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Transconductance amplifiers?


Chris A
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Thanks Carl.  This is actually a pretty esoteric subject, apparently.   Many people really don't know what a transconductance amplifier is.  It's a current-source amplifier instead of a voltage-source amplifier. 

 

What most people don't really realize is that their Klipsch loudspeakers operate off of current--not voltage.  Using higher output impedance amplifiers--such as SETs--is really about pushing their amplification paradigm towards current source rather than voltage source amplification.  The problem is that most people are also not prepared to hear the boost in low frequency output that they will get around the woofers' resonant frequency when using a true transconductance amplifier.  So all they have to do is to EQ their current source amplifiers down in the 40-60 Hz region [EDIT 19 Aug. 2018:...and the upper midrange region (2-7 kHz)], depending on exact loudspeaker model you're using.  A lot of people seem to have trouble understanding how this can be true, and abandon their transconductance amplifier trial without first dealing with this phenomenon.

 

Chris

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William,

 

The First Watt F4 is notable in that it can be used with an existing integrated amplifier that provides the preamp duties, while the F4 provides power (with no voltage gain) to the loudspeakers.  It's an unusual idea, too. Thanks for the description of your SE F4 experience. 

 

I was really looking for experiences integrating current-source (transconductance) amplifiers with horn-loaded loudspeakers.  Nelson Pass apparently used his F1 and F1J designs with full range drivers (Lowther, et al.).  While that's an interesting application and they are high-efficiency drivers, it's not quite the same as hooking a transconductance amplifier to a pair of La Scalas, Belles, Klipschorns, or K-402s with compression drivers. 

 

In particular, I'm looking for experiences with the cleanness of sound--over and above that of any other amplifier topology.

 

Chris

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Yea...I admit it. 

 

But one first has to find a unit that's reasonable in price.  First Watt is very good, but used F1s or F1Js still command prices exceeding $1.5K(US)--when you can find them--which is difficult.

 

The objective is to have an amplifier that's insensitive to all the back-EMF sources within the loudspeaker (i.e., flux modulation, solenoid forces, back chamber-induced microphonics, small room horn-amplified reflection microphonics, etc.).  The advantages of using amplifiers with effectively infinite output impedance--especially in small rooms--should be easy to hear in terms of the presence and cleanness of the resulting output.

 

The flip side, of course, is to use amplifiers with effectively zero output impedance and zero output-stage feedback (which rules out SETs because of their internal feedback)--so that the output power stage doesn't sense any back-EMF induced by the loudspeaker. 

 

Chris

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It's interesting that you've made a subtle insertion of the term "buffer amplifiers" to the subject at hand, transconductance amplifiers. 

 

There is an unmistakable difference, however.  Transconductance amplifiers have output impedances measured in 10s of ohms, while buffer amplifiers typically have output impedances less than an ohm.  See column 6 of the following embedded table to clearly see those differences among Mr. Pass's First Watt amplifiers:  http://www.firstwatt.com/prod.html

 

I suppose that transconductance amplifiers can be made into buffer amplifiers, but certainly not all buffer amplifiers are transconductance amplifiers, at least not using the property of extremely high output impedance, above. 

 

I believe that you'd hear a difference--and it wouldn't be subtle.

 

Chris

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You seem uncharacteristically sensitive William.  This is not a trick question (the one that I asked...at the top of this thread). 

 

Try this:  http://www.diyaudio.com/forums/solid-state/2213-transconductance-power-amplifier-current-drive-loudspeaker.html

 

Pay attention to the responses from "Papa"...

 

The question at hand regards Klipsch horn-loaded loudspeakers...not full-range...and not direct radiating loudspeakers which are typically what others are using on other forums.  The question that I ask here...is apropos for this forum.

 

Chris

 

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6 minutes ago, wdecho said:

I just fail to see at all the point you are trying to make that's all. No offense meant. Point me to in the right direction in the article you have just listed and what you are referring to. 

 

William, that's what I've been doing.   You seem to have an extremely high threshold for acceptance of the subject, however. 

 

Can we agree that you don't have an answer for the question posed above?  I assume that you've not tried a First Watt F1, F1J, or other extremely high output impedance amplifier (i.e., greater than 30 ohms)  on your fully horn-loaded Klipsch loudspeakers...right?

