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Old 14th September 2011, 01:27 PM   #11
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You could change all of the diodes from BAX16 to BAW62. Homemodder should be able to tell you more, but there are some who have changed the input transistors and driver transistors to faster types and heard greater transparency. I have the unmodded Stochino and have yet to try changing transistor types.
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Old 14th September 2011, 02:43 PM   #12
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Going back to the original post, the reason for the high speed aspect of the design is that several stages are held at a constant output voltage, so they avoid Miller capacitance slowdown. The input goes to p-channel JFET's whose outputs are Tr4 and Tr5 emitters which have their base voltages fixed. Thus, the emitter voltages will be fixed at one diode drop above that. The JFET's have a certain amount of gate-to-drain capacitance. In a typical amplifier, the drain varies in voltage in the opposite direction to the gate, causing the gate-to-drain capacitance to be multiplied by the gain. The extra voltage across the capacitor causes the current through the capacitance to increase by the value of gain+1 and thus, the capacitance appears to be greater than it is. This is the Miller capacitance. It acts to reduce bandwidth and slew rate. By preventing the drain voltage from moving, the Miller capacitance factor goes from gain+1 to 1, a substantial reduction.

The design shows this again on Tr10 and Tr12 and also Tr 11 and Tr13. This is a cascode connection where the collector voltage of the bottom transistor (Tr10 and Tr11) is held constant by the fixed emitter voltage of the upper transistor. This also prevents the Miller capacitance factor from being any larger than 1 (whereas it would be gain+1 without the upper transistors). The use of cascode stages (a common source or emitter stage feeding a common base stage) helps improve the high-frequency response and increase slew rate.
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Old 14th September 2011, 04:24 PM   #13
Bonsai is offline Bonsai  Taiwan
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The things you talk about above do not affect the speed (i.e. slew rate) of an amplifer.

The slew rate is set by the LTP tail current and Cdom in a conventional arrangement and nothing else.

On an FBS topology design, it is quite easy to get 100V/uS (and be stable). All you have to do is ensure that the tail current is high enough to slew the comp cap fast enough in order to acheive this rate of change.

If you use some of the more esoteric approaches (see Bob Cordell's book for a definitive discussion on this), you can achieve 2-3 times this rate (and be stable).

Note also, the UG frequency is set independently of the tail current from Cmiller= 1/[2Pi*fc*Rltp*Acl). The bandwidth is set by the input stage gm and Cdom.
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Old 14th September 2011, 04:40 PM   #14
Ron AKA is offline Ron AKA  Canada
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Quote:
Originally Posted by ingenieus View Post
The specifications given includes output power is 90W into 4 ohm with a slew rate better than 50V/us. Although the author gives some detail of construction in the text, there was no PCB design.
It seems to me that any discussion of slew rate needs to be grounded in what we are trying to do. Required slew rate increases with frequency, and also with peak voltage. At a constant power output you need more voltage as impedance goes up, and as a result a higher slew rate. The article at the link below has a handy table that relates this to amplifier power and speaker impedance. To reproduce a 20 kHz sine wave requires a slew rate of about 3.5 V/uS to feed 90 watts into 4 ohms. This makes me wonder if a slew rate of 35 would be as useful as a bandwidth of 200 kHz instead of 20 kHz in audio applications?

Slew Rate Technical Notes
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Old 15th September 2011, 12:49 PM   #15
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Quote:
Originally Posted by Ron AKA View Post
It seems to me that any discussion of slew rate needs to be grounded in what we are trying to do. Required slew rate increases with frequency, and also with peak voltage. At a constant power output you need more voltage as impedance goes up, and as a result a higher slew rate. The article at the link below has a handy table that relates this to amplifier power and speaker impedance. To reproduce a 20 kHz sine wave requires a slew rate of about 3.5 V/uS to feed 90 watts into 4 ohms. This makes me wonder if a slew rate of 35 would be as useful as a bandwidth of 200 kHz instead of 20 kHz in audio applications?

Slew Rate Technical Notes
It also makes me wonder as the human ear especially after the age of 35 is limited to at best 10khz if a bandwith of 200khz is useful. Good questions.......

As is, the 300Vus amp by stochino sounds excellent to me and many high end companies and myself prefer video opamps which have high slewrates albeit Ill admit these usually also have high bandwith.
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