MC Step-Up Transformer using Altec 4722

Despite the extreme convenience of computer-based audio systems, the turntable has not lost it's place in the hearts of many audiophiles. Sometimes that extra effort can be worth it, balancing the plinth, brushing the dust off the record, cleaning the stylus, flipping the record after a few songs, the list goes on. There's just something nice about that analog warmth of that teeny tiny stylus wandering through the grooves of a well pressed record.

One of the requirements for owning a turntable is a phono pre-amp. A phono pre-amp brings the low-level signal from the record player up to a point where it can be run to a standard amplifier. Equipment like CD players and DACs don't have this requirement because their output is already at a suitable level. So, a very small gauge wire runs from the cartridge through the tone arm and goes to whatever jacks are on the turntable, generally RCAs on more modern decks. A pair of RCAs are run from these jacks to the phono pre-amp, which amplifies the signal and passes it on to the main pre-amp or integrated amp in the users system. Occasionally, the pre-amp or integrated amp will have a built in phono input, but many enthusiasts prefer a separate unit as the quality can be better. To complicate matters further, many phono pre-amps only amplify the signal of a moving magnet (MM) cartridge. There are also moving coil (MC) cartridges that have an even lower signal that the phono pre-amp simply does not have enough gain to play at a proper listening level. In order to remedy this, one can use an MC step up transformer.

A common step-up transformer, the Ortofon ST-80 SE

An MC step up transformer is completely passive, no electricity is needed to run it. The signal runs from the turntable to the MC step up transformer which amplifies the signal a small amount, then the signal runs to the phono preamp, which amplifies it further to the point where it can be run to the main-pre amp or integrated amp, where it is amplified yet again before it makes it's way to the speakers. So the flow would be Record Player > MC Step Up > Phono Pre > Pre / Integrated.

Many DIYers have taken to the fact that vintage microphone transformers can be used as an MC step-up device. As JELabs has stated: "Moving Coil cartridges behave very much like condenser microphones - low impedance and low output - requiring a voltage boost. Step-up transformers are passive devices and as long as they are wired properly, they are virtually noiseless. To me a wide bandwidth input transformer is the most elegant way of boosting MC output to MM phono level." There are a variety of units that fit the bill, from the Altec 4722 and 15095, Denon AU320, RCA MI12399A, Tamura TKS83, and Thordarson T-43606-A just to name a few. Below you will see a variety of these units in step-up devices made by JELabs.

A Variety of Step Up transformers

I'll be making use of the Altec Peerless 4722, it's one of the most popular vintage units and commands a price because of that demand, but from the way JELabs described it, it seemed that the units have a bit more of a euphoric warmth than the others, which I always appreciate. The Altecs are in the shape of a vacuum tube with eight pins at the bottom designed to fit into a vacuum tube socket. The chassis was custom-made by Keith, eBay seller po1019, and is a very fetching little unit. Holes were drilled for the RCAs, ground posts and switches and punched for the tube sockets that would be holding the Altecs.

Prepped Chassis

Cardas RCAs (GRFA) and ground posts (GRND) will be adorning the petite chassis. These were secured using a small socket wrench.

Some Cardas bling

More to come...


Need a pair of low impedance cables for your turntable? Contact Zynsonix Audio today.  

The Fine Print:
The above steps detailing the building of a MC step-up transformer are for entertainment purposes only and not to be performed under any circumstances. The owner of this blog and all associated parties can not / will not be held responsible if you attempt the process posted and cause physical harm to yourself, your surroundings or your property. Please keep this in mind.

Steampunk Art & Design

I wouldn't necessarily call all of my electronics work my own creations; many of the amps and preamps were existing circuits or kits that I may have modified the parts list or made a few minor circuit changes. As enjoyable as it is to build them, it's preferable for me to stick with tried and true circuits created by the experts and not come up with new schematics of my own volition. There tends to be more enjoyment in the final presentation, the color scheme and the chassis work and even the way the inside looks. A number of people have told me that my builds have a Steampunk look to them. I'm familiar with the movement, and I wasn't even aware of the fact that my design choices tend to follow that look, but perhaps was subconsciously making the design decisions with Steampunk as an inspiration.

Steampunk?

I suppose my designers manifesto is that I feel that amplifiers with older technology like vacuum tubes should have a vintage look and patina, even if they are being assembled in the 21st century. Vintage has a different look for different people, but I tend to think of copper and polished brass, gold plating, brown and copper hues with a crinkled / hammered texture, wood and metal grating and leather. These seem to be the same facets that find themselves in the psuedo-Victorian era Steampunk movement of design. I'm not sure that future projects will have the telltale brass gears and non-functional pipes that Steampunk gear tends to have, but I may think up other embellishments as time goes on.

