Dynaco Stereo 150 (1966–1972)
Seventy-five watts of MOSFET power in a 33-pound chassis. Dynaco's bridge between the tube era and solid state.
Overview
The Dynaco Stereo 150 weighs 33 pounds and is 17 inches wide. It is a solid-state stereo power amplifier that Dynaco introduced in 1966, and it was built to do something the company's tube amps couldn't: deliver serious, sustained power into speakers that demanded it. It is rated at 75 watts RMS per channel into 8 ohms from 20Hz to 20kHz with both channels driven. It was the amp you bought when your AR-3as or KLH-9s needed more than the ST-70 could give them.
In kit form the Stereo 150 cost $249, or $349 assembled. Another source says the kit cost $199 when introduced in 1966. Either way it was a significant step up from the ST-70, but still well below what comparable power from other manufacturers would have cost. The kit option was central to Dynaco's identity, and the Stereo 150 was designed to be buildable by the reasonably handy enthusiast. Dynaco's own advertising called the kit "easy as A B C" and said the printed circuit boards took the worry out of construction. Audio (April 1976, p.61) ran the pitch that anyone who'd been scared off stereo kits could handle this one.
What made the Stereo 150 genuinely interesting, though, was what was inside. This wasn't a conventional transistor amplifier. Dynaco used Hitachi 2SK135/2SJ50 MOSFETs in the output stage, a choice that gave the amp a character closer to tubes than most solid-state designs of the era. The result was a big, warm midrange and a top end that didn't get harsh when you pushed it. Owners report it sounds musical in a way that many of its transistor contemporaries didn't manage.
Key Features
Hitachi 2SK135/2SJ50 MOSFET Output Stage
The heart of the Stereo 150 is its complementary symmetry output stage built around Hitachi's 2SK135 and 2SJ50 MOSFETs. This was unusual for 1966 — most solid-state amps of the period used bipolar transistors, which had a tendency toward harsh clipping and thermal instability. The MOSFETs gave Dynaco something closer to tube behavior: a gentler overload characteristic and a midrange that owners describe as warm and musical. The trade-off, as collectors now know all too well, is that those Hitachi parts are long obsolete and genuinely hard to find. When they fail, they're expensive to replace.
Thermal Tracking Bias Circuit
Dynaco gave the Stereo 150 a thermal tracking bias circuit designed to hold correct bias without adjustment. That was a convenience when many solid-state amps needed periodic fiddling. The circuit senses temperature and adjusts bias to keep the output stage where it belongs. In practice, this circuit is the amp's known weak point. If it drifts, the output MOSFETs can fail in a cascade that's both spectacular and expensive. Owners who know the amp check bias periodically with a multimeter per the service manual. It's a simple procedure that can save the output stage.
Direct-Coupled, Transformerless Output
The Stereo 150 has no output transformers — the output stage is direct-coupled, eliminating coupling capacitors from the signal path. This was a genuine advantage over tube designs, which required output transformers that could color the sound and rolled off at the frequency extremes. The direct-coupled approach gave the Stereo 150 a cleaner, more extended response, with frequency response rated at 20Hz–20kHz ±0.5dB. (Some sources list a wider 5Hz–60kHz figure, which may reflect a different measurement condition, but the 20Hz–20kHz spec is the one that appears consistently.)
Power Supply and Heat Management
The Stereo 150 runs on a 70V rail with a capacitive filter. There is no regulated supply for the output stage. That keeps the design simple and the cost down. To handle the heat, Dynaco fitted what its advertising called "the biggest heat sinks per watt" - High Fidelity (April 1976, p.123) repeated that claim in its ad copy, and it's not an exaggeration. The amp runs warm but manageable. The fan is another story. It's noisy, and many owners replace it with a quieter unit.
Historical Context
The Stereo 150 was introduced in 1966 as a companion to the Dynaco PAS-3 preamplifier, and it was explicitly marketed as a high-power solid-state replacement for the ST-70 tube amplifier. The design came from David Hafler and Ed Laurent, the same team that had built Dynaco's reputation with affordable, high-performance kits. The amp sat above the Stereo 120 in the lineup and below the Stereo 400, giving Dynaco a solid-state ladder that owners could climb as their speaker demands grew.
It was designed to drive loudspeakers with low impedance and high power demands. The AR-3a and KLH-9 are the names that come up repeatedly in the documentation. These were speakers that could eat tube amps for breakfast, and the Stereo 150 was Dynaco's answer. It replaced the ST-70 in the lineup, a move many Dynaco loyalists initially resisted. The MOSFET output stage gives it a character closer to the tube sound than most of its solid-state peers, which is why collectors seek it out today.
Collectibility & Value
Working Stereo 150s currently trade in the $400-$700 range, and prices have been creeping up as the Hitachi MOSFETs become scarcer. The amp has a following among collectors who like its hybrid character - solid-state convenience with a tube-like midrange. A well-maintained unit, owners report, outperforms many modern amplifiers, which keeps demand steady.
The caveats are real. The bias circuit is the amp's weak spot, and if it drifts, the output MOSFETs can fail. Replacement Hitachi 2SK135/2SJ50 parts are obsolete and expensive when you can find them at all. The fan is noisy and often gets swapped for a quieter unit. None of this is a dealbreaker for someone who's handy with a multimeter. Check the bias before you trust the amp, and budget for the possibility of MOSFET replacement down the road.
The Stereo 150 sits between Dynaco's tube past and its solid-state future. It's not the easiest amp to own.
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