Current Driven Loudspeakers and Tranconductance Amplifiers

Tranconductance Amplifiers

Recently i have found a Italian site where they made great claims about transconductance amplifiers (amplifiers with very great output impedances)...they claim that they are better suited for driving dinamic speakers,as they say that they are current operated devices.
Wath do you think of this,guys??...current or voltage???...;)

Regards

Jorge

PS: the site ishttp://www.ultrasound-hifi.com/
 

Havoc

Member
2002-02-06 9:16 pm
Well, there has been a lot of basic work done on it by MOJ Hawksford. He did theoretical as well as practical work on it, 2 great articles in JAES. The thing is that current drive cancels some of the non-linear terms in the transfert function of dynamic speakers. He measured up to 20dB reduction in distortion of a speaker. Wich is great, concerning that speakers are the weakest link in this respect. On the other hand, JL Hood (I think, it might be D Self) argues that unless everything is extremely well made, a good voltage amp can reach the same levels of total distortion. It can only work with active speakers, no x-overs in the way! Do not forget that you loose all damping of the woofer, the damping factor of these amps makes that of a tube amp look great. Think of output impedances of 20k-100k.

I have build a small 3-way speaker with 10W transconductance amps. No bad, but it were no top speakers either. More a proof of concept. Still have to finish them. Only one is made and has filler applied. It even needs to be sanded.
 
Saying that a speaker is a 'current driven' device is semantically null. Without voltage, you will have no current; it takes both. Any equation that shows current as a variable can be restated to show voltage. For instance, the old standby for power I**2R can also be phrased V**2/R--note the lack of any (direct) referent to current. At all times and in every way, Ohm's Law will prevail.
Now, to the extent that an amplifier might have a high output impedance, I wouldn't be surprised to find that such a circuit 'sounds' different. For one thing, the damping factor will be low, by definition. This may work better with some speakers than others. There will very likely be other factors that come into play depending on the circuit topology. All in all, the amplifier could very easily have a distinctive sound.
Whether that sound suits you is another question entirely.

Grey
 
This thread really belongs into the loudspeaker forum. Will one of the moderators move it and give it a name like "current driven loudspeakers"?


While it is true that a speaker is both current and voltage driven, the terminology is still ok. Voltage driven means that the feedback loop keeps the output voltage proportional to the input signal while current driven means that the current is proportional to the input signal.

Any constant current source will try to keep the current to its ideal value no matter what the load resistance is. By definition, R_out = delta V_out / delta I_out. As the deminator is almost zero, the output impedance approaches infinity.

This does not mean we loose electric damping, it just means that the definition of damping factor is only meaningful for voltage drive. In a voltage drive configuration, the back-EMF generates a voltage that will be shunted to virtual ground by the low R_out of the amp.

In a current driven situation, ideally, the back-EMF causes an error current and the amp corrects for this by outputting an extra current of opposite polarity.
 
It would be nice to light up this thread.

Thorsten mentions current drive at least a couple times in the gainclone thread, and i had a long telephone converstaion with an old RCA engineer who figured that hifi had gone down a blind alley by moving to voltage drive.

He figured the amp should have the same output impedance as the speaker. He was building a small number of bullet-proof PP Class A amps for theatre use (he was in ill health and just disappeared off the map so i was never able to contact him again).

dave
 

ThorstenL

Previously: Kuei Yang Wang
Hi,

Saying that a speaker is a 'current driven' device is semantically null.

True, however what it attempts to state in electrical tems is sound. it implies that the speaker is a current actuated device, meaning that the force being applied to the cone is proportional to the current flowing in the voicecoil and ONLY, STRICTLY and ABSOLUTELY to the current.

Given that Speaker has a voicecoil that is filled with a solid iron plug (causing eddy currents to be induced) and given that the voice coil gets hot and is wound with copper or aluminum (instead of constantan) and thus increases it's resistance it should be PAINFULLY obvious that driving a moving coil speaker from a modulated voltage source (Amplifier with low output impedance) will MAXIMISE the system wide distortion and will cause significant levels of short and long term compression.

Some measures can of course be taken to aleviate the problems in driver design, but in the end there are strict limits as you always trade efficiency for these improvements and thus you loose a significant proportions of the gains when we take a specific given SPL as reference.

