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August 17, 2018

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. 


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.

To be continued!

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. 



July 17, 2018

Cable and Mods for Sennheiser Massdrop HD6XX

Back in 2004 I purchased my first serious pair of headphones, a pair of Sennheiser HD600s. This was an introduction to a number of new concepts for me: headphones that are open, benefit from an amp, are sensitive to different cables, and are intended to be listened to at home (where you could be listening to speakers, sacrilege in some circles! haha).

In these formative days of headphone audiophilea I recall being at a meet in College Park, MD where there were a number of HD600s and HD650s. We were auditioning cables by Cardas, Stefan AudioArt and other DIY concoctions including pure silver cables. It was amazing what a difference the cable could make to the signature.

I'd long since sold my HD600s and HD650s and moved on to other headphones like Audeze, when I saw Massdrop and Sennheiser teamed up to released the HD6XX. The HD6XX is quite literally a more affordably produced HD650 (based on Inner Fidelity's measurements). I figured for $200 it would be fun to have a listen to the Sennheiser house sound once again, and see how good it could sound with a balanced amp and some tweaks.

If you're not familiar with Massdrop, it's an interesting business model where the company has a number of people commit to buy a product at a discounted price. Generally there are a certain number of buyers required to make the "drop" happen, and once it does, it can take months before you have a product in your hand. The upside is you can get some incredibly good deals so long as you're patient. Massdrop also teams up with manufactures to produce one-off products at a sizable discount, such as the Focal Elex, AKG M220 Pro, HiFiMan HE-4XX and other headphones which are a little different from what's on store shelves. The Sennheiser HD6XX is such an animal.

Looking at the HD6XX versus the HD650, you'll note the HD6XX has a dark blue headband versus silver and the screen printed "Sennheiser" on the top of the band is more subtle. I personally think the HD650 color scheme is a little easier on the eyes, but given this relatively superficial difference shaves $150+ off of the street price of the HD650s and still sounds identical, the choice is a veritable no-brainer.

The stock HD6XX

Given you're saving big bucks on the HD6XX, those dollars should go toward making them even better, right? I'll be going over a few tweaks you can do for improvements in sound, comfort, and looks.

One thing I couldn't be happier about is the trend toward calfskin earpads in the past decade. Exposed foam and velour don't feel all that great against my skin. The Sennheiser HD580, 5XX, 600, 650 and 6XX all feature velour pads, but fortunately there are companies producing replacement pads. I've found the Dekoni fenestrated pads are quite comfortable while allowing breath-ability. The pads snap in place on the frame and are quick and easy to install and uninstall.



Even with the new pads, the HD6XXs look a little dull, easily glanced over when looking at newer headphone designs with chrome and carbon fiber. I decided to reach out to Larry at Headphile to see about getting some custom wood cups. 

Talking about Headphile with some of the old-heads at HeadFi will elicit smiles and good memories. Larry's been at it for quite some time, and for a little while there, the "Darth Beyers", which were Beyerdynamic DT770 headphones modified with wood cups, were one of the most popular cans on HeadFi. Every now and then you'll see a pair pop up for sale, and if you like bass, I'd recommend you give 'em a shot. In addition, if you have the scratch, Larry is still building them with a number of wood choices... although I don't know if he's still building the deep versions (which I most favored). 

I chose Padauk (pronounced Pah-duke) wood as it's reasonably priced and has a nice reddish hue, which will pop on the dark HD6XX. Larry offers the cups open with screens or closed. The open versions sound pretty much identical to the stock screens with perhaps a tiny bit more reverb, and the closed versions have notably more reverberations (or a chamber-like effect) depending on what sort of damping is applied inside. 

The installation process is quick and painless. The screens are pressure fit on the frame, and using a spoon or similar utensil (I used an avocado knife) you can slowly pull the screen away from the frame and remove it. 

The screens can be removed easily with a spoon (or avocado knife)

The difference in color and presentation is night and day from the new wood cups and the existing stock screens. 

Headphile wood cups versus stock screens


Installation is completed on each side using a set of 4 brass wood screws. 


You will need to adjust the tension of each screw to ensure it has adequately threaded into the wood, but is not impeding on the cavity for the cable plug insert, otherwise the cable will either not insert at all or will be super tight.  




