Octal OTL DIY Tube Headphone Amp

Another day, another headphone amp :) One of my life's pleasures is digging around on eBay for exciting finds, whether it be obscure vintage tube equipment or interesting/useful PCBs. This article covers the latter, a very nicely made PCB which features an octal (8-pin) tube OTL headphone amp circuit. Specifically, this is an Aikido input with a White Cathode Follower output, based on John Broskie's design. The circuit is designed to power most headphones, including those with a 32 ohm impedance. This is relatively uncommon with OTL headphone amps, which usually get along best with headphones ~300 ohms.

Top and Bottom of PCB

The tube compliment is a pair of  6BX7 or 6BL7 output tubes, a pair of 6SN7 input tubes, and a 5AR4 rectifier... because tube rectifiers are fashionably old school. Using all these tubes requires a hefty transformer, the 8.6lb.Hammond 272JX, which has a 5V, 6.3V and 600V secondary outputs. Yes, 600V, so this is not an amp for beginners. 

One can utilize the 6BX7 or 6BL7 tubes by switching two pairs of jumpers on the board, or the circuit can be configured to only work with one of the two tubes, but you can select between 32 and 300 ohm outputs using the jumpers. I personally chose the 6BL7 tubes and two different impedance outputs. 

One nice thing is that getting pairs of vintage 6BX7 / 6BL7 tubes is pretty reasonable ... $25-30 a pair for many different brands and vintages. Same for the 6SN7. You can get the GTB variants which are built to a slightly higher spec. For the 5AR4 you'd probably want to get new construction from Sovtek or similar. Guitar center is a good source. 

All the stuff on the BOM is of great quality: WIMA film caps, Vishay resistors, Nichicon electrolytics... all the good stuff. I ended up digging around the workroom to see what I had first... mostly Kiwame and carbon comp resistors, hand matched as carbon comp values can vary by 10% or more. Film caps are compact Panasonics. For the power resistors I think I went with Koa Speers as the Vishays were oddly overpriced on Mouser.

PCB populated with the smaller components

Ceramic tube sockets soldered in place

For the larger film caps, I wanted to incorporate some of the EVO oil Mundorfs. The sizing is tough, as the Mundorfs are fairly compact, but wide, and the chassis is about 3" tall, so some unique fitting was required. As most of the area underneath the caps on the circuit board doesn't have any traces running through it, I used a cutting disc and Dremeled away the area on each side... this would give an extra 1/4" to 1/2" height lost from the PCB and standoffs for extra clearance. I drilled a hole on each side of the PCB as well so a zip tie could hold each cap in place. 

The chassis I selected is from IAG DIY Tube Audio Products. He lists his chassis on ebay and on his website. Hand-made in the USA and built like a tank. The chassis gauge is relatively thick, but aluminum so easy to work with. I chose to cover up the pretty polished aluminum with painters tape to try and prevent any scratches.

Using a step drill bit to make holes in chassis

Using a Greelee punch to make the holes for the headphone outputs

Dremelled out area for IEC power inlet

I accidentally inverted the drawing when punching the holes, so we have an extra hole here. No big deal, it can be covered with a 1 1/8" Hillman hole cover.

Interior of the chassis

The populated PCB slips right in place. There aren't too many wire connections needed, just the transformers, pot controlled RCA input, and headphone output. Shielded Cardas 2 x 21.5 was used for the input and output (as they no longer make the 2 x 24 for some reason). I also wired up a pilot light to the 6.3V heater. The Hammond comes with a 115V and 125V primary. Typically you use the one that closest matches your house's voltage. As mine is 120V on the dot, I used the 125V as it didn't really matter. You dial in the voltage via the two adjustable resistors inside anyway.

When adjusting the 300V secondary, I noticed R24 was dissipating a bit too much heat and starting to smoke. Per the seller, R24 has been changed to 22K, so I went ahead and swapped that out.

Ignore the power wiring in this image, the ground is isolated on the board and is wired differently

By default for a tube amp, I wired in a filtered IEC outlet, however these are incompatible with this design as there is a ground isolation circuit built in. Once everything was corrected, I ensured the secondaries were dialed in correctly, but was getting a high DC offset. Chatting with the designer, he suggested just loading the circuit with 32 ohms on each channel. Upon measuring this, all was well. The unit started up without any issue and sounds excellent.

Completed headphone amp rear

I had a nice little brass tag made for the nameless amp, simply calling it "High Voltage OTL Aikido Octal" as the seller's eBay name is HVforless and it employs an Aikido circuit. The front features a 4 pin XLR for convenience with a shorted left and right ground.