I think that the main inspiration for vintage electronics was a company called Moth Audio that produced a number of designs from 1996 to 2007. There are many purveyors of audio gear that make really beautiful work, but in my minds eye, the aesthetics of Moth Audio gear is second to none. Craig Uthus was the head of Moth and at the time, the work was considered to be "retro-styled", the designs were highly distinctive and still highly sought after today. Craig's partner at Moth was Joel Marshall, a Hollywood prop designer, who designed the beautiful chassis you'll see below. 

Moth Audio NiteLite with 15E Radar Tube

Moth Audio Sphinx balanced line-out amp and phono preamp

I almost feel that these amplifiers belong in an exhibit somewhere. Perhaps someone out there could open up an audiophile museum one day, the cable section alone could take up a whole wing ;)

There is at least a few audio builders out there that fully embrace the Steampunk aesthetic; Aevil at Coppersteam Labs put together the best looking S-5 Electronics K-12G I've ever seen. I'd imagine there's more money invested in the copper than the amp itself, it's a real beauty that looks like it came right out of Bioshock.

AEvilMike's K-12G Amplifier 

I also found a lovely "effect-pedal" if you will made by Peter Groenewoud at Hilltree Productions called the Steampunk Doomsday Machine. The top features two 230V DC bulbs that carry the music signal and create the effect. Lots of wood, brass and copper in this Steampunk design. Peter will not share a schematic on this device as the 230V DC going across those lightbulbs is fairly dangerous.

Hilltree Productions Steampunk Doomsday Machine

Also, a new addition after the 2012 Bay Area HeadFi Meet was Frank Cooters amazing looking headphone amplifier. I don't know too many details about this one, but the glorious copper piping and top-plate and nicely figured hardwood really set this apart from other builds. Evidently all the work inside this beast is point-to-point, and Frank has a matching one that powers electrostats. In this photo it looks like someone has an Audeze LCD-2 headphone plugged in.

Frank Cooter's steampunk-styled headphone amp

In conclusion, I'm not sure that my builds are Steampunk. They may look like it to a certain degree, but it's nebulous where the line of vintage and Steampunk merge, intersect and share common facets. What do you think?

Looking for some steampunk-esq audio cables and equipment? Contact your friends at Zynsonix Audio.

A Customized Dynaco ST35 Kit

The ST35, also known as the Stereo 35, was released by Dynaco in 1963. It's the baby brother of the Dynaco ST70 and tends to be overlooked by many vintage amplifier aficionados. I think the main reason this is the case is that there isn't a large price difference between the two units, and with a tube rectified power supply, power choke, bias system and twice the wattage, it's hard not to want to step up to the ST70. For those of us with relatively sensitive speakers, the ST35 is a nice little unit with a small footprint and more than ample power (17.5 watts x 2). I've also read from more than one source that the ST35 is the best sounding of the Dynaco units stock, for whatever that's worth ;)

Because there are a lot less ST35s floating around, there aren't really any boards with modern circuits that fit into the original chassis. Shannon Parks of DIYTube has created a brand new circuit board that is similar to the ST35 circuit with some modern sensibilities if one would like to go that route. I prefer the looks of the original ST35 chassis myself, with the exposed PCBs on the ends. For this, I turned to Dynakit Parts. Dynakit Parts is a company based in Clifton, New Jersey and has an impressive selection of old school reproductions of Dynaco products. The kits are about as close to the originals as you can get, from the can capacitor to the screw-down terminal strips.

Dynakit Parts ST35 Kit

The original Dynaco ST35 Schematic

Rather than purchasing the whole kit, I purchased a number of the parts a la carte, as I was interested in adding a little bit of customization to the kit with a power choke, power switch, triode / ultra-linear switches, a C8 power inlet for a custom C7 removable power cord and a number of boutique parts. I also added the ST35-BCU bias control upgrade kit which fits nicely above the chassis. As stated on the Dynakit site:  The ST35-BCU employs an adjustable fixed bias circuit which allows for user adjustment of the bias setting of each (EL84) output tube via (4) separate on board precision bias pots. This allows you to not have to purchase a matched set of power tubes. And even if you did purchase a matched quad, power tubes tend to age differently, so they can be adjusted after this occurs. 


