Audio amplifier by Esa Merilainen (EDN) is nearly perfect, has one flaw - Page 1
Forum section: electronics / projects

author: unitedatoms
author_rank: Frequent Contributor
timestamp: October 21, 2015, 03:45:40 am
content: There is interesting recent (? 2011..2013 ?) claim about superiority of current driven audio amplifiers. I agree with author, except one fundamental flaw in relevance to practically available audio records. None of contemporary records will reproduce correctly with his amplifier. Also voltage driven amplifiers will try to reproduce correctly, but will also fail for the reasons explained by author in EDN article.http://www.edn.com/design/consumer/4423155/Loudspeaker-operation--The-superiority-of-current-drive-over-voltage-driveI think there is one small block is missing to complete perfect current driven amplifier: it is differentiator.To verify my thoughts, I checked the current basics of acoustic theory. After reading this paper (up to p 2-14), it is correct to say that microphone voltage corresponds to instantaneous value of acoustic pressure, because particle velocity (velocity of weightless membrane) is in phase with momentary pressure. This means that all audio records are records of pressure.http://www.usna.edu/Users/physics/ejtuchol/documents/SP411/Chapter2.pdfTo perfectly reproduce the pressure back from recorded voltage data through ordinary speakers, the velocity of speakers should match. Author (Esa Merilainen) with his approach interprets voltage data as a torque: mechanical force applied by speaker, causing match of acceleration (very good match, better than match of voltage amplifier to velocity), but in actuality the need is in reproduction of velocity to match the acoustic pressure.My claim is: none of voltage or current amplifiers will sound correctly (mostly phase wise) when are driven by unmodified voltage records. To perfectly match the acoustic pressure recorded by microphones the current amplifier needs a differentiator.Edit: added picture

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 21, 2015, 05:05:38 am
content: Power is a much more useful measure than arbitrary voltages and currents in circuit.The transducers have a fundamentally ugly impedance response, except for very specially designed ones (which happen to be very expensive as well). To reproduce low frequencies, a large cone is needed, but the cone warps and flexes at high frequencies due to finite speed of sound in the cone material. Reflections from the cone edges interfere with those from the coil, not only producing strange radiation patterns and changing the efficiency, but changing the electrical impedance as well.So, in general, over a wide bandwidth, neither current nor voltage can be expected to be a reasonable representation of what's really going on, because reactance and resistance in the coupling utterly dominate the coupling to air.RF antennas are a much more dramatic illustration of this, as the coupling between transmission line (or amplifier), antenna and space are fairly similar to begin with, and the efficiency tends to be high, so the notches and peaks in impedance are that much more exaggerated. Still, due to sheer symmetry alone, one cannot radiate from, for example, a dipole antenna at 1 wavelength, even with extreme tuning. (Speakers tend to work anyway, probably because of the cone shape, unevenness in the material and build, change in radiation pattern, and losses along the construction which tend to dampen the reflections, and reduce efficiency significantly. Audio amplifiers are far cheaper than RF amplifiers, so this is a better tradeoff, if still a crappy one.)Tim

author: Marco
author_rank: Super Contributor
timestamp: October 21, 2015, 05:19:05 am
content: Just get a good microphone, capture the transfer function and equalize it. You could even use Volterra series and try to linearize it as well.

author: unitedatoms
author_rank: Frequent Contributor
timestamp: October 21, 2015, 02:40:33 pm
content: Ok. Found theepic 2 years long thread by book author himself. Interesting read .. BTW, I am not advertising the book or the other forum. The whole message of the book can be replaced with 2 words "current drive". Also I am pointing to the missing differentiator block.Quote from: T3sl4co1l on October 21, 2015, 05:05:38 amEnergy conservation. That might lead somewhere.. Especially your analogy to RF transducers. If we could follow the energy of low frequencies, say with audio band splitters, circulators, directional diplexers, terminators, with some DSP: then many claims and theories could be scientifically proved or disproved.Quote from: Marco on October 21, 2015, 05:19:05 amMay be the microphones frequency response imperfections is not the point (author complains mostly about lack of data about non-linearities/intermodulation distortion data about microphones), because audio records already exist.I think that guy (@EDN) found the better way to beat the non-linearities at the end of signal chain. Frequency response model is an idealistic view when signal path is linear. Today we have more or less linear systems, and are used to speak about them in linear terms. The spectral properties of "linear devices" is much easier to understand/validate, than non-linear control loop systems, which real hardware is.This is less common kind of article about how industry is stuck in the past for historical reasons, when early obstacles are no more being obstacles (current feedback was harder to design back in tube era of low currents, than in todays labs). And ironically, most tube amplifiers designed for voltage drive have tendency to be a bit of current sources. That, perhaps is the reason why tube sound is more pleasing (because integrator ups the lower frequencies without phase information loss).