Completed headphone amp front

You can get vintage driver tubes for this amp on the cheap, so that's another nice bonus.

Please remember that 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. 

DIYTube Get Set Go 300B Headphone Amp

It's been a little while since the last post, and there's a good reason for that; I've been gathering parts for a very special DIYTube Get Set Go made specifically for the Audeze LCD-2 headphones. The Get Set Go is a single-ended triode design made first-and-foremost for high-sensitivity speakers that only need a few watts to fill a room with sound, such as the Planet 10 Audio Frugel Horn. I did have the fortune to build the Get Set Go for speakers previously and was impressed with the overall tonal character and quality of the sound.

The Get Set Go Board from DIYTube

You may be wondering why someone would be using a speaker amp for a pair of headphones. A typical headphone amplifier will output relatively a small amount of power compared to the speaker amplifiers (in small fractions of a watt measured in milliwatts). The Audeze, as well as certain other planar magnetic headphones like the HiFiMan, really benefit from some added wattage to sound their best as they aren't the most sensitive drivers. Hense, using a low-powered speaker amplifier makes sense, but there are some changes that are necessary.

The biggest change is the output impedance. While speakers are typically 4, 8 or 16 ohm, headphones often range between 32 and 600 ohm. The Audeze headphones that the amplifier is being made for are 50 ohm (although I have also seen them listed as 60 ohm). Since 50 ohm single-ended output transformers don't really exist on the market, you can either have one custom wound, or try and find something that would have an equivalent turn ratio. There's a handy calculator that Shannon of DIYTube recommended that will give the equivalent turn ratio for this purpose. A 3.5K / 50 ohm output transformer would have a turns ratio of 0.12, or 8.367:1. Gery at Transcendar offers a 1.6K / 16 ohm single-ended transformer, the TT-023-OT, which is a 10:1 ratio and close enough for our purposes. Per Shannon, The reflected load would be a touch more linear and lost max output would be minimal.

Secondly, learned from previous experiences like with the Millet Jonokuchi, and to a much lesser extent, the ECP Audio Torpedo, headphone amplifiers do not like their output transformers to be close to the power transformer. When they are too close in proximity, there is something called flux interaction when the magnetic waves interfere with one-another and cause an audible hum with certain headphones. So not only will the output transformers be moved to the opposite side of the chassis, but rather than using an EI transformer for power, a toroid will be used instead as they create much less of a magnetic field. Antec is one of the few manufacturers out there that builds certain toroids specifically for tube amplifiers. The Antec AS-2T300 would be a good substitute for the PA774 with a bit higher ratings.

So other than the transformer choices and their position, this build will be fairly similar to the previous Get Set Go. To make things a little different, 300B tubes will be used rather than 6B4Gs. Also Mundorf Silver Oil capacitors will be taking the place of the Jupiter wax paper caps. The Mundorfs should be a little more detailed, which the Audeze can benefit from with their warmer/darker presentation.

The first action-item on the list was the chassis. I always like to create a virtual mock-up of the chassis layout to see different set-ups and settle on what I feel will work the best. Note that in this layout, the output transformers are distanced from the power toroid and chokes.

Virtual Chassis Layout

Once this is settled, I forwarded the dimensions over to Keith (ebay seller po1019) to fabricate the chassis. The virtual layout is then used as a guide to lay out the parts, take measurements and make markings, and ultimately start drilling / cutting away. After prepping a number of aluminum and steel chassis, I have a small collection of very handy Greenlee punches, which are ideal for making clean holes for tubes and sockets. The metal area (which will be behind the little red tab on the Neutrik locking plug) to release the headphone plug was recessed a bit by sanding using a Dremel and by hand for adequate clearance.

The prepped aluminum chassis

I decided that this chassis would have a more unique and artistic finish. Something more worn and rustic looking. I was actually inspired when I was scrapping off the dark grease on an old baking sheet. This finish starts with some black engine enamel that is then sanded and scrapped by hand minutes after spraying. It produces and interesting texture in my humble opinion.

Textured Bottom Plate

Texture Detail

The texture is achieved by hand using worn out sandpaper at different times of paint dryness, along with the sharp point of a scissor to get the longer lines. Once the texture was where I wanted it, I gave the chassis two coats of clear satin enamel. The parts can now be dropped in place. I began with the smaller pieces, including the headphone sockets, which were flush mounted and secured with black oxide screws. The switch, pilot light, RCAs, binding posts, IEC, Teflon 300B sockets and hole grommets were all installed next.

