Adcom GFA-1A
The Adcom GFA-1A is a stereo power amplifier that replaced the earlier GFA-1 and came with a number of circuit refinements. It is rated at 200 watts per channel into 8 ohms (both channels driven, 20 Hz-20 kHz) and 300-340 watts into 4 ohms. It was surprisingly compact for its power class at 17 inches wide, 14 inches deep and 7¼ inches tall, and it weighs 55 pounds.
The amp uses a toroidal power transformer, which was unusual for the period, and a fully complementary "balanced bridge" topology. Each channel has two power amplifiers in a bridge, with the speaker connected across them rather than to ground. The output stage runs in Class A at low levels and moves to Class AB at higher output. That design generates a lot of heat. Twelve output transistors (MJ15015/16, six per channel) handle the current, and a fan provides forced-air cooling.
Compared to the GFA-1, the GFA-1A has a separate driver board, larger filter capacitors and higher-rated output transistors. These changes made it run cooler than its predecessor, though it still runs hot by contemporary standards because of its high bias. Adcom positioned it as a high-power amplifier at an accessible price. It earned a reputation for clean, neutral sound and the ability to drive difficult speaker loads. Adcom's advertising in Audio (January 1981, p.13) pushed the value philosophy behind the GFA series, and the GFA-1A later became known among enthusiasts as the "poor man's Krell."
On the used market, prices typically range from $150 to $400 depending on condition. Common problems are failing solder joints on the output transistor emitter resistors, dried-out thermal compound, and noisy or dead fans. All of it is repairable with basic effort. The MJ15015/16 output transistors are still available, as are replacement capacitors from Nichicon and Panasonic.
Known Specifications
- Power output: 200 W/ch RMS into 8 Ω (both channels driven, 20 Hz–20 kHz); 300–340 W/ch into 4 Ω
- THD: less than 0.05%
- IM distortion: less than 0.05%
- Damping factor: 1,000 (8 Ω, 20 Hz–20 kHz)
- Input sensitivity: 1.5 V for rated output
- Frequency response: DC–100 kHz (−3 dB); ±0.25 dB from 20 Hz–20 kHz
- S/N ratio: 100 dB (IHF A-weighted)
- Power consumption: 900 W
- Dimensions: 17″ W × 7¼″ H × 14″ D
- Weight: 55 lb (25 kg)
eBay Listings
As an eBay Partner, we earn from qualifying purchases. This helps support our independent vintage technology research.
Related Models
- Adcom GFA-1 (1979)
- Adcom GFA-2
- Adcom GFA-545 II (1991-94)
- Adcom GFA-555 II
- Eico 3150 (1967)
- Fisher CUSTOM-ELECTRA-I (K-14/K-15) (1957)
- Fisher CUSTOM-ELECTRA IV (E33/E44) (1960)
- Akai AM-2400 (1978)
- Akai AM-2600 (1977)
- Akai AM-2650 (1975)