Mic Preamp (Part 11)
Another module for the Tantek rack from the drawing board of Paul Williams.
Extending the range of the Modular Effects Rack, Paul Williams describes the first of three modules which together offer high quality mixing facilities. The Mic Preamp described here boasts a balanced low noise input with variable gain, pad and phase switches, mix and pan controls and a phantom powering option.
Despite the permeation of electronics into every facet of life, most especially into music, where the keyboard and percussion scene has changed out of all recognition in recent years, the all-time most popular sound source; the human voice, remains relatively untouched by the electronics revolution. It still needs that fickle interface; the microphone, until the day comes when someone finally figures out a way of DI'ing vocals!
Assuming that your vocal sound is up to scratch in the first place, it is with a little care in the design of the equipment, relatively easy to preserve the quality of the signal once it is up at line level. The most critical area, and the one most likely to be skimped on by the small studio on a tight budget is the microphone itself. No matter how good your original vocal sound is, and no matter how much you spend on effects, nothing can compensate for a lousy mic, so get the very best you can afford.
Having hopefully now prodded the reader in the direction of the higher quality microphone, it's likely that you will be looking for a preamplifier to match its performance, with a balanced XLR input, adjustable gain with a pad and a phase reverse switch, an uncompromised specification featuring low noise and wide bandwidth, and possibly phantom powering. Well, look no further than the Mic Preamp for the modular effects rack.
This high quality device accepts a balanced XLR input, and boosts it to line level, watched over by a peak indicator. The already ample headroom is further enhanced by a -20dB pad switch. For use in a multi-mic situation, a phase reverse switch relieves any possible antiphase problems.
Phantom powering is provided either internally for 12 volt microphones, or by means of the separate phantom power module for 24 or 48 volt operation.
The module also sports a host of mixer facilities comprising a separate unbalanced line input, effects send and return sockets and mix level and pan controls, allowing an array of Mic Preamps to combine their outputs onto a stereo virtual earth buss, which is brought into the outside world by means of the special mixer module.
Figure 1 shows the full circuit diagram for the Mic Preamp. The balanced input signal is injected at SK1, where R2 and R3 provide the phantom power feed which is selected by means of a link to be either 12v from the internal rail, or 24 to 48v from the Aux 1 buss, which is fed from the phantom power module. C1 and C2 provide adequate decoupling in either case. SW2 switches in the resistive attenuator formed by R4-7 when the pad is selected, while SW1 performs phase reversal.
The differential pair TR1 and TR2 form the heart of the unit, providing, along with IC1a, all the necessary gain with an excellent noise performance due mainly to the ideal chip geometry of the transistors which although are essentially medium power devices, have the low base region resistance necessary for low noise when operated at low current levels. The stage gain is determined by VR1, the sensitivity control. R10, C11, R16 and C14 provide ample supply decoupling.
The now unbalanced and amplified microphone signal is mixed with any signal appearing at the Line input by IC1b, the composite signal being available at JK1, the Effects Send socket. If a plug is inserted into the Return socket then the internal chain is broken, allowing the signal to pass via external processors, or indeed other modules in the rack. IC2a buffers the Return input, and provides a line output in addition to the virtual earth buss output. IC2b pushes a signal current determined by the Mix control, VR2 onto both the virtual earth busses via R29, the panned position in the stereo field being determined by VR3.
Any signal peaks much over 0dBm are detected by TR3, pushing current into C24, producing a voltage which results in TR4 drawing current through the LED D1 to illuminate it. The peaks are stretched in time by C24 so that even fairly short duration transients are made visible. The 0v rail is kept nice and clean as usual by driving the LED between the rails, which are decoupled by C25-28. Full wave detection is unnecessary in this instance as there is so much free headroom anyway that the odd 6dB of asymmetry will do no harm.
Building the Mic Preamp module using the high quality kit should present no problems especially since, by exclusive use of PC mounting connectors, switches and potentiometers, there is no interwiring to do. The first step in construction is to insert, solder and crop the resistor leads, populating the PCB according to the parts list, and the overlay printed on the PCB itself. Bending the leads outward at 45 degrees prior to soldering will hold the components in place without running the risk of shorting together a pair of pads. Solder the seven links in place using resistor lead off-cuts, at the positions shown dotted on the overlay. An additional link will be required later to select the phantom power voltage. Taking care with orientation, locate and solder the transistors, TR1-4. The IC sockets come next, making sure that they are pressed down onto the PCB whilst soldering, but leaving the ICs themselves out until later. Now insert and solder the capacitors, taking care with the polarity of the electrolytic types. The buss connector and the four jack sockets can then be soldered whilst holding them firmly down onto the PCB. A piece of foam rubber laid on the bench comes in handy for holding connectors and the like in place on up-turned PCBs during soldering.
