DrG said:
Which of course is a function of the speaker's impedance and the voltage across it.
I suppose. But a speaker is also an electromechanical resonant system and whether driven by a voltage source or a current source, it basically boils down to whether the damping of its resonance is electrical or mechaincal. And I'm not sure that I see mechanical damping as being any better than electrical damping.
se
> a roundabout mental analogy with CFB op-amps...
Wrong side of the roundabout. Current Feedback is about the input stage, not the output stage that the speaker sees.
> magnetodynamic interaction between the voice coil and magnet is determined by the induced flux in the coil, which in turn is proportional to the current flowing through it...
Force is proportional to current. But air is very thin stuff. The coil's force goes mostly into mass, not acoustic energy.
Velocity is proportional to voltage, and for cone speakers the acoustic output is mostly a function of velocity, not force. Voltage is a perfectly valid input to a cone speaker, and usually better than constant-current because....
> I'm not sure that I see mechanical damping as being any better than electrical damping.
It is worse. Electrical losses are unavoidable; mechanical losses can usually be made extrememely low. It is more efficient to use those electrical losses for damping, than to add mechanical losses and then feed both the electrical and mechanical losses on your way to a sound field.
Wrong side of the roundabout. Current Feedback is about the input stage, not the output stage that the speaker sees.
> magnetodynamic interaction between the voice coil and magnet is determined by the induced flux in the coil, which in turn is proportional to the current flowing through it...
Force is proportional to current. But air is very thin stuff. The coil's force goes mostly into mass, not acoustic energy.
Velocity is proportional to voltage, and for cone speakers the acoustic output is mostly a function of velocity, not force. Voltage is a perfectly valid input to a cone speaker, and usually better than constant-current because....
> I'm not sure that I see mechanical damping as being any better than electrical damping.
It is worse. Electrical losses are unavoidable; mechanical losses can usually be made extrememely low. It is more efficient to use those electrical losses for damping, than to add mechanical losses and then feed both the electrical and mechanical losses on your way to a sound field.
The chicken or the egg...
Well, look at it this way: voltage drive of speakers has been done a million ways, just as *all* op-amps were VFB until (relatively) recently. Along came the CFB op-amps with many performance improvements, notably in GBW.
So there must exist a possibility that current-drive of a loudspeaker could hold advantages over voltage drive, not so? Which I think would be worthwhile exploring.
Well, look at it this way: voltage drive of speakers has been done a million ways, just as *all* op-amps were VFB until (relatively) recently. Along came the CFB op-amps with many performance improvements, notably in GBW.
So there must exist a possibility that current-drive of a loudspeaker could hold advantages over voltage drive, not so? Which I think would be worthwhile exploring.
Ok. I see your point. How about doing both though? Use the speaker as R1 of a NI feedback loop, with a small R2 of say 0R22. I've actually seen this done somewhere...
Would this not provide current-drive as well as far greater speaker control by including it in the NFB loop? What are the disadvantages, PRR?
VFB vs. CFB
I think the terms VFB and CFB as used for modern op amps
can cause a lot of confusion, depending on what literature
one has read. I had a lot of difficulties understanding the
CFB concept. I tried to relate it to the four types of
feedback schemes discussed in my old electronics bible,
Schilling & Belove: Electronic Circuits - Discrete and
Integrated. First (?) edition, 1968. It made me just more
confused. Eventually, some app notes from TI helped me
on the track, and these plus a lot of thinking about it
finally made me understand CFB. However, it also
made me realize that the terms voltage feedback and
current feedback as used today do not at all mean what
the same terms mean in Schilling & Belove, that uses
the terms to specify if we sense the output voltage or the
output current. The modern terms VFB and CFB are called
voltage error and current error in Schilling & Belove.
We can then combine these into four different feedback
schemes, we can have either voltage or current feedback
and combine this with either voltage or current error. A
richer terminology IMHO.
That is, VFB and CFB as used today refers to the input, as
PRR says, but at least according to some literature it used
to refer to the output.
Please tell me if I am still confused about this. This apparent
clash of terminology has caused me a lot, lot of headache, but
I think I have eventually understood it.
