Summit Audio TMX-420

Four channel tube mixer

Summit Audio TMX-420 Four channel tube mixer

Review by George Shilling

The TMX-420 comes accommodated within the familiar silver-coloured Summit 2U rack housing. It is built like the proverbial brick outhouse, and weighs a stonking 23lbs. Large black and red knobs and industrial strength toggle switches adorn the front panel. There is no denying that the TMX is aesthetically pleasing. However, it has no microphone preamps, no EQ, and just one Send output. Despite the TMX's undeniable charm, why on earth would anyone buy a 4:2 line mixer at the price of a 32 channel 8-buss desk?

When I first received the review model, it did not work. Unfortunately its replacement was only slightly better, with DC showing on the meters. By the third delivery, the problem had been found to lie with the power supply. Apparently it is difficult for Summit to simulate the British 240V 50Hz supply in California, but the problem is now rectified.

I am sure all customer units' PSUs will be suitably modified before sale. Hopefully, a UK mains lead will be included: mine was of the completely useless American variety. The left side of the front panel accommodates controls for the four input channels. Each channel has a large black level knob; two smaller red knobs control panning and send level. None of the pots are damped, which makes accidents likely, and small adjustments difficult, particularly with the smaller knobs.

Summit Audio TMX-420 Four channel tube mixer

There are small toggles for phase change (180*/0*), On/Off (slightly misleadingly labelled In/Out: I initially - wrongly - thought that this defeated the panpot, which has no centre détente), and Send Pre/Out/Post. In the middle, the two busses A and B each have an illuminated VU meter, overload LED, and passive Level trim knob. There is a Send overload LED, a huge Master Level knob (great for fades), and three large toggles for Send Link, Stand Alone/Link, and Power On/Off. The link switches enable an important feature: the ability to chain a number of units together. A small illuminating button enables the unit as Master in a linked situation.

The rear panel features input sockets of the combined Male XLR/Jack variety which have not proved entirely reliable in my experience. However, this solution was doubtless preferred to having dual input sockets for reasons of sonic purity. Outputs have separate XLRs and Jack sockets, and here some clue is given to the design philosophy of the unit: the Buss outputs and Send outputs are on bolt-on panels. Jack connections are -10dBu unbalanced; XLRs are +4dBu balanced. There are two DB-15 connectors for linking Send and Busses (a cable is supplied). A massive protruding heatsink is set adjacent to the IEC mains socket and fuseholder/voltage selector. Unfortunately, the heavyweight nature of the internals means that as well as top and side vents and the aforementioned heatsink, the inclusion of a cooling fan is deemed necessary. It is placed to the side of the box and is noisy, especially if the TMX-420 is not mounted in a rack. Inside the unit, a huge, weighty transformer dominates. Considering the meagre features of this unit, there are a huge number of components on the circuit boards, which are unusually thick.

Each section of the mixer is modular, interconnected by ribbon cables. This enables separate testing during manufacture, and individual selection of valves during construction. Each channel employs a Chinese-made 12AX7A/ECC83 tube, and each of the three outputs a 6922/6DJ8/ECC88 hanging down from overhead circuit boards attached to the Output panels. On mine, the upside-down valves were not seated properly. They had probably come loose during transit as their boards are only held rigidly at the opposite end to the valve sockets. In use, the first thing I noticed is that there is nothing particularly 'valvey' about the sound. It is extremely transparent, and details are kept fully intact. It is claimed that due to the nature of the circuitry, the jack inputs sound different from the XLRs. I found that any differences were fairly subtle, but I thought I detected a slight presence lift and greater dynamics with the jack inputs compared to the XLRs which were slightly smoother. Either way, the clarity is exceptional.

There is no doubting the specialist nature of this mixer. It is aimed at two markets: primarily for recording small ensembles such as jazz, classical or 'à capella', where the choice of mic preamp is left with the user. If recording using a purist four-mic setup, perhaps to a high sample-rate digital stereo recorder, this would be the appropriate mixing device. Alternatively, by chaining a number of units together, a multitracking setup is a viable, if expensive, prospect. Up to four units can be linked, giving 16 inputs. The combination of +4dBu and -10dBu outputs is useful, but there are cheaper ways of achieving level matching. The modular nature of the TMX enables future expansion, and Summit are currently exploring Digital I/O possibilities. This unit certainly does its job well, and is difficult to fault. However, this is big money for something that does so little. While purists may love the TMX-420, I suspect that it may not feature in many best-seller lists..

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