Reality Check Preamp
     Before moving on, I recommend building a reality check preamp. If several hundred dollars worth of expensive parts and ten hours of frustration and work cannot beat the sonics from a twenty dollar IC-based preamp, then we need to go no further. The schematic below shows a preamp I built back in the 80s. It offers 40 dB of gain and is very quiet. I was able to stuff all of the parts and four 9-volt batteries (dual mono power supplies) in a small aluminum box that sat underneath my turntable. The input leads were hard wired in place and only a foot in length. I used a four-pole rotary switch to turn on the unit. Battery life was easily 20-50 hours, which meant weeks of listening; Costco sold a brick of batteries (25) for under $10 back then, which would last many years. (Yet friends who owned $1000 phono cartridges that were only good for 700 hours of use looked troubled by the expense of replacing the batteries!)   

    Tube CAD does the hard math for you. This program covers 13 types of tube circuits, each one divided into four variations: 52 circuits in all. Tube CAD calculates the noteworthy results, such as gain, phase, output impedance, low frequency cutoff, PSRR, bias voltage, plate and load resistor heat dissipations. Which tube gives the most gain? Tube CAD's scenario comparison feature shows which tube wins.

Windows 95/98/Me/NT/2000/XP

     The circuit can be improved by adding negative pull-down resistors to the IC outputs, but at the cost of less battery life or by using a pair of batteries per Op-Amp, thereby doubling the battery life. Now, do not get me wrong; this circuit is not the best phono preamp in the world, far from it. However, whatever tube-based preamp we do build must decisively beat it or we are wasting our money and time.

Next Time
      The second half of this article will cover my preferred equalization method: passive equalization and a few hybrid topologies.

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