author: ConKbot
author_rank: Super Contributor
timestamp: October 21, 2015, 07:20:38 pm
content: Quote from: unitedatoms on October 21, 2015, 02:40:33 pmI guess youre talking about having a finite output impedance, instead of a typically very low impedance that a solid state voltage amp would have?http://sound.westhost.com/project56.htmA brief article and varying from an amplifiers output from 'infinite' to 'zero' to even negative.

author: edavid
author_rank: Super Contributor
timestamp: October 21, 2015, 08:02:13 pm
content: It just seems like regular audiophool stuff... if it's so great, where are the blind test results?

author: calexanian
author_rank: Super Contributor
timestamp: October 21, 2015, 08:05:42 pm
content: Current domain amplifiers have been around for a long time. In fact the output transformer of a tube amp behaves much like one. The problem you get into is acoustic loading problems with low frequency drivers and impedance/phase/frequency variation with higher frequency drivers. It can be designed very well with proper circuit and speaker selection, but most of the time just sounds awful when improperly matched amp speaker combinations. On the surface it appears to actually be the superior way to do things, but in actual practice its pretty hard to do properly. Its just easier with todays speakers to have amplifiers with large damping factors (Low output impedance and voltage feedback virtually right from the output.)

author: unitedatoms
author_rank: Frequent Contributor
timestamp: October 21, 2015, 09:00:57 pm
content: Quote from: ConKbot on October 21, 2015, 07:20:38 pmYes, the higher the output impedance, the closer the voltage amp to current amp. The solid state amplifiers since the beginning of their designs treated the load as a resistor, when in fact it is more like linear motor. If you look at contemporary motion controllers, DC motors controlled with current loop perform more accurate motion than voltage controlled.Quote from: edavid on October 21, 2015, 08:02:13 pmThe article is recent (on historical scale), so the tests are yet to come. I expect the designs and builds to start appearing here and there. Though, the missing differentiator stage (I am talking about my amendment to "current drive" idea) will be even later evolution step.Quote from: calexanian on October 21, 2015, 08:05:42 pmI feel that the "damping theory" is very misleading. It was brought as a word, because "regenerative braking" is not well adopted even in motion controllers, and much less in audio amplifiers. The problem with language is coming from mixing two domains: non-linear control systems with feedbacks vs linear frequency dependent blocks. I feel that "damping" the word is somewhat in middle area, where designers or advertisers of designs could not find a better explanation of what they are trying to do with back EMF of a motor (speaker).High frequency terminology and methods with impedances and harmonics is not very helpful with explaining the control loop systems. Consider mechanical motion of hand, writing on the board. It is precise in-phase motion with near zero torque. Now imagine that writer is very strong and a little kid is hanging on the arm and adds errors by pulling the hand. The error will be compensated with opposite force, and velocity will be maintained with small error.In this case velocity is afterfact of torque (current control). Same in-phase motion control is performed by amplifier against speaker. ... etc.. that book is about all of this. I am not advertising book. I just like that the guy is trying to fix amplifiers.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 21, 2015, 09:39:15 pm
content: Quote from: unitedatoms on October 21, 2015, 02:40:33 pmThey're a bit of both, actually. Triodes are more voltage-sourcey (typically Rp ~ R_L / 3), while pentodes are as good current sources as transistors are. The impedance of the transformer disappears in mid-band, and its inductance is fairly irrelevant at low frequencies where the speaker itself is largely resistive or slightly inductive.So it takes quite a bit of NFB to roll it back to a modest voltage source, which they didn't usually do too much of, because tubes, and therefore gain, was expensive, and [equally cheap] transformers sucked even more (which causes the impedance and distortion to rise at the band edges).Tim

author: unitedatoms
author_rank: Frequent Contributor
timestamp: October 21, 2015, 10:06:00 pm
content: Quote from: T3sl4co1l on October 21, 2015, 09:39:15 pmI am not that old to know it about triodes vs penthodes. The earliest I remember about solid state vs vacuum is one claim that FETs sound similar to tubes. I think the reason was that early FETs had very high channel resistance, much more than intended load resistance.

