Plenty of DIY electronics and reviews including vacuum tube audio, new and vintage gear, custom audio cables, headphones, experiments, and retro video gaming.
Likely the most popular tube amps in existence, Dynaco has
many fans both young and old. Given how well they have stood the test of time,
I hope they continue to be sought after for many decades to come. There are a
number of companies offering kits resembling the originals with modern bits and
pieces, but if you want to build or restore a Dynaco as close to the spirit of
the original as possible, Dynakit Parts offerings are a great choice.
Dynakit Parts is a company based in Paramus, New Jersey and
has an impressive selection of reproductions of all the most popular Dynaco vacuum tube products. The kits are about as close to the originals as you can get, from the
can capacitor to the screw-down terminal strips but have the benefit of using
freshly made transformers and other modern parts.
If you are familiar with the older Dynakits from the 60s,
you will remember they featured RCA inputs that are a little different than
what we typically see today. On the pre-amps and integrated amps, they were
spaced very close to one another and it is hard to find RCA cables that fit
them now-a-days, not to mention middling quality (though good for the time).
There have been several attempts to reproduce the RCA board
on the PAS using higher quality materials, however because of the spacing, not
all the RCAs were able to fit in these offerings. Now that we're in 2020 that is not the case. Dynakit Parts
has created a very nice-looking solution for those of you looking to feature
all the inputs of the original while also providing a high-quality
solution. Best of all, you do not have to cut or modify the existing chassis in any way, the kit fits like a glove.
Above you'll see the items that are included in the kit. You get the handsome brown polyimide board with integrated grounding scheme, nine pair of gold RCAs, two pair of half-watt resistors, a bag of hardware, some detailed instructions, and a nice decal to identify the connections. The cost is a very reasonable $75, especially considering how many gold-plated RCAs are included. Last I checked they run about $6 a pair at PartsExpress, so you can do the math.
Something else that $75 bucks buys you is you don't have to wire up all the ground connections. Just tightening on the RCAs makes electrical connection with the integrated grounding. The image below gives you a better look at the traces.
So what do you have to do to get this puppy ready for install? Simply populate the RCAs onto the board and lightly tighten with a socket tool of your choice. Next you add the resistors and solder in place, and viola! you are done. So simple you can knock it out in 5-10 minutes. You'd then install on your PAS chassis using the provided hardware, or if you prefer a permanent install you can use pop-rivets to fasten the board in place.
Here is the other side of the board when completed.
...and here are a couple of images of the unit installed in a chassis (the nice ground lug is included in the parts):
So what's the verdict? If you are looking for an RCA board that features all the connectivity of the original PAS, this is a no-brainer. It's well made, easy to use and the gold RCAs look like they'll withstand the test of time. You could make a similar board yourself by drilling out a FR4 board, but you'd end up paying the same or more and it probably wouldn't look as nice.
Now, if you're not a purist and don't need every connection available for the PAS, Dynakit Parts makes a nice kit with a couple less RCAs for a very reasonable $50 (as of this writing).
If you're interested in the PAS RCA board or other Dynaco-related items, whether building a kit or performing a restoration, Dynakit Parts should be on your short-list. I've ordered quite a few things from Kevin over the years and everything has been superlative.
Hope you enjoyed the write-up. For more PAS related entertainment, you can check out this restoration I did several years ago.
Review disclosure: Dynakit Parts provided a reviewer's sample of the PAS RCA Board to DIYAudioBlog for an honest review.
I've been quite busy over the last couple of years and haven't been able to build as much as I would have liked, but now that I'm getting back into the swing of things, there are a number of DIY providers who have left the space, so I wanted to put together a list of all the great companies who still offer DIY kits, boards, transformers, etc. I hope it's of use to you, and please try to support the DIY community in any way you can. There's not much money in selling just boards, it's more of a community service in many cases. This is a growing list and I'll be adding more as time goes on. If you have one you'd like to see here, please add in the comments section.
ACTIVE BUSINESSES FOR THE DIYER
AMB Labs (Solid State board and part provider): Ti Kan has lots of great solid state options including headphone amps (desktop and portable), power amps, DACs, power supplies and more. The famous β22 (Beta 22) balanced headphone amplifier is here, as well as the portable Mini³ headphone amp which shouldn't be too hard for beginners. Tons of boards and parts available and a good forum. Antek (Transformers): My favorite provider of toroidal transformers, Antek has fantastic pricing and a large variety of values, most of which are typically in stock. You'll find tube-friendly 6.3V and 12.6V heater secondaries here as well.