I think the terms VFB and CFB as used for modern op amps
can cause a lot of confusion, depending on what literature
one has read. I had a lot of difficulties understanding the
CFB concept. I tried to relate it to the four types of
feedback schemes discussed in my old electronics bible,
Schilling & Belove: Electronic Circuits - Discrete and
Integrated. First (?) edition, 1968. It made me just more
confused. Eventually, some app notes from TI helped me
on the track, and these plus a lot of thinking about it
finally made me understand CFB. However, it also
made me realize that the terms voltage feedback and
current feedback as used today do not at all mean what
the same terms mean in Schilling & Belove, that uses
the terms to specify if we sense the output voltage or the
output current. The modern terms VFB and CFB are called
voltage error and current error in Schilling & Belove.
We can then combine these into four different feedback
schemes, we can have either voltage or current feedback
and combine this with either voltage or current error. A
richer terminology IMHO.
That is, VFB and CFB as used today refers to the input, as
PRR says, but at least according to some literature it used
to refer to the output.
Please tell me if I am still confused about this. This apparent
clash of terminology has caused me a lot, lot of headache, but
I think I have eventually understood it.
> the terms VFB and CFB as used for modern op amps can cause a lot of confusion,
In modern usage: Voltage feedback has hi-Z inputs; current feedback has low-Z inputs (normally just on the inverting input).
The low-Z input also allows open-loop gain to be set by selecting feedback impedance. That's the miracle: we can adjust compensation without a little pile of pFd caps or any extra parts. Indeed usually no computation is needed: the feedback resistor is some fixed value, often 500Ω or 1KΩ, for any gain. The other feedback resistor to ground is adjusted to set the closed-loop gain; at the same time it changes the open-loop gain so the amp has a constant feedback factor and constant closed-loop GBW.
In a voltage feedback amp, the "to ground" resistance is the fixed resistance of the input device junction. To change compensation you have to change that resistance (usually not possible in a chip) or change an external capacitor (not always possible, and if it is then it is a pain). The "simplicity" of fixed-compensation comes with a price: it is fixed at worst-case compensation and performance.
Yes, a 1968 book might describe things different. Transistor op-amps were novel and they were still finding their way around. They settled on approximations of tube op-amps, discovered the 709 and 101 and 741, and stuck in that rut for 20 years (with considerable improvement on the originals). Then when people started thinking outside that box, they forgot (or never knew) the old terminology and wrote it up anew. Some terms got recycled under new meanings.
> Use the speaker as R1 of a NI feedback loop
Been done many times. It was old in 1955: see the old Fisher quad-6L6 console with "Variable Damping". Interesting that, even in integrated amp/speaker units where it might be practical, it is very rare. It works fine to give good damping and then EQ-out any response errors you can afford to over-power. You can't "control the cone better" because of the large coil resistance and the need to let it resonate to lessen the need for box-size and amp-power in the bass.
In modern usage: Voltage feedback has hi-Z inputs; current feedback has low-Z inputs (normally just on the inverting input).
The low-Z input also allows open-loop gain to be set by selecting feedback impedance. That's the miracle: we can adjust compensation without a little pile of pFd caps or any extra parts. Indeed usually no computation is needed: the feedback resistor is some fixed value, often 500Ω or 1KΩ, for any gain. The other feedback resistor to ground is adjusted to set the closed-loop gain; at the same time it changes the open-loop gain so the amp has a constant feedback factor and constant closed-loop GBW.
In a voltage feedback amp, the "to ground" resistance is the fixed resistance of the input device junction. To change compensation you have to change that resistance (usually not possible in a chip) or change an external capacitor (not always possible, and if it is then it is a pain). The "simplicity" of fixed-compensation comes with a price: it is fixed at worst-case compensation and performance.
Yes, a 1968 book might describe things different. Transistor op-amps were novel and they were still finding their way around. They settled on approximations of tube op-amps, discovered the 709 and 101 and 741, and stuck in that rut for 20 years (with considerable improvement on the originals). Then when people started thinking outside that box, they forgot (or never knew) the old terminology and wrote it up anew. Some terms got recycled under new meanings.
> Use the speaker as R1 of a NI feedback loop
Been done many times. It was old in 1955: see the old Fisher quad-6L6 console with "Variable Damping". Interesting that, even in integrated amp/speaker units where it might be practical, it is very rare. It works fine to give good damping and then EQ-out any response errors you can afford to over-power. You can't "control the cone better" because of the large coil resistance and the need to let it resonate to lessen the need for box-size and amp-power in the bass.
