Actually any class (A)B amp stage would rate as 'current on demand'. You have 'VFA'-type opamps that can deliver more than 50mA output current, surely they have to have a cod mechanism somewhere as they may consume only a few mA standing current from the supply.
I am not sure why we all of a sudden need a new term 'current on demand' while we have used class (A)B for that the last several decades. (I exclude any use for marketing purposes of course; that is not bound by logic and consistency
)
Jan
I am not sure why we all of a sudden need a new term 'current on demand' while we have used class (A)B for that the last several decades. (I exclude any use for marketing purposes of course; that is not bound by logic and consistency
)Jan
Last edited:
Well, i was thinking more along the lines of diamond buffers on I/O...... But, regardless, I'm not sure your point is relevant to the question I've been attempting to answere -- apparently, unsuccessfully:
Do You actually know why many mfr are using CFA on their top models only?
Or put another way....... why would design change and not use VFA on all models?
THx-RNMarsh
Do You actually know why many mfr are using CFA on their top models only?
Or put another way....... why would design change and not use VFA on all models?
THx-RNMarsh
Last edited:
No I don't know, and I don't know whether that is the case.
But if that IS the case, I have some ideas. As we look at the audio landscape, now that technically it is a mature industry, new issues have to be found to distinguish yourself from the rest of the pack. CFA or COD has a nice ring to it (even if it is just class(A)B) so this may be a factor.
It is extremely unlikely that the reason is sound quality because there are so many different topologies and methods (class (A) B, class D, SET, Hybrid, et al) which all have their fanatical defenders as giving 'the best' sound quality.
So, invoking Occam's Razor would point to anything but sound quality.
Jan
Last edited:
Diamond buffers are class (A)B stages by another name. Nothing new here.
If you don't have Arto Kolinummi's book yet I can send you one; very enlightening.
Jan
I dont care if it is NEW or not..... why is NEW something to hang a hat on? I am thinking combinations which optimise the performance for audio.
IF it sounds better, why? Of course, if the mfr themselves (Krell and the other main stream mfr i found who use CFA) have no good reason at all and none of them hear a difference either .... then I dont know what to say.... Industry wide collusion ?
THx-RNMarsh
IF it sounds better, why? Of course, if the mfr themselves (Krell and the other main stream mfr i found who use CFA) have no good reason at all and none of them hear a difference either .... then I dont know what to say.... Industry wide collusion ?
THx-RNMarsh
So Many has now become two, only one of whom you can remember the name of?
Apologies to Bob for pulling this so off topic, but there is some mighty fine marketing here. Cables . I particularly like the 'radical current mirror technology'.
There is definately a correlation and causality problem going on here, which Jan has pointed out.
Apologies to Bob for pulling this so off topic, but there is some mighty fine marketing here. Cables . I particularly like the 'radical current mirror technology'.
There is definately a correlation and causality problem going on here, which Jan has pointed out.
Well funny thing is, I had been using for many years a VFA amp of same power output and very low THD etc for several years. On ESL and Dynamic speakers. I really didnt know that those other mainstream mfr made any CFA amps until I searched. After hearing the CFA from DADoD, I bought a new Marantz CFA (7025) expecting it to be better than the VFA. But I was disappointed in the sound, overall. It was better in imaging details (a good thing). But bass was not as accurate and deep etal. I had 4 each 15 inch Cerwin-Vega high effec bass drivers which are amazing.... but not with the Marantz. Not enough current capability for low Z of paralleled bass drivers. So I went back to the VFA PA. So, not wanting to make a DIY version of DADoD amp, I sent it to my friends in Bangkok to make for me. Meanwhile, I upgraded the speakers to JBL M2's.
I think you will have a hard time convincing me the amps heard are all the same sounding. No collusions IMO. Some time soon I will have a pair of DADod CFA to power the M2. I hope i can consider my quest finished... just in time to have a few remaining years to listen and enjoy the sound of music.
I have only found characteristic differences in the test data profiles, which i have told. The differences which stand out are --- some more SR margin, and a flat thd vs freq vs power vs load... all at very, very low distortion. I have mfr working up a full set of specs.
THx-RNMarsh
I'm still in SE Asia... Cambodia, Thailand, Nepal. No good sound for 3 months. :-(
Last edited:
Richard I wish you many, many years of good health and musical enjoyment!
And I am not going to argue with people's personal preferences, however obtained; as I mentioned before, people swear by the most different and completely opposite conceptual amps as you can think of, as being The best Sounding. So that is unfortunately no help to decide which topology is objectively better in terms of accuracy, the original HiFi concept.
So it seems that at this point we just lack the tools to decide that which is coincidental is or is not causal.
So the quest continues....
Jan
And I am not going to argue with people's personal preferences, however obtained; as I mentioned before, people swear by the most different and completely opposite conceptual amps as you can think of, as being The best Sounding. So that is unfortunately no help to decide which topology is objectively better in terms of accuracy, the original HiFi concept.
So it seems that at this point we just lack the tools to decide that which is coincidental is or is not causal.
So the quest continues....
Jan
Hi Scott
-Thanks for pointing on xDSL drivers chips , but for example if I refer to incredible products offering from Linear Technology there is obvious that by comparing internal structure and topology of xDSL drivers IC`s and CFA`s IC`s they are internally very different , so I can only speculate that Toshiba newest car power amp IC is CFA based not xDSL since Toshiba have not showed TCB501HQ even simplified internal structure .
