Zynsonix Link

July 18, 2017

AMB σ11 Sigma 11 Regulated Power Supply

Version 2 of AMB's σ11 (Sigma 11) regulated power supply has been out since June of 2015, but I'm finally getting around to building my own. AMB (Ti Kan) is mostly known for his headphone amp and DAC designs, but his site offers several power supply options as well.  

The σ11 is a top notch DIY single-rail linear regulated power supply known for low-noise, high-current, excellent line/load regulation, wide-bandwidth, and stability. The circuit features all discrete components and high-current MOSFETs for the output. Per AMB, the σ11 is a great choice for DIY headphone amplifiers, pre-amps, DACs, network media players, and other devices needing a single-rail regulated power supply.

AMB o11 Power Supply PCB

This power supply is a pretty quick build with ~40-50 total parts. A typical builder can populate the PCB in an hour or two. AMB offers the PCBs for sale on his site, along with some of the necessary parts. The rest you'll need to get from Mouser / Digikey or your favorite parts supplier. I went with all stock parts aside from an Antek 50VA toroid instead of the Amveco TE62062 25VA as it was much more reasonably priced, albeit a little larger. Because of this, the Hammond 1455N2201 was the best chassis size for the job. 

Antek 50VA toroid

Rather than leave the Hammond case plain, I wanted to give it a little more character. I sourced a piece of wood online which didn't have a species listed, it simply was called Asian burlwood. I cut a front plate with about a 1/2" overhang in each dimension and drilled holes for the screws to line up with the existing screw holes in the Hammond case. Two coats of satin lacquer gave it a nice sheen. 

Asian burlwood front cover

Two holes were cut into the top of the Hammond case and it was sanded down with 180 grit sandpaper to reveal a grain-like anodization pattern. Powercoated aluminum grating was secured with adhesive underneath the holes... then a permanent furniture market was used to give the bare aluminum a bronze-like color. 

In the rear, I've installed a pair of outputs. The board offers up to four outputs if desired, but an appropriately sized toroid should also be used. If you're not sure about the rating needed, the forum on the AMB website can be very helpful as other people have likely already asked the questions you are wondering. 


AMB σ11 Sigma power supply rear

A Fender-style pilot light was installed up front with an Amber jewel and LED. 

AMB σ11 Sigma power supply front with pilot light

All done! Now I have a nice regulated 24v power supply now with a pair of outputs. 

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 11, 2017

Modifying the MHDT Atlantis DAC

MHDT has a cult following in the DAC world, and for good reason. They may not all be featuring the latest chip designs and 768kHz inputs, but new technology doesn't always have a direct correlation with better sound. Case-in-point, the prices of 1960s equipment made by H.H. Scott, Harmon Kardon, Marantz and McIntosh all continue to increase from more and more audio enthusiasts desiring the musical qualities inherent from the Golden-era of audio. Granted if you are a purist and want a straight line response curve, these won't scratch that itch, but if you crave a warm musicality MHDT has few peers that can rival.

The Atlantis is one of the latest offerings from MHDT, and much like last generation's Havana, along with the current generation's Stockholm, features a R/2R Multibit DAC. The Stockholm, Atlantis and Pagoda are unique as they have discrete transistors in the I/V stage, no opamps at all. Opamps are often looked down upon as they can be a "one-size-fits-all" type of solution, and the tasks they perform can generally be done better with discrete components. The Atlantis features a AD1862-J DAC chip, and uses a tube-buffered output stage (GE5670).

MHDT Atlantis with stock internals


Luckbad, a member at headphone enthusiast forum HeadFi, created an AWESOME table that compares the more recent MHDT DACs:

