Hafler Iris preamplifier
The Iris preamplifier is undoubtedly a Hafler product; with its discrete, charcoal exterior, it could hardly be otherwise. Yet it was designed from the start both to be fully remote-controllable and to be the basis of a complete remote-control system. Though David Hafler did have some design input, the Iris is also fundamentally the brainchild of Acoustat's Jim Strickland, a result of the engineering cross-fertilization that the new owners encourage.
Jim was responsible for an all-FET preamplifier design (the Trans-Nova) and a power amplifier design (the TNT-200) which impressed the socks off the venerable J. Gordon Holt a few years back (footnote 1), and it was therefore with a degree of eager anticipation that I approached this review. (It is also not irrelevant that the first true high-end preamplifier I ever heard, back in the days when I thought that amplifiers all sounded alike—1978—was the Hafler DH-101.)
Looking inside the Iris, the full width and depth of the interior is occupied by a single large double-sided printed circuit board that carries the audio circuitry and all of the power supply apart from the transformer, which is physically unprepossessing and bolted to the rear panel. A second pcb is mounted above the main board, this carrying the IR input decoding circuitry, while the front panel push-switches and their associated LEDs are carried on a third, vertical, board into which the IR board plugs via an edge-connector. The rear of the IR board features pins carrying the Iris 8-bit-wide data bus that are accessible through a rear-panel slot. As other Iris components become available—a tuner is imminent and a CD player is planned—this will enable them to be daisy-chained via this bus so that their functions can be controlled by the master remote, which already has the relevant commands programmed.
For example, if the Iris tuner is connected to the preamp, then selecting Tuner with the remote will enable the remote's secondary tuner commands to be activated, thus avoiding the use of a special Shift key. However, to return the remote to controlling the preamp's functions apart from Volume and Balance, the user has to hit the Clear key.
Compared with most high-end products, the interior of the Iris is full. Every product carries a label that says something along the lines of "Do Not Remove The Cover While Unit Plugged Into The Mains: Danger Of Shock!"; accordingly, reviewers and technicians are careful to avoid touching the parts of the circuit that carry the full 110V. But with the Iris, it demands more than a little care if not unplugged: the three utility mains outlets, with uninsulated connections, are mounted at the top of the rear panel, and a small pcb carrying the mains fuse and transformer primary voltage options resides at the right-hand side of the enclosure. Anyone taking the cover off the Iris without unplugging the unit from the mains, to change the cartridge loading for example, must take more than normal care to avoid a shocking experience.
Following the circuit from the phono inputs, these are on the far left of the rear panel (when seen from the front) and feature high-quality gold-plated jacks insulated from the chassis. A twisted-pair connection carries the signal to the board where it encounters a double-pole, double-throw, rear-panel switch that selects either a 47k input resistive loading, a pair of pcb sockets that carry the desired shunt capacitance (220pF is supplied), and sets the input stage for around 40dB gain, or increases the stage gain by 20dB and selects a pair of sockets that carry the resistor to set the MC loading. 100 ohms is supplied as standard, but it is a trivial task to change this to any preferred value.
The phono circuit itself appears very simple but is the subject of US patent #4,496,910. Carefully selected J-FETs are used throughout—Jim Strickland feels that he has yet to build a preamplifier with other devices that sound as natural overall as an all-FET design—and the phono stage uses two n-channel and two p-channel FETs in a complementary-symmetry configuration feeding a further complementary pair. The output is taken from the drain connections of these two, with the RIAA equalization components contained in a negative feedback loop taken from the same point. The circuit gain is increased for MC usage by bypassing one resistor that connects the input feedback point to ground.
The output of the phono stage is AC-coupled to one input of a Motorola 4051 chip, while the seven line-level inputs are DC-coupled to the IC's other seven inputs. The 4051 is a common CMOS IC, made by a number of manufacturers, that is often used to select one output from eight inputs or vice versa, for multiplexing eight signal sources to a single-channel 'scope, for example. Whichever input is fed to the IC's output is selected by a three-bit input word, supplied in the case of the Iris either by a front-panel pushbutton or by the remote decoder card. The 4051 features an on-resistance of typically 80 ohms when powered from a 15V supply, and very low leakage from any of the non-selected inputs, but it does have a potential problem: when used for voltage-switching purposes, as it is here, its on-resistance will be modulated to a small degree by the signal. To avoid this having a deleterious effect, the 4051 output therefore needs to be followed by a high-input-impedance buffer circuit. The Iris takes the selected input signal and feeds it to a simple FET source-follower with a constant-current FET load, thus ensuring that the 4051 is not loaded down in any way.
The advantage of solid-state switching is that there are no mechanical connections to age or corrode, and that the switching chips can be placed close to the rear-panel input sockets, keeping signal-carrying pcb tracks as short as possible.
This buffer sends the selected signal to the two tape-record sockets, each selectable by a latching pushbutton, and is followed by mechanical tape monitor, mono, and EPL front-panel switching, none of which is duplicated on the remote, which would appear reasonable. These switches are connected to the appropriate rear-panel input and output jacks by long pcb traces; purists should rest easier in their beds knowing that these will only appear in the signal path during tape-monitoring operations or if an external processor is connected. The state of the tape switches is monitored by a logic circuit that ensures that the preamp will refuse to select the Tape 1 input if Record 1 is engaged, or the Tape 2 input if Record 2 is engaged. In addition, although the preamp source-select buttons still operate with either Record 1 or 2 engaged, the remote control's source buttons are locked out to avoid the user inadvertently spoiling a recording in process. Remote control of Volume, Balance, Mute, and Normal are still possible, however.
Footnote 1: In Vol.6 No.3 and Vol.5 Nos.5 & 9, respectively.