JBL SG620 (c. 1980)

Name: JBL SG620
Brand: JBL

The JBL SG620 is a high-end control amplifier, basically a full-featured preamp. It came out around 1980 at ¥460,000 in Japan. JBL pitched it as its statement of "sound policy and circuit technology" in a control center. It is one of the company's more obscure pieces. No production years are documented and formal reviews are scarce. What survives is a detailed technical record and a handful of auction listings.

The SG620 is a serious phono-first design. The MC head amplifier runs in Class A push-pull at every stage. The input stage uses large low-noise devices wired in parallel and DC-coupled to preserve signal purity. Gain on the MC input switches between 60dB (low) and 69dB (high). The MM input delivers 37dB. A subsonic filter sits between the first stage and the equalizer stage to cut the 3-7Hz rumble band from disc playback. The equalizer itself is a semi-passive filter design aimed at wide dynamic range. Input resistance switches between 33/47/100kΩ and input capacitance between High/Low, so it can accommodate virtually any MM cartridge. The high-level section uses a Class A push-pull complementary configuration with a floating cascode first stage built around a high-speed dual FET. Tone controls are passive-only, stepped, and bypassed entirely at the flat position. Gold-plated connectors and contacts were used throughout to resist aging.

Measured performance is strong for the era. THD is rated at 0.008% (20Hz - 20kHz, 3V out, 600Ω load) across MM, MC, and high-level inputs. Channel separation is 65dB (MM, 10kHz) and 60dB (MC and high-level). RIAA deviation is ±0.25dB. Frequency response runs from 2Hz to 350kHz (+0/ - 3dB) at maximum volume. Hum and noise are given as input-equivalent values: 0.35µV for MM, 0.04µV for MC (high gain), and 3µV for high-level. The unit draws 40W, measures 432×98×334mm (per audio-database.com; an auction listing gives approximate dimensions of length 10 cm, width 43 cm, depth 35 cm), and weighs 9kg (per audio-database.com; an auction listing gives 6.94kg). Audio-heritage.jp documents the full specification set and design philosophy.

Collector interest is modest but real. A Yahoo! Japan auction closed at ¥59,400 (57 bids, starting at ¥1). Another ended at ¥37,620 (35 bids). A separate listing carried a buy-it-now of ¥120,000, and a sold listing at afroaudio.jp shows ¥110,000. A seller in Osaka tested one unit across TUNER, AUX, TAPE1/2, and both phono inputs with MM and MC cartridges, and reported clean operation of the loudness, bass/treble, balance, and power lamp. An eBay listing describes it as a "Vintage Class A Hi-Fi" control amplifier. A Vietnamese YouTube title calls it "độc hiếm" - rare and unique.

Known Specs

MM gain: 37dB (1kHz, tape out); MC gain: 60dB low / 69dB high
THD: 0.008% (20Hz–20kHz, 3V out, 600Ω load, all inputs)
RIAA deviation: ±0.25dB
Input resistance switch: 33/47/100kΩ; input capacitance: High/Low
Tone control: ±8dB bass/treble, passive, bypassed at flat
Subsonic filter: 15Hz cut; loudness: High/Low
Frequency response: 2Hz–350kHz +0/–3dB
Power: 100VAC, 50/60Hz, 40W
Dimensions: 432 × 98 × 334mm (per audio-database.com; an auction listing gives approx. length 10 cm, width 43 cm, depth 35 cm); weight: 9kg (per audio-database.com; an auction listing gives 6.94kg)