Nelson Pass designed and sold through First Watt a current-drive amp. I remember reading a review of it in Stereophile years ago. Apparently it's major flaw was an output impedance of some 50 ohms!
Are all amps being built wrong?
The power amplifiers that drive our loudspeakers are mostly built as a low impedance voltage source. They have always been ... but why?
Loudspeakers have a (greatly) varying impedance over the frequency range. A current drive amplifier would eliminate the issues that stem from this varying impedance, and at the same time make discussions about esoteric speaker cables that strive for optimal R, C, L superfluous. Although there still would be these un-measurable ’this (very expensive) cable sounds better’ debates and opinions ... and that’s OK, that’s part of the fun. :)
So ... why are amplifiers not built as a high impedance current source?
This is an interesting read: https://www.current-drive.info/
Loudspeakers have a (greatly) varying impedance over the frequency range. A current drive amplifier would eliminate the issues that stem from this varying impedance, and at the same time make discussions about esoteric speaker cables that strive for optimal R, C, L superfluous. Although there still would be these un-measurable ’this (very expensive) cable sounds better’ debates and opinions ... and that’s OK, that’s part of the fun. :)
So ... why are amplifiers not built as a high impedance current source?
This is an interesting read: https://www.current-drive.info/
81 responses Add your response
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Low output impedance probably also overdamps many speakers so you get the opposite effect of a loudness contour.Probably the main reason that many people prefer tube amps is that they have medium to high output impedance which do not overdamp speakers and tube amps are typically at least partially current drive.Some active loudspeaker companies like SGR provide the option of using/upgrading to current drive amplifiers and assert that they sound better than voltage drive.This provides a good overview of the voltage drive/current drive topic.http://education.lenardaudio.com/en/12_amps_8.html |
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They have always been ... but why? Because, for the most part, most speaker drivers are linear to voltage, not current. Nelson Pass has of course at least one if not more famous papers on how this may not be the right approach with some large drivers. If you want to see what a high impedance amplifier, one that is closer to a current source is, then look no further than the Stereophile tube amplifier measurements with a simulated speaker load. As the amplifier's output impedance rises the frequency response of the amplifier tracks with the impedance curve of the speaker system. On the VERY opposite end of the spectrum we now even have the top end Technics digital amplifiers who deliberately compensate for the impedance curves and give a near perfect voltage amplifier transfer function. |
@erik_squires "Because, for the most part, most speaker drivers are linear to voltage, not current." Unless I misunderstood what you tried to say ... I thought linear with current. The loudspeaker uses a moving cone to create air vibrations. The cone is put in motion via a coil (wire length L) inside a magnetic field (strength B). The force (F) that moves the cone is created by the current (I) that flows through the coil, with the linear equation F=BIL. From this it seems only logical to drive the coil with a current source, making the current independent on the loudspeaker impedance? Apparently there are other parameters and phenomenon in play that made/make amp designers choose for voltage output, which does make the current that flows dependent on speaker and cable impedance. |
Unless I misunderstood what you tried to say ... I thought linear with current. Nope. Well, linear is probably the wrong term. Proportional is correct. The output of an ideal driver in dB would be directly proportional to the output in db Volts of an amplifier. That is, if you increase the V by 3 dB, you should measure 3 dB more acoustically regardless of the final speaker impedance (i.e. drivers + crossover). Of course most speakers are not ideal and compression artifacts occur but this is the goal so long as we are not overdriving the speaker. This is also why equalizers work. :) If you reduce output by 6 dB at 40 Hz in your EQ then your speaker’s output will be reduced exactly that much. Because everything is proportional to voltage. PS, db Volts is calculated this way: db = 20 x log( V original / Voltage now) So doubling the amplifier voltage = 6 dB louder in both voltage and sound pressure level (SPL) so long as everything is not over driven. |
