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Fostex MN-15 Mixer/Compressor

When Fostex released the X-15 four-track cassette recorder some 15 months ago, the cost of personal multitracking came down to the most accessible level ever and inspired many people to start recording their own music. Although the X-15 has given thousands of people a taste of multitracking at an attractive price it must be said that Fostex only achieved this by designing down to a price and so compromises were obviously made. One of the main design compromises resulted in a limited mixer section which in fact only allows you to record one sound source per channel at a time.

Fostex have now introduced the MN-15 mixer/compressor. This is a small, low-cost 5 into 1 channel mixer specifically conceived to work in conjunction with the X-15 Multitracker. The MN-15, when used with the X-15, gives added flexibility and will actually allow you to mix up to five separate sound sources into one (mono) output. This is particularly useful when laying down original tracks or when bouncing tracks down, and greatly extends the creative potential of the X-15.

The unit itself, in keeping with the portable idea of Fostex equipment, is battery operated; though a socket which accepts an external 9 volt power supply is also included. Physically the MN-15 is approximately the size of a paperback book and is well constructed with a black plastic control facia firmly attached to a metal base plate. Housed within the case there arte two separate circuit boards, one for the mixer section and one for the compressor.


Figure 1 shows the various signal paths within the MN-15 and as can be seen from the diagram there are two separate sections within the unit. Four line inputs are provided via phono sockets and signals applied to these inputs can be mixed together by use of the fader on each channel and will eventually emerge as a single mono mix at the output phono socket. Although each channel has its own level control there is, however, no overall group level control. This means that if you intend mixing together, for example, a couple of keyboards, drum machine and direct injected bass guitar, you're able to mix the relative levels of the instruments but you can't vary the overall level of the total mix. This apparent problem is, in fact, not a problem at all, but rather a conscious design feature as the overall level control is handled by the input record level fader on the X-15 itself, and obviously there's little point in duplicating control features when designing a budget recording system.

Figure 1.


The second section of the MN-15 is the compressor circuit and it's this section that really makes the unit such an interesting accessory. Within the compressor section you have the choice of using either a -10dB line level input on a phono socket or a high impedance microphone input on a ¼"-jack. When using the input channel, a source selection switch allows you to choose between line, mic or attenuated mic inputs, and the level of the signal is then controlled by the input fader. At this point the signal may take one of two paths, either directly to the output or alternatively to the compressor; the choice of directions being selected by a three position switch whose status is, Power Off, Normal On, and Comp In with the compressor in operation. Whichever option is selected the signal eventually arrives at the output stage where it is mixed with any signal from the four tape (line) inputs before finally making its exit via the output phono socket.

As this unit is aimed at the very bottom end of the cassette-based multitracking market, it's nice to see that Fostex have included a compressor within the design of the MN-15. For most people on a limited budget a compressor is often one of the last things on the shopping list because its operation is either little understood or it is seen as a device which doesn't provide a sound effect of any audible interest. If the truth be known, a compressor should be, along with a reverb unit, one of the first signal processors you should consider buying. It's even more necessary when you're working with budget equipment that doesn't have the sort of headroom you find in more upmarket mixers and tape recorders.

So what is this compressor going to do for you? By switching the compressor in circuit you can control the dynamic range ie. the variation from quiet to loud, of the sound in the input channel. The input fader used for varying the line or mic level is adjusted so that the compressor 'Min' light flashes, which indicates that the signal is just being compressed as it has reached the lower threshold level. A 'Max' light is also provided and this flashes when compression is at its maximum.

Associated with the compression is a variable release time which may be set anywhere between a fast (50ms) and slow (1 sec) release time by means of another slider control. As we are dealing with a cost effective unit, the MN-15 doesn't have a variable compression ratio but rather what is usually referred to as an 'optimised ratio' and in this case a fixed 6:1 compression ratio has been chosen. This means that for a signal input level variation of 6 units, you only get a variation of 1 unit coming out the other end; well that's the theory. In practice, you're never dealing with such a well-behaved signal but you still get good results and a compressor will help you to record difficult dynamic material such as acoustic instruments, vocals and drum machines more easily.


The MN-15 mixer/compressor is very simple to use and can operate in two major areas of the recording process ie. laying original tracks and track bouncing.

When you start to record a song you may want to put several instruments down onto one track of the X-15 simultaneously, which can be achieved by plugging up to four different instruments into the line in sockets of the MN-15 and mixing them together. You will notice that these sockets are in fact labelled 'tape in', but they will also accept instruments operating around the -10dB level and that covers about every keyboard and drum machine on the market. Unfortunately, the MN-15 only has one microphone input and where this could be seen as a minor disadvantage you must not forget that you can use the X-15's own mic inputs in addition.

To return to the line inputs; having plugged in your chosen instruments eg. a drum machine, Roland MC202 sequencer, a monophonic synth and a poly synth; you can now set up the sounds on the instruments and prepare to record.

The X-15 offers a maximum of four tracks to record on and therefore track bouncing becomes a necessary evil if your music requires more than four different instrument sounds. When you are limited to recording only one sound source at a time it's difficult to avoid track bouncing, however, if you do have the ability to mix together several instrument sound sources prior to recording on one of the X-15's tracks, you can retain a good sound recording quality and still have a lot of instruments on the final mix: you could say that it's the ideal situation. The MN-15 will allow you to do precisely this.

Figure 2.