PRR said:
Another way to measure a high DF without problems with meter accuracy is the following:
Do not apply an input signal to the amplifier under test; the output should be zero (except for some noise perhaps). Now take a second amplifier (could be the other channel of a stereo amplifier) and connect the output of that second amplifier via the usual load resistance (e.g. 8Ω ) to the output of the first amplifier. Now apply a signal to the second amplifier so that the second amplifier will force a current into the output of the first amplifier. The DF is now simply the ratio of the output voltage of the second amplifier to the output voltage of the first amplifier. For example, the second amplifier is driven to 10V output (easy to measure) and the output of the first amplifier shows 10mV (also easy to measure). The DF is now 10V/10mV=1000. The measurement does not depend anymore on the difference between two almost equal values.
It is also interesting to have an amplifier with a negative output impedance. The heavier the load, the more output voltage you get. This has been done in the past (maybe still) to tune bass reflex enclosures, if the port is not properly calculated to match the resonance frequency of the loudspeaker.
Steven
PRR,
Yes, I knew about the CFB as used today, since I have finally
understood it I think. I appreciate you explanation/hypothesis
on the different terminology. BTW, I still think that book quite
good, despite it's age. The basics hasn't changed that much,
and it is quite a good book. One has to complement it with
additional more up-to-date ínformation, though.
Yes, I knew about the CFB as used today, since I have finally
understood it I think. I appreciate you explanation/hypothesis
on the different terminology. BTW, I still think that book quite
good, despite it's age. The basics hasn't changed that much,
and it is quite a good book. One has to complement it with
additional more up-to-date ínformation, though.
Thanks PRR and STEVEN (Netherlands) for giving the way to measure damping factor. (PRR, again thank you. I still waiting for your answers in "Very low voltage preamp", but I don't know where to find you). I would like to ask about Steven's method. From his method, the first amp's output is treated like ground for the second amp. And this first amp output is then measured again to absolute ground. Is this right?
From PRR's answers, I think damping factor is also have to do with the power supply. The more bulk and stiff we make, offcourse the dip will be smaller for the same load. So, if the dip is smaller, the damping factor will be greater. Is this analogy right?
Maybe somebody say that it is useless to pursue damping factor >1000. This is the same case to make the frequency response from 1hz to 100khz, maybe somebody out there say that it is useless too. But how come people get more attratracted by these useless figures? From things that we cannot hear?
I'm just a newbies in audio electronics. I just want to "feel" what is the difference if I make the same basic amp, one with normal damping factor and another with damping factor>1000.
Again from PRR's answer, we can make high damping factor by making high feedback. That is to make as big as possible open loop gain, then closed the gain in about 20-40db. From my imagination, in 3 stages power amp, I can make that by eliminating almost all the emitor resistance (since the gain of a single transistor is like collector resistance/emitor resistance, so if I eliminate all emitor resistance, all the gain will be infinite)
Is there any other method or trick (do-able) to pursue high damping factor?
From PRR's answers, I think damping factor is also have to do with the power supply. The more bulk and stiff we make, offcourse the dip will be smaller for the same load. So, if the dip is smaller, the damping factor will be greater. Is this analogy right?
Maybe somebody say that it is useless to pursue damping factor >1000. This is the same case to make the frequency response from 1hz to 100khz, maybe somebody out there say that it is useless too. But how come people get more attratracted by these useless figures? From things that we cannot hear?
I'm just a newbies in audio electronics. I just want to "feel" what is the difference if I make the same basic amp, one with normal damping factor and another with damping factor>1000.
Again from PRR's answer, we can make high damping factor by making high feedback. That is to make as big as possible open loop gain, then closed the gain in about 20-40db. From my imagination, in 3 stages power amp, I can make that by eliminating almost all the emitor resistance (since the gain of a single transistor is like collector resistance/emitor resistance, so if I eliminate all emitor resistance, all the gain will be infinite)
Is there any other method or trick (do-able) to pursue high damping factor?
Are we missing a point about Zout?
It was mentioned earlier in the thread that the speaker cable
( amp to speaker ) and connections and crossover components will be the limiting factor for the total Zout , as far as the driver is concerned ( Z out of amp +inductor dc resistance +connecting cable resistance + connection point resistance ).
The ideal situation is a short across the speaker terminals. The damping actually is done by the speaker's own coil and magnetic field. Not by the amp. The amp only provides a low impedance return path for the speaker. The external resistances - mainly cable and inductor and connectors will be significant compared to the output impedance of the amp( 0.008 ohms?).