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For those interested in learning more about transconductance power amplifiers, here are a few more sources of interest (two JAES articles, and one from Nelson Pass on the First Watt F1 amplifier):

 

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.211.4368&rep=rep1&type=pdf

 

http://www.pearl-hifi.com/06_Lit_Archive/14_Books_Tech_Papers/Hawksford_Malcolm/Distortion_Reduction_with_iDrive.pdf

 

http://www.firstwatt.com/pdf/prod_f1_man.pdf

 

Chris

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William, I've reread what you've written, and I believe that you've made an assumption that's not correct: transconductance amplifiers have gain.  Since you probably didn't click on the convenient link that I posted above to Nelson Pass's First Watt amplifier comparison chart, I've reproduced it here so that you don't have to click on the link:

FW Amp Comparison Chart.GIF

 

If you notice the F4 row, you'll see that it has zero (0) gain, indicating that it's a buffer amplifier.  However, both the F1 and F1J have gain listed by Mr. Pass.  I invite you to investigate further so that you can understand what Mr. Pass is saying and has done with transconductance amplifiers. 

 

His F1 and F1J amplifiers are notable in that they are not using feedback to achieve their voltage-to-current gain...gain that doesn't care if the back emf generated within the loudspeaker tries to resist the current.  This is the key concept that I believe that you've missed (but I could be wrong). 

 

Chris

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2 minutes ago, wdecho said:

I have been told you like to have the last word to prove you are right. I cannot see where my posting more is going to enlighten your knowledge since you think old technology is the way to go. I have see this many times on this forum so it is nothing new to me here.

 

1 minute ago, wdecho said:

So you were wrong about a transconductance amplifier being a current only amplifier but rather a common source amplifier with voltage gain and current gain which is much more common than at common drain amplifier. We have not been comparing apples to apples because of your mistake.

 

Why the ad hominem stuff, William?  Have I offended you in some way?

 

Chris

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Perhaps the concept of a transconductance power amplifier is too far beyond your understanding, William.  My apologies for upsetting the apple cart.

 

I'm still looking for anyone that has tested a Nelson Pass transconductance (i.e., NOT buffer) amplifier... with their Klipsch horn-loaded loudspeakers.  In particular, I'm looking for the changes in frequency response and "transparency" of the sound, as well as the overall presentation. 

 

In the mean time, I'll try some of the techniques that NP tried in his article on transconductance amplifiers and full-range drivers.  That looks to be much lower cost and something that can demonstrate the effect of moving from voltage control to current control, and with low feedback used that re-injects loudspeaker-induced back-EMF into the forward loop.

 

I have a feeling that some of the experiences with other low output feedback amplifier topologies (which excludes triodes, of course) are also approximating some of the benefits of transconductance power amplification. 

 

Chris

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The F4 is not a transconductance amplifier--nor is its operation considered to be equivalent to the F1 or F1J. 

 

It would be nice to refer to transconductance amplifiers, not unity gain "impedance converting amplifiers".  Wikipedia refers to transconductance amplifiers as VCCSes:  Voltage-Controlled Current Sources...

 

F1 simplfied schematic.GIF

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William, 

 

It's now abundantly clear that you do not wish to understand the original question, since NP also made it abundantly clear in the article that you've said that you've read "many times":

 

Quote

Power Transconductance Amplifiers

 

This is a fancy name for a power current source. An input voltage causes the amplifier to deliver a proportional output current. Of course this same sort of thing would occur with an ordinary amplifier driving a pure resistance...but a loudspeaker circuit is not purely resistive. It possesses numerous reactive elements, some due to inductance and capacitance in the electrical circuit, some from the reaction to motion of the voice coil in a complex mechanical system. Fed by a voltage amplifier, the current through the driver’s voice coil is not directly or instantly proportional to the input to the amplifier.  Ordinarily, loudspeakers are designed around this assumption but the “piston model” of loudspeaker design assumes that the acoustic output mirrors the acceleration of the voice coil/cone assembly over a specific range and this is reflected by the current through the voice coil.

 

The most precise way to develop that specific current is with a current-source amplifier. Such an amplifier ignores the impedances in series with the circuit, the resistance and inductance of the wire and voice coil and the back electromotive force (EMF) produced by the cone motion. As I said, most speakers are designed around voltage sources but there are few instances where a current source can be used to advantage.

