The Norman Koren Dynaco PAS Preamp

Some of you may have seen Geek's rebuild of a Dynaco PAS using some specially designed boards based on  Norman Koren's SPICE design back in May of 2010. Below you can see his very nicely appointed PAS preamp.

Geek's PAS rebuild using the Norman Koren boards

Geek, who runs ClassicValve, is now offering the Norman Koren boards for sale, calling them the Norman Koren PAS Purist Mod as it eliminates the tone controls from the original PAS 2 / 3 / 3X design. From what I read, these boards are a much more modern sounding design, with more detail and clarity than the stock boards. The Dynaco PAS has a warm and endearing sound, but it really isn't the last word in detail. Both the line and phono boards make use of three tubes each rather than the stock boards that only use two, so an additional tranformer is necessary for the 12X4 rectifier tube and pilot light to prevent overloading the PA211 transformer. You can also mix and match other PAS boards with any combination of the Norman Koren boards by changing a few parts and running the leads in a different configuration. Below are the board schematics from ClassicValve.

Norman Koren PAS Line Board Schematic

Norman Koren PAS Phono Board Schematic

If you've kept up with my earlier blogs, you've seen that I've already done a complete rebuild on a Dynaco PAS, but after all that I've heard about the Norman Koren design, I just couldn't resist building another. Geek offers the two purist boards, along with his own PAS regulated power supply for the special package price of $75 (as of this writing). He also has the tube sockets and MOSFET & heatsink for the power supply board. Below, you will see my mostly populated set of boards with the obligatory Kiwame and Takman carbon film resistors and Obbligato Gold caps. It took a little bit of time to find the correctly-sized polypropylene film caps on Mouser, if using something like a square polyester film cap, it would likely be difficult to squeeze in place. The silver mica caps are all 1% for the sake of accuracy in the phono board. There's a fancy pants pair of Mundorf caps that will be occupying the left and right capacitor slots on the line stage, but will have to be installed after the boards are mounted due to spacing.

Norman Koren Populated Line Board

Norman Koren Populated Phono Board

Regulated PAS Power Supply in Progress

Rather than going the route of using a PAS donor chassis like I did in a previous build, this special PAS will be built using a brand new PAS chassis from Dynakit Parts. Up until now, no such thing was available on the market, but Kevin @ Dynakitparts has been working for most of this year on this offering. The chassis is the same identical size to the original, but is made of thicker Zinc plated steel. Two different color covers are available in polyeurethane textured brown or black, and three different faceplates are available, on that resembles the original, a full-function version, and a black "purist" faceplate that eliminates any unnecessary controls. 

The new chassis feels much more solid and durable than the original PAS chassis, and there are a number of new mounting options to mount the transformer to the back or add an additional filament transformer in place of the multicap. The fourth power outlet was removed, which is ideal as there was really no room for it in my other build. The outlets are included, which appear to be from Radio Daze, so they are ever-so-slightly larger than the ones included in the original PAS. The cover includes some much needed vent holes on the left and right sides, as the PAS can get rather hot on the inside. The textured black paint on the cover looks excellent. The original plan was to sand it down and powdercoat it, but the finish excellent and it will be kept as-is. Overall this is a very thoughtful offering, it's obvious that a lot of time and energy went into the planning. Color me very impressed (in the interest of full disclosure, I did receive a small discount on the chassis from DynaKitParts, since this is a semi-review). The cost isn't much more than a vintage PAS on that auction site, and often times vintage PASs have some dents and dings to worry about. 

One thing I'd like to see Kevin offer in the future is a gold faceplate like the original. A gold faceplate and brown cover would look vintage-chic for sure. 

This build will be using the PAS Purist Faceplate in black. The Purist faceplate only has slots for tone controls, volume, balance and power. Most modern boards for the PAS do not offer the tone controls anyway, so I'd imagine the majority of modern builds will make use of this faceplate. 

The Faceplate is very attractive, featuring engraved fonts that are color-filled light blue and white. One minor quibble, the same issue I take with Front Panel Express panels, is that the sides are not anodized black. I'd imagine that would only bother anal people though ;) A quick trace around the border with a Sharpie is really all that's needed to make it black as well. 

PAS chassis with cover off

PAS Purist black faceplate

In the above photos you will notice a Dynakitparts reproduction PA-211 transformer. This unit has the same dimensions as the original, but appears to have a little bit more headroom in the specs: 120 vac, 50/60 hz primary. 330-0-330 vac secondary @ 15 mA DC rectified current plus 10.9 vac secondary @ 1.25 A supply for tube filaments and pilot light.

The power supply board has now been completed. Due to a shortage on a few of the parts, some substitutes were used which will require a half ohm power resistor in line with the heater section.

Completed PAS power board from Classic Valve

In the interest of having the interior and back match the exterior, the chassis will be taken to the powdercoater for a nice textured black finish. Before that happens, the RCA back panel will be fashioned out of FRP material and I would need the chassis to verify the measurements and hole placement. There are already a number of replacements for this board pre-made by Tubes4HiFi and Dynakitparts, but I wanted to challenge myself and save a few bucks in the process.

The FRP board was purchased at ClassicValve for about $10 shipped. A few lines and circles were drawn in with a black marker which would be a guide for the drill press. These will be rubbed away with isl-propyl alcohol after the fact.

