Articles

Stereo Explained!

Realizing that many of the pieces of equipment in our system are complicated and sometimes based on difficult to understand principles, we thought that it might be valuable for you to read an overview of some of the more popular issues and equipment used in audio video systems today.

What's inside a preamplifier?
A preamplifier can be broken down into its component sections quite easily. Typically, there is the input selection stage first. The input selection stage is usually nothing more that a number of input connectors, either RCA or Balanced (XLR), and a switch arrangement set up to choose one of the inputs (such as phono, tuner, tape, etc.). Once an input has been selected, it next goes to the volume and balance control.

These simple variable resistive elements (potentiometers or pots) vary the resistance the signal goes through thus making the level of the signal go higher or lower depending on the direction the control is turned.

If the preamp in question is a passive preamp (or 'pots in a box'), the signal is then routed either through an electronic buffer or straight out to a set of jacks to meet the outside world. This type of arrangement has no gain. In other words, whatever level of signal goes into the passive preamp is the highest level that can come out. even if the volume control is turned all the way up.

A standard preamplifier, though, has a gain stage. This can be either a tube gain stage or a transistor gain stage, and there are literally hundreds of different designs to accomplish this. These gain stages provide amplification of signal, so output signals can be stronger than the signal fed into the inputs, depending on the volume control settings.

The new GC series of preamps, amps and Control amplifiers are the world's first variable gain devices that change the gain of their amplification stage rather than attentuate the volume.

What's the difference between Class A and AB?
Most people understand that Class A runs hotter, generally sounds better, and is typically more expensive.

The term class A refers to biasing of the output stage (in a transistor power amp). Bias is an engineering term that actually means what it implies (i.e. Leaning in one direction). By that, we refer to the amount of current flowing through the output stage.

Class A is when both devices conduct at all times while Class B is when only one device is on at any instant. These are both limiting cases and there is thus a setting between these two limits where, for small signal levels, both devices conduct but for larger levels, only one conducts. This is termed Class AB.

Class AB is not a true class, but is a very common term applied to the biasing levels used in most audio amplifiers. The output stage is biased to carry a quiescent current significantly less than half the maximum output current, (as needed for full Class A) but sufficient to keep both devices running in Class A for small output signals. However as the output signal increases, the amplifier becomes Class B with one device cutting off on each half cycle. This biasing scheme effectively moves the linearity curves toward one another resulting in a transfer curve that is more linear as it passes through the origin. This scheme approaches the efficiency of class B yet offers class A distortion levels (nearly) especially for small output levels where class B suffers most.

It is particularly significant that the distortion is reduced for small signal levels, as it is at low levels that the human ear is most sensitive to distortion.

With the exception of Nelson Pass's (Pass Labs) amplifier that essentially has one 'sex' (polarity) of output

device handling the entire signal, all transistor amplifiers have (in essence) two output devices. One output device handles the positive (plus) half of the signal and the other handles the negative half of the signal. If you can envision such an arrangement, a possible problem might come to mind � the transition area. Like a relay runner handing off the baton to the next runner, the first half of the two-device output stage 'hands over' the signal to the second half of the output pair ;this transition is the problem. As the signal goes from the positive device, to the negative device there is a moment when neither device is handling the signal, creating a 'gap' in the music.

Class A biasing in its truest form means that there is a lot of current running through these two devices all the time. When you run current through the devices, it creates heat, and the more you run through the stage the greater the amount of heat. A true Class A amplifier runs the same amount of current through its output stage as it is expected to deliver to the load (the speaker). So, for instance, a 100 watt (rated) class A amplifier draws 200 watts per channel at all times (even when there is not a signal). When the amplifier is asked to deliver 100 watts of power to the loudspeaker, 100 watts (or half of the current) goes into the load (speaker) and the other half continues to go through the output stage.

One interesting fact is that under full power delivery conditions (when the amp is putting its full 100 watts into the speaker), the Class A power amp actually runs cooler. This is because only half of the power is being converted into heat in the amplifier, while the other half is busy driving the loudspeaker.

How does digital work?
We all understand that digital is made up of 1's and 0's. So, how does a 1 and a 0 become music?

First, let's consider how music is expressed in analog terms. When a microphone pickups up sound, it converts it into electricity or voltage. Voltage is what comes out of a battery. So, let us envision that we have a battery with 10 volts available, and that it is hooked up to a microphone (somehow). Let us further envision that when we speak or play music into this microphone, it takes the voltage out of the battery in small or great amounts. Small amounts when we whisper into the microphone, and large amounts when we yell. Therefore, the louder we speak into the microphone, the more voltage is produced out of it (amplitude).

Now, we have a rising and falling level of voltage (amplitude) out of our battery caused by any change in volume of speech into the microphone. Also, this voltage is turning on and off faster when we hit a high note (AC), slower when we hit a lower note, therefore the speed at which this voltage moves (frequency) is determined strictly by the pitch of one's voice.

Putting it another way, we have a voltage that its bigger and smaller with the level we speak (amplitude): those same voltages are turning on and off (AC) in rythym with one's speech; and the speed at wh

ich those voltages turn on and off (frequency) varies in direct proportion to the pitch of one's voice.

If we then took this varying voltage and plugged it into a power amplifier that was connected directly to a pair of loudspeakers, we would hear ourselves over those same speakers. And, if we put this varying voltage (the output of our microphone) into a tape recorder, it would record all of these changes in amplitude and frequency so we could play it back later.

OK. Let's go digital. In its simplest form, we're going to change the higher and lower voltages into numbers, and then record those numbers. That's basically it. The rising and falling voltages (amplitude) are electronically measured and those measurements converted to their numeric value, and that numeric value then recorded computer style.

