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Old 2nd May 2002, 10:00 PM   #1
syl is offline syl  Sweden
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Default The sound of cascodes

I have tried cascodes. I have tried them in input differentials, in driver stages, in line stages, in powerstages. All with the same result - an initial luring illusion that the sound is better, cleaner and more focused because the resolution seems better. After a week of listening things have always changed. The problem is that the amp is not silent when it should be. The sound of a cymbal clash has two distinct phases - the initial strong clash transient followed by the decay phase with its very rich harmonic content. With cascodes the clash transient becomes exaggerated and extends well into the decay phase, masking the instrumentīs natural timbre. The sonic signature of this transient behaviour is very fatiguing, a bit like TIM.

Iīve spent hours thinking on why the sonics does not correlate with the theory, it should be better but it is not. From my experience, good interstage linearity always makes good audio performance. The cascode is a compound circuit build with TWO transistors. The question is if something about the interstage linearity affects the sound. I am pretty sure no one would seriously use a stand-alone CB stage as a gain element in a power amp front-end because the local overload margin would be next to nil (due to the high gain). But we hook it up with a CE-stage collector and figures are now great. Question to the board: what about the interstage linearity ?

I once tried to "improve" a very good power amp by cascoding the input differential but the audio magic was now gone and after just 10 minutes of listening the stuff was lifted out again. It just doesnīt work for audio AND I DONīT KNOW WHY. It is annoying that I cannot explain the reason in technical terms.

"You are wrong - cascodes are better" is not the reply I seek. I have heard those words many times already but the sound still does not change to the better. It would be much interesting to hear someone explain technically why transients seems to be mangled in a cascode stage.

Kind Regards

Syl
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Old 2nd May 2002, 10:26 PM   #2
jam is offline jam  United States
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Question Cascode Quandry!

Have you tried running more current through the cascoded stages?

How did you bias the cascodes? Did you use resistors, zeners or current sources. Did you put a capacitor from the base of the cascode device to ground?

Did you reference the cascodes to ground or the emitter of the cascoded device?

You raise some good questions but raise a few others and maybe we can come up with some answers.

Jam
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Old 2nd May 2002, 11:50 PM   #3
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Should have known Jam would get here before I did...
I am suspicious of cascodes and use them only when their use allows me to dispense with another gain stage later. Yes, they are better on paper, but that doesn't always pan out in the listening room.
Jam's points are valid, however. There are a million variables that can be toyed with to improve the performance of a cascode. By all means, jiggle the parameters back and forth.
But do you really need all that gain? You're just going to have to burn it off later with NFB or whatever. There are times when a cascode is just the ticket, but they aren't as common as many people suppose.

Grey

(Jam and I can jog each others' elbows about using resistors vs. current sources 'till the cows come home. I prefer resistors [in most cases--current sources do have their uses] and he prefers current sources. I feel pretty much the same way about current sources that I do about cascodes...use only when absolutely necessary...and that ain't often.)
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Old 3rd May 2002, 12:01 AM   #4
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I don't know much about SS cascodes, but a tube cascode doesn't have very good power supply rejection so needs really good regs.

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Old 3rd May 2002, 12:29 AM   #5
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It very much depends on what you are doing and why.

It has been my experience that cascoding does not always
improve the sound. It's just another tool in the box.

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Old 3rd May 2002, 12:58 AM   #6
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While I can't agree with most of the comments so far.................

I have noticed that cascodes can do some odd things, visible on a good 'scope, depending on how the cascode section is biased and bypassed.

In case jam, et. al., are wondering............

Yes, I like them, and current sources too. So there.

Jocko
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Old 3rd May 2002, 01:16 AM   #7
AKSA is offline AKSA  Australia
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Default Cascodes

Syl,

Interesting comments on the sound. I followed it clearly; decay is a big issue, it's one of the final frontiers to get it right, but it should not be the main event.

