in the voltage at the amplifier output/speaker input varying as a function of frequency. If the amplifier's output impedance is a good match for the design of the particular speaker, that variation in amplifier output will result in minimal variation of the output of the speaker, as a function of frequency.

Audiolabyrinth,

I didn't say that amps operate in a current mode and a voltage mode. Actually, I don't know what that would mean, if anything. What I said is that amplifiers amplify both current and voltage. In other words, the voltage at their output is greater than the voltage at their input, and the current supplied by their output is greater than the current going into their input.

You are probably referring to Krell's CAST concept, as described here. Note the statement in the first paragraph that "A Krell system connected in CAST transfers the signal in the current domain, throughout the signal path to the amplifier output stage where only one current to voltage conversion (I-to-V) takes place."

Following that current to voltage conversion, at the output of the amplifier, both voltage and current would certainly be greater than at the input of the amplifier, or it wouldn't be able to drive the speakers to anything approaching reasonable volume.

I see in the manual for the 700CX that its gain is spec'd at 26.4 db. That corresponds to an increase in voltage of about 20 times. And its 700 watt capability into 8 ohms certainly represents vastly more output current (actually, about 9.4 amperes, when the full 700 watts is being delivered into 8 ohms) than any preamp could ever supply to the amplifier's input.

Best regards,
-- Al

Hi Bombaywalla,

Yes, agreed.

As you certainly realize, any design involves a zillion or so tradeoffs and competing considerations, but it's still striking how some speakers can perform well with very diverse kinds of amplifiers, while other speakers cause the choice of amplifier to be very critical.

One of the reasons I chose the Daedalus speakers I have is that their combination of a pretty much flat impedance curve, high efficiency, and high power handling capability makes them suitable for use with just about any amplifier out there, except for the really flea-powered types. The Coincidents that Charles uses are similar in that respect.

Charles & Audiolabyrinth, thank you most kindly for the nice words.

Best regards,
-- Al

Bifwynne, by adding negative feedback to an amplifier you do indeed move the amp towards the Voltage Paradigm. This is because as you surmised the 'output impedance' is lowered. I paraphrased the term because it is misunderstood however, and that is where the clarification comes in. Unsound, you might want to pay attention to this.

Under the Voltage Paradigm, you have the term 'output impedance'. The term has a definition which is not intuitive. It refers to the amount of servo gain that the amplifier has which allows it to react to a load. It does not refer to the actual output impedance of the amp, as measured by any other field of endeavor in the world of electronics.

How can we know this? The answer is simple. If the output impedance were indeed lowered, the amp could drive a progressively lower and lower impedance. It might even make more power. But we see by adding feedback to an amplifier that the output power into lower impedances does not change.

IOW, what is happening is that the feedback gives the amp the ability to adapt to its load within certain limits by taking samples of its performance and using that as an error correction. The only way you can really get a lower output impedance is with bigger output transformers, more tubes or more transistors. The extra ability to drive a lower impedance does not come out of thin air or feedback- to do so would violate a fundamental rule of electronics known as Kirchoff's Law.

The difference between the Voltage and Power Paradigms has more to do with feedback then tubes/transistors.

Ralph, thanks for your characteristically erudite inputs. I think I understand this, and it seems to make sense to me.

At the same time, though, referring to the conventional and intuitive definition of "output impedance," isn't it true that the output impedance of a tube amp for its 4 ohm tap will be about one-half of its value for the 8 ohm tap? And if so, regardless of whether the amp uses feedback from its output or not, won't that reduced output impedance reduce the frequency response variations at the amplifier output/speaker input terminals that would result from the interaction of that output impedance with the impedance vs. frequency variations of the speaker? Albeit at the possible cost of compromising the performance of the amplifier itself, depending on how well the amp can handle the mismatch that may result. That often being a subjectively more significant consideration, as you pointed out in your second post.

Best regards,
-- Al

05-09-13: Swampwalker
I'd love to hear Atmasphere and Almarg comment about how that type of amplifier's inherent electrical characteristics interacts w speaker loads.

Hi Michael,

I'm not especially familiar with Class D amplifiers. But in terms of output impedance they are of course firmly in the voltage source camp, with some of them having extraordinarily low output impedances/high damping factors. In terms of feedback, I don't have a particular feel for how much negative feedback most of them use, but I'd imagine that it is not insignificant in most cases.

