Preamplifiers (page 2)

As I described previously, my experience with preamplifiers is they degrade over time. They eventually had to be replaced because the switches and volume controls developed noise and problems. Over time, there was sound degradation that I didn't notice. It was sort of like a light bulb loosing its brightness over time. I didn't notice the degradation until I replaced the preamp. The new preamplifier had far more life and sounded more detailed.

Actually, it was not so much that the new preamplifier was doing anything than rather it wasn't doing anything. The new preamplifier was not adding noise or distortion. At this time, I also acquired the highly respected Plinius M16 preamplifier. The Plinius preamplifier has been praised in several international magazines.

It is quite possible to build a preamplifier simply buy buying some connectors, op amps, selector, volume control and power supply. In fact, these are the basic components for preamplifiers whether they are cheap or expensive.

It is quite possible to hook a CD player or other audio device directly to an amplifier. The problem is that without some way of attenuating the signal, there is only one volume-loud. You could use the headphone output jack from a portable amplifier and run it directly to an amplifier and use the volume control for the headphone. In this way, you wouldn't need any other. A nice office system might be a pair of nOrh 4.0s, a small amplifier (such as the ZEN Triode Amplifier from DECWARE) and a portable CD player. In fact, the some Portable CD players have FM tuners. Such a system would cost about $600 but would have very high quality sound.

Most people have more than one audio device. Multiple device require that you must be able to switch from one device to another. The switch is a very likely point of failure. The contacts can wear or become dirty. A few switches are available that have gold contacts but these are usually quite expensive.

Remember the voltage from the music source has to pass through the connecting wire, past the volume control then through the switch and out to the amplifier. In a passive system, the signal is simply passed through with no additional amplification. Receivers, some integrated amplifiers and some preamps also have facilities for tone manipulation. Most high-end preamplifiers do not have facilities for tone manipulation because most audiophiles prefer the sound be as authentic as possible. Some people might feel they can compensate for room weaknesses or other system deficiencies with their tone controls. The truth is, they can't. Any accurate correction would require an equalizer with the ability to measure the before and after effects from equalization.

The least expensive switches are the push button type. You can buy very inexpensive switch boxes from places like Radio Shack that allow you to switch between multiple inputs and outputs. My first Preamplifier was a kit made by Hafler. It cost about $200.00. It used this type of switch. At $200, the Hafler was certainly a good value but it was very prone to getting noisy.

How is it possible that some preamplifiers can be so horribly expensive?

The volume control is the first component in the audio chain. It therefore is the first component to add a sonic character to the overall system. The technical name for a volume control is an attenuator. There are two types. The most common is a potentiometer and the second is a switched attenuator.

The least expensive potentiometer (POT) uses carbon. Basically the POT is a variable resistor that increases or decreases the amount of resistance. The problem is that for stereo, the POT has to track two channels at the same time. Common POTs don't track all that well. Worse yet, the technology used to build POTs hasn't changed much in the past fifty years. Many are riveted together and can easily get out of alignment.

Many high quality preamplifiers use the ALPS BLUE. It is a very good plastic POT that sells for about $12.50.

The best volume control is a switched attenuator Switched attenuator are very expensive (when compared to POTs). Switched volume controls used precision resistors in a stair step arrangement so that both channels track precisely. Probably the best volume controls in the world are made by DACT (Danish Audio ConnecT). I used one of these in my Chinese preamplifier to replace the ALPS BLUE that came with the unit. How does it sound? It doesn't sound at all. It attenuates the signal without adding any character of its own.

The cost of a DACT volume control is about $185.00 USD. The unit is made primarily for individuals upgrading
equipment as virtually no manufacturers would consider using such an expensive volume control. Our partner here in Bangkok, audioform, uses the DACT volume control in the 5i integrated integrated amplifier.

Before the volume control, the signal must pass through a switch--the input selector. Once again, most companies will use rather inexpensive switches. Switches can add their own sonic signature and degrade over time. There are differing grades of switches available for switching audio sources. ALPs is once again the source for one of the better switches. The ALPs has a provision that allows the switch to be cleaned simply by turning it back and forth. The best switches are made by DACT.

Another approach is to use relays. Each device switched requires two relays for stereo, to switch signal and ground. Each relay costs about $8.00 or $16 per channel. Assuming you are going to switch seven devices, the cost for only the relays is $112.00. Of course, if you are using relays, you will need a power supply to drive the relays. It is unlikely relays would be used in a passive system as you have the expense of the power supply already.

The next thing we need is some form of preamplification. Three options include tubes, transistors and integrated circuits. Virtually any source of audio will pass through an op amp. The voltage from your CD most likely is generated by an op amp. Op amps have continuously improved to the point that they sound nearly the same as straight wire. Bipolar op amps are probably best for preamplifiers. Some of the best op amps are made by Burr Brown. Mark Levenson and other very expensive preamplifiers use Burr Brown op amps.

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