Mhdt Labs DAC Families ​
                      Models​
Havana​
Havana 2​
Stockholm 2​
Atlantis​
Pagoda​
Steeplechase​
      Spec
Input Capacity USB (Max)​
16bits/48kHz​
24bits/192kHz​
24bits/192kHz​
24bits/192kHz​
24bits/192kHz​
24bits/192kHz​
Input Capacity SPDIF (Max)​
24bits/96kHz​
24bits/192kHz​
24bits/192kHz​
24bits/192kHz​
24bits/192kHz​
24bits/192kHz​
Output  Format​
16bits​
16bits​
16bits​
20bits​
24bits​
24bits​
Digital Receiver Chip​
CS8414​
CS8416​
CS8416​
CS8416​
CS8416​
CS8416​
USB Chip​
CM102AS+​
CM6631A​
CM6631A​
CM6631A​
CM6631A​
CM6631A​
DAC Chip​
PCM56P-J​
PCM56P-J​
PCM56P-J​
AD1862-J​
PCM1704​
AK4396​
DAC Chip Construction​
R/2R Multi Bits​
R/2R Multi Bits​
R/2R Multi Bits​
R/2R Multi Bits​
R/2R Multi Bits​
Delta-Sigma 1 Bit​
I/V Stage​
Voltage out
 DAC's Internal OPAMP ​
Current Out
Discrete Transistors I/V,  No OPAMP, No feedback​
Current Out
 AD847AQ as I/V​
Tube Buffer​
Tube Buffer with 5670/2C51 ​
Output level​
2.6V​
2.6V​
3.0V​
3.0V​
3.0V​
2.8V​
Output impedance​
32 ohms​
32 ohms​
32 ohms​
32 ohms​
32 ohms​
32 ohms​
Inputs Available​
3 Inputs -- USB/ RCA/Optic​
4 Inputs -- USB/RCA/BNC/Optic​
USB Input Topology​
USB1.0​
USB2.0​
USB Driver for Win XP/W7/W8​
No Needed​
Yes, Needed​
USB Driver for Linux/Mac​
No Needed​
Dimensions clear (WxDXH) ​
260X150X60​
276 x 150 x 60 mm​
Dimensions w/ socket ​
280X170X60​
295 x 170 x 60 mm​
Weight​
1.8Kg​
2Kg​
Box Color Avialable​
Black Only​
Black/Silver Selectable​
Silver Only
It's been fun to see the progression of MHDT DACs over the years. The chassis used to be made completely out of acrylic. Now only the front cover is acrylic (a nice thick acrylic) and the rest of the body is make of aluminum panels that screw together. In addition, the op-amps are going away and there is more utilization of 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.

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.

MHDT includes a larger toroid in the Stockholm, so I wanted to upgrade the power supply in the Atlantis. I checked Mouser and Digikey for a larger toroid and no one seems to make a unit with the two secondaries needed, so you'd either have to have two toroids (which wouldn't fit in the case) or snag one from MHDT directly. The larger unit from the Stockholm fits perfectly well in the Atlantis, you don't even have to drill a new hole to mount it. While they look similar in size, the larger one is twice the volt-ampere rating.

Comparing the larger toroid from the Stockholm (black) to the stock Atlantis unit (white)

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 ordered from Partsconnexion



The Jupiter 2.2uF 600V caps had to be shoehorned to fit. While they fit fine in the Havana (albeit tightly), the Atlantis opening was a little narrower despite the slightly larger chassis footprint. Reviewing my options, I decided to stand the caps off the board slightly to clear the small Oscons and sand down the heatsink to allow the capacitor to wedge up next to it. It doesn't get terribly hot, so we'll see how that goes.

While I don't go out of my way to get audio fuses (I'd be pretty grumpy if a $70 fuse broke, and I'm a little skeptical as the resistance in a fuse is minimal), PCX had a few on sale for $12 from HiFiTuning so I figured I'd give it a shot. $12 is a drop in the bucket for audio gear as you know. Swapping the 400ma stock fuse with the gold fuse seemed to increase the higher frequencies (by a small degree), so it may be counteractive when you're after a warm-sounding DAC. I'll be testing more, but that's my initial finding. I may try in some other equipment to see if that's consistent if they're still cheap for my next order ;).  

Click to zoom in

The DAC sounds excellent. More resolving and clear than my Paradisea and less forward than my Havana. It's my favorite sounding MHDT so far and very easy to listen to. I hear the Stockholm is even better (based on forum banter around the web), but this is a DAC for a few bucks less. 

UPDATE: I found some 30mm aluminum feet with a damping ring on Aliexpress that are a nice step up from the stock plastic feet. They're  quick and easy replacement and are less than $10 for a set of 4.