Sayonara

PS, driving moving coil speakers designed to require external electrical damping will not work of course, we must in effect design drivers with a Qm in the region of 0.3 to 1 as the only possible damping is mechanical.

A good choice would be a seamless Aluminum coil former, this would both kill some of the eddy current problems in the polepieces and reduce the drivers voicecoil inductance drastically and it would give inherent damping to the driver that would in effect appear as very low Qm. Of course, such a speaker when driven from a normal amp would have absolutely no bass...
 
capslock said:
In a current driven situation, ideally, the back-EMF causes an error current and the amp corrects for this by outputting an extra current of opposite polarity.
Or might we say that the back emf generated by the moving coil (which will be highest at resonance) will be of a polarity that tends to oppose the drive emf. Therefore the actual coil voltage that makes the current flow is effectively (drive_emf) - (back_emf) and so the current would try to drop in proportion to the amount of back emf, so the amplifier gets busy and puts out a higher voltage to overcome the back emf make sure the current stays the same as it would be if there were no back emf.
**********

Another thought, a subwoofer with current drive (i.e. from a high electrical impedance) and motional feedback (making the electro-mechanical impedance lower) might be an interesting thing to try.
 
Al, thanks for moving the thread.

Kuei, I am not sure I can follow your argument about requiring mechanical damping. Is there any flaw in my argument that Circlotron picked up, that if the output current is controlled by the feedback loop, this very loop will provide the electric damping?

I realize, of course, that getting pure current feedback with a conventional voltage in/voltage out op amp (which is what most power amps are) may be impossible due to stability issues...
 

AndrewJ

Member
2001-09-05 6:37 pm
The use of high output impedance amplifiers (so called current drive) for driving speakers is well documented in the literature. I worked on such a combination for a research project at KEF, Dr Mills at Tannoy has published work on current driven subwoofers, and as already mentioned Dr Hawksford has published work on the subject.
Current drive greatly reduces midband distortion in the speaker, by up to 20dB, and also elimanates thermal compression (where as the coil heats up, the coil resistance increases and reduces the driver sensitivity and hence the output level). The downside is that the low frequency response is now dominated by the mechanical Q factor rather than the electrical (or total) Q factor. Conventionally, the mechanical Q is kept as high as possible, typically around 5-7 compared to 0.5-0.7 for total Q when voltage driven (low output impedance amplifier). The result is that the system now has a large peak in response at the resonant frequency in excess of 10-15 dB. Although one could consider putting in equalization to correct for this, the mechanical Q factor is typically not well controlled, varying from sample to sample, and changing with drive level. Mechanical damping is not normally linear enough to be used as the only form of damping.
The solutions are some form of motional feedback, or apply acoustic damping immediately to the rear of the cone. This can be made quite linear. This can lower the total Q to a value where combined with modest EQ the system response can be made quite stable.

Andrew
 

ThorstenL

Previously: Kuei Yang Wang
Hi,



Kuei, I am not sure I can follow your argument about requiring mechanical damping.


I think we are sorta talking crosspurposes.

My point is that Speakers of the conventional sort are current conrolled devices. This means if you feed the speaker with a voltage, meaning from a low impedance source (theoretically a zero impedance source), then the current flowing through the Speakers voicecoil will be a result of the voltage applied and the drivers impedance. Here comes the trickey part. The Drivers impedance (even if we discount mechanical resonances) is highly non-linear and signal dependent.

For one, the drivers voicecoil is normally filled and surrounded with a solid iron/steel chunks. If we take such a coil and apply a voltage from a low impedance source we will observe a distortion of the current flowing through the coil. This means that current through the vociecoil will be no longer directly proportional to the Voltage applied to the driver, but will contain a further non-linear element following in fact the cube (exponential function) of the current through the voicecoil.

Most common drivers will have motors (Magnet & Voicecoil) exhibiting 0.1 - 0.5% 3rd harmonics caused mainly by this phenomaena (eddy current distortion from here on) for around 1 Watt RMS applied power. As the distortion follows the cube of the current it also means this distortion follows the square of the power. Thus a tenfold increase in power will be accompanied by a hundertfold increase in eddy current distortion. Near full rated Power many speaker driver have (motor) distortion levels exceeding 50% composed of ODD ORDER (or in musical terms dissonant) Harmonics!!!!

