PAT 4 Preamp Tech Specs

“The PAT-4 was Dynaco’s first solid state preamplifier. Like the tube-based PAS-3 that preceded it, the solid state PAT-4 circuit was very simple, consisting of two discreet transistors in both the phono preamplifier and the line amplifier (per channel). The line amp included bass and treble controls. Although there have been many modifications offered for Dynaco tube equipment over the years, the PAT-4 has been largely ignored. Most upgrades for the PAT-4 focused on power supply and individual component changes while leaving the audio circuits themselves relatively unchanged. This upgrade takes advantage of the advances in semiconductors since the PAT-4 was introduced, utilizing a low noise op amp specially designed for audio applications — the LME49740. The LME49740 combines four op amps into a single package which makes for more efficient use of the space available on the circuit board.
The phono preamplifier circuit design is straightforward. There are two op amp gain stages, with the first feeding a passive composite RIAA filter. The output of the filter is then fed to the second gain stage. Because the passive RIAA filter attenuates the signal by a factor of 10 (i.e., has an insertion loss of 20 dB), the two gain stages must provide a total gain of x1,000 (60 dB) in order to achieve the desired overall gain of x100 (40 dB). The op amp, with its extremely high open loop gain, is ideal for this type of application. Gain at 1 KHz is nominally x100 (40 dB), and RIAA playback equalization is accurate to within +/- 0.01 dB across the 20 Hz to 20 KHz audio band.
The tone control line amplifier (TCLA) retains the tone control functions of the original PAT-4 but implements them using the time-tested active Baxandall circuit. There are four stages in the TCLA, two of which are buffers which isolate the frequency sensitive stages from the gain stages. The first stage provides all of the gain for the circuit, which has been set at x4 (12 dB). This value was chosen as a reasonable compromise for modern sources which can be expected to deliver inputs on the order of 1/4 to 1 volt. This will allow the TCLA to drive nearly any power amplifier to full output. The second stage is a buffer that provides a low impedance source for the active Baxandall circuit which follows. The third (Baxandall) stage is a unity gain feedback circuit which provides the tone control functions. This circuit is designed to offer flat response when the controls are centered and provides up to 12 dB of boost and cut at the audio frequency extremes. This range is sufficient to compensate for most recording deficiencies and/or room acoustics. The final buffer stage provides a low impedance source to the power amplifier or other device which follows to minimize interactions with that device which might affect the frequency response of the TCLA.
Although this board is designed as a direct replacement for the Dynaco PAT-4, its performance exceeds other solid state designs of its era including the popular Hafler DH-100 control amplifier. It rivals modern audio equipment costing thousands of dollars and there is no chassis modification needed. The board comes with hardware, a wiring diagram, four tone controls (with center detente), a volume control, and email support. A bare board is available for $59.95 + shipping. (Including a parts list)