The 1's and 0's you hear so much about are merely a counting scheme used to record bigger and smaller

numbers. How? By a counting method that is bonehead easy to understand. Let's use a simple 4 bit system to count, for example, and note the drawing below. There are 3 vertical lines and 16 horizontal lines. Along the top of our box (now full of squares) going in a horizontal direction we've marked #'s 1,2, 4, 8 (one number in each box). These 4 numbers (1,2,4,8) are the digital'meaning' or representation of the selected squares below. The squares below use 'X' and '0' to represent '1' and '0' that we hear so much about. Using only '1's' and '0's', let's start counting in binary fashion.

In the first horizontal row (below our written numbers) note 0,0,0,0 in each of the 4 horizontal squares. This represents zero.

The next row has three '0's' and one 'X'. Note the 'X' is in the same column as the digital representation for 1 (one).

If we need 3 (three), then we'll place an 'X' in both the '1' and the '2' column. The computer knows to add these two together, so we get '3'. Study the chart below and it will become obvious.

See how, using only 1's and 0's a computer can count? Instead of a 4 bit system that can only count 16 numbers, 16 and 20 bit and 24 bit systems can count numbers as in the millions.

What's a DAC?
Digital to Analog Converter. Simply stated, it is a separate device used to convert the digital signal, created

by a CD transport, into an analog signal. The first separate DAC used for High End Audio was invented by PS Audio. Later introductions were by Arcam (the black box) and Theta.

DAC's have four main component groups that make up their workings: a receiver that takes in the digital data and separates out the clock signal from the transport, the digital filter that gets rid of unwanted digital signals, the D to A processors where the actual conversion from digital to analog takes place, and the analog output stage that amplifies the converted signal and gets it ready for the outside world.

There are two types of DAC's, one bit and multi bit, with the standard being the multi-bit.

Long speaker cables, short interconnects?
Certainly the conventional wisdom is to keep the speaker cables as short as possible and the interconnects long enough to make up the difference.

This philosophy works well as long as the interconnects aren't longer than six feet (2 meters). Once the interconnects exceed this length, their capacitance becomes a factor (it increases with cable length), and will start to make some preamplifiers sound worse (not all).

You can make a simple experiment to find out if your long interconnects are affecting the preamp or not. Using the extra set of outputs found on most preamps (or even a Y connector if you have to), attach your interconnect cable to the preamp's output but do not terminate it (or plug it into anything on the other end). Let the cable hang in space. Listen to your system with the long interconnect attached in this fashion and with it not attached. If you hear no difference, then you're OK!

Balanced or TRUE balanced?
There's a lot of confusion when it comes to balanced inputs. You hear lots of terms, like true balanced, being bandied about. True balanced is not really an engineering term, but rather a marketing term.

Balanced simply means that there are two equal halves of a circuit operating on a waveform to amplify it. To view this easily, picture two independent amplification stages, tied together in parallel. One half of this two-part circuit operates on the positive half of the waveform and the other on the negative half.

But, having said that, there is one key difference that (strictly speaking) has to happen before we can truly call ourselves 'balanced'. That is an engineering term known as common mode rejection. When a circuit displays common mode rejection it means that any signal presented to it that is COMMON to both inputs is rejected.

Common Mode Rejection (CMR) means that if you have a balanced input on a preamp (for example), and if you put the same signal in both the + and the � inputs at the same time, no signal will appear at the output (balanced or XLR connectors have 3 wires inside them, a ground and a + wire ((just like an RCA connector)) with the addition of a � signal wire). Why is this important? Because, on an input, this feature will reject

noise. Any hum or high frequency noise that gets into an interconnect cable (for example) will be on both wires (or all three wires in the case of a balanced cable). When the signal goes into the preamp, anything common (like the noise) will be eliminated.

However, if we do the opposite, put in a signal that appears (out of phase) on both the + and � inputs simultaneously, the output of the amplifier or preamplifier will double (6 dB rise).

Further, when a circuit inside an amplifier is balanced, distortion components that are common to both halves of a circuit will be reduced or eliminated by this same phenomenon (CMR).

Not all products that suggest they have a balanced input actually have a proper one.

Balanced output on a power amp?
We used to call a solid state power amp with a 'balanced output' a bridged amp. Bridged amp means that there is no ground, but rather there are two power amps, one out of phase with the other. So, the + terminal of your power amp is the output of one amp and the � terminal of the power amp is the output of a second, and out of phase, amp. Bridged amps were taboo, because they have a lot of extra circuitry in the path (twice as much in fact), and they would blow up if connected to something with a

common ground (because you would be shorting to ground one of the power amps).

Do not confuse this with a balanced amp design, which is entirely different. See the section on balanced vs. true balanced.

Tube amps are almost always balanced on their outputs because they have an output transformer. Transformers are naturally balanced devices.

Is it better to leave equipment on or off?
Sonically, it's better to leave equipment on. The problem is tubes. While tube gear certainly sounds better after it's been on for a while, tube life will be shortened if it is left on all the time.

With solid state gear (transistor), it is best to leave it on all the time, but a lot of people are afraid to leave their power amps on for fear of something happening that might blow up their speakers or amp or both.

We recommend leaving everything BUT your power amp on.

Also, bear in mind, anything with a remote control is probably not off anyway. It certainly isn't COMPLETELY off; if it were, there would be no way for the signal from the remote control to be read.

Tubes: leave it off and warm up about 30 minutes before use. Transistors: leave it on always, unless it's a power amp.

Join our newsletter group and get insider information once a month featuring the latest articles and product information from PS Audio!

Can you receive HTML email? Yes No

Email Address: