Current amp vs Voltage amp

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.

hi Al, often it seems that a speaker manuf tries to find an amplifier that has an approximate H^-1(f) transfer function of that manuf's speaker so that the amplifier input-to-speaker output (which is sound) transfer function is more or less flat over the audio bandwidth.
As you would agree this is a very limiting way to design a speaker - change the amp (or another variable) & the performance falls off the cliff....

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

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...

indeed good qualifications for a speaker to have. Empirically you will find that the people obsessing the least about amplifier-speaker interface are those that have understood the need for the speaker to have a relatively flat impedance & phase (let's not forget that phase response!) curves vs. frequency. Often in these speakers the curve is -3dB at 20KHz which I have been told by certain speaker manufs that this provides better in-room acoustics (vs. a speaker that has no turn-down at 20KHz). Such flat impedance speakers speak about the manuf ability to understand playback requirements & the ability to produce a product that is largely agnostic of the amplifier. Few such speaker manuf exist.....
I heard the Daedalus at RMAF one year - I remember that their sonics were very good. Best regards.

Al and Bombaywalla, it's a shame that only now am I starting to appreciate the import of impedance and phase angles plots. Had I known this before, I would have approached my speaker and amp purchases differently.

As I posted above, even the "mighty" Revel Ultima Salons have roller coaster impedance curves, albeit more tube friendly than my S8s. At least Revel had the curtesy of including in their sales literature advice that their speakers sound best when driven by a high current/high power SS amp.

At this point, I'm standing pat for now. As I mentioned, I switched over to the 4 ohm taps on my ARC VS-115 tube amp. I think the S8s sound less forward because the midrange is toned down. Although the bench test reports I posted on the ARC Ref 150 and VS-115 suggest that using the 4 ohm taps smooth out the FR variations at the amps' outputs, I'm not sure how that translates into speaker output.

My understanding is that the S8s are ruler flat when driven by a solid state amp. Perhaps, using the 4 ohm taps lowers the amp's output impedance (maybe .5+ ohms) enough to smooth out the speaker's actual FR to approach a near zero output impedance of a SS amp. Dunno.

If my wife would let me switch out gear, I'm not sure whether I would flip the speakers or the amp. The S8s are the darlings of the reviewer community, especially because of the Be tweeters and top grade engineering, R&D and QC that goes into their manufacture. The VS-115 also has garnered high grades.

If I was to flip speakers, I probably would opt for the Revel Studio 2s, but would need to also flip for a high quality tube amp, maybe the new Ayre VX-5. Dunno???

Sorry I got to this late.

It seems there are some things that need clarifying. If I am not beating a dead horse too much...

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.

With regards to Current Source, Power Source and Voltage Source:

If the amp has a lot of voltage feedback it could become a Voltage Source. Most Transistor amps attempt to be Voltage Sources (and most of them run a lot of feedback). What this means is given a sine wave input signal, and a load of variable impedance (like a typical box speaker), the output voltage will always be the same. Note that with this model (the Voltage Paradigm) the *current* will vary as the load impedance varies. This is how such an amp can double power as impedance is cut in half, as with half the impedance the current will be double.

Under the Power Paradigm the amplifier will attempt to make constant power into any load rather than constant voltage (there is a little math of note here, Power = Voltage X Current). IOW a Power Source. This is the behavior of a tube amp without voltage feedback. Depending on the load variation, both output voltage and output current will vary.

Under the Power Paradigm, the output impedance of the amplifier is in fact the actual impedance of the output section of the amp.

An amplifier can also be a Current Source, although there has never been a paradigm of design, test and measurement that has evolved around it. This type of operation is done by using current feedback rather than voltage feedback. Constant Current amps tend to have a very high output impedance by any measure, often well above 50 ohms. This does not mean that they cannot work with conventional speakers. Nelson Pass as done a lot of work in this area in recent years.