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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Upgrading the Dynaco ST-80 power amplifier

After the success of my Dynaco SCA-80Q and PAT-4 treatment, I decided to tackle the ST-80. This would prove to be a fairly easy to do job as I used many of the upgraded parts from the SCA-80Q. I've always been a separates guy, so I had no problem cannibalizing the parts for the amplifier upgrade.

The Dynaco ST-80 was introduced in 1969 and is roughly a pared down version of the venerable ST-120. The '80 features a single rail power supply, quasi-complementary output, dc blocking output capacitors and the use of only 12 transistors. However, unlike the ST-120, the ST-80 does not feature a regulated power supply, which may have improved the sound since the amplifier would clip more gently when running hard.


I bought my Dynaco ST-80 on Ebay for a paltry $45. It was in working condition, but you can see by the auction picture, the parts are aged and in need of replacement. I decided against any serious modifications and just went for a straight parts replacement. The resistors tested within spec, so that left the capacitors to upgrade.

Note that modern values are a little different than the days of yore. For example, I used 47uF instead of 50uF. Radial capacitors fit in axial PCB locations without issue. However, modern capacitors are about 1/3 the size of the originals, so new clamps for the standalone capacitors also need to be purchased. Also note that the foil on the circuit boards is flimsy - use a low wattage iron and take care when pulling out parts! The stock 'wire-around-the-capacitor' inductors were replaced by modern toroid inductors I bought from an Ebay seller who specializes in Dynaco amplifiers. Do a search on "Dynaco inductor" if you want to buy a pair for yourself.



For the power supply, I merely replaced the old capacitors with larger values. The four 400 ohm resistors on the power supply PCB were replaced with 390 ohm / 10W units. Make sure to keep the resistors directly off of PCB-19 as the SCA-80 and ST-80 are known to start on fire here as the board takes a lot of heat.



For testing, I hooked the amplifier up to a pair of test Radio Shack Minimus 7 loudspeakers and fed a signal in from my Threshold preamplifier combination and VPI turntable. This is a pretty intensive front end for such a budget amplifier and speaker. I was very surprised how good this strange combination sounded, which just proves (at least to me) how important a good front end is to getting the best out of your system. Having passed the smoke test, I decided to bring the amplifier upstairs to replace my McIntosh MC-250 amplifier.


Initial results has an amplifier that sounds pretty classic solid-state. It thas great bass and is very fast sounding, but in comparison to the McIntosh 250, the upper mids and treble are a touch 'hard'. However, the ST-80 has very nice detail and it sounds a little less grainy than the McIntosh. The ol' Mac does win out on slam and warmth, sounding much like a poor man's tube amplifier. Hopefully some extra burn in time will smooth the Dynaco ST-80 sound out and make it a viable replacement.


Second System:
Dynaco PAT-4 modified
Dynaco AF-6 tuner
McIntosh MC250 amplifier
Panasonic linear tracking turntable
Pioneer DVD-V7400 cd player
various budget cables


Update on 06/09/10. Some break-in had smoothed out the more aggressive aspects of this amplifier and the end results are very pleasing. Though not warm like a tube amplifier or the McIntosh 250, the reborn Dynaco ST-80 is now a good budget amplifier with a cohesive dynamic sound. Good for efficient speakers and rock n' roll, it's not bad at all for a $45 amplifier with $50 worth of parts.

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