The closest one can come to the ideal is to have the power amp right next to the driver ( on the rear wall of the cabinet ?) and a direct connection to the amp without protection relay and an active crossover.
I like the dc protection on some amps with a SCR crowbar across the power supply. So if there is dc on the speaker the fuse blows.
No devices across the speaker or in series with it.
So in reality we will probably never see a damping factor ( from the speakers point of view) of around 80 or less. I suspect this must be down to 20 or worse in most cases !
Directly driven drivers may fare a bit better.
Cheers.
It was mentioned earlier in the thread that the speaker cable
( amp to speaker ) and connections and crossover components will be the limiting factor for the total Zout , as far as the driver is concerned ( Z out of amp +inductor dc resistance +connecting cable resistance + connection point resistance ).
The ideal situation is a short across the speaker terminals. The damping actually is done by the speaker's own coil and magnetic field. Not by the amp. The amp only provides a low impedance return path for the speaker. The external resistances - mainly cable and inductor and connectors will be significant compared to the output impedance of the amp( 0.008 ohms?).
The closest one can come to the ideal is to have the power amp right next to the driver ( on the rear wall of the cabinet ?) and a direct connection to the amp without protection relay and an active crossover.
I like the dc protection on some amps with a SCR crowbar across the power supply. So if there is dc on the speaker the fuse blows.
No devices across the speaker or in series with it.
So in reality we will probably never see a damping factor ( from the speakers point of view) of around 80 or less. I suspect this must be down to 20 or worse in most cases !
Directly driven drivers may fare a bit better.
Cheers.
It is always astonishing how people believe in the magic that a high DF should be capable of doing to speaker control.
And there is ab solutely nothing gained (apart from an amp with LESS load stability) to move from an already extremely high DF of 1000 to one of 20000.
Lets do some math:
Load is 8 Ohms, we connect a cable with a total resistance of 20 mOhms (for cable AND connectors, which is a damnblodygood value).
The DF of 1000 would now be reduced to 444 and the DF of 20000 is reduced to 769 !
I don't disagree with the fact that some of these amps have indeed good LF control. But these are usually very generously dimensioned amps and I think it is the SUM of all this aspects that makes for good woofer control.
You can't have more control than the driver's TSPs allow unless you are going the NFB way or you are using an amp with negative output resistance (and don't forget to take VC heating into account when doing the latter !).
As long as your amp's output impedance is real and positive the difference in damping between an amp with a DF of 10 and a DF of 1000 is just a few % !!!
Regards
Charles
And there is ab solutely nothing gained (apart from an amp with LESS load stability) to move from an already extremely high DF of 1000 to one of 20000.
Lets do some math:
Load is 8 Ohms, we connect a cable with a total resistance of 20 mOhms (for cable AND connectors, which is a damnblodygood value).
The DF of 1000 would now be reduced to 444 and the DF of 20000 is reduced to 769 !
I don't disagree with the fact that some of these amps have indeed good LF control. But these are usually very generously dimensioned amps and I think it is the SUM of all this aspects that makes for good woofer control.
You can't have more control than the driver's TSPs allow unless you are going the NFB way or you are using an amp with negative output resistance (and don't forget to take VC heating into account when doing the latter !).
As long as your amp's output impedance is real and positive the difference in damping between an amp with a DF of 10 and a DF of 1000 is just a few % !!!
Regards
Charles
Regarding the question of voltage versus current driving a single dynamic loudspeaker:
In the vicinity of the loudspeaker resonance frequency, the voltage across the loudspeaker terminals is largely motional voltage, that is, the back EMF of the moving voice coil. Therefore, there is a pretty direct relation between terminal voltage and cone velocity for frequencies close to the resonance frequency.
For frequencies far from the resonance frequency, the voltage is mainly determined by the product of the current and the impedance of the voice coil. Voice coil resistance changes significantly when the coil heats up and its self inductance can also vary when it moves back and forth, because the surrounding iron gets more or less influence depending on the coil position.
So, near resonance, there is a close relation between the voltage and the cone movements, and it makes sense to voltage drive the loudspeaker. Far from resonance, it makes more sense to use current drive. If you use voltage drive at a frequency far from resonance, the voltage is first converted into a distorted and compressed current by the non-linear voice coil impedance, and then the current is converted into a force driving the cone.