Marked FRP board for RCA panel

Since the six inputs on the original PAS is a little overkill in my humble opinion, I will only be including four on this example, the phono, which is off to the left, and three other inputs. The output RCAs will be fastened directly on the chassis using slightly larger Cardas RCAs. The RCAs included below are very reasonably priced gold-plated RCA jacks with Teflon insulation made by Philmore, the MTG12. The two dremelled-away areas on the left of the board will be to allow clearance for the Cardas RCAs.

RCA panel front

The grounds on the three inputs (not including the phono) are all connected using a single piece of UP-OCC copper wire. The ground solder tab is twisted slightly on each so the wire can be fed through without issue.

RCA panel back

It's common knowledge that the original PAS chassis could benefit from some additional airflow. I was very pleased to see that Kevin opted to add side ventilation to the cover, which can be seen below:

Ventilated PAS Chassis Cover

Similar to the previous PAS rebuild, I opted to add even more ventilation to the chassis by punching 6 holes in the rear of the chassis using a Greenlee die punch. These would be right behind the power supply and centimeters away from the transformer. A perforated metal sheet was added in combination with the holes which would prevent fingers from touching the power supply unit. The RCA panel was also secured to the back of the unit using copper pop rivets. Cardas rhodium-plated RCAs were installed next to the RCA panel to act as the output RCAs. The chassis conveniently includes some hole covers, plastic grommets and snap-in power outlets which should be visible in the photos below. 

PAS Chassis Side

Rather than run a lamp cord directly into the unit, a removable C7 power cord would be used. A little bit of filing of one of the power outlet holes was all that was necessary to fit a C8 inlet with a pair of pop rivets.

PAS Chassis Back

Finally those beautiful circuit boards could be installed. Since the 0.47uF capacitors were oversized, they had to be installed after the board was fitted. The Dynakit Parts transformer was then dropped in place with a set of rubber washers underneath to ward off excess vibration. A fuse holder was also installed to the front panel next to where the power switch will be located. 

Chassis interior with PCBs in place

The front controls could now be mounted. Selected for the task was an Elma 6 position switch, DACT CT2 stepped attenuator, Audio Note balance control and a DPST silver cadmium power switch from Dynakit Parts. 

DACT CT2 stepped attenuator and Elma 6 position switch

Also installed was an LED power indicator board which takes the place of the incandescent pilot light from the original PAS. These boards can be purchased from a number of places including ClassicValve and DynakitParts and put a lot less strain on the transformer than a pilot light would. 

Controls and LED board installed

The pre-amp could now be wired up. Chosen for the task was Neotech 24 gauge UP-OCC copper in Teflon for signal wiring and Kimber 19 gauge TCSS for power / heater wiring. Cardas 2x24 shielded wire was also used for the phono input wire and pre-amp output, as both traveled a little closer to the power supply.

Norman Koren PAS fully wired up

I didn't bother wiring up the convenience outlets as they'll likely never be used, which keeps the interior wiring a little cleaner on the bottom. As can be seen below, only the power wire to the fuse & transformer, 12V wires to the LED, ground connections and output wiring were run in the lower part of the chassis.

Bottom of chassis

In some instances, wires were soldered directly to the board rather than using the screw down terminals. I tend to prefer a solder joint over a screw terminal when possible, just seems more secure ;)

Interior side view

So, the Norman Koren PAS is fully up and running. The sound is a real delight, definitely reference-level in my humble opinion. Everything is very crisp, clear and engaging, no detectable noise either, and this is with low-cost Sovtek long plate 12AX7s. It will be interesting to try some NOS tubes at some point. I want to thank Gregg over at Classic Valve and J.P. as they were able to answer my questions during the build process.

One minor quibble I have with the build is the gain is quite high, and you reach full volume just a few clicks in on the attenuator. Gregg mentioned there is a way to remedy this with a small modification to add more NFB to the line stage, so I will be performing this next.

Replacing the 120K resistors on R6C (the cathode follower load) with a series of a smaller value resistor and a capacitor will assist in reducing the gain. The reactance of the cap in series should be equal to the series resistor at 10Hz, then multiplying that resulting value by 10+ is necessary to avoid LF instabilities. Choosing a 62K resistor, then calculating the value of the capacitor and multiplying the result by 10 led to a value of 2.7uF, since this value is not critical and can be increased without issue, 3.3uF caps were used as Obbligato does not offer 2.7uF. Since there is little real estate left in the PAS chassis at this point, this little mini circuit would be mounted where the tone controls normally reside. In order to mount the mini circuit, a small turret board was fabricated to contain the two caps and two resistors. Fly leads would run to and from this to the board where the existing 120K resistors resided.

DIY turret board with Obbligato caps and Kiwame resistors

Mounting this small board proved to be a little tricky. After a couple of tries, the final mounting place for the board was directly behind the LED power indicator and power switch using a pair of standoffs. The existing 120K resistors were removed and Neotech UP-OCC wire was run to the board in their place.

Final interior Norman Koren PAS

Indeed the mod reduced the gain substantially. I'd have to say this is one of the best sounding pre-amps I've heard, and an extremely good value for the cost of parts. Highly recommended!

The Fine Print:

Please remember that building circuits and performing circuit modifications can be dangerous to you and/or your surroundings and should only be performed by a certified technician. The owner of this blog and all associated parties can not / will not be held responsible if you attempt a build or modification posted above and cause physical harm to yourself or your surroundings. Many electronics contain high voltages that can kill, and mods, if performed improperly, can be a fire hazard. Please keep this in mind.