author: edavid
author_rank: Super Contributor
timestamp: October 21, 2015, 10:40:18 pm
content: Quote from: unitedatoms on October 21, 2015, 09:00:57 pmHe published his book in 2010, and it was hardly a new idea.Here's an informative article from 2001:http://sound.westhost.com/z-effects.htmAnd here's a long, tedious discussion from 2003:http://www.diyaudio.com/forums/multi-way/3889-current-driven-loudspeakers-tranconductance-amplifiers.htmlThe idea has been tested many times, and it just hasn't caught on. So, it seems highly implausible that it offers any real advantage.

author: calexanian
author_rank: Super Contributor
timestamp: October 21, 2015, 10:42:59 pm
content: Amplifier damping in the parlance of audio typically means the amplifiers ability to overcome the natural kinetodynamics (Sometimes called ballistics) of the loudspeaker diaphragm, or in other words making sure the cone moves how the input signal tells it to and lessen the effect of the speaker suspension and resonances and dynamic impedance.

author: Marco
author_rank: Super Contributor
timestamp: October 22, 2015, 12:12:08 am
content: Quote from: unitedatoms on October 21, 2015, 02:40:33 pmThe current vs voltage drive makes no difference for non-linearity, it will keep throwing out harmonics and intermodulation regardless.

author: dom0
author_rank: Super Contributor
timestamp: October 22, 2015, 06:29:08 am
content: Quote from: unitedatoms on October 21, 2015, 10:06:00 pmNo, it's about the clipping behaviour. A source follower output stage has much softer clipping than an emitter follower, since the threshold voltage and the linear region is much broader in the FET, thus it clips "softly"(-er than a BJT). This is somewhat similar to the behaviour of a tube output stage.Of course this is totally irrelevant in 'Hi-Fi' amplifiers (if such an amplifier clips your setup is wrong, period.)

author: rjeberhardt
author_rank: Regular Contributor
timestamp: October 22, 2015, 02:50:13 pm
content: The proponents of current driving loudspeakers seem to concentrate on non-linearity distortion. They ignore the effects of the lack of damping and the resultant poor transient response giving ringing around the bass resonance.Russell.

author: Circlotron
author_rank: Super Contributor
timestamp: October 23, 2015, 01:51:56 pm
content: If you current-drive a loudspeaker, pretty sure you would raise the Qes and therefore the Qts.With a bass reflex box, when you get down to the frequency where the port + box starts to really load the driver, it would then pretty much fall on it's face I expect.

author: tszaboo
author_rank: Super Contributor
timestamp: October 23, 2015, 02:02:27 pm
content: I'm yet to see an audio amplifer with 4 wire connection to the speakers. That would be more interesting I think.http://www.nandblog.com/wire-audio/Quote

author: dom0
author_rank: Super Contributor
timestamp: October 23, 2015, 02:21:05 pm
content: 1.) In the quote non-linear distortion and attenuation are mixed up. Attenuation is one kind of linear distortion, and honestly, the human that can hear 0.5 % level difference(0.043 dB)hasn't been born yet.2.) The idea in itself is interesting. It could turn out to be quite tricky to get it right without impeding stability and linearity of the amplifier.

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 23, 2015, 02:30:23 pm
content: Quote from: NANDBlog on October 23, 2015, 02:02:27 pmThere were some attempts back many decades ago (1960s or 1970s?)Probably abandoned for being too fiddly.It would require both amplifier and speaker manufacturers to implement Kelvin connections.Negative cost/benefit ratio.

author: max_torque
author_rank: Super Contributor
timestamp: October 23, 2015, 10:04:41 pm
content: Good music is good music, it doesn't matter if you listen to it on a $3 transistor radio or a $100,000 "hi fi".Getting tied up in knots over minor details is missing the entire point imo!(and besides, the BEST music is live, and at least 50% of the "information" in such performances isn't audible at all. You could have a totally 100% precisely reproduced sound, played, sat on your own in your listening Room, and it would still be nothing like listening to that self same music live, with an audience, at a gig.............)