Trim each pot shaft to 8mm from the bush using a hacksaw, whilst holding the pot shaft in a vice, or just use a pair of cable cutters. Fit a PC bracket to each pot and locate into their respective PCB positions, but don't solder at this point. After determining the correct orientation of the LED, bend its leads down at right angles, 4mm from its body and locate into the PCB without soldering. Screw one nut onto each toggle switch and locate into the PCB, again without soldering. Locate the XLR socket PC pins into place, then put shakeproof washers on the switches and pots, and offer the front panel up, feeding the pot and switch bushes, XLR socket rim and LED dome into the appropriate panel apertures. The panel is then fixed in place by means of the pot nuts which should be fully tightened. Only finger tighten the front switch nuts however, leaving the final securing to the rear nuts, which should be screwed up against the rear of the panel. Fix the XLR socket to the panel using black M2,5 screws and nuts. The pots, brackets, switches, XLR socket and LED can now be soldered, after making sure that they are all fully home, and that the panel is square to the PCB.
Spend some time now to check over the assembly very carefully, especially on the track side where dry joints and solder splashes are all too common, even for the experienced constructor. When you are completely satisfied with the assembly, load the ICs into their sockets, being careful with orientation. Finally, fit the knobs and caps so that the marker line of each covers the scale evenly, with equal 'dead-band' at each end, then push on the toggle switch lever covers.
Once you have determined what phantom power requirements your microphone has, the phantom power link can be provided appropriately. The PCB is marked to show the link position for either internal 12v or external 24-48v. Many microphones will in fact work quite happily at 12v, although the reduced headroom may show itself when close-miking drums, or when recording a powerful singer. If the link is placed in the 24-48v position, it will be necessary to use the Phantom Power module. If several preamps are used simultaneously with microphones of differing phantom power requirements, then this is possible by providing the appropriate link on each module.
Unbalanced microphones may be used by making up a short lead with a male XLR plug one end, and an in-line jack socket the other. The module is unsuitable in its standard form for high impedance microphones, but these should generally be avoided if you really value your sound quality.
The sensitivity control sets the gain from the XLR input, and is adjusted either to achieve the required output level as determined by a meter such as that on the Input module, or in accordance with the built-in peak indicator, which shows when 0dBm has been reached. This is not a disastrous, or even significant level as far as the Mic Preamp is concerned, it merely acts as an indication of level. Plenty of gain is available, so if you find that the sensitivity control has to be turned fully clockwise, this would be an indication that the Microphone should be closer to the source. Close Miking with high output microphones can sometimes lead to headroom problems, which can be remedied by switching the -20dB pad into effect.
The Mic Preamp modules are best placed at the left hand end of the sub-rack, to the left of any Input module. The line output from the unit is, in addition to being available from the jack at the rear, also passed on to the rack's linking system, passing its signal on to the module to its right, unless a plug is inserted into the line out socket, or if the module to its right is an Input module.
Up to nine Mic Preamp modules may be used in conjunction with two Mixer modules, or eight if the Phantom Power module is used. When used in this way, the system is effectively a high quality 9:2 (or 8:2) mixer; the Mic Preamps being the input channels, and the mixer modules being the output groups. The balance of the mix is affected by the Mix controls on the Mic Preamps, stereo position being dictated by the Pan controls. When using the line input to get a higher level/impedance signal into the mix, the high gain amplifier is not used, so it is advisable to turn the sensitivity control anticlockwise in this case to maximise the signal-to-noise ratio.
Effects either within the rack or external to it can be patched into any Mic Preamp by using the effects Send and Return sockets. If, for example EQ is required on every Mic Preamp in the rack, these can be permanently patched in, placing Parametric EQ modules in a convenient position such as to the right of each Mic Preamp. Since use of the send socket does not break the chain, it can be used to 'sniff' the signal if the effects processing is to be mixed in elsewhere, or if the channel is to be monitored.
Rather than using the modules to form a mixer, they are also ideal as a supplement to a mixer whose microphone inputs leave something to be desired, in which case the line output from he Mic Preamp would be fed to the line input of the mixer.
Next month we will describe the Mixer module.
The Mic Preamp module is available from: Tantek, (Contact Details) either in kit form for £38.95 or ready to use for £54.95. Prices include VAT and postage within the UK.
Further information on the Modular Effects Rack system can be obtained from the above address, or by 'phoning (Contact Details).
Feature by Paul Williams
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