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. 


July 7, 2017

Fred's Amplifiers Portable 12AU7 Tube Headphone Amp

I was browsing around the net looking for a quick and fun little project when I discovered Fred's Amplifiers. Fred has a small site with a number of PCBs and kits, most of which are portable headphone amplifiers, along with a portable guitar amp kit and a couple of crossfeed units.



I decided to order the 12AU7 Valve headphone amp PCB. It's a single sided PCB and was only $10, with the BoM (bill of materials) posted on the site. I went with the PCB versus the kit as I have tons of parts lying about, but the kit is only $45 if you want to go that route, just note you need to buy the chassis (Hammond 1593) and A/C adapter (12v 1A Regulated DC supply, center pin positive) on your own. The unit only requires around 25 parts to complete, so you'll be done in no time. The Op-amp in the kit is the venerable JRC 4556 used in most CMOYs / RA1 clones and similar portable low-power headphone amps.

I went with the translucent blue version of the Hammond case (1593KTBU). Trimming into the plastic Hammond case requires you to go slowly on the drill press or the plastic will crack. You may want to buy two cases (they're $4 each) just in case your holes don't line up perfectly.





My unit features Takman carbon film resistors, a diffused 5mm amber LED, a milled aluminum knob from Kilo, and 470uf 16v Elna Silmic II capacitors. Elna Silmic II are widely accepted as the best electrolytic available for the signal path. They are a tough fit, and will require you melt down the PCB standoffs on the bottom of the Hammond case to get everything to fit nicely. You could also drill holes at the top of the chassis for clearance, but that reduces the portability in my opinion. The AC adapter I went with was a $10 Meanwell unit - SGA12U12-P1J. I'm sure there are cheaper, I just plugged the specs into Mouser and bought the first one on the list. 

Here are a few more photos of the amp:    










There's nothing challenging or unusual about this build, so if you'd like a fun and affordable little portable tube headphone amp, put this one on your short list. 


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. 

June 23, 2017

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. 


The Lessloss Power Conditioning Firewall Module

I've heard many positive impressions from fellow audio enthusiasts regarding LessLoss power conditioning products and have always been curious about how they stack up. The unique construction of the previous Firewall using exotic materials like Panzerholz, a plywood-like bulletproof material with high damping properties, and carbon fiber, had left me very curious. The new Firewall is a much more affordable endeavor, starting as low as $198 for the DIY version, $385 for the USB version and $410 for the power outlet version.

As nearly all of my gear is completely DIY, I took a special interest in the DIY version of the Firewall. I've come accustomed to the typical simple resistor/capacitor/inductor networks in discrete filtering circuits (those you commonly find in an audio power conditioner, the filtering IECs that are sometimes installed in DIY tube amps, and other DIY implementations), but have never seen an alternative to these.

The Firewall DIY module is relatively small, roughly 2" by 4" and has and input and output for L (line/hot) and an input and output for N (neutral). There are unique ripples/patterns in the surface of the solder mask which cover the copper underneath.

Click to enlarge the photo to see the solder mask

Five Firewall units lined up


Per the instructions, The Firewall units can handle up to 1000VDC and can be stacked and/or run in series to increase the level of the filtering. Kapton tape or discs is recommended to prevent the inputs and outputs from shorting.

Four LessLoss Firewall Modules stacked in series

There have been tests of up to four parallel stacks of eleven boards if you want to go super crazy ;) Do note that "LessLoss will not be held responsible for the fate of your equipment or health or other personal property should you choose to use our product in your own designs", so stick with the off-the-shelf units unless you know what you're doing.

More to come...

Burson Audio Opamp Review - Supreme Sound V5 / V5i

Op-amps, or operational amplifiers, are ubiquitous in solid state amplifiers. These little integrated chips are simple little building blocks designed to work in a wide range of electronics such as PCs and other devices. For those of you not familiar, op-amps are like vacuum tubes for the solid state world. Most manufacturers use sockets so the op-amps can be swapped, and each have a difference sound signature (e.g. warm, revealing, etc.). Some have voltage requirements that make them only compatible in certain designs, etc. Some are affordable (the LM4562 which runs less than $1), and some are expensive like the popular $25 Burr Brown OPA627.