author: neotesla
author_rank: Unknown
timestamp: October 24, 2015, 04:29:09 am
content: That might be so in the case of (garage/pub) rock, however it doesn't apply to all music. I certainly wouldn't recommend listening to, say, Pink Floyd on a $3 transistor radio. As for live music being "the best" - listening to, say, Miles Davis or Arthur Rubinstein in 2015 can still constitute about the best musical experience imaginable, though it's a bit hard to catch them live these days.(Sorry for being off topic).

author: TerraHertz
author_rank: Super Contributor
timestamp: October 24, 2015, 08:41:35 am
content: I wonder if anyone ever tried putting an accelerometer on a speaker cone, and using that as the source of negative feedback?Then it wouldn't matter if the amp output was voltage or current drive. The system would be producing speaker cone acceleration matching the original microphone signal. Isn't that the ideal? (Within limits. Unless you wanted to launch the cone across the room.)Or rather than a accelerometer, perhaps a small secondary coil in a magnetic field? Giving a velocity proportional signal.I wonder if audiophools treat the visual part of music videos with the same degree of fanatical irrationality?Speaking of music videos, this is nice.My Tâm - LIKE A DREAM

author: dom0
author_rank: Super Contributor
timestamp: October 24, 2015, 09:20:07 am
content: Quote from: TerraHertz on October 24, 2015, 08:41:35 amThis has been done since the 60s (with other sensors, laser and ultrasonic, though).

author: Pjotr
author_rank: Frequent Contributor
timestamp: October 24, 2015, 09:47:35 am
content: This is quite old stuff and an old debate, the EDN article is merely an advertorial for Meriläinen's book. (Which seems not available any more.)It is proved long time ago (by others than Meriläinen) that current drive can lower distortion by up to 12 dB. This is because damping due to the back EMF of the loudspeaker is inhibited. This damping by itself has some non linearities. However this damping needs to be replaced by another means, otherwise we get a highly under damped loudspeaker system. It is usually done with a velocity sensor as a feedback mechanism. This can have much lower distortion than the back EMF mechanism.

author: Circlotron
author_rank: Super Contributor
timestamp: October 24, 2015, 10:30:45 am
content: Motional feedback.https://www.google.com.au/search?q=motional+feedback&complete=0&biw=1448&bih=981&site=webhp&source=lnms&tbm=isch&sa=X&ved=0CAYQ_AUoAWoVChMIooWg7PHayAIVA2OmCh2jwgmA&dpr=1One method that works well enough for low frequency drivers is to place a small electret microphone capsule about 10-20mm away from the cone dust cap and use it's signal for feedback. Phase shift can set an upper limit on frequency, also the limit at which the cone moves like a piston, without ripples appearing on it's surface aka "cone breakup".My spidey senses tell me an interesting setup would be to current drive the speaker to have a drive =force= directly proportional to the instantaneous signal, and then close the loop with some motional feedback setup. Must have been done by somebody.Edit -> didn't read this from the previous post...Quote from: Pjotr on October 24, 2015, 09:47:35 am

author: Don Hills
author_rank: Regular Contributor
timestamp: October 24, 2015, 11:11:38 am
content: Quote from: dom0 on October 24, 2015, 09:20:07 amPhilips used an accelerometer (piezo type) in their MFB range (www.mfbfreaks.com,www.mfbfreaks.nl).Velodyne use an accelerometer in their subwoofers.Lots of links to more here:http://www.servospeaker.com/documents_and_links/documents_and_links.html

author: max_torque
author_rank: Super Contributor
timestamp: October 24, 2015, 11:31:08 am
content: Quote from: TerraHertz on October 24, 2015, 08:41:35 amFar better to tap into the cochlear nerve in ones head, capture the electrical data there, and have afullycompensated system.............

author: TerraHertz
author_rank: Super Contributor
timestamp: October 24, 2015, 12:27:12 pm
content: Quote from: max_torque on October 24, 2015, 11:31:08 amYES! Even better, one could simply imagine hearing perfect audio, without any of that messy real-world uncontrolled distortion.Then there'd be a whole new lucrative market for imagination-improvers. Crystal hats, and things like that. Or some sort of electrically impelled diaphragm, to produce imagination-boosting air vibrations. These could probably be most effective if bilaterally head-mounted.

author: Pjotr
author_rank: Frequent Contributor
timestamp: October 24, 2015, 12:39:11 pm
content: Yes, meta-audio is endless (and good business)The Philips MFB (Motional Feed Back) system in those days was merely targeted at getting good low frequency response from a relatively small cabinet than having audiophile properties.