Kevin of Dynakit Parts was working on a brand new brown PCB board for the ST35 when I contacted him. I really appreciate it when a company isn't resting on their laurels but investing in new products and innovations. The new PC-13 boards from Dynakit is made from aerospace grade 240 degree Centigrade brown Polymide mil spec material and features silk screened component designations. The brown will go very nicely with the vintage patina powdercoat scheme that I had planned. I wish the bias board was made of the same nice brown material, but we can't have it all, can we? ;)

Brown Polymide boards with a few parts in place

I elected to populate the small driver / power boards with Kiwame carbon film resistors, Obbligato and Mundorf Supreme film capacitors and Teflon and ceramic tube sockets (the Teflon sockets even have beryllium copper contacts). The caps fit pretty much perfectly, although the Mundorfs will have to be mounted once the boards are mounted due to sizing constraints. The Dynakit build calls for nicer silver mica capacitors in place of the ceramic disc capacitors that were in the older units. These can be found just about anywhere (Mouser, Digikey, Partsconnexion, Handmade, etc.). 


I chose to go with the cadmium metal chassis as opposed to the newer stainless steel version as the chassis is going to be powder-coated; no need to pay extra for that chromed-out finish when no one's going to see it ;) Below you will see the chassis, I've enlarged the holes to fit the Cardas binding posts and RCAs, and the center hole has been enlarged to hold a piece of FR4 material that will then hold the C8 inlet in place. I then gave the chassis a nice sanding and labelled it for the powder-coater. 

A few chassis mods and a bit of sanding prep

One thing I would be painting myself was the power choke. A choke is a nice cheap upgrade for the ST35 and helps reduce ripple in the power supply. A number of people have used the C354 to replace the 50 ohm resistor, I chose a Hammond 156R with 56ohms DCR, which is pretty much the equivalent. Chokes tend to be on the ugly side, so if they aren't hiding in the chassis (there's absolutely no room in this case), they need to be dressed up a bit. I was prepping two at once, one for the ST35 and one for the Millett Jonokuchi. 


The first step in this process is to sand down the metal frame on the choke. Then painters tape is cut and applied to mask off the inside. I've seen some builders just paint the whole thing, but I don't really care for that look. 

Chokes masked off with painters tape

Next I apply two layers of primer, one per hour, then multiple coats of Hammertone paint. It can be difficult to get the Hammertone paint to lay correctly without making the coat too thick. Any over-spray is corrected using black permanent ink. 

Chokes painted with copper Hammertone paint

Once dry, I measured and cut some strips of black leather and used some epoxy to keep them in place, wrapping around the spool of the choke. I use a couple layers of leather when possible to give the spool a fuller look, it's more aesthetically pleasing.

Completed chokes with leather wrapping the spool. 

UPDATE (6/19/11): The parts arrived for the ST35 power / driver boards, so I went ahead and populated everything except for C4 and C5, which will be 0.1uF Mundorf Supremes that will hang off the board just a bit and will need to be installed once the boards are fastened to the chassis. C1 was not used per Bob Latino's suggestion as its purpose is DC input blocking and nearly all modern-day preamps already have a DC blocking output cap, so I used a pair of thick leads to short C1. As you can see below, the Teflon tube sockets have blue PCB boards installed to ease the soldering process. The sockets had a rather long ground pin that I trimmed down and sanded smooth as there would have been a clearance issue in the height challenged chassis. 

ST35 boards mostly populated and ready for installation

UPDATE (7/8/11): The chassis is pack from the powder-coater so I was able to fit the boards in place. I opted to install them rotated 180 degrees as I wanted the RCAs on the rear of the chassis and there wasn't sufficient clearance with the power tube sockets blocking the way. I took some generic gold-plated RCAs from my parts bin and trimmed off the back with a cutting disc, rendering them nonfunctional but small enough to squeeze between the tube socket and the chassis front, simply for the sake of aesthetics. I was also able to position the painted Hammond filter choke, attach the Cardas copper binding posts and install the Cardas rhodium plated RCA jacks. Also installed was the large silver multicap and the bias control board from Dynakitparts. 

Getting started assembling the parts on the chassis

In the interest of making things convenient, I installed a small 6A SPST toggle switch on the front panel of the chassis so the amp can be powered on and off conveniently.

A 3 Amp power switch installed on the front of the unit

The ST35 Bias Control Unit from DynakitParts makes use of an adjustable fixed-bias circuit, allowing for adjustment of the bias setting of each EL84 power tube via four separate precision bias potentiometers. 