Burson Audio is a company on a very short list of providers of discrete op-amps. Rather than having a bunch of teeny tiny components shoe-horned in a chip the size of a fingernail, Burson creates devices that plug into op-amp sockets that use a fully discrete set of diodes, resistors, etc. I remember my first purchase of several of their discrete op-amps  roughly six years ago, trying to breath some additional life into a Music Hall DAC. While their first iterations weren't super easy to install, they made a world of difference in the sound department. Everything was much more natural and effortless sounding than the stock op-amps which I believe were mid-grade models from Texas Instruments.

Fast forward to today and we have the Supreme Sound V5 and V5i Opamps. The V5 features fully discrete components including 0.5% tolerance metal film TKD resistors, hand-matched FET transistors, and a sleek looking red cover.

The Burson V5 (left) and V5i (right) in their packaging



Note the size difference between the two units

A fixed 8 pin DIP socket makes installation much easier

It's also a smaller form factor than the originals, which is crucial for installing in tight circuits / spaces. The V5i is a hybrid of sorts which includes both IC and discrete components, but is much smaller and should fit in virtually any build. Do note that these units can only replace op-amps as noted on the Burson site:

Dual Op-amps:
AD823, AD823AN, AD8066, AD8620, AD712, AD827, C4570, JRC4556AD, JRC4580, JRC5532, JRC5532D, JRC5534, LF353, LM4562, LME49860, LM833N, MUSES8920, NE5532, NEC4520, NEC4570, NJM2068D, NJM2114, NJM2214D, NJM4558, NJM4558D, NJM4560, NJM5532, NJM4558P, OP275, OPA1612, OPA2277PA, OPA2132, OPA2134, OPA2604, JRC4558, RC4558D, RC4558P, TL052, TL072, MUSES01, MUSES02, MUSES8820, MUSES8920, MUSES8832, BA15532

Single Op-amps:
NE5534, LT1122, TL071, OPA134, OPA627, AD811,AD829, AD844, OPA604, AD8610, AD711, AD797, LME49990, LME49710

Installing them in a unit that is not compatible could cause undue stress on the components / oscillation, etc.

More to come...

May 24, 2017

Four Pairs of Headphones for Any Studio Setup

This guest post is brought to you by my friends at Headphone Ninja. Anyone following the blog knows I love headphones, and Jo-Ann and company have culminated four tried-and true classics to discuss.

Studio headphones fill the market today. However, many of them are either far too expensive or far too inaccurate when it comes to meeting the referencing demands of producers and audio engineers. While everyone would love to own a pair of the sleek Sennheiser HD800s or the beautiful Japanese cherry birch wood Fostex TH 900s, we don’t all have a Hans Zimmer’s budget.

In this post we explore four of the more preferred headphones available in 2017 that are well suited to the modern day studio.


Closed-back headphones


Audio-Technica ATH-M50X


Comfort is key when it comes to long studio sessions. The generously padded earcups and adjustable headband of the ATH-MX50 provides comfort, and while the headphones do clamp down reasonably hard, they do not physically strangle the life out of your temples. They are slightly heavy at 10-ounces, but this should not be a deciding factor for those looking to use these in a studio environment.

The ATH-M50X is fitted with large 45mm proprietary drivers that deliver exceptional clarity. Some individuals who have used these thought that the bass is slightly high, but the mid frequencies are fairly well represented. The higher frequencies also appear to be slightly bright, but not to the point of fatiguing the ear. The 90-degree swivelling earcup is great for single ear monitoring and the frequency response ranges from 15 - 20,000Hz.




Sony MDR7506


Introduced back in 1991, the Sony MD7506 has remained a firm favorite amongst many looking for high quality and reliable headphones. Their durability is as decent as a pair of plastic headphones can get, with the only real issue of deterioration being the earpads on the cups, but these are replaceable. At around 8-ounces, they are light enough to not burden the wearer much.

