JBL SG520 (1964)

JBL's first serious solid-state preamp. It had a smoked window, a face full of sliders, and a flip-down door hiding half the controls.

Overview

The SG520 is a solid-state stereo graphic controller, which is a tone-control preamplifier with more switching and convenience than the company had ever attempted. JBL introduced it in 1964. That was a bold move for a firm then known mainly for loudspeakers and transducers. The unit carries the James Lansing Sound Inc. name and was made in the USA. Owners spell it three ways - SG520, SG-520, SG 520 - and there is a European variant, the SG 520E / SG-520E, that shows up in listings with a 220-volt supply.

The layout is what people remember. Tone, volume, and balance are all straight-line sliders instead of rotary knobs. Input selection is done with individually illuminated pushbuttons: a row of eight across the top, six more below for mode-of-operation selection, plus a loudness switch. Inputs are on the front as well as the back, so a portable tape deck or a temporary source can be patched in without crawling behind the rack. A flip-down door hides the less-used facilities: headphone jack, fuse, filters, and system gain, level, and balancing controls.

The smoked window on the front sets the SG520 apart from a generic black box. The aperture looks nearly black until power is applied and the label on the circuit board behind it lights up. It is pure showmanship, the sort of thing a design group does when it has been given its head.

Electrically, the SG520 is a hybrid of silicon and germanium transistors, a detail owners cite when explaining the sound. The published frequency response is 20 Hz to 20 kHz, with bass shelving quoted at ±17 dB at 20 Hz and total harmonic distortion of 0.15%. A built-in 1000-cycle test oscillator is provided for speaker balancing. The manual describes the feature set as a wide range of input and output options together with tone controls, a scratch filter, and that test oscillator. Both 120-volt and 240-volt versions were offered, and the European 220-volt variant appears in the marketplace under the E suffix.

Production years, original price, dimensions, and weight are not documented in the available record. The start date is 1964. When the line ended is not established.

Key Features

Straight-Line Slider Controls

Tone, volume, and balance are all vertical sliders rather than rotary pots. The manual's operating notes explain why. The four tone sliders can be grasped together and moved as a pair, giving identical bass and treble settings to both channels, or split apart when one speaker needs more bass boost than the other. The volume slider is a straight-line attenuator controlling both channels at once. The balance slider shifts emphasis left or right, with identical channel gain at the center detent. With the tone markers at the zero position, the response curve is flat.

Illuminated Pushbutton Source and Mode Selection

Source selection is a row of eight pushbuttons, each individually illuminated. Some are set up for high-level inputs and others add gain for low-level devices such as microphones and phono cartridges. Below them sit six mode buttons controlling how the stereo channels feed the loudspeakers. A TEST button works with the F-22 Remote Relay assembly to let the operator balance the whole system quickly. The manual is explicit that all inputs except TAPE PLAY stay shorted unless the appropriate source button is depressed. That keeps unused high-level sources from injecting noise.

Concealed Control Panel

The lower part of the face is a bottom-hinged panel that hides the infrequently used controls and connectors while keeping them accessible. Behind it are the fuse (a Slo-Blo, 3/10 amp for 110-120 volt operation, 15/100 amp for 220-240 volt), the Channel A microphone jack, the rumble filter switch, the auxiliary front inputs, the scratch filter, the test tone switch, the tape monitor switch, the out-to-recorder jacks, and the headphone jack. When the panel is closed, the tape monitor switch automatically returns to OFF. Plugging in headphones disconnects the signal to the power amplifier and mutes the speakers.

Built-In 1000-Cycle Test Oscillator

A 1000-cycle test oscillator is built into the unit for aural balancing. The manual's circuit description calls it a special circuit. The power supply provides -24 volts for the amplifiers and oscillator and +24 volts for the F22 accessory. The oscillator has its own component series on the schematic - capacitors, resistors, a 5K potentiometer, a 100 mH inductor, a 2N2614 transistor, and a pair of 1N2090 diodes - and its parts are listed separately in the replacement schedule. The test tone switch is normally left in the up or ON position when the TEST button in the mode bank is used.

