The most popular multi-voice guitar amp configuration is comprised of a 2-channel preamp and an effects loop. Each preamp voice can be combined with effects or not, resulting in four distinct system voices overall: A, A+FX, B, B+FX. Random access from one voice to any other with a single switch actuation is easily accomplished using the Q-LATCH 4-voice Selector. The actual interface to the audio path is where things can be awkward – or graceful.
One can use Q-FET or Q-RLY to perform the switching of the channels and the effects loop, but both of these boards have more switching capability than needed – and yet not quite the switching that we truly require for a sophisticated loop. If the loop is simple, then these cards will work.
A sophisticated effects loop is one like our BFX Best All-tube Effects Loop, which allows series or mixing modes and has ‘send’ and ‘return’ controls. The circuitry is as transparent as traditional tube circuits can be. The switching “complexity” comes in because of the ‘series’ mode. In this mode, the signal leaves the amp entirely, passes through the effect and then returns to the amp. To bypass this in the usal manner would result in no sound. Meanwhile, the ‘mix’ mode has to allow the ‘dry’ sound to be present all the time and have just the ‘wet’ effect sound add or disappear. Two switching points are needed in this loop, and they must switch in the correct way with respect to the operating mode of the loop.
London Power designed the Q-MNR as a unified control board to handle both preamp channel selection and correct control of a sophisticated loop like BFX. It is the heart of the Q-MINI system – the ‘MN’ in MNR. All switching is handled by mechanical relays – the ‘R’ in MNR. Add two RLYs for FX control and one or two more for preamp selection, as required. Q-MNR provides two symmetric relay driver outputs for the channel control block, to accommodate any preamp configuration. The ‘series’ switch from BFX is wired into Q-MNR instead of into the loop, to make the ‘series’ mode controllable by Q-MNR.
Programming which voices control the preamp change and which control the FX selection is achieved by soldering diodes into specific circuit locations on the Q-MNR board. Because the 4-voices above require only a limited range of access to the channel selection portion and to the FX loop portion, only four diodes are provided for the actual programming. However, there are four positions available for each, to allow maximum flexibility.
LEDs indicate each preamp selection, as well as the FX status and ‘series’ mode. The LEDs supplied are standard brightness round 3mm green LEDs, but any other type may be used.
In a typical system, Q-LATCH will be mounted remotely as a foot switch. For this use, add four Q-FSWs to complete the foot switch box. Q-LATCH will drive the LEDs at that location. Q-MNR mounts inside the amp chassis, and it will drive the amp’s panel LEDs and control the circuit switching.
Alternatively, a second Q-LATCH can be mounted inside the amp chassis along with Q-MNR. The two Q-LATCHes will be wired for 2-way communication, as their kit notes illustrate, providing synchronization of all LEDs at the amp and at the foot switch regardless of which end has had a switch actuated.
Power for Q-MINI can be plus or minus 12Vdc. Current should be about 70mA to accommodate the LEDs and four RLYs. Add 12mA per RLY added beyond this.
PCB construction makes assembly quick and easy.
Q-MNR 3.7 x 1.5″ (94 x 38mm)
Q-LATCH 3.7 x 2″ (94 x 51mm)
each RLY 1 x 1″ (25 x 25mm)
For more information:
– see our page, About Our Switching Kits
– download our informative PDF: London Power Switching Kits Selection & Application (135 kB, 22 pages)