There are four reasons I don't go for cascodes. One is complexity/tube count, two is floating the upper filament, which often creates noise, three is the very high impedance output, and four is lousy PSRR. If you want more gain, use a different tube, or cascade two medium mu tubes, preferably with direct coupling - and much lower Zout, which at these levels of signal is always required to drive output tubes anyway. And I have to say that I don't like mu followers either (although Zout and PSRR are not issues with this topology).

I too have often wondered why cascodes are not always good performers. Perhaps, like cathode followers, they are hypersensitive to operating point. But one thing has always worried me; the huge impedance mismatch between the drive of the bottom tube and the input of the upper tube. Impedance mismatches are often the source of many problems, and this could be it.

It's strange that most good designs are very simple, with plate loaded triodes and the odd cathode follower.


Great post!

Cheers,

Hugh R. Dean

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Old 3rd May 2002, 11:28 AM   #8
dice45 is offline dice45  Germany
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Syl and all,

1st,
thanxalot for the great input !

2nd,
had the opportunity to listen for an extended period to two preamps suffed with cascodes. One was single-ended, the other was fully differntial. Cascodes were hybride: bottom MOSFET. top tube.

While those preamps, designed by a dedicated cascode lover, were the best HIFI i ever had in some respects, it was not music to my body and soul. I suffered long-term listening fatigue with both preamps, it went that far that i preferred to have no music playing in the background (remember my last posts in the "Going Pro" thread ? ). I was considering building that differntial preamp for me but to incorporate some own ideas and i wondered how close i should stick with the original design; so i discussed a lot of toplogogy issues with my friends and cascode-or-not was the most important (2nd being fancy output CF or OPT).

Both friends, one a tube wizard, the other a SS guru with tube experinece, complained about "spiritual" nastinesses of cascodes i also felt uncomfortable with (and it turned out i could describe their unease better thant hey themselves could). We agreed sonics were sterile, not involving, artifcial, excellent for the brain but not for the heart. It was very interesting for me to find the SS guru (hifidaddy) to complain about the same in SS circuits i had to complain about in hybride in pure tube cascodes.

I do not condemn cascodes at all, I see it like Nelson Pass, they are another tool in the box.. They just seem not to sing for me. And my preamp design ended up in having neither cascode nor CF (but a lot of iron, personal taste). And i found it quite a challenge to get the gain low enuff and still maintain the count of stages needed for a proper split phono EQ. Two choke-loaded long-tailed-pair gain stages together withan 1:12 MC input transformer too easily have the total gain i need .... a cascode would be the last i need here.
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Old 4th May 2002, 09:32 PM   #9
syl is offline syl  Sweden
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Thankīs for all replies.

It seems like some people use them only if they have to. I too use one in my 100W power amp. I use fetīs for input and they cannot cope with 53 Volts. This amps drives my woofer so only bass matters.

Jam,
The way I use them I want to maximize the local feedback of the CB stage, so I use non-bypassed resistors connected to the base. The reference is ground, not the emitter of the CE-stage. In your view, what is the sonic difference between referencing to ground and referencing to the emitters of the CE-stage ?


Regards

Syl
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Old 5th May 2002, 12:22 AM   #10
sonnya is offline sonnya  Denmark
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Default Re: Cascodes

Hi Hugh.. You wrote "There are four reasons I don't go for cascodes. One is complexity/tube count, two is floating the upper filament, which often creates noise

three is the very high impedance output, and four is lousy PSRR. "

I think that this is not a drawback. High impedance output => Less interference from the the BJT/FET on the signal. It is only a "linear" resistor who is the output impedance not a "nonlinear" transistor.

Lousy PSRR .. I think this comes from a pour base/gate drive of the Cascode BJT/FET ?? I cannot see another way of entering the circuit.

As JAM writes "How did you bias the cascodes? Did you use resistors, zeners or current sources. Did you put a capacitor from the base of the cascode device to ground? "
This could possible effect the PSRR!?

I know you where talking about tubes, which i do not know anything about.

I would as the second/third person in this thread agree with Nelson "It has been my experience that cascoding does not always improve the sound. It's just another tool in the box."

Sonny
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