All of this is a direct, unavoidable and absolutely NECCESARY result of driving the speakers voicecoil from a near zero impedance voltage source. We may apply zero distortion zero ohm source impedance voltage to the driver, all we get back for our efforts in making an amplifier with 0.000001% THD and a damping factor of 10,000 is that we have succeeded in maximally distorting the current in the drivers voicecoil and thus to maximise the distortion of the Driver/Amplifier system!!!!

If we now design an Amplifiers that feeds the driver from a very high impedance (theoretically an infinite impedance) we will directly control the current in the voicecoil and the current in the voicecoil will become directly proportional to our input signal. If we where to measure the voltage on the speaker terminal we would find the voltage to be now VERY HEAVILY distorted, but as the voltage is completely and utterly inconsequential to the forceexcerted on the cone and thus the movement of the cone we would attain an acoustical signal essentially free of certain types of distortion.

Just to complete the list of issues, another source of distortion in the motor (assuming of course a magnet gap geometry that ensures a magnet field that remains strictly constant strength across the whole intended voicecoil travel) is the "stiffness" of the magnetic field. This is related to the "willingness" of the magnet material to change it's magnetic properties under the influence of external fields. In our case the external magnetic fields are from the voicecoil.

Modern cermaic magnets are pretty bad in this respect, being able to accept much less sheer fieldstrength and being quite "soggy", being very notably modulated by the voicecoil mangetic field. The classic Alnico (Aluminum Nickel Cobalt) Alloys are by a factor of around five (depends upon the exact alloy) better than ferrite/ceramic magents with modern Neodymium magnets being better by a factor of up to twentyfive.

Just for kicks, using a currentsource fed fieldcoil would result in a magnetic field that cannot be externally modulated and also allows much greater fieldstrength than any permanent magnet....

Anyway, the distortion caused by this "magnet field stiffness" follows the square of the magnetic field modulating and thus the square of the current. Current drive cannot do anything about this. Luckily the square law favours even harmonics whch tend to lend warmth and musicality to the music if not being in too great a proportion.

Another happening in speakers is the so called thermal compression. I shall re-quote my text from above:

"given that the voice coil gets hot and is wound with copper or aluminum (instead of constantan) and thus increases it's resistance"

What this means is that with more applied voltage (and thus power) the voicecoil gets hotter and thus will result in less current being drawn (as an aside, the Re and thus Qe in the T/S Parameters will be effected too - that just for those misguided souls that assume T/S parameters are constants).

This means that for a given (say 6db) increase in drive voltage the actaul SPL produced by the driver will increase by less than increase in level of the driving signal. Socalled "High Fidelity" Speakers showing 4 - 6db thermal compression at full rated power are not uncommon, this means that near full power the speaker is actually 4 - 6db LESS sensitive than at low power. This is very audible, trust me.

What has all the above missive to do with the discussion here?

Ultrasound claims a Damping Factor of Zero and thus employs what is best described as current drive. It matters zip how this current drive is achieved (feedback or not), what happens is that current through the drivers voicecoil is controlled, not the Voltage across the voicecoil.

Having experimented with similar configurations (using chipamps) in the mid 80's of the last century I can attest to the rather surprising transformation of the sound. I ended up using current drive only on wideband midranges and tweeters (so from around 200Hz upwarsd), ending up with negative feedback via electret mike capsulas for the woofers (this was a fully active 3-Way speaker).

IF and that is a BIG IF I could design my "dream" Speaker driver from scratch I suspect it would look like this:

1) Fieldcoil Magnet with constant current actuation

2) Linear Magnet Field Geometry (Similar to JBL's SFG style arrangement)

3) Qm of the woofer cone section controlled by electromagnetic breaking (eddy current in the aluminum voicecoil former) and a voicecoil winding that makes the Qm the woofers dominant Q, not as usal Qe.

4) Coincident (in the same magnet gap) ICT style 2" Tweeter Dome, induction from the Woofer voicecoil coupled to the tweeter dome acting in effect as transformer to drive a very low resistance "voicecoil". The tweeter requires a form of phaseplug to extend the treble past 10KHz by avoiding cancellation, maybe a ring radiator arrangement would be good.