Wires run to the Bias Control Unit from DynakitParts

The initial wiring was completed using Kimber TCSS 19 gauge stranded copper in teflon dielectric. I opted to drill out the area originally designated for the fuse holder and the cable in the interest of fitting a C8 power inlet. Because the re-purposed real estate on the chassis back was a little too large for the C8 inlet, I mounted it to a piece of FR-4 material and then mounted the combo to the chassis. This adds a degree of convenience, allowing a removable C7 power cord to be used to power the amp. The fuse was then moved inside the unit and mounted to one of the chassis braces, really a perfect place for it in my humble opinion. Grounding lugs were added to both chassis braces to connect to the RCA grounds (the Cardas RCAs are insulated from the chassis unlike the stock Dynaco RCAs) and the negative connector of the speaker binding posts. 

Initial wiring of the ST35 complete

UPDATE (7/28/11): The transformer bell-ends arrived back from the powdercoater and were reattached to the iron with brass hardware. Rather than using the cut transformer wire, I prefer the Kimber TCSS Teflon wire for all extraneous point-to-point wiring. The dielectric doesn't melt, it's a little thinner so it's easier to route, and I'd imagine the copper's of a higher purity. I opted to add a set of three 1uF 630V Solen film capacitors as bypass caps for the can capacitor. The Solen film caps are only a couple bucks a pop and have the potential to add refinement to the power supply. It's a narrow fit in the ST35 case, no room for zip tie mounts for the caps. 

Transformers wired in place, Solen bypass caps added

The wiring was finalized by adding a pair of DPDT switches to manage the triode and ultralinear switching. This was wired based on the following advise from Bob Latino and Tazsmonn on the DynacoTubeAudio Forum: Remove green or green white wire from pin #9 of each tube and solder to separate poles of DPDT switch on the same end, Solder wire from pin #9 to center pole of DPDT switch, Solder 100 ohm [resistor] from pin #7 to DPDT switch opposite end from green or green white wire, All wiring to and from switch side must be to same tube.

Triode mode switches wired up

Now that the wiring is complete, a final checkout of the circuit is performed, tubes and installed, the unit is powered on and the tubes biased using the ST35-BCU. The tubes installed were Mullard EL84s and Phillips EGC 12DW7. The sound is quite excellent, very resolving and holographic imaging. It actually sounds quite "hifi" for a circuit from the early 1960s. I could use a tiny bit more bass, but I was listening on a small set of Tektons with 4" Fostex full range drivers. Here's some additional progress pics:

Dynaco ST35 front view

Final Dynaco ST35 back view

Dynaco ST35 side view

UPDATE 8/8/2011: I was able to have a pair of custom engravings made for the unit. A 1" gold circle with the word "Dynaco" engraved in black and a small rectangle with a Dynaco Stereo 35 logo that I made in Adobe Illustrator. These are the final touch.

Final Dynaco ST-35 front view

Final Dynaco ST-35 top view

Engraving detail on quad-cap

Final Dynaco ST-35 rear view

To recap, the customizations to the kit were as follows:

• Teflon tube sockets with Beryllium copper contacts
• Ceramic gold plated driver tube sockets
• Mundorf and Obbligato driver board capacitors
• Solen power bypass caps
• Kimber TCSS internal wiring
• Kiwame / Koa Speers carbon film resistors
• Hammond 156R Choke - 56ohms DCR
• Dynakit Parts ST35 BCU (Bias Control Unit)
• Front power switch
• Triode / Ultralinear Switches
• Cardas RCAs and Binding Posts
• C8 power inlet jack with custom C7 power cord
• Custom powdercoat color-scheme
• Alternate fuse holder and placement
• Custom Brass Engravings

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. 

A new upgrade for the Bottlehead Seduction Phono Pre-amp

In the interest of a little tweak for the Bottlehead Seduction Phono Pre-amp, a pair of Hammond 155J chokes were added to the power supply in place of the two 1K resistors. On paper, they are 1026 ohms +/- 15%. In practice, they measured about 900 ohms each, which should be close enough for this purpose. Chokes allows better filtering in the power supply (less hum in the output of the amp and less voltage drop) which should translate to better sound. There's a nice quick lesson on the purpose of chokes here. Since the stock Bottlehead design lifts the transformer with a pair of standoffs for noise reduction, I decided to use a pair of standoffs for the chokes to physically move them away from the tubes. As typical with Bottlehead kits, once you start dropping in some fancy parts, the real estate tends to go pretty quickly. The most accommodating place to mount them in my case was right next to the C4S constant current upgrade board.

Pair of Hammond 155J Chokes next to the C4S

 The upgrade required about a half hour of time to mount the chokes and solder them in place, along with about $30 plus shipping for the chokes themselves. I'm listening away to the new Radiohead The King of Limbs vinyl as I write this, the sound is warm and smooth, just as a record should sound.


Please read the full story here: http://www.diyaudioblog.com/2011/02/bottlehead-seduction-phono-pre-amp.html 

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. 

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.