Complete Restoration of a Dynaco PAS Pre Amp

This is a story of rags to riches; turning an abused Dynaco PAS 2 into a very modern PAS 3.

The Dynaco PAS 2, for those who are unaware, is a pre amp kit introduced by Dynaco in 1960 to compliment the Stereo 70 (ST70) that came the year before (1959) and other various other power amplifiers in the company's lineup. The Dynaco PAS has developed a cult following, and there are an immense amount of boards being produced for those interested in restoring such archaic technology :) CurcioAudio, Tubes4HiFi, AudioRegenesis, ClassicValve and others have created drop-in boards to restore the PAS, whether you want to maintain the same circuit or drop in something completely different.

I chose to use reproductions of the original PAS 3 boards and rather than recreate the rudimentary selenium rectifier with standard capacitors, I chose to make use of the high-tech ClassicValve PAS Regulated Power Supply.

ClassicValve Regulated Power Supply

The PAS is a design that was built to fit a budget, so a number of other things are recommended to be replaced. The 2 pole, 6 position switch is rather archaic compared to what is being produced today. A $10 switch would likely be an improvement. I opted to go with an Elma 2 pole, 6 position switch 04-1264 ($60 SRP) so that I wouldn't have to replace it down the line.

A Stock PAS Switch

The rear input and output board also leaves something to be desired. Many consider this to be one of the most drastic areas that Dynaco cut costs on the kit. Cheap Nickel plated RCAs on an old PCB board don't really inspire confidence.

Stock PAS IO Board

So... let's have a look at the splendid PAS 2 example that I was able to source:

Original PAS Back

Original PAS Back Detail

Original PAS Insides

Original PAS Outside and Bottom

The previous user left me with a lot more work that I would have had if I had a pristine PAS 2 / 3. There were a number of unusual mods, including plexiglass mounted RCAs, a 9 pin tube mounted in the middle, a non-descript transformer that was halfway mounted, some additional holes cut out of the back, and a powdercoated front panel that was rubbed with armor-all. No worries, one has to rise to the challenge.

The PAS was completely taken apart, a few parts were set aside, most were disposed of as they had been pillaged by mods. The chassis was sanded down and prepped for powder-coating.

Chassis Prep

The new boards were sourced and populated with my favorite parts, Kiwame and Takman resistors and Sonicraft caps. 

Old boards versus new boards

Curcio Audio has a number of nice tutorials on the PAS, including one to make the Phono board a bit more accurate. I performed those mods, along with bypassing the tone controls. I sourced a new input output board from Tubes4Hifi to connect to the Elma switch. I also picked up an Alps volume control (100K) and balance control, along with PCBs to make soldering a little easier.

Mostly done

The wiring was completed using Neotech PCOCC in teflon wiring, Kimber wiring and SPC in teflon. Russian teflon bypass caps were added to the main board (mounted to the front), and Russian PIO were used on the phono board. A power LED board was sourced from ClassicValve, and a new transformer was sourced from TriodeElectronics. After all was said and done, I sent off the PAS to Sal at Dynaco-ST70.com to look over all my wiring and fire it up. I wanted to have an expert give it the final eye and do the final wiring.

Here's the final build:

PAS guts from the right

PAS guts from the left

Inside Detail

Inside Detail

Bottom wiring

Outside with new gold front panel from ebay

Outside Photo 2

Rear, notice rear vent and Cardas output RCAs

Some final details were the Cardas RCAs added to use the holes made by the initial owner, a power supply vent to eliminate the ugly holes in the chassis, the bottom slots were covered using slightly larger than stock power receptacles from Radio Daze, and the front panel was replaced with a gold anodized version from Michael Nipomo on eBay (he typically only sells black and silver, but had this gold one as a one-off).

This little baby was an amazing amount of work, but it came out well and I'm really happy with it. Good luck to everyone out there restoring their PASs :)

Wondering where I got all my parts for this build? Check out a list of my favorite online vendors here.

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. 

Recabling the Venerable Koss KSC75 Headphones

The Koss lineup of portable headphones is truly legendary, from the Portapro that was released in 1984, to the KSC35 that features the same computer-optimized neodymium iron boron magnets in a clip on version, and the slightly newer KSC75 with more comfortable ear clips and a titanium coating on its diaphragms. All three of these products can still be purchased, all of which are under $40 street price (although it's a little more difficult to locate a retailer carrying the KSC35 than the other two). 

The Koss KSC35, KSC75 and PortaPro

Of Koss' three legendary offerings, the best value has to be the Koss KSC75. Surprisingly detailed highs and thick bass reproduction make for a fun presentation and one can purchase this little guy for about $15 on Amazon, likely the best $15 you'll ever spend. Below is photo of some newer packaging for the KSC75 (circa 2012) selling them as "Pulse Clips", same thing, different package. 

KSC75 aka Pulse Clips

Of course, an audiophile can never be satisfied with things as they are, and will tweak and overhaul to get the maximum amount of performance out of something, even a $15 headphone. So, why not put a cable on the headphone that costs several times as much as the headphone itsself? :)

Once removing the plastic shell and ear clip, the solder tabs are exposed. Note below a Xev headphone cable soldered to the KSC75 driver and secured with heavy duty adhesive. 