 

Taken from: http://www.firstwatt.com/f1.html

 

In order to do what NP said, the output impedance of the amplifier must be ~10x the nominal input impedance of the loudspeaker (rule of thumb), or the effects of the reactances of the loudspeaker driver(s) must be significantly reduced (i.e., by 50 dB or more from their present levels) by shunting significant external resistances, capacitances, and inductances across the loudspeaker load to restore their frequency response due uneven input impedances and passive crossovers designed for voltage amplifiers.

 

NP designed and built 100 units of what he called the "First Watt F1".  That amplifier boasts an output impedance of 80 ohms.  Later, NP upgraded the existing F1s to take better FETs, slightly lowering the output impedance but increasing the gain (i.e., transconductance or VCCS gain)--and called that upgrade of those 100 amplifiers the "F1J", for JFET (instead of MOSFET power transistors that the F1 used)--in the year 2012...four years ago

 

NP never released the PCB designs for the F1, so I'd imagine that he still has plans for this type of design in the future, i.e., this isn't an "old design" (in the way you imply that it shouldn't be taken seriously nowadays).  This is neither an F4 buffer amplifier, nor any other First Watt amplifier.  It is unique...and is used for a unique purpose that NP stated--not something else that you seem to be fastened upon for reasons that escape me.  I fail to understand why you continue to try to convince others that "it doesn't make any difference".  NP himself--the man who you called "the master" and would have no other source of information to answer your questions (and apparently dismissed as "old" after that information generated by him hand fed to you) said the F1/F1J was the most "truthful" amplifier that he has ever produced, as you have also provided, above.

 

The reason why I created this thread (...please read carefully...) still is:

 

I'd like to hear from someone that has used one on something like a La Scala, Belle, Klipschorn, or Jubilee for their subjective listening experiences since those are the only Klipsch loudspeakers that would also benefit from this type of amplifier--i.e., those with horn-loaded bass. 

 

That's the question...not some debate on amplifier device configuration equivalences that render the F1's design "obsolete and old".  If you don't have direct listening experience using one of those two amplifiers, I would respectfully request that you refrain from attacking the premise of NP's F1's design in this thread. 

 

If you actually know nothing on this subject and are still driven to pooh-pooh the idea in this thread, I strongly recommend that you first read and understand the sources on "transconductance power amplifiers" that are provided above...particularly the book that I referenced above--without resorting to blaming others for not hand feeding exact pieces of information (i.e., the "right" information...in a fashion that I've yet to fathom) that can get through your preconceived ideas that "...this isn't any different...".

 

_______________________________________________________

 

Well...having thrashed the above--yet again, I can still say that my interest in hearing from someone that has personally heard the First Watt F1 or F1J, in particular, or perhaps another extremely high output impedance amplifier...with effectively infinite output impedance...is still piqued. 

 

I have a hunch (based on physics) that the best quality fully horn loaded Klipsch loudspeaker designs will benefit greatly.  In fact, I'd be willing to wager that some would call the F1 the best amplifier that they've ever heard driving these loudspeakers. 

 

Peace to all...and good will towards men.   :emotion-51:

 

Chris

 

 

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1 hour ago, wdecho said:

I am not going to post on the topic anymore as it serves no purpose.

TIA.

 

Well...moving on.

 

One of the interesting comments that is found in the above-referenced book:

 

Quote

(Pg. 112) The output impedance of transistor amplifiers is typically less than 0.1 ohm, which denotes pure voltage feed for the speaker.  In tube amplifiers instead, the output impedance varies rather widely [in their various implementations]; from tenths of an ohm to even more than five ohms (with 8 ohm loading).  A source impedance of even a couple of ohms is able to weaken the speaker's EMP currents so that the effects are observable; and as the value exceeds 5 ohms, the speaker may function at some frequencies even halfly current-driven. 

 

It is indeed interesting, although not surprising, to notice how in various sources the listening evaluations published for amplifiers correlate with measured output impedance.  When output impedance exceeds 3 ohms, positive comments about sound clarity and spatial impression increase remarkably.  And with [amplifiers] exceeding 5 ohms, "better than ever" style characterizations are common, provided of course that the speakers used are suitable and that their frequency response is not too confounded due to the frequency dependence of their impedance.

 

Chris

 

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