Audio Synthesis ADEQ phono stage

ADEQ PCB

This was introduced as part of series written by Ben Duncan, published in Hi-Fi News and Record Review in October/November 1989 and released as a kit by Audio Synthesis. I bought one of the last available PCB sets before AS discontinued the kit, and bought my own parts for it.

Each channel uses an SSM2016 preamp IC for the input stage, and a pair of AD845s for RIAA correction and output buffering. Apart from the slightly exotic choice of ICs, the main circuit is quite unremarkable. The 75us EQ is carried out passively, and the rest actively around the second op-amp stage. There is an optional IEC filter at 20Hz or so, which can be switched in or out via a relay. The gain of the input stage and the cartridge loading are selected by plugging in appropriate DIL headers, which is a nice compromise between external switching (with its concomitant potential for noise and poor contacts) and the need to solder in a new set of resistors each time you change the cartridge. The only problem in use is that the AD845s have a high quiescent current in their output stages, presumably to reduce crossover distortion, so they dissipate a lot of heat. The insides of the preamp boxes became worryingly warm, so I drilled holes in the box to improve ventilation.

There are three separate dual rail voltage regulators per channel on a separate board from the equalisation board. I get the impression that Ben Duncan put a fair amount of effort into the choice of decoupling in the regulator circuits, which are designed around LM317/337 type regulators (actually Ben recommends the better-specified Linear Technology versions, but I didn't try them). I use outboard dual mono power supplies, with 160VA transformers, 10,000uF of reservoir capacitors per line, each made up of paralleled 1000uF SMPS capacitors, and pre-regulated DC supplies to the ADEQ's regulators at 18V and 22V.

At the moment the ADEQ boards lie inside my preamp, with the equalisation board sitting on top of the regulator board in Audio Synthesis' recommended piggy-back arrangement. The ADEQ shares its power supply with the other preamp circuits, although I plan to put it in a separate box with its own independent PSU to give it some more space - I may physically separate the boards, but then I may not.

I am using an Ortofon MC30 Supreme, a medium-output (0.5mV) moving coil cartridge with a neutral balance and good detail. It is relatively undemanding compared with some of Ortofon's low-output moving coils (for instance the MC2000 and 3000), but still needs a fairly low-noise phono stage. I use an load impedance of 20 ohms, as recommended by the manufacturer (Ortofon UK were unable to specify a load capacitance, so I used none), and a gain of 65dB, the latter giving a theoretical output of just under 900mV for an input of 0.5mV. This combination gives very low hiss and hum - easily swamped by record surface noise.

I have used quite a few phono stages in my time, including the ones in the Croft Super Micro and Super Micro A valve preamplifiers (UK hairshirt budget esoterica), the Exposure X (years ago), and one or two I built myself. I moved up to the ADEQ from one based on John Linsley Hood's design in Electronics Today magazine, and found a big improvement in clarity and dynamics, which may or may not be related to the less sophisticated power supply I used in the latter. It certainly sounded more detailed and firmer in the bass than even the better of the valve preamps.

I still occasionally long for the lushness and body that I've heard from the best valve phono stages, but on the whole I'm very satisfied with my ADEQ.

Alex Megann, March 2000

Update - May 2000

I heard the effects of a PS Audio Power Plant 300 in my system. This is a mains regeneration unit that can supply up to 300W, with fine control over the frequency and voltage. It had little (if any) audible effect when I fed my turntable from it, which wasn't much of a surprise, since the deck already has an external quartz-locked AC supply and hence is substantially isolated from the mains. I was more surprised to hear a big improvement in clarity and ease to the sound when I used the PP to supply the preamp (including the ADEQ). I already have a mains filter and substantial reservoir capacitance in the power supply, so I wasn't expecting what I heard. I'm planning to rebuild the phono stage and preamp in separate boxes, and I have included the option of an external DC power supply. I'm looking forward to experimenting with a battery supply, which ought to better even the effects of the Power Plant.

Update - September 2000 My ADEQ: inside view

I've heard recently that Audio Synthesis have just released a new phono stage (the first since they discontinued the ADEQ some years ago) called the Passion Phono. This is a very different design from ADEQ with totally discrete components and a simpler circuit, which David Heaton of AS tells me is vastly superior to the ADEQ (though he still has the ADEQ in his own system!).

The Passion Phono is not strictly available as a kit, since the PCB is supplied ready-built and calibrated, and it costs a fair bit more than ADEQ.

Update - May 2001

I have just rehoused my ADEQ in its own box: this is the first more-or-less completed unit in my long-term system rebuilding plan. The ADEQ and regulator PCBs are now mounted inside a 0.5mm copper subenclosure, and are compliantly mounted to the chassis. The power supply is similar to the original, although now it only supplies the phono stage instead of the whole preamp, and it also uses SBYV27-150 soft-recovery high-speed rectifier diodes rather than the original generic 1N4001s. The box includes switching which will allow an external power supply to be installed, as also will the line preamp and the active crossover.

My ADEQ: front view I have added balanced outputs by connecting up a further AD845 to each channel as an inverter, and connecting this to the cold pin of the XLR plug. There is also an XLR input to allow balanced connection to the cartridge - I was already using a balanced configuration on the header plug which selects the gain, but the SME interconnect doesn't quite constitute a balanced connection since the shield is connected to the cold pin of the RCA plug.

I can't say heard a marked difference from the original arrangement, apart from a slightly lower overall noise level. However, I'm still only using my Maplin power amps, which aren't very transparent - I'm looking forward to finishing the Stochino power amps and reinstalling my three-way speakers.

At some point I intend to replace the SME lead from the base of the tonearm with a balanced interconnect chosen to have thin, flexible signal cores, and then to run the ends of the signal wires "nude" from the P-clip on the plinth of my PT TOO to the SME DIN plug. The latter changes should minimise the mechanical coupling, which I feel is a weakness of my current setup - the armlead certainly affects the bounce of the subchassis. This idea came from seeing the "flexi-link" on a PT Anniversary, which has extremely thin flexible wiring between armbase and RCA sockets on the plinth. This remarkable (but sadly discontinued) turntable has the least coupling between the subchassis/platter assembly and the outside world that I've ever encountered. In addition, the mounting of the motor on the subchassis means that the suspension is almost completely undamped, which leads to an amazing degree of mechanical isolation.

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