Audio Note Kits (Tube amplifiers, Preamplifiers, DACs, etc.): A little higher end than the typical DIY kit, ANK has quite a few nice offerings, most of which are based around universal chassis that work with several different configurations. Also one of the very few kit providers that offer a DAC, perhaps the only tube-based one I am aware of. Prices range from $1,300 - $6,850 per kit, so these may be out of reach for the budding audiophile, but I still consider them a good value for the parts that are being provided. One thing to note is that the tubes are hidden within the chassis on some models (many of us like the tubes out in the open because they not only look nice but it's easier to roll them).
AudioSector (Chip Amps): Nice simple circuit PCBs designed around Texas Instruments LM3875 and LM4780 chips.
Beezar (Tube Amp Kits): Tom (Tomb on HeadFi) runs Beezar and offers great, proven designs from ECP Audio (The Torpedo I and II headphone amps) as well as other DACs and headphone amps. Tom puts together a great kit and the documentation is unbelievably good. Every step is illustrated similarly to the Bottlehead kits. Highly recommended and beginner friendly.
Bottlehead (Tube and Solid State Kit Provider): Bottlehead provides mostly tube builds, from headphone amplifiers, to low-wattage speaker amplifiers, to pre-amps, etc. This is a good company for the budding DIYer as the directions are very explicit with color photographs and the support is quite good. Most of the builds are point to point with a few small circuit boards here and there for constant current upgrades usually. I'm certain they've sold a truckload of Cracks, a very affordable headphone amp offering. Below is Tyll Hertsens of InnerFidelity (great site) building a Crack.
Cinemag (Transformers): Very nicely made signal conversion and moving coil step up transformers.
DigiKey (General Supplier): Carries all the various passive and active components you'll need to fill a circuit board (resistors, capacitors, op-amps, sockets, etc.). If they are out of a product, try Mouser.
DIYAudio Store (Solid State PCBs, Kits, Chassis): Never to be overlooked, DIYAudio.com has its own store with some great little accessory PCBs like the soft start board, a universal power supply, as well as some legendary Nelson Pass designs and big hulking chassis you'll need to house them in. Pricing is very reasonable.
DynakitParts (Tube Kits, PCBs, Transformers): Kevin at Dynakit parts is an incredible guy. You can build Dynakits just like you used to back in the 1950s thanks to his incredible efforts. Dynaco, if you aren't familiar, is likely the most legendary audio kit manufacturer of all time. The ST70, their 35 watt-per-channel amplifier is a ubiquitous favorite, and their other models, including the ST35, MKIII, MKIV, and PAS pre-amp all have a very strong following. Dynakitpart's uses the original circuits, which are excellent, but perspective buyers may also use alternative boards from a variety of producers (and will need to in the case of the PAS pre-amp and SCA35 integrated). The newly designed boards have some nice attributes in some cases, but are becoming harder to obtain lately. The Quad-caps are very cool and vintage looking, but some people prefer cap boards on the inside instead. The beautiful thing is that this is DIY, and you can do whatever you like.
Edcor (Transformers): Affordable custom-made "E I" transformers with iconic blue bells. Everything is made to order at Edcor and takes 6 weeks to get produced. If you need something immediately, Hammond may be a better bet. ElectraPrint (Transformers): Well-made custom "E I" and moving coil transformers at reasonable prices and excellent customer service. Check out the ebay page for ready made units.
Glass Jar Audio (Solid State Kit Provider): Jeff is kind enough to assemble kits with designs from AMB, Cavelli, Toole and others. It's much easier to order through him than sourcing all the parts all over the place and the pricing is very reasonable.
Glassware Audio Design (Tube PCBs and Kits): Glassware has an impressive collection of designs, mostly around the Aikido circuit. You'll find pre-amps, phono pre-amps, headphone amps, volume attenuators, and power rectifier boards. Nice really thick PCBs that are well laid out and some offer the ability to fit bypass capacitors and large coupling capacitors (for DIYers who have strong opinions ;) ). Manuals and schematics are included with purchases. Not as friendly for beginners, start with Bottlehead unless you have some experience under your belt.