A Xev cable soldered to the KSC75 driver

Below are a couple of different finished KSC75s:

Xev wire and soft nylon multifilament covering

Xev wire and black chrome covering

How's it sound? Much more Hi-Fi; highs are crisper and more detailed and bass is much more controlled; it's really like a new headphone. Worth the extra cost? That depends entirely on your wallet and sanity level ;) Want a recabled KSC75 of your own? Contact Zynsonix for details.

Using Silver Plated Copper for Speaker Cables

We've all seen the restrictively expensive speaker cables out there on the market. Most of them look incredibly awesome and have sophisticated, patented geometry, but sometimes you'll notice that there was some scrimping done with the quality of the conductors. When you're paying good money for cables, why settle for Polyethylene or Poly-Vinyl-Chloride (PVC) dielectric when Teflon (PTFE) is better?

I went into a brief explanation regarding the dielectric constant of Teflon in my post regarding Kimber TCSS wire, but briefly, it's a synthetic fluoropolymer that was developed by DuPont and has a dielectric constant of 2.1, lower (better) than all other noted cable dielectrics.

Silver-plated copper is a type of wire covered with Teflon that's widely available for industry applications. Belden offers it, among a number of other companies, and you'll find a number of military spec varieties of different vintages. The cost is relatively low compared to audiophile boutique wire and the quality is generally high. You do have to keep an eye out for tin covered wire though, it looks quite similar but sounds different, and not preferable in my experience. The reason these wires have silver or tin plating has to do with the extrusion process of Teflon, it's much easier to extrude it over silver or tin than copper from what I've read.

I've built a number of interconnects and speaker cables using silver-plated copper. While there is a vast variety of Teflon coated interconnect wire floating around out there (Neotech, Mundorf, Kimber, Legenburg, etc.) but when it comes to big, thick, burly speaker wire, that variety really dwindles away, I'd imagine due to production costs. So enter the silver-plated copper; you can snag some nice thick 10-12 gauge wire under a buck a foot, whether you're running one or two conductors per terminal, it's still works out to a lot cheaper than fancy boutique wires.

You can always dress it up and make it look nice too if you want ;) Here's a few I've put together:

2 x 10 Gauge - Black Nylon Covering, Rhodium Bananas

4 x 12 Gauge - Copper Bananas and Spades, Carbon Reflex at the "Y" Split

Above in Detail

2 x 10 gauge - Cotton Reinforced Hose covered with black Techflex

If you're looking for some nice quality bananas and spades, I've found that Homegrown Audio has a nice selection for reasonable prices. The sound from a silver-plated copper speaker cable is crisp and clear. Give it a shot before you lay down some serious dough on boutique wire.

The Millett Jonokuchi Headphone & Speaker Amplifier

Our industrious friend Pete Millett has released another build of note, the Jonokuchi headphone / speaker amp. The Jonokuchi, like the Engineers Amplifier, makes use of a compliment of very reasonably priced Edcor transformers. Unfortunately, or fortunately depending on your tastes, the Edcor iron is only available with Royal Blue powdercoated bells, so if you'd prefer another color, be prepared to pay for your local shop to sandblast it off or purchase some unpainted bells of the same size.

The Jonokuchi, like the Engineers Amplifier, makes use of old television tubes that are cheap and readily available. It's a single-ended affair that uses 13EM7 tubes with 13V heaters. As stated on Pete's page: the amp design was optimized for headphone use, but still makes a fine amp to drive small speakers to reasonable levels. This is a relatively low cost build at approximately $200 in parts, just add a chassis. Assuming you make use of the PCB, you'll want to go with an 8" x 12" chassis as it should fit everything perfectly. Hammond makes a cheap one for ~$30, or you could get something more exotic like...

... a Hammond Chassis with Walnut Side Panels

...or a custom chassis from ebay seller po1019

Pete also put up plans for a Front Panel Express Chassis that comes with all the holes already trimmed out and labeled, but for nearly $300 for the chassis alone, I think I'll pass ;)

Jonokuchi with custom FPE panels

I was able to pick up my red board just recently from Pete and I've dropped a few parts on it so far.

The lovely red Jonokuchi board

Mostly populated, a few things left to source

I'll update in the coming weeks as the remaining parts arrive. Should be a fun little project. Now which film caps should I use... Obbligato, Mundorf, Sonicraft... oh the possibilities :)


UPDATE (3/05/11): I decided to go with the Hammond Chassis and AmpOhm Paper in Oil caps. Yes... the AmpOhm caps are a bit large, but with some creative mounting, they didn't pose a problem. Pete has the hole layouts in PDF on his website, so download and either print out yourself on a couple pieces of paper and line them up with tape or let Kinkos or Staples handle it on larger format paper. 

Chassis with drilling holes laid out on top

Drilling out the holes on the drill press.

I had Staples print the CAD drawings on the 11x17 paper, but despite saying not to resize in the instructions, they did anyway and I killed the first chassis, so be sure to measure the drawings first no matter what.