So, if we return to the original instrument list we could envisage the following (Figure 2): the MN-15 is plugged into one track of the X-15 recorder, the drum machine is plugged into channel one of the MN-15, and the fader adjusted so that you can hear the sound. Next, the Roland MC202 has been programmed with a sequence on each of its two tracks and synchronised to the drum machine. The sound output of the MC202 is plugged into channel two of the MN-15 and the other track of the MC202 sequencer is linked to a monophonic synth which provides the sound output and is plugged directly into channel three of the MN-15. Finally, a polyphonic synth is plugged into the fourth channel.

With all four instruments plugged into the MN-15, each of the channel faders can be adjusted until the desired overall balance between the instruments is achieved until finally you have the basic rhythm sound for your song all mixed together and ready to record onto one track of the Fostex X-15. Before actually recording, the overall recording level on the X-15 can be carefully adjusted for the optimum setting. An interesting feature of the set-up described here is that because the examples chosen are all synthesisers and can control each other, all four instruments can be played simultaneously by one person.

Once you have recorded the backing track, you can rewind the tape in the X-15 and then sort out your sounds for the next part of the song you're going to record. The next stage might be to add a guitar track so you could try the following method.

Set up the X-15 to play back the previously recorded track and monitor this on headphones. Take the guitar and put it through any effects pedals you normally use to get the sound you want and then take the output from the last effects pedal into the line input of the MN-15. Select line input with the input source switched to the compressor mode and set the release time fader to 'fast'. Play the guitar and adjust the input fader to around half way, then set the record level on your X-15 so that the meter peaks around the 0VU mark.

At this point you can start experimenting with the MN-15's compressor. If you play the guitar and increase the fader level of the input channel you will notice the 'Min' light starts to flash at the loudest points as the guitar is played. This indicates the onset of the compression and means that the sound has crossed the threshold point at which the compressor starts to work. What the compressor is actually doing is squeezing the natural dynamic range of the guitar's sound, in effect reducing the difference between the quietest and loudest parts of the sound. The reason for doing such an apparently drastic thing is that by compressing the guitar sound into a smaller dynamic range you can record it at a higher level on the tape. This has two major advantages: first of all, it allows you to record your sound at a healthy level and well away from the tape noise, and secondly, because the level of the guitar's sound doesn't wander about too much, its clarity within a mix is enhanced and results in guitar finger picking techniques and strummed chords being of a fairly similar volume level.

You will notice that as you increase the input level and the compressor comes into operation the actual maximum signal level on the VU meter will start to drop, this is because the dynamic range is becoming narrower and so the variation in the meter's movement is less. By increasing the compressor's release time you will hear the effect it has upon the sound quite clearly and also if you observe the VU meter you will notice that on fast transient sounds the level indicator shoots up but comes down more slowly. The release time control can be used to help thicken up certain sounds and experimentation by listening is always the best approach. When you have experimented with the compressor, you can now listen to the previous backing track via headphones and record the new guitar on another track of the X-15.

To summarise, a compressor is a very useful recording aid and should be seen as a means of controlling the dynamic range of your sound so that it fits within the dynamic range of your tape recorder resulting hopefully in a well-recorded sound.

The compressor in the MN-15 is ideally suited to a wide variety of applications such as vocals, guitars, basses and any other sound that you can pick up with a microphone; it's particularly useful for keeping drum machines under control, especially snare and cymbal sounds which can often overload the tape recorder and end up sounding very slushy.


As outlined before, the MN-15 can be used in both track laying and bounce down modes. When using the unit in a bounce down situation all you have to do is connect the outputs from the various tracks of the X-15 eg. tracks 1 to 3, to the 'tape in' channels of the MN-15. Then mix the relative levels of the instruments and take the output from the MN-15, plug it back into track 4 of the X-15 and bounce the three tracks down to track 4. You can, of course, add another sound at the same time by plugging another instrument into the spare channel of the MN-15 for a straightforward recording, or alternatively you could plug into the compressor input channel for additional processing. If you're really ambitious you can connect instruments to both of the spare inputs and mix five separate sounds together.

As can be seen from this example, it's quite easy to build up tracks on the X-15 which contain several instruments per layer and are relatively noise free. Common sense tells you that the more times you bounce sounds from one track to another the more the tape noise will build up. By using the MN-15, a marked improvement in the overall recording quality can be achieved purely by virtue of the fact that you can mix several sounds together before recording them, instead of the usual method of track bouncing every time you need to add another instrument.

Concrete Conclusions

The MN-15 mixer/compressor is a splendid little accessory and represents exceedingly good value for money at only £40.95 - you can't even buy a compressor alone for that price never mind getting a mixer thrown in as well.

In operation the unit was quiet, clean sounding and simplicity itself to use. It must be pointed out that although this review has concentrated on using the MN-15 with the X-15 Multitracker, it can, in fact, be used with any other similar piece of equipment. The MN-15 mixer operates at the standard semi-professional signal range of -10dB and so it's compatible with a wide range of cassette and open spool tape recorders as well as a great many direct outputs from musical instruments. There are no real competing devices on the market that can compare with the MN-15, and it must be said that Fostex have a real winner here.

Review sample kindly supplied by the London Rock Shop. RRP £40.95 including VAT.

Further details from London Rock Shop, (Contact Details) or from Bandive Ltd, (Contact Details).

Previous Article in this issue

Cutec Reverb/Delay

Next article in this issue

Sync or Swim?

Home & Studio Recording - Copyright: Music Maker Publications (UK), Future Publishing.


Home & Studio Recording - Sep 1984

Donated & scanned by: Mike Gorman


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Mixer > Fostex > MN-15 Mixer/Compressor

Review by Paul Gilby

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> Cutec Reverb/Delay

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> Sync or Swim?

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