The HD650 (and HD6XX) is a warm-sounding headphone, no doubt about it. It can be a little too warm and rolled-off in the highs for some. This is where a cable comes into play. I opted to build a Zynsonix Xev cable for the HD6XX. While the most affordable in the line, it features silver-clad conductors, low-strand count and a PTFE (Teflon) dielectric, giving it a forward and detailed sound signature. It's exactly what the HD6XX needs, pushing forward the treble, resolving the bass, and giving the headphones a far more dynamic presentation. 




I wasn't sure if I would keep the HD6XX, it was more of an entertaining experiment, however with these mods the enhanced comfort and sound really make it a winner and a definite keeper. Headphone enthusiasts these days are quite lucky, as it would be very hard to reach this level of sonic performance at this price years ago. 

If you'd like a Xev cable made to your specifications (whatever length, sleeving, and termination you'd like), please reach out to Zynsonix today and take your Sennheisers to the next level. 


Zynsonix Audio, LLC does not have any relationship or financial interest in the companies mentioned in this post, including Headphile, Massdrop, Sennheiser, Inner Fidelity, Dekoni, etc. 


May 24, 2018

DIY Headphone Switchbox

It's a head-scratcher why this hasn't been requested before... a switchable headphone output box. I know so many people in this hobby who have more headphones than fingers and toes, so something like this should be commonly on headphone enthusiasts desks across the world! Haha, well maybe not...

Anyway, I had a client approach me who wanted a switchbox so he could switch easily between four headphones. Obviously you don't want to have all four headphones connected at once due to various drive and impedance issues, hence the switch. In addition, the unit should be as transparent as possible as sound quality is paramount in this hobby... so we have a point-to-point 1 to 4 headphone switch, so up to four headphones can be connected simultaneously to an amp, and switching between them is as simple as the click of a switch (and swapping them on your head).

For the box I used a Hammond extruded aluminum box with aluminum endplates, anodized black (everyone likes black). It needed to be a wider model for enough panel real estate for the switch and four 1/4" connectors.

While 4 pole, 3 throw switches are as plentiful as flatulent passengers in airplanes, 3 pole, 4 throw are a little more difficult to find. I used one made in the U.S. by Electroswitch, found on Mouser. If funds permitted we would have gone with Elma/Goldpoint, but this is a great switch with smooth operation. For the 1/4", I went with the super common Neutrik NMJ6HC-S, which is the more attractive jack from their M-Series and is very reliable. It has switching ability as well, but that wasn't needed in this application.  

Here's a few progress pics below: 

Lining up the jacks 

Drilling the pilot holes

Mounting the jacks and switch


Installing the Kilo aluminum knob

Everything was wired up using solid core silver-clad wire in PTFE. I highly recommend this wire for internal chassis work. It sounds excellent and is easy to work with. Just be sure not to be too aggressive when stripping it as if you nick the solid core wire, it can break off and you'll end up with an open circuit. You'll note below the ground is tied to the chassis via a screw and locking solder lug. This is important to ensure the metal chassis has active shielding from noise no matter which output is selected. 

Interior wiring

This is a simple, effective switchbox which hopefully the client will enjoy. 



Final Product


Hopefully you have fun reading this, and who knows, maybe someone will want an 8, 12 or 16 headphone switchbox in the future! You guys will be the first to know ;)


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. 


May 2, 2018

WHAMMY Pass Labs DIY Headphone Amp

Wayne Colburn of PASS Labs was kind enough to release a new DIY headphone amp circuit into the wild, the WHAMMY, standing for Wayne' Headphone Amplifier Must Make Yourself. It's a nice simple build, no need to match parts or make adjustments, so it's perfect for the budding DIY builder. It's also inexpensive. The PCB, parts and chassis should be under $200 without much effort. Jim Tiemann aka 6L6 of DIYAudio has been kind enough to coordinate the project and has been instrumental in making it accessible. Having a headphone amp of this caliber available at this price is an incredible opportunity and I hope many can take advantage of it. If you're not familiar, Pass Labs is very

You can read more about the project here and boards can be purchased for a very reasonable price at DIYAudio. And you can learn more about the circuit design by watching this Youtube video:

Wayne Colburn of Pass Labs, the designer of the WHAMMY


I'll be briefly walking through my own build. I encourage you to also check out Jim Tiemann's build as it is more granular. My parts choices are also going to be outside of the standard BOM to some extent and will be more expensive, but DIY is fun because you have the freedom to make some unique choices without deviating from the original circuit.

As I knew I would have a little bit of extra headroom in the chassis I'd be using, so I added a piece of Dynamat between the board and the transformer just for a little bit of extra vibration damping. There's electrical tape over some of the larger contacts to prevent any shorting with the aluminum on top of the Dynamat.

