PowerDrive-S

Build a Better Mousetrap and the World will Beat a Path to Your Door

Back in 1978 when NAD introduced the inexpensive 3020, a 20 watts per channel amplifier designed by Bjorn Erik Edvardsen, it revolutionized amplifier design by sounding like amplifiers costing up to ten times as much. Even more amazing was it's ability to drive difficult high-end speakers. Other amplifier manufacturers had to rethink their designs after the NAD 3020 and consumers are still talking about it! Today NAD launches the C315BEE amplifier with PowerDrive-S™ - it's our better mousetrap and we invite you to learn what a bright idea it is!

The Challenge

Develop a less expensive amplifier that does not compromise NAD's renowned audio performance in any way. Bjorn Erik Edvardsen, NAD's Director of Advanced Development, met this challenge with a simplified version of the PowerDrive™ design that provides high dynamic power combined with very low distortion even when driving difficult low impedance loads. The C315BEE is the result of this challenge and features PowerDrive-S™ - offering the benefits of PowerDrive while minimizing cost. This breakthrough allows for unprecedented power and performance in the "budget amp" category. Here's performance, value and simplicity exemplified!

Historical Precedent

Many will fondly remember earlier generations of NAD amplifiers (the 3225PE and others in the series) that featured a Power Envelope design. This design delivered extraordinary reserves of tone-burst power for music from an output stage delivering dual voltages roughly in the proportion 1.4:1. In the case of the C315BEE both supplies are fed from a low resistance 150VA toroidal transformer but the higher voltage winding has PTC (Positive Temperature Coefficient) resistances in series. In the early generation "simple PE" amps, we used polymeric positive temperature coefficient thermistors, but these had some undesirable characteristics and were expensive, pushing us further away from the budget sector.

Fast-Forward to Today A Light Switched On, Actually Two

Amazingly, two modern long life quartz halogen light bulbs meet our needs more ideally than these thermistors providing cost effective, dynamic power (10- 100msec bursts) to a typical music envelope for a given maximum output stage dissipation. The light bulbs have very low resistance (<1 Ohm) when cold so drop very little voltage on short peaks. At 7 V, after a few hundred milliseconds, the resistance rises to an ideal 4 Ohms.

Dynamic Power Performance

For short peaks of 5msec, the C315BEE's undistorted peak power is > 110W in 8 Ohms, 190W in 4 Ohms and 270 W in 2 Ohms. This is from an amplifier rated for 40W continuous in 8 Ohms! This gain in usable dynamic power on music is very nearly equivalent to a conventional amplifier rated at 80W continuous at 8 Ohms! IHF Dynamic Power (20ms) with and without PowerDrive-S™ is shown in Figure A.

Reliability

We all know that light bulbs don't last forever and we obviously consider reliability to be a very major consideration for our customers. The light bulbs are rated for 5,000 hours but in the C315BEE circuit the light bulbs are used at a maximum of 60% of rated voltage (35% of rated power) and that is only reached on sine wave testing, or in music on sustained high-level notes, and even then at a low overall dutycycle. Under these conditions the life conservatively extends to 15,000 hours. Thus if played at high volume levels for 3 hours a day every day of the week it would take 14+ years before replacement is required. In the unlikely event of a bulb failure, the amplifier will still give 30W in 8 Ohms and 40 W in 4 Ohms - or in other words, perform like an ordinary amplifier of its size and price. This would keep you operational until you can get the unit serviced and the replacement of the bulbs is simple using a part that is commonly available for a few dollars.

S is for Simplicity

NAD's critically acclaimed PowerDrive circuitry, as used in the more expensive NAD amplifiers, has a highly sophisticated impedance, power and temperature sensing of the high voltage supply. While still a performance bargain, this adds cost that moves models like the C325BEE to the next retail price level. Figure B compares the parts required for PowerDrive-S™ and PowerDrive circuits.

Supply Rejection

Typically, using such a "soft" supply in an amplifier is very demanding on its ability to reject interference and distortion by power supply modulation and back electro-magnetic fields from the loudspeaker. As was the case with the 3020, careful engineering results in a supply rejection ratio that is very high (>75dB) across the audible frequency range ensuring tight bass and no measurable distortion from supply modulation. Furthermore, the idling current is extremely tightly stabilised and kept constant with temperature and supply voltage changes. This eliminates the major limiting factor for Class AB amplifiers: transient crossover-over distortion.

A better mousetrap at a better price too - Figure C compares key performance characteristics of the 3020 and the C315BEE. 30 years later and the C315BEE delivers more power, less distortion, better signal to noise and more real current. You would think all this would be more expensive? In fact, it is almost £100.00 less expensive when you compare the corrected inflation price. Now that is a bright idea!

Conclusion

Nearly thirty years ago, NAD's 3020 amplifier redefined performance and value by keeping things simple and to the point. As time progressed subsequent generations of NAD amplifiers continued to raise the ante for the competition using technological innovation to continually improve the really important aspects of amplifier performance. The NAD C315BEE with PowerDrive-S™ is sure to light up the life of music lovers on a tight budget. Yet again Bjorn Erik Edvardsen succeeds in building a better mousetrap at a bargain price!