Someone asked in another thread why there would be interest in yet another big box amplifier. And by implication, why isn't there anything new as far as approaches to amplifications. That reminded me that I was long overdue to pen an article on the Mark Levinson No 53. I have had to explain what it does over and over again and thought it is best to put down my thoughts on the device. And here it is: Mark Levinson No 53 Audio Amplifier Review.
Please forgive the slight informercial aspects of it .
Here is the introduction to it:
The power amplifier has been around for decades. Countless companies manufacture them at all price points and performance levels. In this crowded field it is hard to stand out. But Mark Levinson No 53 does, literally from its vertical configuration and circuit design point of view.
In a traditional amplifier design, the output transistors are operated in the so called “linear” region where the output of the device is more or less proportional to the input. Unfortunately transistors are not very efficient when operated in this mode and as a result the total power output relative to the input is roughly 50% in practical implementations.
This is more of a problem in the US with our home electrical voltage being 120 as compared to other countries which use 240 volt power. This means the maximum amount of power we have is half as much. For low power electronics this is OK. But our high power stereo amplifiers can starve for power when the low frequency transients arrive in our music. There is only so much power that can be stored inside the power supply of the amplifier after which, the output power must reduce and with it, additional distortion created.
Efficiency also hurts us with respect to modern speaker designs. Today, the trend is toward more attractive speakers in diminutive enclosures. This reduces the footprint of the speaker letting it blend better into our home décor but has the down side that it takes more power to produce the same level of loudness than it used with the boxy designs of yesteryear.
The solution in the professional sound reinforcement industry (e.g. audio for live concert concerts) has been to use a newer class of amplifier with far more efficient design called switching or class D amplifier. The efficiency of switching amplifier comes from its name. The output transistors are not working proportional to the input signal as they are in traditional amplifiers. But rather, they are either on or off. This sharply reduces transistor losses as the device is not being asked to produce and intermediate output level.
But how do you amplify a signal by just jumping from extreme low to extreme high voltage? The answer has two parts. First, in the input stage the analog waveform is converted into a set of pulses whose width depends on level of the input signal. The louder the input, the more packed the square wave pulses and vice versa. The plus train is then used to drive the output transistors in switching mode as they toggle between zero and maximum voltage to get our high efficiency.
Alas, we can’t drive our speaker with that signal or we would destroy it in an instant. The square wave has infinite bandwidth and hence would instantly damage our tweeters. Fortunately we can get back to our original signal if we filter out the switching frequency before sending the output to the speaker. This is easier said than done because the output of the amplifier is high power and hence, we cannot use active filtering techniques (such is used in tone control or EQ). Instead, we have to resort to passive components such as used in speaker crossovers. Without going through the circuit design details, let's say it is challenging to create a passive filter that filters out the switching frequency but leaves the audio band perfectly flat. You see the ramification of this in response anomalies of some class D amplifiers that “ringing” at higher frequencies (frequency response has oscillations to it).
Output filter design becomes simpler if we can push out the switching frequency way higher than the audio band. That way, even a gentle filter that starts to roll off after the audio band will effectively suppress the switching frequency. Laws of physics interfere yet again because increased switching frequency can stress the output transistors and reduce their reliability or cause outright destruction. Designers attempt to balance the two factors and usually wind up with a switching frequency somewhere in the 300 KHz to 400 KHz range.
Read the rest here:Mark Levinson No 53 Audio Amplifier Review.
Please forgive the slight informercial aspects of it .
Here is the introduction to it:
The power amplifier has been around for decades. Countless companies manufacture them at all price points and performance levels. In this crowded field it is hard to stand out. But Mark Levinson No 53 does, literally from its vertical configuration and circuit design point of view.
In a traditional amplifier design, the output transistors are operated in the so called “linear” region where the output of the device is more or less proportional to the input. Unfortunately transistors are not very efficient when operated in this mode and as a result the total power output relative to the input is roughly 50% in practical implementations.
This is more of a problem in the US with our home electrical voltage being 120 as compared to other countries which use 240 volt power. This means the maximum amount of power we have is half as much. For low power electronics this is OK. But our high power stereo amplifiers can starve for power when the low frequency transients arrive in our music. There is only so much power that can be stored inside the power supply of the amplifier after which, the output power must reduce and with it, additional distortion created.
Efficiency also hurts us with respect to modern speaker designs. Today, the trend is toward more attractive speakers in diminutive enclosures. This reduces the footprint of the speaker letting it blend better into our home décor but has the down side that it takes more power to produce the same level of loudness than it used with the boxy designs of yesteryear.
The solution in the professional sound reinforcement industry (e.g. audio for live concert concerts) has been to use a newer class of amplifier with far more efficient design called switching or class D amplifier. The efficiency of switching amplifier comes from its name. The output transistors are not working proportional to the input signal as they are in traditional amplifiers. But rather, they are either on or off. This sharply reduces transistor losses as the device is not being asked to produce and intermediate output level.
But how do you amplify a signal by just jumping from extreme low to extreme high voltage? The answer has two parts. First, in the input stage the analog waveform is converted into a set of pulses whose width depends on level of the input signal. The louder the input, the more packed the square wave pulses and vice versa. The plus train is then used to drive the output transistors in switching mode as they toggle between zero and maximum voltage to get our high efficiency.
Alas, we can’t drive our speaker with that signal or we would destroy it in an instant. The square wave has infinite bandwidth and hence would instantly damage our tweeters. Fortunately we can get back to our original signal if we filter out the switching frequency before sending the output to the speaker. This is easier said than done because the output of the amplifier is high power and hence, we cannot use active filtering techniques (such is used in tone control or EQ). Instead, we have to resort to passive components such as used in speaker crossovers. Without going through the circuit design details, let's say it is challenging to create a passive filter that filters out the switching frequency but leaves the audio band perfectly flat. You see the ramification of this in response anomalies of some class D amplifiers that “ringing” at higher frequencies (frequency response has oscillations to it).
Output filter design becomes simpler if we can push out the switching frequency way higher than the audio band. That way, even a gentle filter that starts to roll off after the audio band will effectively suppress the switching frequency. Laws of physics interfere yet again because increased switching frequency can stress the output transistors and reduce their reliability or cause outright destruction. Designers attempt to balance the two factors and usually wind up with a switching frequency somewhere in the 300 KHz to 400 KHz range.
Read the rest here:Mark Levinson No 53 Audio Amplifier Review.