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. 

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. 

Modifying the MHDT Havana DAC

MHDT has a cult following in the DAC world, and for good reason. They may not be featuring all the latest chip designs and 24bit oversampling converters, but new technology doesn't have a direct correlation with better sound. The equipment designed in the 1960s from H.H. Scott, Dynaco, Harmon Kardon, Marantz and McIntosh is still well sought after today for it's excellent sound qualities. Granted if you are a purist and want a straight line response curve, these won't scratch that itch, but if you want to enjoy the music they'll fit the bill perfectly well.

The MHDT Havana comes after the venerable Paradisea 3, which is an excellent sounding example of a NOS (non over-sampling) DAC with a tube buffer. Both the Paradisea and the previous Constantine make use of a Philips TDA 1545A DAC chip and come in a handsome translucent dark acrylic. (See the posts covering the modification of the Constantine and the Paradisea.) Unlike the previous two, the Havana uses no op-amp for I/V conversion. Opamps are often looked down upon as the tasks performed can generally be done better with discrete components. The Havana features a 16 bit R-2R Burr Brown PCM56P DAC, and uses a tube-buffered output stage (GE5670).

A stock MHDT Havana DAC

It's interesting to see the progression of MHDT DACs over the years. Not only are the op-amps going away, there's more aluminum panels and less acrylic body panels, as well as the utilization of more surface mount components on the PCB. The circuit boards have also become more robust, which is nice when exchanging components. Other components generally stay about the same, from Nichicon Fine Gold electrolytic caps in the power supply, Nichicon Muse caps and Sanyo/Panasonic OS-CON caps elsewhere, as well as MHDT brand film caps which I imagine are sourced in Taiwan. 

The stock PCB removed from the chassis

I generally like to replace the Muse caps with Elna Silmic II for a little more warmth, as well as Soniccraft 600v 0.1uf and 0.22uf Sonicaps for the smaller film caps, as Sonicaps are a great value and are small in form factor compared to most other audio capacitors. For the output caps, replacing with the best quality that fits that you can afford is generally the best strategy, as they are directly in the signal path.

Below is a partially modified unit. Note the Nichicon FG power supply smoothing caps have been bumped up to 3,300uF (there's plenty of room), most film caps have been replaced, and I had just started installing the Elma Silmic II caps.

Partially modified PCB, note larger Nichicon FG filter caps, Sonicap film caps

The RCAs on the unit are perfectly fine, but I was able to source some nice ones with teflon insulation, so I installed those. The unit on the left is the teflon insulated one, it looks a little bit different but fits perfectly. 

Teflon RCA left, stock RCA right

The RCA for the digital coax was replaced with a Vampire BNC (about $13-14). I chose Jupiter HT paper in wax caps for the output. They've served me well in other applications and have a nice natural presentation. I happened to have several Kiwame resistors that I installed in the tube buffer. 

The completed mod

The Jupiter 2.2uF 600V caps just narrowly fit in the Havana chassis. I tombstoned one of the Sonicaps to ensure there was enough clearance. One thing I found odd was MHDT never grounds their DACs. I understood there wasn't much of a point with an acrylic chassis, but now that they're brushed aluminum, might as well use it as a shield. The electroplating is very thick on the chassis and needs to be sanded away to make contact. I used a Dremel on the metal under each screw and put everything back together, checking for continuity with a multimeter. I had to re-hit a few of the holes but eventually got there. I ran a wire from the ground on the IEC to circuit ground and connected to chassis ground, so now we have a safe and shielded DAC. 

Boxed back up

Everything is sounding very nice so far. The op-amp-less design is a little more forward than the Paradisea (which is very warm and syrupy), but still very natural sounding and makes for an enjoyable listen. When I say forward, I simply mean related to other MHDT DACs. It's still warm compared to 95% of DACs on the market. I still have to let the Jupiters burn-in, which in my experience just takes a couple of nights of audio running through them, but everything will open up once they do. 

The Fine Print:

Please remember that 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 posted modification 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.