LAST EDITED ON Oct-17-03 AT 09:44 PM (PST)
The photos above show the front and rear views an 8 channel GTM-822 "master" mini-mixer. An 8 channel "add-on" unit and a 16 channel + 2 output fader panel are optional extras and can be set into a beautifully made wooden console designed by Professional Audio Designs Inc. who will be the sole USA distributors of this comprehensive mix system.
So what led me to design a mini-mixer? A yearning desire to get back to my time at Neve when I was the Special Orders Manager in the custom division and racks like this one were my very "bread and butter".
It may be thought of as a mini-mixer... a device for mixing multiple channels of analogue audio down to a stereo bus... but I wanted this unit to be something extra... in effect, a baby console.
I had several basic ground rules for the design:-
1. Top quality components like Swiss made Elma rotary switches, SFER conductive plastic potentiometers, E-Switch and NKK push button switches, Sifam meters, and Neutric connectors. My philosphy is that the use of top quality components, although expensive at first, will repay that investment many times over with the long service life and M.T.B.F. (minimum time before failure)
2. Classic single rail discrete transistor topology. I was asked at the recent AES show whether the unit had any capacitors in it. "Yes," I replied, "Bloody hundreds of them!" If you want to recreate an SSL9000 sound, with servos and zero capacitors then this is one direction to go... but if you want a classic "British" class A sound, then go for single rail and lots of capacitors! One huge advantage of this type of circuit is that every electrolytic has a 12 volt bias across it... this ensures correct polarisation and a long life. Both power supplies (the mixer uses one internal supply for the audio and another for the LEDs and relays... just like a vintage Neve console) are very slow start, taking around 10 seconds to reach the full dc potential. This way all of the capacitors are treated with tender loving care and allowed to polarise gently, rather than having a speaker thumping immediate turn on.
3. All external circuits (except for the headphone outputs) are balanced and line (+4dBu) level... just like a proper console. Connections are by Cannon D 25 pin plug and socket for Pro-Tools (or equivalent) in and out, Neutric XLRs for Line input and post-fade Direct Out, and Neutric 1/4" TRS jack sockets for the remainder of the connections. I would have loved to have used more XLR's but there just wasn't that much room on a 3U rear panel! 
4. I wanted this unit to be both simple to operate and to provide the user with the maximum of information. Each input channel has a ten segment colour LED bar graph VU meter. The stereo output has Sifam VU meters and a Sifam phase meter. You can monitor on two sets of speakers and headphones and the solo-in-place lights all the cut LED's in the affected channels. The control knobs and push buttons are colour coded... the input gain is a traditional Neve maroon Marconi knob, the Aux sends and masters are blue and the remainder of the knobs are grey. The L/R bus assign buttons are also grey and the Aux send buttons are black.
So let's go on a guided tour....
Starting with the line input, a red Marconi knob sets the input gain. There are numbers silk screened around the knob but you don't need to look at them... the level indication is provided by the LED VU meter.
The LED VU has ten LEDs, 6 green, 3 yellow and one red. You just turn the red knob so that the level is hitting the broad yellow region of the scale. The VU has a range of 23dB and the switch moves in 10dB steps. The mixer has an additional 10dB gain in the Channel fader.
I have located the source of the LED VU at the pre-fade insertion return point. My reasoning being that the user may patch an EQ into this point and the EQ may have several controls set to boost, thus raising the level fed back to the channel. The VU will catch this information and will, no doubt, start lighting the +3 red LED. The user now has two options:- Reduce the channel input gain so that the EQ operates at a lower region.... or leave it as a hot input signal and turn down the fader to reduce the signal hitting the bus.
These alternatives allow the user a choice in the nature of the sound in the channel.... an option not provided on other, more basic mixers.
It's worth mentioning at this point that there are two trim pots accessible on the front panel that allow for the VU 0VU reference level to be set and to align the level of the channel. This assists in any calibration tasks without having to heave the mini-mixer out of its rack location!
The channel has four bus sends... the main Left and Right stereo bus and a pre-fade pair of Auxes with their own level control. The Aux busses are intended to be used as a Cue (headphone) Mix though they could also be used as an effects send. Over on the right of the mixer is a stereo effects return path... effectively Channels 9 and 10 to the L/R busses.
The Channel L/R bus assigns can either be assigned individually or, if both are pressed, the pan-pot automatically is selected and a small green LED indicates this. The pan pot is centre-detented and provides a +3dB to infinity swing at its extremities.
The large grey knob at the foot of the channel is the Channel fader with 10dB signal level "in hand" at fully clockwise. Below it are two illuminated switches, a yellow Solo switch and a red Channel Cut switch. All muting is by opto-isolator so that the mutes are quiet... I chose this system as preferable to clunky relays or FETs and, if a Solo button is pressed, the yellow LED in the Solo switch will light as will all the red LEDs in all the affected channel cuts. Pressing more solo's will release the cuts.
The Mini-mixer has a 56 pin Elco on its rear panel into which can be fitted a multi-core cable snake to a remote fader panel, complete with Solo and Cut switches.
To the right of the mixer are the master output level pots and the monitor/meter system. Cut switches are provided for the Stereo output and the Stereo effects return input. The Stereo L/R output has a pre-fade insertion but the Aux/Cue send outputs are direct, post fader.
The meter system follows the monitor and can be switched between Stereo O/P, Stereo Playback return (to A/B the quality of your recording) and Stereo Aux Sends. A Phase meter accurately measures the degree of phase relationship between the L and R outputs and warns instantly of an error. Using the channel solos should quickly locate the faulty source and enable an external correction to be made.
Two loudspeaker outputs are provided post the monitor level pot and an independent headphone output has its own volume control and follows the same source as the Monitor/Meter. The headphone output is 2 watts into 8 ohms and more than enough drive for a pair of computer speakers, as well as 600 ohm headphones.
Well, that's basically it. I'm sure that you'll agree that the mini-mixer is really a baby console. 9 pin D connectors on the rear panel enable Add-on units to be linked together making it a 16, 24, 32 etc channel system. All the solo info is linked through to make the unit function as if a large console.
I had a lot of fun designing it!
Geoff
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