-Yes those specs are not so representative , but others manufacturers car power amps IC`s a falls in the almost same`nothing to write home about` performance ball park ,
-some link to Jan DSL based PA ?
and yes , I think that we should continue this specific disscusion on some other place since I don`t want to abuse this Bob Cordell specific topic .
See, we are talking about high level properties of the CFA before defining the concepts we are operating with.
In my book, "current on demand" is the property of a circuit topology to provide to the parasitic and/or compensation capacitor(s) enough current to avoid the current starving slewing limitation. A topology with current on demand has, by definition, no fundamental slew rate limit. Not much to do with the class A(B) output current. See for example
http://www.analog.com/media/en/training-seminars/tutorials/MT-034.pdf
http://www.analog.com/media/en/training-seminars/tutorials/MT-057.pdf
https://books.google.ca/books?id=dz...Current on demand" "current feedback"&f=false
It happens that the classic CFA topology (diamond buffer style) inherently implements "current on demand", but other topologies (some in pure VFAs) can implement this feature as well.
In your book, yes. i too can define anything I like if I can put it in my personal book.
Jan
You just created your own private definition of "current on demand", Mr. Didden. Nobody likes to be proven wrong, does this fall under the moderation rules and should be avoided?
I don't, those references I quoted are crying out loud, I guess it would help opening the links and reading a little.
If you have any references that the class A(B) output current is "current on demand" I'm all ears. I'm always happy to learn something new, Mr. Didden.
Could somebody please explain to me this new buzz phrase 'Current On Demand'?
Rather than explaining the meaning of the phase explain what it is to not have current on demand.
In the past I would have attributed this phrase to under designed power supplies that can't keep up with current demands. But not the amplifier or even just the output stage.
Rather than explaining the meaning of the phase explain what it is to not have current on demand.
In the past I would have attributed this phrase to under designed power supplies that can't keep up with current demands. But not the amplifier or even just the output stage.
Not at all actually most DSL drivers were/are straight CFA's don't be confused or distracted by details of implementation. The output stages for instance differ a lot and there were several techniques for setting common mode quiescent current, but many were classic diamond input/gain stage/AB output. We had great success with the Gosser two stage CFA, CFA's using the Monticelli biased CE output stage. And best supply current to output current one used a CFP output. -70dB over the 2Mhz BW with 600uA total bias and 400mA peak output
current.
EDIT - If you look at Jan's link the AD815 was the first DSL driver and is a brute force classic CFA in a SIP power package. This was OK to roll out the first round of subscribers but fire codes and temperature limits limit the dissipation in telco cabinets so over the years drivers were tailored to the high crest factor application only.
Last edited:
The simplest example of *not* current on demand is the classic VFA with a long tail pair without degeneration, biased by a current source.
The current available to charge the Miller compensation cap connected across the so-called VAS is limited by the current source capability, or twice the long tail pair bias current. When the input is driven hard, half of the long tail pair saturates, the other side blocks and the current source flows in the Miller compensation cap. Assume a 2mA current source and a 100pF Miller cap, the voltage across is I/C V/s or 2E-3/100E-12=5E7 or 20V/uS. This is a much simplified model, ignoring the long tail pair degeneration (good) effect on the slewing, etc...
This example also illustrates something that is largely ignored: the slewing characteristic is a large signal, nonlinear behavior. When the slewing limitation occurs, the global feedback loop is already broken, since there is little to none forward gain to speak of. Slewing/slew rate is, in general, different and largely independent from the small signal/linear rise time, which is directly related to the amplifier bandwidth Tr=0.35/BW.
You can get "current on demand" even in a classic VFA with long tail pair, if you bypass the current source (or the common emitter resistor) with a cap much larger than the compensation cap. This cap will bypass the resistor limited current and provide extra current to charge the Miller cap during fast transients, providing some sort of non slewing characteristic. Again, see the Stochino amp.
Now if you look into a classic CFA topology, you'll note that the Miller (or any other parasitic cap) charging current is limited only by the feedback resistor and the output voltage swing. Since the available current depends on the output swing the more harder you drive the amp, the more charging current you get (therefore "current on demand"), hence the non slewing characteristic of the classic CFA topology. In a long tail pair VFA, the Miller cap affects both the slew rate and the amplifier bandwidth, while in a CFA classic topology it affects mostly the amplifier bandwidth (and not the slew rate).
Last edited:
Scott
my reference was actual Linear Techology OP-amps products list Operational Amplifiers (Op Amps) - Linear Technology
where I can see two different OP-amps groups on that pretty long list , one group of High Speed OP-amps and second group of CFA OP-amps ,
just for example, LT1739 fall in High Speed OP-amps and is intendet as xDSL driver ,
but LT1206 is CFA OP-amp intented to drive some multiple cables in video system , main difference between those two device are in they internal structure, where by LT1739 both non-inverting and inverting input are high Z , but by LT1206 inverting input is low Z typically for CFA ,
L.T. as manufaturer have provided simplified internal schematic for both IC .
my reference was actual Linear Techology OP-amps products list Operational Amplifiers (Op Amps) - Linear Technology
where I can see two different OP-amps groups on that pretty long list , one group of High Speed OP-amps and second group of CFA OP-amps ,
just for example, LT1739 fall in High Speed OP-amps and is intendet as xDSL driver ,
but LT1206 is CFA OP-amp intented to drive some multiple cables in video system , main difference between those two device are in they internal structure, where by LT1739 both non-inverting and inverting input are high Z , but by LT1206 inverting input is low Z typically for CFA ,
L.T. as manufaturer have provided simplified internal schematic for both IC .