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Old 17th June 2012, 09:25 AM   #1021
Elvee is offline Elvee  Belgium
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What improvement does it actually bring compared to "regular" cascoding, on the other leg?
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Old 17th June 2012, 09:36 AM   #1022
keantoken is offline keantoken  United States
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Lower distortion, greater speed. It's Q4 that sees all the voltage swing after all. The Vce of Q3 is relatively constant.
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Old 17th June 2012, 12:49 PM   #1023
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by keantoken View Post
Lower distortion, greater speed. It's Q4 that sees all the voltage swing after all. The Vce of Q3 is relatively constant.
OK, but the speed argument is probably spurious: the Ccb participates to the overall compensation, and if it disappears something else will have to make for it.

Can you post simulations comparing both options on equal terms?
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Old 17th June 2012, 01:43 PM   #1024
keantoken is offline keantoken  United States
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Quote:
Originally Posted by Elvee View Post
the Ccb participates to the overall compensation, and if it disappears something else will have to make for it.
This is not necessarily true. Ccb is miller compensation. The VAS compensation is shunt compensation. One probably works better than the other, and has less performance cost. Furthermore, the application of Miller compensation to the VAS when not cascoded is lopsided, creating an imbalance which causes Q3 to output more current, with Q4's gain lowered by the miller.

Dan has the most recent sims, maybe he can do a quick demo.
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Old 17th June 2012, 02:33 PM   #1025
danielwritesbac is offline danielwritesbac  United States
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Quote:
Originally Posted by Elvee View Post
What improvement does it actually bring compared to "regular" cascoding, on the other leg?
More specifically, 8x lower distortion, than without a cascode.
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Old 17th June 2012, 06:07 PM   #1026
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by danielwritesbac View Post
More specifically, 8x lower distortion, than without a cascode.
OK, I made a quick test: without any cascode, distortion is 0.0045% (+/-20V supply, 32Vpp out on a 4Ω load).
With a theoretically ideal cascode, it becomes 0.0035%
With Daniel's cascode, it is 0.0044%
With Ken's cascode, it is 0.0040%

Ken's version is marginally superior, but more importantly it doesn't harm anything and plays the role of the zener.
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Old 17th June 2012, 07:30 PM   #1027
keantoken is offline keantoken  United States
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In my simulations the improvement was at most 2x less THD. Maybe Dan's figures are from his high-bias simulations... The effect depends a lot on VAS component, implementation and so on. I noticed in Dan's schematic the VAS was cascoded at 16V - which could explain his better THD specs, but this really limits the negative swing. VAS devices which aren't overpowered don't benefit much from high cascode voltages (hint hint). May I suggest what is potentially the most reasonable, well-performing and hazardless option, attached.

Maybe NXP has models for the BC337-40, I haven't checked. The BC550C might work better but with less Ic margins.
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Old 18th June 2012, 03:01 AM   #1028
TonyTecson is offline TonyTecson  Philippines
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Quote:
Originally Posted by Elvee View Post
OK, I made a quick test: without any cascode, distortion is 0.0045% (+/-20V supply, 32Vpp out on a 4Ω load).
With a theoretically ideal cascode, it becomes 0.0035%
With Daniel's cascode, it is 0.0044%
With Ken's cascode, it is 0.0040%

Ken's version is marginally superior, but more importantly it doesn't harm anything and plays the role of the zener.
does it really matter? can you actually hear the difference?
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Old 18th June 2012, 06:10 PM   #1029
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Tony View Post
does it really matter? can you actually hear the difference?
To be honest, I don't think so.
But when you have two completely equivalent options from a cost perspective, I think it is better to choose the superior one, even if the difference is marginal.

And anyway, by accumulating numbers of marginal improvements, you end up with better designs.
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Old 18th June 2012, 07:40 PM   #1030
keantoken is offline keantoken  United States
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I like to make it a practice to go for a superior design even in the face of diminishing returns when cost is not prohibitive. After all, how do we get better at what we do when we aren't striving for improvement? There has to be some testing ground for creativity and experimentation. It may not sound better but a gain in skill is always a good thing, and helps us in the other endeavours where it actually matters.
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