What all of this adds up to, is that with respect to distortion and compression in the loudspeaker, it may not be such a bad idea to equalise the low-frequency part of the loudspeaker's impedance characteristic with one or two parallel LRC tanks and then current drive the whole thing.
If the loudspeaker is designed to have a more or less flat response at high frequencies with voltage drive, but its impedance rises due to voice coil inductance, then a first-order low-pass filter in front of the amplifier will be required to prevent an increasing response at high frequencies.
In the vicinity of the loudspeaker resonance frequency, the voltage across the loudspeaker terminals is largely motional voltage, that is, the back EMF of the moving voice coil. Therefore, there is a pretty direct relation between terminal voltage and cone velocity for frequencies close to the resonance frequency.
For frequencies far from the resonance frequency, the voltage is mainly determined by the product of the current and the impedance of the voice coil. Voice coil resistance changes significantly when the coil heats up and its self inductance can also vary when it moves back and forth, because the surrounding iron gets more or less influence depending on the coil position.
So, near resonance, there is a close relation between the voltage and the cone movements, and it makes sense to voltage drive the loudspeaker. Far from resonance, it makes more sense to use current drive. If you use voltage drive at a frequency far from resonance, the voltage is first converted into a distorted and compressed current by the non-linear voice coil impedance, and then the current is converted into a force driving the cone.
What all of this adds up to, is that with respect to distortion and compression in the loudspeaker, it may not be such a bad idea to equalise the low-frequency part of the loudspeaker's impedance characteristic with one or two parallel LRC tanks and then current drive the whole thing.
If the loudspeaker is designed to have a more or less flat response at high frequencies with voltage drive, but its impedance rises due to voice coil inductance, then a first-order low-pass filter in front of the amplifier will be required to prevent an increasing response at high frequencies.
MFB is one topolgy that benefits strongly from current-drive. There would be increased stability of the loop simply due to the elimination of the voice coil inductance's effect.
OTOH it should be possible to use a mixed form of current- and voltage- drive as a compromise. Qtc would be higher, but still well defined so that it could be EQed out.
The other possibility is to make it frequency dependant: Voltage-drive around fs and current drive for any other frequencies.
Regards
Charles
With an LRC series circuit in parallel with the loudspeaker and a current source driving the whole thing, the loudspeaker sees a relatively low driving impedance near resonance and a high impedance everywhere else.
I now see that one of my previous posts is a bit confusing. When I wrote "parallel LRC tank", I actually meant an LRC series circuit connected in parallel with the loudspeaker.
> Another way to measure a high DF .... take a second amplifier
There's no "correct" way, and this avoids several issues. It does require another amplifier. My main concern is that some amplifiers' output Z varies with level. For cool-running Class AB transistor, Zout is usually highest at zero voltage, so this gives a low number for DF. If you are writing the specs for the ad, you want a higher number to impress buyers. If you actually want to damp the speaker, you want a low Zout over the whole swing. If the speaker is in resonance, some variation in Zout with swing doesn't matter much; but it does increase distortion.
My way has plenty of objections too. But it needs only standard test setup (oscillator, dummy load, scope, and good AC voltmenter) plus maybe a 50Ω 1.4W resistor. It tests over typical signal swings (you can try several amplitudes to see if thy all give similar answers). It also has a conceptual simplicity, easy to understand.
FWIW: in simulation I use both techniques: unloaded/loaded and external source.
> I think damping factor is also have to do with the power supply. The more bulk and stiff we make, offcourse the dip will be smaller for the same load.
No. Or, not with any significant amount of feedback, and assuming the amplifier (including the power supply) is not overloading and distortiong the signal. You can put a 5534 chip on two 9V batteries and demonstrate a damping factor over 1,000 in 8Ω. You will need a very good meter, because the peak output of a 5534 into 8Ω is less than 0.3 volts, possibly o.1 volts if the 9V batteries are not very fresh. But it will damp an 8Ω load. Power supplies are for power, not control.
> Again from PRR's answer, we can make high damping factor by making high feedback.
I had assumed that was obvious to anybody designing an amplifier. You can always reduce Zout with more feedback.
Without feedback: a pentode's Zout is always higher than a good-power load impedance. A triode Zout is usually about hald the Z of a good-power load, though as an extreme you can make it 1/10th. A transistor collector is always very-high impedance compared to a good-power load. A transistor emitter impedance is usually low compared to a good-power load.
