AKG BX-10

Name: AKG BX-10
Brand: AKG

Two independent spring-reverb channels in a 47-pound box that AKG built for the road as much as the studio.

Overview

Forty-seven pounds, 17 inches wide, 12 high, 19 deep. That's the AKG BX-10 as Audio (December 1977, p.28) sized it up. Those dimensions say who it was for. AKG's own service manual describes the BX-10 as designed for small studios, mobile studios, and professional musicians, "particularly suitable where small space, transportability, robustness and simplicity of operation is required." This is a spring reverb built to be carried around, not to sit in a rack and look pretty.

The two channels are mechanically and electrically isolated from each other. This is not two outputs fed from a shared tank. The signal paths are separate. Each channel has its own decay-time adjustment, its own high- and low-frequency shelving, and its own reverb/dry mix control. Per the same Audio review (December 1977, p.28), each channel offers three reverb periods: 1.5, 2.5, and 3.5 seconds. So one source can get a short, bright ambience while another gets a long, dark tail, at the same time, with no crosstalk between them. That is handy for a mobile rig or a small studio tracking two sources at once, and it is a big part of why the BX-10 stayed in service after cheaper single-tank spring units were retired.

The nominal input level is +3 dBm per channel, per the Audio review. The service manual is more specific about what the unit wants on test: a +6 dBm (1.55 V) signal at the input, with the input-sensitivity switch on the preamp boards set to match. The switch matters. The manual opens its general section with a blunt warning that incorrect loading impedances will produce unsatisfactory results. Feed the BX-10 the wrong level and it will sound wrong, and turning the mix knob won't fix it.

The reverb chamber is a spring system, and the manual treats it as a serviceable assembly, not a sealed black box. The spring windings must not touch each other. The springs must not contact the deviation points, the supports, the movement limiters, or the magnet in the air gap. The small coils should sit in the magnet gap at 45° ± 5°. Coil resistances, measured at the reverb-unit connector, should read 260 ohms across pins 2 and 3 and 5 and 6, and 180 ohms across 7 and 8 and 7 and 9, both ± 10%. Owners report that the spring assembly is repairable by careful lifting, twisting, or bending when contacts occur. That beats the potted-tank designs that turn a $40 part into a $400 repair.

The electronic module is a plug-in card that comes out of the outer case, and AKG Vienna would sell a replacement if you gave them the serial number and channel number. The manual's replacement procedure is specific. Match potentiometers R21 and R51 on the new card to the positions on the defective one, then check with low-frequency impulses in the 200 Hz to 500 Hz range. The manual notes that clapping your palm against a connected microphone is "an excellent improvisation" for this test. Then adjust R21, and if that doesn't do it, alternate with R51 until the low-frequency echoes are minimized, while keeping as little direct signal as possible superimposed on the reverb signal.

Power consumption is modest: 22 mA typical, 34 mA maximum at 11.5 volts working voltage, per the service manual's measurement procedure. The electronic circuit terminates with 200-ohm resistors between terminals 2+3, 5+6, 7+8, and 7+9. The DC voltage at point J should be 0 V ± 1 mV, adjustable with R70. Points A, G, H, 8, and 9 should sit at 0 V ± 30 mV, and points E and F at 0 V ± 300 mV.

Reverb level between the two channels can be trimmed by about ±2 dB with R63. Frequency response is shaped with the tone controls. The manual points to a graph in the technical-data section instead of quoting a flat number, which was typical of the era and of AKG's documentation style. If the output voltage measures only 1.15 V during the AC test instead of the expected 1.5 V, the manual says to replace R88 and R89, going from 120 kΩ to 150 kΩ, and re-trim point J.

The BX-10 is not a tape-echo replacement or a digital reverb. It's a spring. When it's healthy, the decay is smooth and without spurious echoes, per the manual's own fault-finding criteria, but it is still spring-shaped. The Audio review noted that the statistical variations of winding and wire diameter, combined with reflection points introduced along the helical spring, produce a high degree of diffusion for a spring design. That means better diffusion than a cheap tank, but not the same thing as a plate or a chamber. Recording Engineer (August 1979, p.116) noted the BX-10E2-Short variant was available on special order at 5% over the standard cost.

The BX-10 shows up in period studio listings alongside serious outboard. Mix (March 1980, p.59) lists one in a studio inventory next to an EMT 140 plate and an MXR Digital Delay. It wasn't the plate. It was what you used when you needed two independent reverbs, or a reverb that could go on the road. Used listings in Modern Recording (March 1982, p.82) show the BX-10 and BX-20 trading on the secondhand market by the early '80s.

Collectors note that the BX-10 occupies an odd niche. It's not as famous as the BX-20, not as rare as some of the tube-era AKG gear, and not as compact as the later BX-5. What it offers is a two-channel spring reverb with real service documentation, a repairable spring assembly, and a form factor designed to be moved. It doesn't get the attention AKG's microphones do.

Buying one today means taking spring reverb on its own terms. The units are old enough that the electronic module may need recapping, the spring assembly may need re-tensioning or cleaning, and the input-sensitivity switch needs to be set correctly for whatever is feeding it. The documentation exists, and the parts are largely standard.