Dynamat mounted on the PCB


After this, the power supply components were populated on the board. Normally, you would populate the small components all across the board such as the diodes, resistors, etc., however in this case populating the power components first allows you to test the power supply and ensure it is running properly and within tolerance before moving on. Review Jim's write-up for additional information on the measurements.

Please note the snubber capacitor (C20) should be an X-type, so you might see X1 or X2 in the model number. Before buying the capacitor, check the datasheet to ensure it is X1 or X2 rated. The lead spacing is 22.5mm. I personally used part 594-2222-336-10224 from Mouser. The PSU caps are Nichicon "For Audio"... for whatever that's good for ;)

Power supply populated and ready for testing


After the power supply is tested, fully disconnected, and the caps allowed to fully discharge, I moved along to populate the smallest parts first. If you've read my blog in the past, you'd know I really like Kiwame/Koa Speers resistors. These can be found at Mouser under SPR2 and are 40 to 50 cents each, rated at 2 watts which is far above what's actually needed. Not all values are there, so you can get the remainder at a boutique provider like PartsConnexion or just get some Vishay Dales at Mouser. 

Adding the resistors


Then the rest of the parts are added. While Wayne mentioned that the Silmic IIs, which are now being produced in China, aren't as reliable as they used to be, I still like the sound of them so I went ahead and used them. They're over-rated too @ 50V, so hopefully they'll last a while. You'll notice they're a bit large, so some creative mounting is needed. Wayne suggested using Nichicon BPs (bi-polar caps, they're iridescent green) if you'd rather not use Silmics. C1 and C5 are the input coupling caps and should be of high quality as they're directly in the signal path. I personally used some vintage Sprague paper-in-oils, but you can likely fit some of the smaller narrow Mundorf EVO caps or Clarity Caps, either of which can be found at PartsConnexion. The lead spacing is roughly an inch. 


All PCB parts populated


I hate this picture as it looks terrible from all the flux (this is before I cleaned it with some isopropyl alcohol and a toothbrush), but It shows where the film bypass caps are. The units in the audio section are polypropylene and the units in the power supply are polyester. The twisted pair is pulling off the 22V secondary for a pilot light.  

WIMA film bypass caps on bottom of PCB


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. It would be a slightly tight fit so I needed to see where items would be mounted to the front and back of the chassis so they wouldn't bump into any parts on the inside. The IEC filter in particular needed to be offset from the center so it didn't bump into the PCB mounted transformer. I decided to mount the 0.5A fuse and socket on a blank part of the PCB so there'd be one less hole drilled in the chassis. 


Lining up the chassis-mounted parts


Once everything was lined up and the holes cut, the PCB was secured in the enclosure using 1/2" standoffs. The front panel has a four pin connector in addition to a locking Neutrik TRS as most of my headphones are wired that way, so it saves me from having to use an adapter.

Wiring is scraps of Cardas I had in my wire bin, 4x24, 2x24 and 2x21 and some 24AWG, all litz in teflon. The potentiometer is the $40 100K Audionote with solder tags. It's nice and small so I didn't have to worry about it bumping into a discrete opamp. The IEC filter is from Furutech, gold plated. I normally wouldn't spend the money on it but it was discontinued and quite a bargain. RCAs are Cardas rhodium plated. The switch is a simple 3A rated toggle I picked up from Radioshack when they were clearancing everything. The LED pilot light is one from a guitar shop typically used for an effects pedal. Everything else is from the BOM. 


Wiring everything up


Here's a few more images of the chassis with the wood panels.

Front of the WHAMMY amp in the chassis

Back of the WHAMMY amp in the chassis


I used a very simple circuit from ClassicValve for the LED. 

Pilot light circuit to protect LED




Here's an image with the bottom panel and feet installed. 

Bottom panel with vents


...and here's the WHAMMY almost done, just missing the knob. 

Nearly complete

It's quick and easy to swap the op-amp on the unit to slightly alter the sound. Dropping in a Burson Supreme Sound V5 soundeda bit less constricted than the Texas Instruments op-amp that's in the BOM. The difference was less pronounced than if you use a Burson in a DAC, but still an improvement and worth the price of admission (you only need one). I found the V5i picked up a little more noise in this particular amp, so stick with the V5 if you get one. I also swapped in and out the optional capacitors just in case they improved the sound with the Bursons, but wasn't able to hear any difference in any of the configurations. 


Final pics coming soon. 


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.