The above driver would be technically speaking "full-range" (or rather wideband) without electrical crossover as such, fully phase/impulse coherent. I'd probably aim at around 105db/2.83V/1m with a range covered of around 125Hz-12.5KHz, meaning it would likely have to be a 12 - 15" unit.

Together with that it would be nice to have a 20" or larger Subwoofer driver similar in constrction to the above and of course to drive the speaker drivers actively with "current drive" on open baffles. Add a nice ceramic or ribbon supertweeter....

Well, we all can dream.

Sayonara
 
Hi!!

As the proponent of this thread i fell that is time to make some considerations.

Efectively a laudspeaker respond to the current in the voice coil.but a laudspeaker is the reverse of a microphone.I will try to explain...in a microphone the sound produce a voltage that will be the replica of the sound wave...in a laudspeaker the electrical signal will be converted in the oiriginal sound wave. That will be the idial situation!

Now back to business...if you send to the laudspeaker a current signal(from a tranconductance amplifier)...well. if everything is correct then we will have a voltage in the terminals of the loudspeakers similar to the original sound wave...that voltage is the motional voltage of the loudspeaker movement(because the output impedance of the amp is very large)..but this will -not be the case.

Once a make experience: as i have a car loudspeaker with two voice coils laying around i have this idea:idea: i sent a signal in one of the voice coil...and with the osciloscope i see what hapens in the other coil (it will be the motional voltage proporcional to the movement of the voice coil).
So i send a square wave first with a typical voltage amplifier...after that i send the some square wave from a transconductance amplifier(current drive)..well the resultant wave forme in the sensing coil is much more a square wave with de voltage amplifier(voltage drive) than that with the transconductance amplifier(current drive).

Well at the end my felings are:
-Current drive...more eficiencie but let the loudspeaker free to move out of tune..as the amplifier do nothing to stop is misbehaviour.
-Voltage drive..less eficience but make corrections of spurius movement of the loudspeaker...it make the loudspeaker follow the input signal with more fidelity!

Well this is just my opinion!

This is a very controversial subject!!

Cheers :drink:

Jorge Santos
 

ThorstenL

Previously: Kuei Yang Wang
Hi,



Efectively a laudspeaker respond to the current in the voice coil.but a laudspeaker is the reverse of a microphone.I will try to explain...in a microphone the sound produce a voltage that will be the replica of the sound wave...


Does it REALLY, I mean REALLY within the constrains of conventional electronic theory, not my extended "electrons are an illusion, so are current and voltage" sense. Is it REALLY TRUELY a voltage being induced?


Once a make experience: as i have a car loudspeaker with two voice coils laying around i have this idea:idea: i sent a signal in one of the voice coil...and with the osciloscope i see what hapens in the other coil (it will be the motional voltage proporcional to the movement of the voice coil).
So i send a square wave first with a typical voltage amplifier...after that i send the some square wave from a transconductance amplifier(current drive)..well the resultant wave forme in the sensing coil is much more a square wave with de voltage amplifier(voltage drive) than that with the transconductance amplifier(current drive).

Actually, you made a TRANSFORMER (two coils wound on a common former filled with a magnetic material), where high(er) frequencies are concerend, NOT a sense coil that sensed the actual motion. Now this unintentional "transformer" would be pretty bad and thus driving it from a low impedance will OBVOUSLY improve the the HF response. But you have not at all made any relevant measurement as to any possible improved linearity of the cone movement, as you have not measured cone movement.

Such a sense coil can work partially at low frequencies, but in my own motional feedack experiemnts I found that sensing anything with the voicecoil, with a coil wound within the voicecoil or anything near this (even a small sense coil wound a good deal away from the voice coil) all would pick up significant signal levels with the voice coil epoxied in place!!!! I ended up using Micorphone capsulas as sensor as these could actually tell me what was going on really....


-Current drive...more eficiencie but let the loudspeaker free to move out of tune..as the amplifier do nothing to stop is misbehaviour.