Input and Output Flexibility

The back panel carries a chassis ground terminal, Phono 1 (standard RIAA equalization, for any high-quality cartridge rated at one millivolt or more), and Phono 2 (a second low-level cartridge or a tape head, selected by an internal slide switch behind the front panel output level controls). It also has microphone inputs for both channels, tape play, an auxiliary rear input for any source producing 0.3 volts or more, two tuner inputs, out-to-recorder jacks, and outputs to the power amplifier. The front panel duplicates the out-to-recorder jacks for portable machines, and the signal at those jacks is unaffected by the volume and tone controls. Monophonic sources can be run into Channel A with the A+B button engaged, or through a Y connector.

Hybrid Silicon and Germanium Transistor Circuit

Owners describe the SG520 as a hybrid amplifier using both silicon and germanium transistors. The manual's service notes say replacement transistors are not a casual swap. All transistors were selected for current gain, low noise, and other characteristics. Gain varies enough from one device to the next that some will not work well in a given circuit, so every transistor carries a special part number and must be ordered from JBL, with a color code dot noted when ordering. The same caution applies to the capacitors, most of which are specially made, and to the resistors, which are deposited carbon film. Ordinary molded composition carbon resistors are explicitly forbidden as substitutes.

Historical Context

The SG520 arrived in 1964, when JBL was best known for loudspeakers and transducers. A solid-state preamp with this much switching, this many illuminated buttons, and a smoked window was a statement. The design group behind the smoked-window detail was the same group responsible for the SG520, and the model is listed among the products in JBL's "A Remarkable Legacy."

The unit was reviewed in Audio magazine in 1965, and it appeared in the magazine's preamplifier roundups in August 1967 (p.32) and August 1968 (p.27) alongside units from Dyna, Marantz, CM Labs, Pioneer, and Sony. The 1964 advertising in Audio (August 1964, p.65) billed it as "a tour de force of creative engineering" and stressed the fresh-slate design approach. The same ad ran again in October 1964 (p.81). That is marketing copy, not a verdict. It does explain the pitch: straightline controls, pushbutton selectors, inputs front and back, and a flip-down door for the occasional facilities.

JBL paired the SG520 with the SE 400 SE power amplifier as a combo, and the European-market SG 520E variant appears in listings alongside SE400 S equipment. The service manual survives. Sources give the page count as 34 pages, though one account says 28. A separate repair case document exists for the SG-520.

Collectibility & Value

The SG520 has a small but committed following. One owner, Iwan Arjanto, writing in Resistor Magazine, ranks it alongside the Marantz 7 and McIntosh MC22 as one of three legendary preamps of note, and describes the germanium sound as having warm bass and being incredibly dynamic. A Japanese commenter quoted on diyAudio called the SG-520 one of the best-sounding preamps ever made, and noted that a Japanese audiophile magazine editor used it as a reference preamp in the 1990s. A phono preamp clone based on the SG520 circuit circulates on YouTube.

Prices are all over the map. A Reverb listing carries an asking price of $2,126.77, and an eBay listing for a tested "Silver control amplifier transistor AC100V" unit asks $3,101. Documentation is its own market: an installation and operation manual listing runs $69.95, product literature plus 1966 equipment reports $44.95, and an installation sheet for the SG520 and freestanding accessory case $19.95. Condition notes in listings range from "original JBL SG520 in great working condition, fully functional, original components" to "working normally with every button and knob intact."

Buyers should know what they are getting into. The transistors and most of the electrolytics are JBL-specific parts. The manual warns against substituting inferior electrolytics or ordinary carbon resistors, since lower capacitance in any circuit will impair performance and lower working voltage ratings will cause premature failure. That makes a complete, unmolested example more desirable than a modified one, and it makes the service manual worth having. There is also at least one owner report of a volume-control issue at the top of the range, though the account is truncated and no failure pattern is documented. The manual does have some good news: unlike vacuum tube amplifiers, the SG520 is described as normally not subject to gradual deterioration or intermittent hum and noise problems. The solid-state circuits were considered reliable enough that the rest of the system should be checked before the preamp is brought into the shop.