The most popular output transistor affair is the emitter follower. What is the impedance of an emitter? Remember Shockley? The dynamic resistance of the emitter at various currents is:
30Ω at 1 milliAmp
3Ω at 10 milliAmp
0.3Ω at 100 milliAmp
0.03Ω at 1 Amp
0.003Ω at 10 Amp
This is true for ANY Silicon bipolar transistor.
Note that a simple Class A emitter follower working at 1 Amp has a damping factor in 8Ω of over 200, without any added feedback (and ignoring some very practical details of bias).
However that runs hot. We more often run Class AB, bias about 50mA, Zout about 0.6Ω, DF is about 10 for small signals changing to about 200 for large signal peaks. That is sure to distort. Also we have ignored thermal stability bias resistors. You can use bias resistors to also stabilize output impedance, but Zout stabilizes around 0.1Ω-0.5Ω, DF around 20.
> the gain of a single transistor is like collector resistance/emitor resistance, so if I eliminate all emitor resistance, all the gain will be infinite
No. And never trust any thought that says gain can be infinite.
There is always emitter resistance. If you don't have an actual resistor, you have Shockley's junction relation above: the emitter always has a dynamic impedance.
The collector has an impedance too. It is usually 300 to 3000 times larger than the emitter impedance.
And you have a LOAD impedance. In power amps, we never think about the collector impedance because the load impedance is much lower than collector impedance.
Go back to a Class A amplifier, biased at 1 Amp, but make it a common emitter (collector follower) instead of the usual emitter follower. The emitter impedance is 0.03Ω plus zero external resistance. The collector impedance is maybe 30Ω. The load impedance is 8Ω. So the collector load is 30 in parallel with 8. 30||8= 6.3Ω. The emitter is 0.03Ω. 6.3/0.03= 210 voltage gain.
But: look at the input impedance of this transistor. It is Beta times emitter impedance. Say 50*0.03= 1.5Ω! How is your previous stage going to drive 1.5Ω? It can be done, but you won't get any voltage gain.
It -may- be possible to get DF=1000 into 8Ω with three stages. I think you have to sacrifice a lot of other important audio goals to get there.
> I still waiting for your answers in "Very low voltage preamp"
It seems to me you know what you want, and I have nothing to add.
There's no "correct" way, and this avoids several issues. It does require another amplifier. My main concern is that some amplifiers' output Z varies with level. For cool-running Class AB transistor, Zout is usually highest at zero voltage, so this gives a low number for DF. If you are writing the specs for the ad, you want a higher number to impress buyers. If you actually want to damp the speaker, you want a low Zout over the whole swing. If the speaker is in resonance, some variation in Zout with swing doesn't matter much; but it does increase distortion.
My way has plenty of objections too. But it needs only standard test setup (oscillator, dummy load, scope, and good AC voltmenter) plus maybe a 50Ω 1.4W resistor. It tests over typical signal swings (you can try several amplitudes to see if thy all give similar answers). It also has a conceptual simplicity, easy to understand.
FWIW: in simulation I use both techniques: unloaded/loaded and external source.
> I think damping factor is also have to do with the power supply. The more bulk and stiff we make, offcourse the dip will be smaller for the same load.
No. Or, not with any significant amount of feedback, and assuming the amplifier (including the power supply) is not overloading and distortiong the signal. You can put a 5534 chip on two 9V batteries and demonstrate a damping factor over 1,000 in 8Ω. You will need a very good meter, because the peak output of a 5534 into 8Ω is less than 0.3 volts, possibly o.1 volts if the 9V batteries are not very fresh. But it will damp an 8Ω load. Power supplies are for power, not control.
> Again from PRR's answer, we can make high damping factor by making high feedback.
I had assumed that was obvious to anybody designing an amplifier. You can always reduce Zout with more feedback.
Without feedback: a pentode's Zout is always higher than a good-power load impedance. A triode Zout is usually about hald the Z of a good-power load, though as an extreme you can make it 1/10th. A transistor collector is always very-high impedance compared to a good-power load. A transistor emitter impedance is usually low compared to a good-power load.
The most popular output transistor affair is the emitter follower. What is the impedance of an emitter? Remember Shockley? The dynamic resistance of the emitter at various currents is:
30Ω at 1 milliAmp
3Ω at 10 milliAmp
0.3Ω at 100 milliAmp
0.03Ω at 1 Amp
0.003Ω at 10 Amp
This is true for ANY Silicon bipolar transistor.