Mouting the AmpOhm PIO Caps

UPDATE (4/18/11): I finally finished up drilling the Hammond chassis, there were quite a few holes for tubes, transformers, a pilot light, the IEC inlet and the PCB mounting standoffs.

Front of the Chassis

Rear of the Chassis with oversized holes for Cardas RCAs

Bottom Plate

All holes were drilled with a trusty low-cost drill press and de-burred with a dremel tool. The rectangular opening in the back of the chassis was cut using a dremel metal cutting disc and then filed manually. Once complete, I gave the whole thing a once-over with 180 grit sandpaper.

UPDATE (7/8/11): Finally got the chassis back from my overly busy powder-coater. Now things are finally starting to come together. In place of the 5 watt 220 ohm resistor I placed a Hammond 157J filter choke (205 ohm) that was primed, painted and wrapped with leather. Cardas RCAs were installed on the back panel, along with some nice thick gold-plated binding posts that Parts Connexion was clearancing. I didn't want to splurge on Cardas posts as this amp will likely see a lot more duty as a headphone amp.

Board fitted in the powdercoated chassis

I designed a nice custom plate with the Japanese Kanji from Pete's Front Panel file and copperplate font spelling out "Jonokuchi". These little touches make the amp feel a little more polished and complete. The screws connected to the aluminum standoffs surrounding the tubes were depth-mounted as I'll be placing some decorative metal surrounds around them.

Custom plate embellishes the top of the chassis

UPDATE (8/2/11): The bell-ends are done being powdercoated in lovely a copper color and have been reassembled to the Edcor iron using my typical brass hardware and acorn nuts. I actually take the time to trim down each screw with a rotary tool to ensure that it's the proper length for the acorn. A few extra minutes of effort here and there goes a long way on the finished product.

The transformers were then mounted and the set of copper colored tube surrounds were screwed in place using very short screws as there is very little clearance between the chassis and the board. A jewel pilot light with a 12V LED was mounted on the left and connected to the tube heater pins on one side.

One the Jonokuchi was completely assembled and measurements checked for D.C., the amp was plugged in to verify the LEDs were lighting and the tube heaters were warming up. I then plugged in my trusty Panasonic tester headphones and had a listen. Unfortunately there was a bit of static noise depending on the position of the potentiometer. I ran a connection from the back screw of the Alps Blue Velvet pot to the chassis ground to ensure proper grounding, but that did not solve the issue. In Pete's design, the JP1 connection is used to connect AC ground to signal ground. After shorting JP1 with a short piece of buss wire, the noise was gone.

The amp itsself is on the lively side in my humble opinion, it possessed a detailed sound with taut bass and no rolling off of the high frequencies that I could detect. Unfortunately I was only able to source one pair of 13EM7 tubes (Raytheons), they tend to be a difficult tube type to find in the usual places. I feel that with the right compliment (some warmer tubes), I'll be pretty happy with the Jonokuchi.

UPDATE (8/8/11): I was able to get some custom engraved brass plates for the front of the unit detailing the operation of the push buttons, so those will be the final touch. Here's some final photos:

Jonokuchi Front

Jonokuchi Rear

Jonokuchi Side

Jonokuchi Inside

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. 

Kimber TCSS - The perfect marriage of Teflon and copper

I've been using Kimber TCSS for years. It's an excellent wire made from low-strand count high-purity copper (as Kimber calls it: Hyper-pure VariStrand™ copper) and the insulating dielectric is high pressure, low temperature extruded Teflon. It's 19 gauge, but the dimensions are small for 19 gauge, so it's easy to work with as interconnect wire, internal hook-up wire, and Kimber even uses multiple strands of the TCSS for it's higher end speaker cables (4TC, 8TC and 12TC). You'll also find it being used in Russ Andrews power supplies and other various cables.

The reason that Kimber TCSS stands out as much as it does is that most of the copper you find with extruded teflon is silver plated. Sometimes a silver plated wire isn't desired for an application, but Teflon, being the pretty much the best dielectric aside from air, is. Teflon, a synthetic fluoropolymer, was developed by DuPont and has a dielectric constant of 2.1. This is slightly better than Polyethylene and notably better than PVC and older materials like Bakelite and Formica. Between the low strand count, high purity copper and the Teflon dielectric, the Kimber wire exhibits a crisp and clear sound.

I've seen some comments on Audio Asylum and the like that the clear and white TCSS variants sound the most clear. Since this has spread around the campfire, Kimber now offers their speaker cables and interconnects using a braid of white and clear. I personally find the clear to be the most attractive, as you can see the copper strands on the inside.

I've made use of Kimber TCSS in a variety of cables, including:

Kimber TCSS and Solid Silver in Teflon Interconnects with Cardas RCAs

Headphone Cables - This one on a Denon Headphone with ViaBlue Connector

Headphone Extension Cables

Phono Ground Cable

TCSS and Solid Silver Mini to Mini Cable

...and another Headphone Cable

Making use of the Kimber wiring as a headphone cable can be a little stiff, but it's generally not too bad on a longer cable. As always, custom cables making use of Kimber TCSS can be requested at Zynsonix.com.