If it sounds good that’s all that counts. Coda was a pioneer in high current and still do ,120 amps on tap short term , huge regulated power supplies doubling down in power as the impedance drops with a very low noise potted 3 KVA transformer ,not many out there can match this especially at any decent price range , made-in USA 10 year warranty and very competitive with anything In its price class and above. |
The answer is economics. Amplifiers are voltage sources and practically all speakers are designed around being powered by a low impedance voltage source. Change the output impedance of an amplifier and then it sounds better with some speakers and worse with others. The industry is not going to get together and do an Avalon-Spectral-MIT marriage of specification. The ones making current source amplifiers are mostly the ones making active speakers. In short, to optimize a current source amp you must convince the speaker maker to tailor his specs to your amp. Not gonna happen. |
Audioman and andrewkelly I believe you are confusing voltage drive amps that have high current delivery capability with current drive amplifiers.The two types are more like opposites.The first type can drive low impedance loads and tend to have high damping factors and low output impedance whereas a current drive amplifier will suit higher impedance speakers [8-16 ohm] and will have a lowish damping factor and highish output impedance. |
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Translate BS… My 100W SS and then the 100W tunes powered my Vandersteens fine.Or maybe nit the PSE died 16 years in, and the VTLs 15 years later, and the latest one and a NAD before that worked fine. I would not trade them except for a 36 year new version. And even then maybe not. But the OP has a point about current. Speakers with a more uniform impedance, would get a more uniform current at a fixed voltage. Maybe there is something in it?Or We fix wild impedance swings with a current source amp? |
The power amplifiers that drive our loudspeakers are mostly built as a low impedance voltage source. They have always been ... but why?This is mostly incorrect. The voltage rules haven't been around forever; MacIntosh and EV were proponents of this in the late 1950s but it took until the 1970s before the industry had really switched over from the older power rules. The idea was plug and play with no adjustments on the loudspeaker. You can read more about it at this link: http://www.atma-sphere.com/en/resources-paradigms-in-amplifier-design.html Any time you see adjustments on the back of the speaker like a tweeter level control, the speaker is designed around the older power rules- the control(s) allow the speaker to be adjusted to the voltage response of the amplifier and are not there to adjust to the room. Of course a current source amp would be even more sensitive to impedance variation than even a power source amp! But there are speakers that worked best this way; most of them are open baffle with very tight suspensions on the drivers. The problem for many years with the voltage rules has been the to get the amp to be a voltage sources (especially tube amps) you need feedback, and the simple fact has been that most amps made in the last 70 years that have employed it simply do not have enough. As a result the feedback that they do have has caused all of them tube or solid state to be brighter than they should be, since the feedback wasn't enough to allow the amp to clean up the distortion added by the feedback itself. Since the ear perceives distortion as tonality, and since the distortion added by feedback (while suppressing the innate distortion of the amp otherwise) is entirely higher ordered harmonics, the ear perceives that as brighter and harsher even though the amp might be flatter frequency response. That is why there are still power source amps around today (SETs being an example). Tube amps that don't use feedback (and so are not as harsh and bright) will act like a power source. The trick with feedback of course is to have enough, which is usually over about 35dB or else don't use any at all. Its not really been possible to put that kind of feedback in an amplifier until recently. The typical brightness of solid state (which is why tubes are still being made) is caused by insufficient feedback. |
a Bob Carver made amps under the Sunfire brand that had speaker outputs selectable for voltage or current drive. I’m told the current drive was created by the insertion of a series resistor. He did but it wasn’t absolute. He added a little resistance so it behaved more like a tube amp. This means it was LESS like an ideal voltage source and MORE like a current source, but even then there are limits to how well this can ever be done or how badly it would sound. Tube amps are still mostly voltage sources and deviate only due to the relatively small output impedances. I say relatively because a single multi-way speaker may vary from 3 to 30 Ohms. An amplifier with a 2 ohm output impedance won’t be near an ideal current source. For that you’d need say 300 to 3,000 ohms of output impedance. A perfect current source amp would perfectly track the impedance curve of the speaker, which would sound awful. I believe these models use the CJ Premiere 12 (maybe another model, the 8?) so we can use the Steophile measurements as a guide of what to expect. https://www.stereophile.com/content/conrad-johnson-lp125m-monoblock-power-amplifier-measurements On the top chart you'll see a frequency response called "Simulated Speaker Load." That squiggly line will show you how much this, more like a current amp, would behave. |