Note that a simple Class A emitter follower working at 1 Amp has a damping factor in 8Ω of over 200, without any added feedback (and ignoring some very practical details of bias).
However that runs hot. We more often run Class AB, bias about 50mA, Zout about 0.6Ω, DF is about 10 for small signals changing to about 200 for large signal peaks. That is sure to distort. Also we have ignored thermal stability bias resistors. You can use bias resistors to also stabilize output impedance, but Zout stabilizes around 0.1Ω-0.5Ω, DF around 20.
> the gain of a single transistor is like collector resistance/emitor resistance, so if I eliminate all emitor resistance, all the gain will be infinite
No. And never trust any thought that says gain can be infinite.
There is always emitter resistance. If you don't have an actual resistor, you have Shockley's junction relation above: the emitter always has a dynamic impedance.
The collector has an impedance too. It is usually 300 to 3000 times larger than the emitter impedance.
And you have a LOAD impedance. In power amps, we never think about the collector impedance because the load impedance is much lower than collector impedance.
Go back to a Class A amplifier, biased at 1 Amp, but make it a common emitter (collector follower) instead of the usual emitter follower. The emitter impedance is 0.03Ω plus zero external resistance. The collector impedance is maybe 30Ω. The load impedance is 8Ω. So the collector load is 30 in parallel with 8. 30||8= 6.3Ω. The emitter is 0.03Ω. 6.3/0.03= 210 voltage gain.
But: look at the input impedance of this transistor. It is Beta times emitter impedance. Say 50*0.03= 1.5Ω! How is your previous stage going to drive 1.5Ω? It can be done, but you won't get any voltage gain.
It -may- be possible to get DF=1000 into 8Ω with three stages. I think you have to sacrifice a lot of other important audio goals to get there.
> I still waiting for your answers in "Very low voltage preamp"
It seems to me you know what you want, and I have nothing to add.
> with respect to distortion and compression in the loudspeaker
Distortion will be unchanged. Voltage, current, same thing just factored by impedance.
You should never be running speakers into significant thermal compression in non-commercial work. All the parameters shift, performance droops. Get bigger speakers.
In commercial work, some now consider it "necessary" to run in thermal compression.
With constant voltage drive, as the speaker gets hot, it takes less power. Burn-out is delayed.
With constant current drive, as the speaker gets hot, it takes more power. It goes into thermal runaway (except in practice, with practical amplifiers, it will first clip like hell).
> one or two parallel LRC tanks
An expensive and slightly wasteful way to do what constant voltage drive does naturally.
You can drive at any impedance, and then EQ-out the response. It won't even take more power, if you just EQ to the constant voltage response. You will get in trouble if you run in thermal compression.
The synergy between a dynamic small cone loudspeaker and a constant voltage amplifier is a very practical and beautiful thing. People have tried every other thing, but for 85+ years low-Z drive has held its ground (except in millions of cheap pentode radios). Any other plan requires the amp to be mated to the speaker, adds a lot of complication, and almost never gives better results.
Distortion will be unchanged. Voltage, current, same thing just factored by impedance.
You should never be running speakers into significant thermal compression in non-commercial work. All the parameters shift, performance droops. Get bigger speakers.
In commercial work, some now consider it "necessary" to run in thermal compression.
With constant voltage drive, as the speaker gets hot, it takes less power. Burn-out is delayed.
With constant current drive, as the speaker gets hot, it takes more power. It goes into thermal runaway (except in practice, with practical amplifiers, it will first clip like hell).
> one or two parallel LRC tanks
An expensive and slightly wasteful way to do what constant voltage drive does naturally.
You can drive at any impedance, and then EQ-out the response. It won't even take more power, if you just EQ to the constant voltage response. You will get in trouble if you run in thermal compression.
The synergy between a dynamic small cone loudspeaker and a constant voltage amplifier is a very practical and beautiful thing. People have tried every other thing, but for 85+ years low-Z drive has held its ground (except in millions of cheap pentode radios). Any other plan requires the amp to be mated to the speaker, adds a lot of complication, and almost never gives better results.
According to a paper I own the voltage-transfer function (motion vs voltage) has an expression of (B*l)^2 in the denominator where you don't have this for current drive.
This would give less distortion for current drive.
Regards
Charles
This would give less distortion for current drive.
Regards
Charles
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