Hammond (Transformers and Chassis): Hammond offers a great array of transformers, inductors and chassis which are perfect for small to large DIY projects. Hammond is carried by a number of suppliers such as Angela, Digikey, Mouser, Allied, and others.
Magnequest (Transformers): Some of the best custom wound "E I" transformers for audio. Magnequest transformers are used in designs by Wavelength Audio, Curcio Audio, Alessandro, and other well-known makes. Check out the site for Bottlehead upgrade transformers as well.
Mouser (General Supplier): Need to populate a PCB? Mouser likely has everything you need aside from tubes and traditional tube sockets. If they don't... try Digikey.
Neurochrome(Tube and Solid State PCBs): The Modulus86 seems to be a popular design based around the LM3886 chipamp with some performance enhancements. Very low total harmonic distortion, if you're into that ;) Also a very cool 300B SET (single ended triode) PCB set is available.
Oddwatt (Tube Kits): Oddwatt is a two person team (Bruce and Rodney) offering a number of tube amplifiers and a preamplifier with simple but effective circuitry. The kits include Edcor transformers and custom chassis and PCBs. I don't know too much about the company, but there is a nice write up on DIYAudioProjects.
Pete Millett(Tube PCBs): Tons of great designs for tube amplifiers, pre-amplifiers, headphone amplifiers and the like. Some beginner friendly designs like the Butte and Jonokuchi headphone amps, as well as expensive, esoteric amplifiers designed around unusual tube compliments. Definitely worth checking out!
Transcendant Sound (Tube Kits): I havent purchased a kit from Transcendant Sound (yet), but have heard many good things. From what I see they look to be a tremendous value. The Grounded Grid pre-amp is quite popular, and the OTL (output transformerless) tube amp designs are said to offer treble clarity rivaling solid state.
Triode Electronics (Tube PCBs and Kits): Dynaco-based amp kits as well as some guitar amplifier kits. Very nice selection of parts and tubes as well if you are building a vintage amp. Directions not quite as perfect as Tubes4HiFi or Beezar, but still very good.
Tube Audio Lab (Tube Amp Kits and PCBs): Great and not overly complex tube-based point-to-point kits and PCBs using the classic 300Bs, 45s, 2A3, etc. Parts selection based on your preferences.
Tubes4HiFi / Bob Latino (Tube PCBs and Kits): Roy and Bob run a really great DIY amp and preamp site. There are models based on the Dynaco ST70 and MKIV, as well as some higher wattage versions if you have some pretty demanding speakers or a big room to fill with sound. Lots of great kits, excellent directions for most offerings (some of the pre-amps have more basic schematics, etc).
Great products at reasonable prices from my experience.
NO LONGER PRODUCING DIY PRODUCTS / IN HIATUS
ChipAmp (Solid State PCBs and Kits): Allen at Chipamp has a fantastic array of chipamp / gainclone builds based on the LM1875, LM3886, and LM3875. He also has some great power supply, volume and switcher PCBs. Very simple and easy builds. Beginner friendly except the power amps use high amperage and would be dangerous if safety measures aren't followed carefully. UPDATE: per communications at DIYAudio.com and HeadFi.org, it seems there may be issues with order fulfillment in 2016-2017.
Classic Valve (Tube and Solid State PCBs): Geek at Classic Valve is getting out of the PCB business to shift his attention to building turn-key amplifiers. I encourage you to buy up what he has left. There are a number of very unique Dynaco boards for the PAS, ST70, MKIII and MKIV as well as power filters and other great stuff. UPDATE: It looks like the supplier CK Magnetics is also clearing out their leftover PCBs, so grab em while you can: http://www.ebay.com/usr/ckmagnetics
DIYTube (Tube PCBs): Shannon at DIYTube is getting out of the PCB business to shift his attention to building turn-key amplifiers. You will still find excellent documentation on DIYTube.com and a semi-active forum, however the boards are no longer available.
Transcendar(Transformer Producer): Once an extremely high value "E I" transformer producer, this maker appears to no longer be producing new products.
The Fine Print: DIYAudioBlog and Zynsonix Audio, LLC does not represent or have any business affiliation with any of the above businesses or individuals. Building/modifying circuits 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 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
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.
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.
