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Curio's - The "Practical Research" File, of Mike Skeet - Paper No.7



The Mic Pre-Amp is the first part in the recording chain’s path to the destination recorder and there are a number of aspects related to it, and to some others further down the chain. This could be with a stand alone Mic Pre, a Mixer or a Recorder with its own mic amps.


A) Lets start with the Gain Range available from the front end IC, such as the Analogue Devices SSM2019. This can be a stepped gain arrange or a continuously variable one. Due to the wide range of sound sources, from a quiet acoustic guitar to a group of percussionists, there needs to be at least a 60dB range of adjustment.


The primary purpose of the Gain range is to have it set it to avoid clipping of the mic pre-amp, with a sensible working position of the faders, sending on the signal to the recorder. There is a ‘trade off’ factor, involving the headroom versus the basic noise floor, which will be discussed later.


Another factor is the range of output levels of different mics. You have the low output levels from ribbons and moving coil mics, and the higher differing levels from capacitor mics and now also with the newer ribbons, with their on-board ICs producing higher output levels, giving lower noise floors.


B) There is a case for Linear faders, as opposed to Logarithmic types, for use to control the levels sent to the destination recorder. With a linear fader you have a more ‘delicate’ control over any trimming of levels, should un-anticipated peaks occur. In the wide dynamic range world of classical music recording, it proves to be much easier, to in-audibly trim the levels.


There is another factor in the choice of faders, especially for us DIY constructors of our recording kits. This is the use of ‘stereo’ faders, with the two tracks wired in parallel. The reliability is very considerable enhanced!


C) All Mic-Pre / Mixers should have a built-in line-up oscillator to properly relate their outputs to the destination recorder’s controllable line level inputs. The idea is that the overall noise floor and headroom of the mic pre-amp can be sensibly utilised, as mentioned earlier.


There is indeed a trade off between the two aspects. For instance, should the recorder have its input too low, you could be using too high a gain setting with the mic pre-amp/mixer, and therefore increase the chance of ‘clipping’ the mic amp’s input and reducing the head room available. In the opposite direction you could have the recorder’s input too high and the noise floor extracted from the mic pre-amp/mixer’s output will be higher..


The solution is to have a 1kHz output directly available from the mic-pre/mixer output, activated via push button, installed out of easy reach. In the writer’s DIY case, this is intended to be received at neg. 12dBfs by the recorders, getting the best compromise between headroom and noise floor. A 90dB noise floor can be obtained.


D) There is a case for the metering on recorders to have the neg.12dBfs point at half way on its scaling. The main thing we are interested in is avoiding signals going OTT. The two particular Olympus LS10 SD card recorders, shown in Pic: 1, do at least have their neg.12dBfs point half way across the small space that they have available.


However, the writer does currently judge levels with a HHB MDP500 recorder (Pic. One), due to its much wider metering display, and its neg.12 dBfs point is indeed half way. With the same line-up tone, the SD card recorders are set a couple of dBs lower with the same tone. The MDP500 could also be a third backup, as it has balanced mic inputs, should the DIY kit’s mic amps ‘implode’!


As it happens the MPD500 is also used for playback as you can quickly jump between tracks, which show the Take Number you have called. The writer also lists in the Session Log, the last three digits of the automatically selected Wav File numbers, shown on the LS10 recorders.


E) With headphone monitoring, and also in cases where there is a feed to loudspeakers, there should be mono buttons available to produce a mono signal. This allows both monitoring systems to be checked for centre signal position accuracy. This is also useful if the speakers happen to be out of phase, however this should be noticeable with the music anyway!


As a final point, at least two or even three headphone sockets should be provided on any mixer and even some recorders, so that ‘the talent’ can come to the control room and be able to listen at the same time and more quickly make common judgements, as to how they think they are getting on.


Up-dated 15/05/11


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