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HOW TO SELECT THE RIGHT PREAMP
Oct 2004 by Steve Deckert
I’ve had many phone calls from potential customers having troubles trying to decide which preamp they should buy. It occurred to me that with 3 preamps of our own, this would be an ongoing question as more and more people find themselves in this position of choosing.
First let me say that the goal with the addition of any preamp is to achieve synergy between your source(s) and amplifier(s). Any time you have an opportunity to audition several preamps in your system, it goes without saying the one will usually sound and work better then the others. The reason one sounds better generally has a lot less to do with price and more to do with the circuit design of that preamp.
To select an ideal preamp you must know two things:
The output voltage of your source (most stock CD players are 2 volts)
The input sensitivity of your amplifier (most amplifiers are around 1 volt)
If your CD player or other source has an output level less then or greater then 2 volts, you need to know this. On the same note, if you power amplifier has an input sensitivity less then or greater then 1 volt, you need to know that too. Once you know for sure what your voltages are you are well on your way to picking the right preamp.
Lets look at these two terms so we understand exactly what they are.
The output voltage of your source is a constant level, and does not change unless your source has a "variable output". This 2 volts of signal (music) drives the input stage of your power amplifier or it drives the input stage of your preamp which in turn drives the input stage of your power amp.
The input sensitivity of an amplifier simply means how many volts are required to bring the amplifier to full power. Any amount of voltage beyond this figure will make your amplifier try to put out more power then it actually has, the result is called "clipping".
So it stands to reason that in all cases a preamp is used to control the voltage from the source. You can figure that when the preamp volume is all the way down, you have ZERO volts – hence no sound from the amplifier. As you turn the volume knob up the voltage increases as does the sound you hear from your amp. The ideal working range on a volume control should be somewhere between 1/4 and 3/4 of the way up to hit your normal listening level . This means your preamp would actually never be adding any voltage to it’s input signal – also known as "gain".
When do you want "gain" in a preamp? There are two reasons you might want gain in a preamp:
When your power amplifier needs more then 1 volt to come to full power.
Or when your source has less then 2 volts of output.
Some amplifiers can need up to 5 volts to bring them to full power, Decware amps are all set to reach full power with 2 volts. On the flip side I have seen amplifiers that needed only ½ volt to come to full power.
Some modified CD players could have less then 2 volts, and some DACs could also have less then 2 volts. (Although most DACs have at least 2 or slightly more volts – in some cases up to 5)
Vinyl lovers know that it can be sometimes difficult to find a cartridge and phono stage combination that they like with a full 2 volts of output. I’ve seen many with closer to 1 volt.
In the case where your source has say only 1 volt, and your amp has an input sensitivity of say 2 volts you will absolutely need a preamp with some gain, otherwise you’ll never be able to play your amplifier near as loud as it could go. Even if you have super high efficiency speakers and listen at very soft levels, your music will still suffer from a lack of dynamics and weight.
Once you make the determination that you either need gain or don’t need gain, you will have narrowed down your choices considerably. In either case, gain or no gain, there is one more thing to consider and that is called "impedance".
All sources and preamps have what is called "output impedance". Think of it as the units ability to drive difficult loads, like might be experienced with super long interconnects, or obnoxious amplifiers.
On the flip side, all amplifiers have what is called "input impedance". Think of it as the difficulty level the amplifier imposes on the preamp or CD player.
As a general rule, the lower the output impedance is, the better it will drive difficult loads. And, the higher the input impedance is the less difficult it is to drive. A good example of a happy situation is a preamp with an output impedance below 1000 ohms driving an amplifier with an input impedance of 100,000 ohms.
Most stock CD players have a fairly low output impedance, but unfortunately power amplifiers are all over the place, ranging between 10,000 ohms and 500,000 ohms. The most common figure is around 50Kohms.
50Kohms is a load that most sources and preamps can drive without problems. The exception might be when a preamp with a fairly high output impedance trys to drive a 50Kohm amplifier through extra long interconnects. The result can often be either rolled off bass response or lack of dynamics or both. You don’t want it to sound thin, so you run short interconnects. If the preamp had an advertised low output impedance then it wouldn’t matter if the power amp was clear on the other side of the house, it would still have solid dynamics and weight without rolled off bass response.
Of our 3 preamps, the top of the line model, ZTPRE, has either lots of gain or no gain at all and either a normal output impedance or low output impedance depending on how you set the switches on the back. This ensures it will work with anything and always sound right.
The ZSLA-1 model has no gain, and a low output impedance making it ideal for most systems.
The SE84CSP model has lots of gain making it ideal for sources with lower output, or amps with higher input sensitivities (like ours) or both.