An exception would apparently be the ARC amps that have switching output stages. Those are described as using no global feedback, and are spec'd as having 1% THD. Which would seem to raise the possibility that their behavior is somewhat tube-like.

Also, the Spectron writeups on their Musician III indicate that although negative feedback is used its adverse effects are claimed to be essentially eliminated as a result of the fact that transit time (aka propagation delay) through the signal path is exceptionally fast. I see no reason to doubt that claim.

As I say, though, I have no particular knowledge of the degree to which feedback may be employed, and the degree to which it may be an issue sonically, with Class D in general.

05-09-13: Bombaywalla
"In the Power Paradigm the box design puts the peak at a lower frequency to take advantage of the extra energy- but again netting fairly flat frequency response, but with the additional benefit of bass extension, which might well be up to half an octave."
Atmasphere, what does this mean? The box resonance peak is where it is. How does the Power Paradigm amplifier move that peak to a lower frequency?

I believe that Ralph was referring here to speakers that are designed per power paradigm principles, not to amplifiers. One of his underlying points is that both must be designed in accordance with the same paradigm, if they are to work well with each other.

05-09-13: Bifwynne
I've picked some LP dogs that are so bad, the neighborhood dogs howl. And I don't think that's because of NF or impedance peaks or negative phase angles. Some recorded music just stinks.

Truer words were never spoken :-)

Best regards,
-- Al

05-10-13: Atmasphere

I wonder what happens if one listens to nicely produced music, say a big band with lots of brass and high frequency energy live at 105DB? Is it as comfortable as listening to the no NF tube amp at the same level? Its a fair question, I think.

It is. If you sit close up in a concert hall it will easily be that loud.

I once sat in the very first row at Tanglewood for a performance by the Boston Symphony Orchestra of Prokofiev's "Romeo and Juliet." I have no doubt that peaks reached at least 115 db, perhaps even 120 db. I would have to say that it came close to the point of being uncomfortable, but for me didn't quite cross that threshold. There were a couple of times, though, that my wife briefly put her hands over her ears.

I've measured the Sheffield direct-to-disk recording of that work as reaching about 103 to 105 db at my listening position. The acoustics on that recording are dry and excessively bright, though. If it had a tonal balance that is more typical of well engineered minimally mic'd orchestral recordings, I suspect I would be setting the volume control such that peaks would approach 110 db.

Out of curiosity, a while back I examined the waveforms of that recording on a computer, using an audio editing program. Its dynamic range, the difference in volume between the loudest notes and the softest notes, was around 55 db!

Best regards,
-- Al

Why aren't more manufacturers making powered speakers, and why is it that people don't seem to buy them.

Lots of reasons come to mind. Charles mentioned an important one.

Let's say that a passive speaker is designed such that it will perform optimally, in a typical room, with an amplifier having a certain output impedance, a certain amount of feedback, and a certain power capability.

Separate power amplifiers meeting those criteria might range in cost from say $500 to $100,000. Their size, weight, and design philosophies will vary enormously. If the speaker manufacturer were to design a speaker incorporating an amplifier meeting the impedance, feedback, and power criteria, how would he choose among all of those kinds of possibilities without greatly limiting the number of potential customers?

Also, the size and weight of the speaker is likely to increase considerably. Its physical complexity and its appearance will change. WAF is likely to decrease considerably. Microphonics would have to be addressed in the design. Finally, the expertise required to design speakers is obviously very different than the expertise required to design amplifiers. As might be expected, there don't seem to a lot of high-end companies capable of, or even interested in, producing both kinds of products to a high standard. Israel Blume of Coincident being a notable exception.

Regards,
-- Al

I don't mean to sound picayune, but I would once again call attention to the first of my posts dated 5-5-13 in this thread, dealing with terminology. IMO the terms "voltage amp" and "current amp" are incorrect and misleading.

A "voltage amp" would be one that amplifies voltage. A "current amp" would be one that amplifies current. But all or nearly all amplifiers amplify both. What is really being referred to are amps that act as voltage sources, and amps that act as current sources. But that isn't right either, in most cases, because most of the amps that are being referred to as acting as current sources only act in that manner to an extremely loose approximation.

What would be better, IMO, is to refer to amps that have negligibly small output impedance (i.e., most solid state amps) and amps having significant output impedance (i.e., most tube amps and a few solid state amps). Or, alternatively, using Atmasphere's terminology, amps conforming to the "voltage paradigm" and amps conforming to the "power paradigm."

Regards,
-- Al