Dynaco PAS-2/2X Preamplifier
Photo courtesy of James Giske.
The preamplifier essentially has five parts. The power supply is one part and is common to both channels. This power supply contains a full-wave rectifier and a power transformer. A separate rectifying system is used to provide direct current to heat the filaments of the tubes. This is one of the reasons why there is practically no hum in the Dynakit.
The remaining four sections have several functions. There are equalizer-amplifier stages: high-gain circuits designed to bring up the level of such devices as magnetic phonograph cartridges and at the same time to impose the proper playback characteristic on the frequency response. There is one of these stages for left and another for the right stereophonic channels. THe remaining two sections consist of the two amplifying-tone control stages, one for left and one for right channel. High level signals such as those from a radio tuner are fed through these stages, and the functions of volume control, tone control, and similar control functions are accomplished at these points in the circuit.
3 stereo low level: RIAA phono, NAB tape head (7-1/2"), "Special" (optional second phono, microphone, etc.).
4 stereo high level: FM-AM, FM-Multiplex, Spare, Tape.
Outputs: Tape output. Audio output.
Controls: Selector, Volume, Balance, Blend, Individual Treble and Bass for each channel, Tape Monitor, Loudness, Scratch Filter, Power.
Tone Control Range: ±14 dB at 20,000 cycles; ±20 dB at 20 cycles.
Distortion: Less than 0.05% intermodulation at 2 volts (sufficient output to drive any amplifier). Distortion does not increase at low settings of volume control. Harmonic distortion below the measurement capabilities of regular commercial grade test equipment.
Response: ±.5 dB 10 cps to 40 kc. Response not affected by position of volume control.
Transient Performance: Passes square waves without ringing and with minimum deformation from 20 cps to 20,000 cps. No overshoot or bounce on pulse type signals. Instantaneous recovery from overload.
Hum and Noise: 2 microvolt equivalent noise input on RIAA. 74 dB below level of 10 millivolt signal.
60 dB at 1000 cps on RIAA;
60 dB on Tape Head input.
20 dB on Radio and other high level inputs.
Output impedance 1,000 ohms.
Terminating impedance 100,000 ohms or higher.
Low level input impedance 50,000 ohms;
high level input impedance 250,000 ohms.
Tubes: 4 12AX7 (ECC-83); 1 12X4; 1 selenium stack.
Accessories: PK-30 aluminum knob and panel kit; $13.95
Dyna's first stereo preamp; similar to two PAM units with a common power supply. Later updated with improved tone control circuitry and high level stage mods to drive a wider variety of power amplifiers including the new solid state units starting to become popular. Ultimately received a cosmetic upgrade and was formally renamed to the PAS-3X.
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Copyright © 2000,2001 Greg Dunn