Is it the amplifiers job to stop the MECHANICAL resonance of the driver at the cost of increasing distortion ten to hundertfold simply so the Speaker designer can pig design (or rather not at all design) the acoustic load for the speaker? Is it the Job of the Engine in a car to provide the breaking?

Just because it is common practice it is not "right" or even "good".


-Voltage drive..less eficience but make corrections of spurius movement of the loudspeaker...it make the loudspeaker follow the input signal with more fidelity!

What you comment on applies ONLY and STRICTLY aroound the system resonance AND at low signal levels. Moreover, there are other ways of dealing with the system resonance, using the Amplifier to "brake" the speaker cone is a pretty stupid way.

Please really consider the complexissues here. Furthermore, in active multiway speakers you can operate a midrange and treble driver sufficiently above their fundamental resonance and with sufficient mechanical damping in the enclosure that Current Drive is a simple possibility and if you actually measure the cone movement of these drivers and then compare the same drivers voltage driven you will find that the cone motion is much closer to the source signal with current drive.

Please do try, I did so many moons (drinks, shirts and Girlfriends) ago...

Sayonara
 
I think that in an ideal speaker with zero resistance, the CURRENT would control the instantaneous voice coil FORCE, and the VOLTAGE would control the instantaneous voice coil VELOCITY. Neither of these by itself will do exactly what we want. We want control over the voice coil displacement. Mechanical and acoustic loading effects make this uncertain.
 
The voice coil transformer!

Hi Kwey!

Of course the two voice coils can be seen as a transformar...but not a good one...i try to explain:

IF you use current drive you will have a voltage as i stated in the free voice coil...but lets see if that voltage is only a transformer product!:cool:
Stop the loutspeaker menbrane...and "voilá" the voltage diminsh and change shape in the sensing coil and at the sending coil.

If you use voltage drive and you do the some thing you will see also a change in shape of the wave forme in the sensing coil and you will not see this time no change in the sending coil


That prouves that obviously the voltage at the sensing coil is a sample of the input signal via transformer action plus the voltage produced by the motion of the loudspeaker
NOT ONLY A TRANSFORMER ACTION!

;)

The rest is a question of perspective!

Cheers

Jorge Santos

PS.i also have tried this many moons ago(and the others things...also)...i'm 49!
 

ThorstenL

Previously: Kuei Yang Wang
Re: The voice coil transformer!

Hi,

Hi Kwey!

IF you use current drive you will have a voltage as i stated in the free voice coil...but lets see if that voltage is only a transformer product!:cool:
Stop the loutspeaker menbrane...and "voilá" the voltage diminsh and change shape in the sensing coil and at the sending coil.

If you use voltage drive and you do the some thing you will see also a change in shape of the wave forme in the sensing coil and you will not see this time no change in the sending coil


That prouves that obviously the voltage at the sensing coil is a sample of the input signal via transformer action plus the voltage produced by the motion of the loudspeaker
NOT ONLY A TRANSFORMER ACTION!


You somewhat missed my point. I did not say that the "sensing coil" was ONLY giving output from the transformer effetc, but that tansformer effect made any measurements at higher frequencies (which includes squarewaves) invalid, as you where measuring a combination of the current induced into the "sense coil" by the voicecoil movement and of the current induced by the transformer effect. The result will hence be severely inaccurate and thus cannot be used as an indication of performance.

Sayonara
 
The current-drive subject has also been discussed within a damping-factor thread : http://www.diyaudio.com/forums/showthread.php?s=&threadid=4536&highlight=info+on+damping+factor

The main advantages of current drive are (in no particular order):

1.) Compensation of sensitivity changes due to VC temperature rise.

2.) Elimination of the effects of the voice-coil inductance.

3.) Added linearity.

The main disadvantages:

1.) Loss of damping around fs.

2.)Some additional circuit complexity


A workaround for the damping loss can either be a mixed mode (i.e. current drive above fs and voltage drive through the fs range) or MFB. Though MFB is also possible with voltage drive, it would be easier to do it with current drive because the aforementioned advantage # 2.) removes one lowpass pole. This is exactly what the German manufacturer Silbersand does.

The disadvantage can best be tackled using an inverting amp and current feedback (one more reson to use inverting amplifiers :cool: ).

Regards

Charles