Dynaco PAT-4 Preamplifier (1967)

Name: Dynaco PAT-4
Brand: Dynaco

Dynaco's first solid-state preamp: eight transistors, a 600-ohm output built for long cable runs, and a phono stage that punched far above its price.

Overview

The PAT-4 was Dynaco's first solid-state preamplifier, introduced in 1967. It was developed as the second series of the company's solid-state line, following the Stereo 120 power amplifier, and it was the companion preamp to that amp. Dynaco's own documentation calls it an all-silicon solid-state control center that must be used with a basic power amplifier - the Stereo 80 or Stereo 120 - and a pair of loudspeakers. The manual is plain about the design intent. Distortion and noise, up to levels well beyond those required to drive any conventional amplifier, are said to be comparable to the finest tube designs and below what commercial-grade test equipment can accurately measure. That was a bold claim for a company whose reputation rested on tube gear.

The circuit is lean. As few as four transistors per channel satisfy the required function, and the coloring of reproduced sound is suppressed by refinement of the circuit rather than by complexity. Each channel uses two pairs of npn transistors in similar configurations. One pair handles low-level amplification and equalization for phono cartridges, tape heads, and microphones. The other comprises the tone control and filter stages. The input transistors are selected low-noise types. The low-level phono input accepts signals up to 100 millivolts without overload, and the high-level phono input takes up to 800 millivolts. Both pairs use DC feedback loops to stabilize operating conditions and AC feedback loops to optimize audio performance. The full semiconductor complement is eight transistors and two diodes.

The output impedance is 600 ohms, which lets the PAT-4 drive long output cables and makes it relatively noncritical of load impedance. Specifications are based on a load of 10,000 ohms or higher, which covers virtually every tube or transistorized amplifier of the era. With a lower-impedance load there's some reduction in the maximum output level obtainable with low distortion. The output stage can deliver 10 volts into high impedance or 5 volts into 600 ohms.

The frequency response numbers were impressive for 1967. High-level inputs are rated ±0.5 dB from 10 Hz to 100 kHz. The low-level inputs are ±1 dB from 20 Hz to 20 kHz, equalized. Total harmonic distortion at rated 2-volt output is less than 0.05% from 20 Hz to 20 kHz, and intermodulation distortion is less than 0.05% with any combination of test frequencies. Hum and noise sit 70 dB below a 10 mV magnetic phono input signal, and 85 dB below a 0.5 volt high-level input signal. Gain is 54 dB at 1000 Hz for magnetic phono and 20 dB for high-level inputs. Dynaco's manual says the etched circuit modules were pretested in-circuit so that every unit, after assembly, would meet specifications normally associated only with laboratory prototypes.

The PAT-4 came out during the transition from tube to transistor technology, and the fact sheet describes it as exceptional performance at a fraction of the cost of competing brands. Its affordability and fidelity made it a cornerstone of what collectors call the "Dynaco Sound" system, typically paired with the ST-70 tube amplifier or the solid-state ST-120. It was a key component in many high-fidelity systems of its era, and it played a real role in bringing high-fidelity audio to more buyers during the late 1960s and 1970s.

Key Features

High-Level Inputs Flat to 100 kHz

The ±0.5 dB high-level response from 10 Hz to 100 kHz is the headline measurement, and it separates the PAT-4 from the pack of mid-1960s preamps. Most competitors of the era were content with 20 Hz to 20 kHz. Extending flat response an octave and a half past the audio band keeps the phase behavior in the audible range clean, so the preamp isn't introducing the kind of phase shift that smears transients. The phono section is more modest on paper at ±1 dB from 20 Hz to 20 kHz, but that's the equalized figure, and it's the one that applies to vinyl playback.

600-Ohm Output Stage

The 600-ohm output impedance is a practical choice. A preamp with a 600-ohm output can drive shielded cable of any length up to 50 feet, according to the manual. That's why Dynaco supplied two 6-foot audio cables with the unit. The manual also notes that the PAT-4 can be used at 600-ohm load, so it's designed to withstand professional use. For a consumer preamp of 1967, that's an unusual spec.

Feedback Tone Controls with True Flat Center

The tone control system is an exclusive Dynaco development, and it's clever. When the controls are set to the normal mid-point of rotation, they are effectively out of the circuit and have no effect whatsoever on performance. That "center-flat" setting comes from special Dynaco-designed potentiometers. The treble controls have a discontinuous taper, and the bass controls use a dual wiper design. When operated away from center, the frequency response is varied by changes in the amount of feedback at the frequency extremes, rather than by inserting lossy filter networks into the signal path. The bass and treble controls are a double-channel type that can operate the left and right channels separately. Tone control range is ±16 dB at 50 Hz and ±12 dB at 10 kHz. The manual says the tone control circuit is designed to have a completely flat characteristic at the midpoint.

Three-Stage High-Cut Filter

The high-cut filter is switchable in three stages at 15 kHz, 10 kHz, and 7 kHz, and it sits at the output of the preamplifier. Dynaco's marketing calls it a "highly effective sharp-cutoff treble filter." It's designed to get rid of noise and distortion from imperfect program material, which in 1967 meant worn records, marginal FM reception, and the general noise floor of source material. The manual includes 100 Hz and 10 kHz square-wave and tone-burst test results showing that the tone bursts at the output of the PAT-4 are indistinguishable from the generator output. That's a claim about transient behavior, and it's a notable one for a feedback-based design.

Blend Circuitry for Four-Speaker Playback

The PAT-4 incorporates special blend circuitry that facilitates four-dimensional playback with any two-channel amplifier and the requisite speaker systems. The system uses sum and difference matrixing to recover ambience information, which the manual calls "hall sound," from two-channel recordings. The back speaker reproduces the difference information in the two channels. That recovers random-phase ambience effects, most evident in live recordings where a distant microphone was used. No additional electronics are needed, and the manual describes the connection of four speaker systems. Best results occur when all speakers have identical efficiency, impedance, and sonic characteristics, since separate level controls aren't feasible. The manual notes that small rooms frequently benefit most, and that the back speaker is best placed above the listener at moderate level.

