VAS cascode to direct drive high power MOSFET OPS - diyAudio
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Old 13th April 2015, 12:24 PM   #1
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Default VAS cascode to direct drive high power MOSFET OPS

Hi guys

I'm working on a prototype of a simple but high power amplifier with a complimentary MOSFET output stage, operated from 56V rails and designed to handle difficult 4 ohm speakers (tested into a 1.3R resistive load). I would like to drive the output transistors directly from the single-ended VAS.

I would like to run the VAS to at least 100mA standing current, so a single KSC3503 is hopelessly underpowered in this application.

Has anybody successfully cascoded a transistor such as the KSC3503 with a high power MJE15032 (or similar) to good effect to direct drive several MOSFETS with large gate capacitance?
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Old 13th April 2015, 12:34 PM   #2
sajti is offline sajti  Hungary
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Location: Százhalombatta, Hungary
Hi,

I did same, using IRF output devices. The biggest amplifier used 6pairs of IRF640/9640, running +/-65V. The VAS used MJE15032/33 pairs, running with 200mA bias (really hot!).
Generally it was OK, however I have to use double EF between the input stage, and the VAS to provide enough current gain, and speed.
I have to use power darlington for the Vbe multiplier, to handle such high current...
This high current stages needed very good rail decoupling to avoid oscillation.


Sajti
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Old 13th April 2015, 02:29 PM   #3
AndrewT is offline AndrewT  Scotland
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the power bandwidth of many commercial power amps is roughly half the frequency response, i.e. 4Hz to 100kHz +0dB, -1dB with a power bandwidth extending to 50kHz at half maximum power.

If you calculate the current required to charge and discharge the output stage to full power at your chosen power bandwidth, you will know roughly what peak current the FETs will draw from the VAS.
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Old 13th April 2015, 05:04 PM   #4
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Think there is no musical signal at 100kHz . Very few loudspeakers (Specialized ultrasonic) can reproduce such frequency . The cream of the best audio mike can't achieve that . Shanon said "you need 200kHz sampling rate to achieve 100kHz" and you can add "very weak" . Your hearing limit is far lower . Simply said the range from 30 to 100kHz is filled with garbage and distortions .

This said high power ultrasonic need very stable comp . The tweeter is the first victim of instability . If you use the capacity of the output FET in // with the compensation this is not a good stability factor . This capacity is variable (fonction of Vds and Id) . It is better to isolate VAS from the output FET with a simple buffer stage this achieve more reliable compensation and better phase margin .
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Old 8th December 2016, 05:03 PM   #5
kees52 is offline kees52  Netherlands
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Location: Sprang-capelle Holland.
Quote:
Originally Posted by Simplex2 View Post
Think there is no musical signal at 100kHz . Very few loudspeakers (Specialized ultrasonic) can reproduce such frequency . The cream of the best audio mike can't achieve that . Shanon said "you need 200kHz sampling rate to achieve 100kHz" and you can add "very weak" . Your hearing limit is far lower . Simply said the range from 30 to 100kHz is filled with garbage and distortions .

This said high power ultrasonic need very stable comp . The tweeter is the first victim of instability . If you use the capacity of the output FET in // with the compensation this is not a good stability factor . This capacity is variable (fonction of Vds and Id) . It is better to isolate VAS from the output FET with a simple buffer stage this achieve more reliable compensation and better phase margin .
It is old thread but see bandwidth as a tube who move water through it, the bigger it is how more easy water get through, make it small water still get through but with resistance, see music the same way, bandwith is what make music easy sounding with much details, make it small like the tube and things get nasty.
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