Folded Cascode Sound?

[eLarson]

Originally posted by sreten
I'm certainly no expert on circuit topologies but NP's comments
confirmed my dimmed with time observation (which I wasn't
now sure about) that a folded cascode is effectively a re-
arrangement of an LTP pair.

With respect to a re-arranged LTP, were you thinking of an emitter-follower/common-base cascade? That would result when driving one half of the LTP (only) and taking the output from the opposite side.

You'd avoid the Miller capacitance-multiplying effect in a similar way to a cascode, too.

Erik

[thoriated]

This isn't directly on subject, but what do people think of the vacuum tube LTP differential crosscoupled cascode amplifier?

[wrenchone]

Please don't hijack my thread. It is so easy to start a new thread in the vacuum state section - you might even get some replies...

[lumanauw]

FYI, here's an older thread, dealing with the sound of cascode:http://www.diyaudio.com/forums/show...3398&highlight=

That is a very good thread, but unfortunately it stops without any answer to the original question. The guru's are debating about the definition of cascodes instead. I wonder what will be the conclusion of that thread if it was continued to finish.

After reading that old thread, it shakes my believe again. First, I learn that distortion comes from fluctuating voltage and fluctuating current.

The fluctuating current is remedied by putting big standing current (bias current), so the percentage of signal is small compared to bias current (class A).

The fluctuating voltage is remedied by cascodes, that is to lock the operating voltage of the gain transistor.

I tought that is it. But that old thread discuss about how cascodes gives "FATIGUING" sound for long time listening. Actually I still have questions, why Mr. Nelson Pass seldom use cascode, inspite he has his own patent on cascodes? He only uses it for small FETS or dividing thermal dissipation.

Is it true, while cascode can help with measurement figures, it is bad for listening? What is the reason behind it?

The emphasis is on properly done; The CCS which supplies both the input gain device (the one being cascoded) and the cascode device has to be high quality, and there has to be
adequate voltage margins provided for it - which is not always
done.

What is a high quality CCS looks like? How much is adequate voltage margin?

[jcx]

Originally posted by wrenchone
... The second has probably been done by someone else too, though I haven't heard about it.

probably because positive feedback in linear amplifiers fell out of fashion as pentodes became popular - the positive feedback (known as "regeneration") was useful to cancel the internal negative plate resistance feedback of triodes when larger external gains were desired

its going to be difficult to do a “apples to apples” comparison of folded cascode vs current mirrors given the number of inversions necessary to create a negative “current feedback” connection not being the same

you could try adding a common source output amp to circuit #2 but then you are also testing nested vs global feedback schemes as well

[wrenchone]

The source follower buffer on the first circuit did little if anything to the simulated distortion figure. I would only use it when the circuit has to interface with the cold, cruel world of uncertain impedances. The current mirrors in the second circuit are only there for the bias, to make sure that the two current source loads are reasonably equal, and as benificial fallout, also to provide bias for the cascode transistors. The same thing could be done in countless other ways, but bipolar transistors are cheap...

[wrenchone]

So sad, I took a closer look at the circuit on the right, jcx, and it does have positive feedback. For some reason, it didn't flop over to one side in simulation like it should. Shame on me for not staring at it harder...It could be fixed by adding another inverting stage or by returning the feedback to the input, but I'm not too interested in either approach. It looks like my folded cascode measurements will be confined for the time being to my power amplifier, once I get all the mods done and coax it back into its case. Now that one has proper negative feedback...