Headphone Output and Front-Panel Jack

The PAT-4 provides sufficient power for a pair of 600-ohm headphones, and when headphones only are to be used, a power amplifier isn't required. The front-panel output jack provides a normal 600-ohm output in parallel with amplifier Output 1 on the back panel. Connecting to it mutes Output 2 on the rear, so inserting headphones automatically cuts out the speakers connected to that output. Headphones should be medium impedance, nominally 600 ohms or higher. Low-impedance 4-to-16-ohm headphones need a matching transformer to preserve high-fidelity results. One quirk: because of the load imposed when headphones are connected, the high filter switch operation is altered, and in effect only the 15 kHz position has any marked effect, albeit at a lower frequency.

Plug-In Circuit Boards

The PAT-4 used plug-in circuit boards for easy servicing and upgrades, a novel concept at the time. The etched circuit modules were pretested in-circuit to ensure every unit would meet spec after assembly. The modular approach is the reason so many PAT-4s are still running today, since a failed board can be swapped rather than traced. The manual's service section notes that the components are of the highest quality to protect against failure, and that all parts are operated conservatively with close tolerances. The design will deliver unchanging performance with the maximum variations in AC line voltage - 110 to 130 volts - permitted in normal use.

Flexible Input and Output Complement

Inputs cover Mag (Hi, Low, one system each), Ceramic, Tape head, Tape amp, Tuner, Spare (2 sets), and Special (Mic, Phono 2). Outputs include two sets for audio, one set switched, plus a front-panel output and a tape recording output. The front panel has a spare input terminal for connecting a tape deck, microphone, or similar source, which was unusual for a consumer preamp of the period. The manual says thoughtful planning has provided exceptional flexibility to accommodate future requirements for such sources as a second phonograph or a microphone, television sound, or musical instruments requiring electronic amplification.

Historical Context

The PAT-4 was developed as the second series of Dynaco's solid-state amplifier line following the Stereo 120 power amplifier. It inherited the advantages of the PAS-3X and was designed with more functions. It was Dynaco's first solid-state preamplifier, and it arrived as the consumer hi-fi market was moving from tubes to transistors. It was one of the first widely adopted solid-state preamplifiers in that market.

Dynaco's kits allowed audiophiles to assemble their own gear, which cut costs and fed a DIY culture that the PAT-4 exemplified. The factory-assembled PAT-4/A carried a one-year warranty. Kit parts were warranted against manufacturing defects for one year, with defective parts replaced at no charge. After the warranty period, Dynaco would service a completed PAT-4 for a maximum charge of $10.00, plus the cost of parts after one year. Factory service wasn't available for kits that were incompletely wired, wired with other than rosin-core solder, or physically or electrically modified without prior authorization.

The Stereophile review of the PAT-4 was first published December 1, 1968, with the review system using a Grado Model A phono cartridge and Electro-Voice Model Two and Model Six loudspeakers. Production is generally given as 1967 to 1972, though some sources tie pricing to around 1970 and around 1973, so the end year should be treated as approximate. Audio (April 1970, p.39) called the PAT-4 unsurpassed and unmatched at anywhere near its low price. Original Japanese-market pricing was ¥54,000 around 1970 and ¥52,000 around 1973. A wooden case common to the PAT-4 and PAS-3X was sold separately at ¥3,900 circa 1970. No US MSRP appears in the available documentation.

By 1989, the PAT-4 was still interesting enough to warrant an upgrade article, and the aftermarket has kept supporting it. A Phono Preamp Upgrade Kit increases signal-to-noise ratio by between 4 and 8 dB and brings the phono section to the next level. The upgrade requires the PAT4PWR power supply to be installed first.

Collectibility & Value

The PAT-4 carries a collectibility rating of 7/10. Unmodified working units typically trade in the $150-$250 range, while fully restored or upgraded examples command $250-$300. Original kit form with documentation carries a premium for collectors, and units that have been professionally recapped and tested command higher prices. Original packaging and manuals also increase desirability. Dealer listings have been seen as high as $575, and Reverb listings in the $339 range turn up periodically, though those represent the top of the market rather than the norm. eBay listings range from a $400 kit-to-assembler example down to $29.95 for a single Hi Filter control selector switch, with a Stereo 120 and PAT-4 combination listed at $175.

Common issues are well understood. Electrolytic capacitors degrade over time, leading to hum, distortion, or channel imbalance, and recapping is often necessary. Early germanium and silicon transistors can drift or fail, and testing with replacement by modern equivalents may be needed. The volume control and input selector switches are prone to noise. Contact cleaner can help, but replacement may be required. Some units develop DC voltage at the output, which can damage speakers if connected directly, so check for DC offset before use. Poor grounding or shielded cable issues can introduce hum, especially in modified units. One specific service note: C16 sits right next to R20, which runs quite warm, so the fix is to offset C16 and extend one leg.

Maintenance advice is straightforward. Replace all electrolytic and coupling capacitors to restore performance and reliability. Clean potentiometers and switches with high-quality contact cleaner. Test all transistors and replace any out-of-spec units. Verify no DC voltage is present at the output jacks. The PAT-4 was designed for Dynaco power amps, but it works well with most line-level inputs, though caution is warranted with sensitive equipment.

Owners report that these units have held up well and that they're very moddable and repairable. The original design is sound, the failure modes are well documented, and there's an active upgrade community. The PAT-4 isn't rare, it isn't exotic, and it isn't expensive.