Monday, June 21, 2010

Building the Audio Sector Phono Stage

(Picture by Audio Sector)

I've had some turntable upgrades in mind, starting with a low output moving coil cartridge and a new tonearm. With my current setup I was worried about having enough gain for a .3mv MC, so I decided to build the Audio Sector Phono Stage. This is based on the 47 Labs Phono Cube, but utilizes better parts and a shunt regulated power supply. Audio Sector just provides the PCB board, while you provide the sweat, enclosure and passive parts selection. The board quality is excellent and is designed so the power supply transformer can even be mounted within the same enclosure. For my build, I mostly stuck with the recommended parts, but opted for RelCap Teflon and Polysterene film caps over the specified (and expensive) V-Caps. Instead of Blackgates, I chose Nichicon for power supply capacitor duties. Building was done over the course of a few days as I have a hard time soldering such small parts onto the PCB board.

The signal path is unique as there is no cartridge loading per se - the input impedance is zero, which is pretty much a straight current-to-voltage converter. Gain and frequency response are determined by the cartridge voltage and output impedance. There are also no signal capacitors in the signal path as RIAA equalization is done in the feedback loop. Feedback? Oh no! Did I mention the OPA627 op-amps that are used? Now things are getting weird for a discrete parts builder like myself.

Don't let the integrated circuits and feedback put you off as the end results were beyond my expectations. This the most transparent phono preamplifier I have ever heard. Hyper detailed, it shows every flaw in your turntable setup and the recording quality. It's not a kind, mellow sound at all - but is revealing, quick but still grain free. Bass is also oh-so fluid, deep and bouncily rhythmic which started my feet a-tappin'. With my Magnepan speakers, vocals are eerily real and float wonderfully between the two panels. This is a very forward sounding preamplifier and pulls the listener right into the soundstaging. In comparison, my Threshold FET-10/PC sounds a touch muffled and dark.

I can't stress patience enough as the upper-midrange energy is extreme until some time has been burned into the circuit. I don't think I've ever heard any audio component break-in like this. It literally takes hours for everything to settle down and start sounding good.

As I'm using a high output Dynavector 10X5 phono cartridge, I'm expecting some better results with a low output moving coil. Stay tuned!


System:
preamp: Threshold FET-10/HL

phono preamp: Threshold FET-10/PC
amplifier: Threshold S/500
analog: VPI HW19 MkIII - Rega RB300 with Incognito wiring - Dynavector 10x5
speakers: Magnepan 1.6/QR
speaker cable: Kimber 4PR/8PR Bi-wire with banana jacks
Interconnects: Cardas Cross and Cardas Quadlink 5C


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