He did but it wasn’t absolute. He added a little resistance so it behaved more like a tube amp. This means it was LESS like an ideal voltage source and MORE like a current source, but even then there are limits to how well this can ever be done or how badly it would sound. Tube amps are still mostly voltage sources and deviate only due to the relatively small output impedances. I say relatively because a single multi-way speaker may vary from 3 to 30 Ohms. An amplifier with a 2 ohm output impedance won’t be near an ideal current source. For that you’d need say 300 to 3,000 ohms of output impedance.@erik_squires Most of this is not quite correct. If you add a resistor to the output of a solid state amp it will indeed simulate some sort of tube amp that uses feedback. Most transformer coupled tube amps with 15dB of feedback will act as a pretty good voltage source. You might have to play with the taps on the output transformer. Between Voltage source and Current source there is Power Source, which is how a tube amp will behave if it has no feedback or if it has voltage and current feedback of equal amounts. Most tube amps do behave as voltage sources. You need current feedback to get the amp to behave as a current source. The only amps commercially available that ever did that had variable damping controls and were made in the 1950s. Fisher made a few as did EV. I'm not sure how many others. But the damping control really allowed you to set up the speaker incorrectly since it did cause the amp to misbehave it you had the control set incorrectly- which was usually towards the 'Current source' side of things. If the amp has a 50 ohm output impedance it can act as a current source. Some speakers can sound amazing if driven by a current source- Nelson Pass did exactly that with some OB speakers he had a RMAF about 10-12 years ago. They played bass far better than you would have expected considering how small the OB speakers were! But for the most part the approach is impractical. |
My answer to the original posed question is "not at all, not even close" I think Eric contributed some good stuff above. As am amp designer, i think they ought to be voltage sources, with low output impedance. Now, for the record mine are not zero, because i don't like to employ global control (feedback) loops nor expose those loops to external elements that create unknown stability issues. But we are talking small fractions of an ohm. I realize that a higher output impedance may result in a warmer 'bloom", but that's a coloration, whether you like it or not. A current source would likely result in very distorted sound. Also note that the impedance of a speaker changes on many dimensions - with frequency, with excursion of the electromagnets, with volume, ....so why would we want to make the gain impedance dependent? G |
It would be very interesting to learn about the intricacies of the Bakoon amplifiers.They are reputed to be current drive and have high output impedance and yet seem to be able to drive many mainstream speakers without problems although they definitely sound best with genuine 8 ohm speakers rather than 4 ohm.It is surprising what speakers even the base model 15 watt will drive and they sound superb.I have owned every type of amplifier other than an OTL and the Bakoon is by far the best sounding.There is just a fundamental rightness to the sound and that is hard to unhear. It would be interesting to know exactly how the patented SATRI circuit works but if that information is available it is probably in Japanese. |
re: Vandersteen's being hard to drive. I had 2Cs and then 2Cis and 2Ces. They were the easiest, least fussy speakers to drive i ever owned. Drive them with a 20 W NAD 3020 that could NOT drive Spica TC or SC-50s. Drove them with a pair of tiny Lux tube amps. They are very resistive, basically 8 ohms, and modest sensitivity (87 dB???). Look at the impedance curves for heaven's sake. Do they like good amps? Sure. Do they like highly capable, high current amps? Sure, i guess. Do they need them to make music? Nope. |
... why are amplifiers not built as a high impedance current source?Mainly because speakers are designed to be driven by a low impedance voltage source. When amps and speakers are co-designed though there exists the possibility to use current drive - I believe Bruno Putzeys has done this on his actives (Kiis) over at least part of the frequency band. |
