www.tubecad.com   Copyright © 2001 GlassWare   All Rights Reserved

NEXT >

< PREVIOUS

<<  March 2001                                   ^^ TUBE CAD JOURNAL ^^              Volume 3  Number 2  >>

21

Pg.

    The other query I have is that in all the other reading I have done regarding tubes driving the capacitive gates of MOSFETs the point is always made about requiring a low driver impedance, to enable sufficient interstage bandwidth - thus not limiting slew rate capability. In your suggested single ended hybrid circuit as shown last month (somewhat similar in layout to the circuit mentioned) this does not appear to be an issue. I would have imagined that in this type of hybrid design that the lower output impedance of a SRPP ( at the least) or cathode follower would need to be used.

    Your analysis of the current mirror and the driver impedance issue would be a great help to my tube learning curve.

 

Wayne


    First off, let me thank you for the link, as I enjoyed exploring the circuit. Now, I don't wish to appear disrespectful, but I got a few giggles when I visited the site, as both the advertising copy and circuit design held a few bloopers. But first a quick recap of what I saw: the amplifier is a hybrid design with a tube differential amplifier input stage that finds a transistor current mirror as an active load that then directly cascades into a pair of parallel P-channel MOSFETs (IRF9540), which are loaded by N-channel MOSFETs (IRF540) used as a constant current source.

    In other words, the amplifier runs a pure Class A, single-ended output stage. And here is where the advertising blooper enters. The power supply rail voltages are ±35 volts, which means if all of the rail voltage could be delivered into the loudspeaker (it can't), the maximum power output would 76 RMS watts, not the 90-100 RMS watts the copy claims. Or if we take the maximum idle current they specify (4A), we end up with only 64 RMS watts. On the other hand, 100 RMS watts requires an idle current of 5A, as the idle current equals the maximum current that can be delivered into a load with a current source loaded Class A amplifier. Furthermore, the maximum theoretical efficiency of a current source load Class A amplifier is 25%. Now 70 volts against 4 A equals 280 watts, which even if we could realize the maximum efficiency of the amplifier (we cannot), the  maximum output is limited to 70 watts.

    This practice of advertising imaginative watts as RMS is widespread. My best guess is that manufacturers claim as many watts as they think the appearance of the amplifier suggests. This also seems to be the technique used by transformer makers. For example, if a single-ended transformer is rated as having an idle current limit of 100 mA and a reflected impedance of 3k, what is the maximum possible RMS power into the load? The math is trivial:

    Power = (Rpri x I²) / 2.

In this case, 15 RMS watts is the answer, not the 35

The original tube/MOSFET hybrid amplifier