QRK-MINI - 4-voice amp control package


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QRK-MINI is a discount package containing QRK-LATCH and QRK-MNR.

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. To have direct random access from one voice to any other with a single switch actuation is easily accomplished using the QRK-LATCH 4-voice Selector. The actual interface to the audio path is where things can be awkward or graceful.

One could use QRK-FET or QRK-RLY to perform the switching of the channels and the effects loop, but both of these boards have more switching capability than needed yet not quite the switching that we truly require if the loop is in any way sophisticated. If the loop is simple, then these cards will work.

A sophisticated effects loop is one like 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 and 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 QRK-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 QRK-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. QRK-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 QRK-MNR instead of into the loop, to make the 'series' mode controllable by QRK-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 QRK-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, QRK-LATCH will be mounted remotely as a foot switch. For this use, add four QRK-FSWs to complete the foot switch box. QRK-LATCH will drive the LEDs at that location. QRK-MNR mounts inside the amp chassis and it will drive the amp's panel LEDs and control the circuit switching.

Alternatively, a second QRK-LATCH can be mounted inside the amp chassis along with QRK-MNR. The two QRK-LATCHes will be wired for 2-way communication, as their kit notes illustrate, providing sychronisation of all LEDs at the amp and at the foot switch regardless of which end has had a switch actuated.

Power for QRK-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.

PCB sizes:

QRK-MNR 3.7 x 1.5" (94 x 38mm)

QRK-LATCH 3.7 x 2" (94 x 51mm)

each RLY 1 x 1" (25 x 25mm)


About London Power

London Power was founded in 1990 by Kevin O'Connor, after two decades of research into innovative audio amplification techniques. Kevin is an audio designer, author and speaker, known for his proprietary methodology called Power Scaling, and for his eleven books on audio subjects. Whether you seek amps, kits or information, you'll find useful answers here on not just the "how" but the "why" of audio amplifier design. Please enjoy the information on this site, and don't hesitate to contact us with questions.

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