Mainly because speakers are designed to be driven by a low impedance voltage source.Aside from what i already wrote, here's also the practical issue. A low impedance, hgih current amp will drive anything. Others might (might) sing with speaker A and suck with speaker B. You know, like lots of high end :-) I'm always fascinated by this. I cam plug my amp into about anything and it will work very well and sound very good. Will it produce some synergistic, euphonic magic effect? Nope. |
Aside from what i already wrote, here's also the practical issue. A low impedance, hgih current amp will drive anything.This isn't quite correct, just so you know. Try putting your amp on a Sound Lab ESL sometime. That speaker is 32 Ohms in the bass and maybe about 2 Ohms at 20KHz (depending on the position of the Brilliance control). Most voltage source amps tend to sound too bright on this speaker; they struggle to make power at bass frequencies. Its MO isn't based on the Voltage Paradigm. Anytime you mix the two paradigms (Voltage and Power) you are at risk of a tonal anomaly. SETs and other zero feedback tube amps are not meant to be used with difficult speakers (and I argue that no amp should be used with such speakers since the last thing you want to do is make any amp work hard for a living- it will make more distortion which is audible). So no amp can really work with all speakers. |
Julius Futterman solved the tube amp feedback problem by eliminating the output transformer. His OTL amps can have 60 db feedback with low output impedance and low harmonic/IM distortion. Yet remain 100% stabile into complex loads! The drawback is that they are best used with speakers that are 8 ohms and up due to the fact that power/watts declines into low impedances - but rises with impedance increase! |
I have three Futterman OTL's. Two mono's and a stereo chassis version that Julius made and sold as the Harvard Music H3 in the mid-Sixties. That one got a review in the old High Fidelity mag! The H3 I bought has an added pair of Altec autoformers because the first owner used it to drive a large custom-made pair of panel speakers that had ten oval cone drivers per side! Consequently these presented a low impedance that the stock H3 couldn't drive - hence the Altec autoformers! |
PS, db Volts is calculated this way: Technically dB(V) is 10log(V) Since power is V*I… and with a constant impedance is also equivalent to V^2/R…Then we use dB(W) as 20Log(V). (I am pretty sure of it?) |
Because a current drive amp is only useful with a speaker that has a flat impedance. Otherwise the amp will act as a tone control and follow the dips and peaks of the speaker's impedance curve. This can hardly be called "accurate"! How useful would a current drive amp be with a 1 ohm speaker that has a flat impedance? |
Julius Futterman solved the tube amp feedback problem by eliminating the output transformer. His OTL amps can have 60 db feedback with low output impedance and low harmonic/IM distortion. Yet remain 100% stabile into complex loads!This statement is false. No Futterman amp ever had anything like 60dB of feedback!! The most any had was more like 20dB, and because the amp had very wide bandwidth, oscillation (caused by its phase margin being exceeded by the feedback) was sometimes an issue. So to your closing comment here- they are well-known to **not** be stable into complex loads. If you doubt any of what I'm saying, just for the record I've been manufacturing OTLs longer than anyone else on the planet (over 46 years); I'm not being hyperbolic. |
This is an interesting article on the subject, © 2019, Rod Elliott (ESP) https://sound-au.com/articles/current-drive.htm#s1 One of the conclusions (scroll to the bottom of the long article): "There have been many claims over the years that current drive is the best, and some may claim it's the only) way to drive loudspeakers, as it reduces distortion and allows the speaker to work the "way it was intended". While there is some discussion of this on the Net (see [ 2 ] as an example), there is little real evidence that the benefits are anywhere near as great as claimed. Tests I've run have shown little improvement, and this is expected given that loudspeaker systems and the drivers used therein are designed specifically with the understanding that they will be driven with a voltage amplifier. By definition, that means the output impedance is low, always below 0.2Ω, and often much less." |
This is an interesting statement: " Tests I’ve run have shown little improvement, and this is expected given that loudspeaker systems and the drivers used therein are designed specifically with the understanding that they will be driven with a voltage amplifier." However small, apparently there is an inprovement with current drive. Which could grow larger when speakers would be specifically designed for current drive. One can wonder why it is not used more often ... any improvement is good, even more so knowing people are willing to spend thousands on their (power, speaker, interconnect) cables alone to gain just a very minute improvement. |