Running out of inputs on a preamplifier or integrated amplifier is a problem for audiophiles. Collectors who play all types of disc records often have two turntables — one for LPs and 45s and another for 78s — but only one phono preamplifier. Others may have two different turntable/arm/cartridge setups just for LPs. A few high-end turntables enable the mounting of more than one tone arm. However, switching audio signals at phono cartridge-levels can be fraught with problems if it’s not correctly done.
High-quality switches and connectors are essential, but the length of the output leads between the switchbox and the phono preamplifier input is a potential source of trouble. A switchbox needs to be within easy reach, which often means a half meter or more of output cable. This increases the total capacitance of your tone arm cabling, and can lead to hum and noise problems.
The KAB Electro Acoustics’s RSX-1 remote analog switch was designed to address these problems by providing a clean, noise-free signal path in a remote-controlled device that can be placed close to your phono inputs (see Photo 1).
The switch will work equally well with line-level signals. The small remote control is wired, supplying 3 V to the switching relay. It is connected to the switchbox with a stereo mini-phone interconnect cable. The supplied cable is 6’ long, so the remote can be placed in a convenient spot. The remote has two buttons and an internal CR2430 coin-type, 3-V lithium battery.
Functionally, the RSX-1 was designed to be more than just an input switcher. There are three pairs of RCA jacks labeled J1, J2, and J3, and three modes of operation, configured using three DIP switches on the PC board. The switches are accessible by popping three black plastic caps on the top of the case. The illustration painted on the case indicates each mode’s switch positions. In Stereo mode the unit functions as a switch box, with J2 and J3 functioning as inputs or outputs and J1 as common.
Remote Button 1 connects J1 and J2, and Button 2 connects J1 and J3. This enables you to route either of two inputs (J2 or J3) to output J1. Or J1 can function as an input, with the output routed to either J2 or J3. In Stereo/Mono mode J1 is the input, J2 is the output, and J3 is unused. J2 is a stereo output when Button 1 is pressed on the remote control.
Button 2 combines the two channels, providing a mono output at J2. In Stereo & Mono mode, J1 is common, J2 is connected to J1 in stereo, and J3 is connected to J1 in mono. Remote Button 1 selects the stereo connection and Button 2 selects the mono. With this configuration, a source connected to J1 can be routed to output J2 in stereo or J3 in mono. Conversely, stereo and mono sources can be connected to J2 and J3 and routed to the J1 output.
KAB Electro Acoustics said the RSX-1 can be used to switch two turntables between one preamplifier, switch one turntable between two preamplifiers, compare two turntables or line sources, configure as a mono/stereo switch, configure to deliver separate mono and stereo outputs, or get good mono sound from your stereo phono cartridge.
The RSX-1 is a completely passive device. KAB Electro Acoustics has used premium parts to ensure the highest possible signal integrity (see Photo 2). The six RCA jacks are Cardas-type GRFA PRT, which are right-angle, PC-mount connectors made with nonmagnetic eutectic brass, plated with rhodium over silver. The jacks cost $25 per pair and are the most expensive components in the device. The switching relay is a Panasonic ASX22003, which is guaranteed for low-level switching capability. The stationary contact is silver plus palladium, gold clad (AgPd+Au clad) and the movable contact is AgPd. The switch has a 100-m maximum static contact resistance.
The ASX22003 is a double pole, double throw (DPDT), two-coil latching relay. Voltage is momentarily applied to one coil to move the contacts to one position. Once in position, the contacts remain latched without any applied voltage until they are moved to the opposite position by momentarily applying voltage to the other coil. With only intermittent current drain, the battery should last years before needing replacement.
The unit is housed in an all-steel enclosure, which provides complete shielding. To avoid ground loops, the left and right channel grounds are isolated from each other and from the chassis. The chassis ground post is exactly the type I use on equipment that I build—a 6-32 machine screw and a knurled thumb nut, ensuring a solid connection and longterm durability. The RSX-1’s ground post should be connected to the ground on your preamplifier.
Connect your tone arm ground wire directly to the preamplifier. Do not connect the tone arm ground wire to the RSX-1, since daisy-chained ground schemes will usually cause hum problems, especially with the low signal levels produced by magnetic phono cartridges. Remember, the RSX-1’s chassis is not connected to signal ground. The ground connection’s purpose is to enable the chassis to function as a shield. In many line level applications, the ground connection will be unnecessary.
