1. What makes a digital amp (class D) digital?

1. What makes a digital amp (class D) digital?

Hi Gary,

Hope you had a good Thanksgiving and that the business is going well. I'm continuing to love the 800! Thanks again for the great service when I had those early minor problems.

I have a quick question that a number of bass players and I have been discussing. If you have time to answer this great... if you are totally swamped, don't worry about it! The question is.... what makes a digital amp (class D) digital? I always thought it was the switching power supply like in the Walter Woods and Stewart amps. However, based on your website explanation, the 800 is a digital amp with what sounds like a traditional analog power supply? I can't quite figure it out from the technical write-up. You guys talk about 'linear' versus 'switch mode' power supplies. I assume Walter Woods is 'switch mode' and the 800 is linear?

Anyway, we surely aren't losing sleep over this, but if the question makes sense and is easily answered, I'll share your response with my gear-head colleagues.

Thanks again,

Ken

Hello Ken:

Thanks for your interest and support. v The iAmp800 has a conventional linear power supply using a toroidal power transformer. There is nothing unusual about the power supply other than it has a generous amount of filter capacitance and current reserve which accounts for the solid low end response and good transient response. Other companies skimp on the power supply capacitors and transformer design to save money. There is no substitute for good capacitors and a good transformer design. I design the transformers specifically for the EA products.
The amplifier section of the 800 is where the magic lies. The circuit is more accurately called a "switching" or "pulse width modulated" amplifier because the audio is modulated on a square wave signal and then extracted later at higher power. Because the square waves are reproduced at the output transistors, and square waves are "off" or "on" in general, the term "digital" has been adopted by the industry because of the "off" and "on" nature of computer bits. The Class D amp has nothing to do with computers; rather, the square waves used by the switching amp only resemble the clock waveforms found in computers, hence the term digital.

Because the output transistors of a switching amplifier are generally off as much as they are on, only a small amount of heat is generated. This is the beauty of the design. Plus, the switching nature of the design makes for a circuit that is 85-90% efficient. So, to make 800 watts of RMS power to a speaker, the power supply has to make about 950 watts, and only 125 watts is lost due to heat. To get the same power to the speaker using a linear amp which has about a 50% efficiency, you have to generate 1600 watts in the power supply, 50% of which is lost due to heat. That is why linear amps get so hot and have cooling fans. This efficiency increase is a big deal, and only now are players starting to understand the advantages of Class D designs.

The switching amp approach is probably 50-60 years old. However, when the technique was first discovered, there were no semiconductors available which could switch at a fast enough speed to give good audio quality. Even the second and third generation products were only good for subwoofer applications where you didn't need high bandwidth. The 800 switches at 150Khz and gives good bandwidth response to 20Khz. We can make it go as high as you want, but for MI applications, there is no reason to stress the components. Plus, the human ear can only hear to about 13Khz anyway. The reason why EA amplifiers sound so good is a secret. And, I'm not about to reveal it.
If you wanted to get rid of the big power transformer, you could use a switching power supply which works in a similar manner. Switching power supplies modulate the power line voltage much like the switching amplifiers modulate the audio signal. However, because the currents and voltages are so high, a switching power supply to replace a power transformer would be very expensive. You could probably get the 800 down to about 6 pounds total if you had a big enough bankroll. As it is at 18 pounds, it still gives you a great performance to weight ratio.

I am not that familiar with the Walter Woods or Stewart products. However, I understand that Walter uses a very clever switching power supply coupled to a linear amplifier. I know Stewart has been marketing amps with switching power supplies for quite a while.

So, there are switching power supplies and switching amplifiers. And there are linear power supplies and linear amplifiers. We hook a linear power supply to a switching amplifier. Walter hooks a switching power supply to a linear amplifier. You can use whatever combination you want depending on your goal.

I hope this answers your questions and I have not bored you. Thanks for your continued support.

Gary, EA, Inc.