Part installation on the chassis

Next comes the fun part, the population of the PCB. Varying from the BoM, we have Elma Silmic II caps, Nichicon FineGold caps, Kiwame and Mills resistors and Mundorf Silver/Oil Caps. Slight changes to the circuit includes the Wima 0.1uF snubber cap and a pair of Solen 630V 5.6uF bypass caps in the power section.

The populated Get Set Go Board

On the bottom of the board, at Shannon's suggestion, we have a few of the Mills power resistors to help distribute the heat a little better. 

Bottom of the board

Now comes the long process of preparing the wood side panels. I decided on an antique silver leaf finish for these pieces. Since there is no off-the-shelf option for antique silver leaf that was readily available, I'm using some Speedball brand silver leaf pieces and some Old World Art brand leafing and antiquing materials.

Leafing Materials

Below are the wood side panels for the chassis before any prep.

Wood Chassis Panels

The first step in the antique leafing process is a red basecoat. This is typically used with a gold foil, so we'll see how it performs with silver. Below is the wood with the basecoat applied.

Red antique basecoat

 After two basecoats and a layer of adhesive size, the wood is covered with the silver foil sheets. This is a very unusual process, the foil is incredibly thin and will tear apart if it catches your fingers, so you have to be delicate. each foil sheet is laid down with a little bit of overlap.

Silver foil sheets laid in place

The foil is then burnished with a cheesecloth to have it form the curves of the wood. This is a messy process and the foil flakes will end up everywhere. They are so thin that you'll find them floating in the air. Once this is complete, a layer of sealant is used to protect the foil. 

Silver foil after burnishing

The antiquing kit included an antique glaze that is made for gold so it's a brownish hue. It didn't look right on the silver. Instead I chose to rub it down with powdered graphite before sealing it again. Then two layers of satin clearcoat were added for additional protection of the finish.

More to come

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 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. 

The Latest Updates - Bottlehead and MHDT

Like an artist may say that a painting is never done, or the guys at PIXAR saying a movie is never finished, some of us electronics nuts feel like a project is never done. There is always another tweak, another piece to be added, substituted or upgraded. In the interest of keeping all information on a particular project in one place, I regularly go back and edit existing posts to include updates.

The first update to cover is the MHDT Constantine NOS DAC. When I was placing an order for some nicer caps in the MHDT Paradisea, I opted to get a few for the older Constantine as well, replacing the four 0.10uF stock caps with Obbligatos. A little bit of drilling of the soldering eyelets was required for the thick leads. Luckily, the Constantine board is one sided and I didn't have to worry about the conductive shells of the Obbligatos touching any traces. I did opt to add Teflon sleeving to the leads so that they wouldn't short across the shell though.

A few more of those beautiful gold Obbligatos for the Constantine

You can find the rest of the story here:
http://www.diyaudioblog.com/2011/02/modifying-mhdt-constantine-nos-dac.html

The second update covers a few additional modifications for the Bottlehead Crack headphone amplifier.  I managed to squeeze in a Solen 220uF film cap in the final electrolytic power cap location, sitting on a pair of standoffs. I also took the time to bypass the other two power caps with a pair of low-cost Audiophiler 2.2uF film caps. A Triad C7X was selected with a rating of 270ohms to take the place of the final resistor in the power supply (also 270 ohms). Since pretty much all the real estate has been used up on the plate, two 2" aluminum standoffs were used to raise the choke above the capacitor near the RCA inputs. This required adding 1" spiked feet to the bottom of the wood base for adequate clearance, just like in the Bottlehead S.E.X. build.  

Large Solen power cap and Triad choke

You can find the rest of the story here:

http://www.diyaudioblog.com/2011/02/bottlehead-crack-otl-headphone.html

Finally, just a few days ago a MHDT Paradisea NOS DAC modification project was finished up. A variety of nice items were installed including Takman and Kiwame carbon film resistors along with Obbligato and Mundorf Silver Oil film capacitors. Next on the list for this mod is a Burr Brown op-amp to replace the stock Analog Devices one. 

Some creative mounting of the Mundorf Silve Oil Caps

You can see the whole story here:

http://www.diyaudioblog.com/2011/05/modifying-mhdt-paradisea-nos-dac.html  

Be sure to keep an eye out on other posts for new updates. I hope to be finishing up the Millett Jonokuchi headphone amplifier in the next few weeks, along with casing up a completed AMB Mini Cubed portable headphone amplifier. Of course it will be unique looking like my other projects ;)

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 modification posted below 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.