The PC board layout has been designed to add minimum capacitance across the signal path. I used a digital LCR meter to measure the capacitance, carefully zeroing the meter to compensate for the short test leads before making the measurements. In Stereo mode, each channel measured a very low 13 pF. In the Stereo/Mono mode, capacitance was 12 pF per channel in stereo, rising to 20 pF when the unit is switched to mono. In the Stereo & Mono mode, capacitance is 13 pF in stereo and 17 pF in mono. Even a short cable between the RSX-1 and your phono input will add some capacitance, so you should consider the tone arm cable’s capacitance, the RSX-1, and the output cable when determining how much additional loading capacitance, if any, is needed in your phono preamplifier. In most line-level applications the capacitance will not be an issue.
KAB Electro Acoustics proprietor and designer Kevin Barrett said that he was mainly concerned with the RSX-1’s performance with high-output moving-magnet phono cartridges, where high impedances create the greatest potential for noise problems. Although he did not specifically test the unit with low-output moving-coil cartridges, he noted that, despite the high gain required in the phono preamplifier, the RSX-1’s noise performance should remain excellent due to the low impedances involved.
Testing the Switchbox
I tested the RSX-1 in the system I use in my studio at work to do digital transfers of LP records. The system consists of a Rek-O-Kut Rondine 3 belt driven turntable fitted with a Rek-O-Kut S-240 Fluid-Damped Tone Arm (actually a Jelco SA-750E) and a Grado Prestige Gold1 cartridge, all purchased from Esoteric Sound. Esoteric Sound revived the Rek-O-Kut name a few years back for its own line of turntables, arms, and accessories—these new designs should not be confused with the old Rek-O-Kut clunkers from over a half century ago!
The Rondine 3 has a unique feature—a removable spindle, which enables precise centering of off-center pressings (if you look and listen carefully, a perfectly centered pressing is the exception rather than the rule). The phono preamplifier is a Parasound Z-Phono, which feeds a Benchmark ADC-1 with RCA-to-XLR interconnects wired to correct the Z-Phono’s inverted absolute polarity. Analog interconnects are made with D.H. Labs Pro Studio cable fitted with Canare F-10 RCA and Neutrik NC3MXX-B XLR connectors. The D.H. Labs cable has a Teflon-foam dielectric. The Canare F-10 is insulated with polytetrafluoroethylene (PTFE) and both connectors have gold-plated contacts. From there, I can connect the Benchmark ADC to a Tascam CD-RW901SL CD recorder or a high-resolution Tascam DV-RA1000HD digital recorder.
This LP transfer system is extremely quiet. If I advance my preamplifier volume control to maximum (without music playing, of course!), there is a trace of very low-level hum picked up by the unshielded Grado cartridge. At anything close to a safe playback level, the system is silent. When I put the RSX-1 in the signal path between the tone arm cable and the Parasound phono preamplifier, there was no increase in noise. I played several LPs, using Stereo Switch mode and Stereo/Mono mode and found that the RSX-1 did an excellent job maintaining signal integrity. I would have no reservations about using the RSX-1 in the phono cartridge signal path when making digital transfers or simply listening.
The RSX-1 is a versatile analog switcher that will solve many audio switching problems without degrading the signal and introducing noise. Its unique, remote-controlled design enables it to be placed close to a preamplifier’s phono input, so cable lengths can be kept at a minimum. It works equally well with line-level signals. Considering the high quality parts and construction, the price is more than fair. The RSX-1 is another fine product from KAB Electro Acoustics. ax
This article was originally published in audioXpress, July 2014.
Wired remote control with CR2430 lithium coin cell battery (included)
Cardas GRFA PRT RCA jacks
Panasonic ASX22003 latching relay
Fully isolated left and right channel grounds
Isolated chassis ground
Dimensions: Switch box 7.5” × 4” × 1.4”; Remote 2.5” × 1.75” × 0.75”
Weight: 1.75 lb
Warranty: 1 year
KAB Electro Acoustics
Plainfield, NJ 07062
Benchmark Media Systems, Inc. | www.benchmarkmedia.com
Rek-O-Kut Rondine 3
Esoteric Sound | www.esotericsound.com
Prestige Gold1 cartridge
Grado Labs | www.gradolabs.com
Jelco Tone Arms
Ichikawa Jewel Co., Ltd. | www.jelco-ichikawa.co.jp
Parasound Products, Inc. | www.parasound.com
CD-RW901SL CD Recorder and DV-RA1000HD digital recorder
TEAC Corp. | www.tascam.com