The MDR7506’s frequency response ranges from 10 - 20,000Hz. It has revealed a solid bass response in tests, albeit slightly bumped up in the 60 -100Hz range. Mids remain fairly flat while the low treble to the high treble frequencies are quite inconsistent; the sibilance range is slightly boosted and can be slightly piercing to the more sensitive ears. The soundstage is decent for a closed-back pair of cans. They have an 63-ohm rated impedance and their low leakage is also suitable for recording purposes.
The coiled 9.8-inch cord is more than enough for studio environments and it does come with a ¼ inch adaptor. The Sony MDR7506 gives many mid-range referencing cans a run for their money.




Open-back headphones

Sennheiser HD 650


The Sennheiser HD 650 are slightly pricier, but it seems important to add these cans for those who enjoy the spacier sounds that open-back circumaural headphones offer. The suede-like fabric is a nice touch on the large, encompassing cups that house the efficient neodymium magnets.

The slightly under-emphasized low bass frequencies with the minor boost on the high bass gives the headphones a more natural sounding low end. The mid range responses are pretty much flat throughout and the treble frequencies are consistent. Their impressive frequency response coupled with the spacious sound created by the deep drivers and open-back design makes these excellent for critical listening.

Needless to say, their isolation is pretty much non-existent, making these more suited to quieter environments. They are also less suitable for recording purposes, because they do leak quite a bit of additional noise, which in turn will bleed into your microphone. 


The Sennheiser HD 650 is an excellent referencing headphone and offer an awesome frequency response, though they are less suitable for recording purposes however.

Editors Note: The 650s can sound a little dull in the treble with the stock cable in some setups. A replacement featuring silver-clad copper can bring out the highs. Check with Zynsonix for additional details.


AKG K712 PRO


The AKG K712 PRO are another great choice for mixing and mastering purposes. The design is slightly bulky, but they are purposefully built for studios, so that is completely understandable. The memory foam earcups captures the shape of your head, but they are rather large. Sound wise, they deliver a spacious and accurate audio reproduction, but the sound is a bit warmer in the upper bass. Still, it is well controlled overall. The mids are represented accurately.

Sensitive ears may find the slightly boosted (and somewhat inconsistent) higher frequencies a tad overwhelming, as they can sound harsh. The frequency response ranges from 10 - 39,800Hz in its entirety and it has a low harmonic distortion.

As a whole, the AKG K712 PRO have a natural sound and are well suited to intensive referencing sessions. The open-back design means they are more suited for mixing and mastering in studios.

Wrap up

This is by no means a comprehensive guide to selecting studio headphones, but a look at some of the more popular headphones around at the moment for any type of studio. While they may mostly be mid-range headphones, they perform well within their specific categories and are well suited for studio use.


May 22, 2017

Millett NuHybrid Headphone Amp Using Korg NuTube

In Mid-March, the prolific DIYer Pete Millett introduced a new hybrid headphone amp called the NuHybrid using the Korg Nutube 6P1. The design is similar to the original Millett Hybrid, which spawned both the exceedingly popular Millett MAX and stripped-down Starving Student builds. The Starving Student was super-popular as it could be built for as little as $35, which is unheard of for an amp with two tubes.

To shake things up (Millett designs are rarely status quo), the old car radio tubes were eschewed for the new Korg Nutube. The Nutube is a low-power, directly-heated dual triode tube built using a process originally used for Vacuum Fluorescent Displays (VFDs) and emits a bluish-white glow when the tube is powered up. The Nutube has been popular with DIYers in Japan since it was released at the tail-end of 2016, but hasn't really caught on with DIYers elsewhere, at least yet. The cost of a Nutube is around $50, which may seem a little steep, but two noval signal tubes will set you back at least $30-35 these days, along with another $5 for sockets and the potential to have to replace them in a few years, which makes $50 not seem all that bad.

The Korg NuTube

One of the nice things about the design is you don't have to feed it plenty of amps. The whole thing runs off of a 24V power supply, so it is much safer than a typical tube-based build. A pair of OPA551 op-amps to drive the headphones, but others can be rolled in for a different sound signature. Pete has made the design so it can be built cheaply (~$120) and placed in a Serpac plastic case. Much like Pete I don't like status quo, so I'll be building a case from scratch for my NuHybrid amp. I wanted to make the amp look like an old tabletop radio, so a wood frame would be needed.

A quick trip to Home Depot yielded a 6ft long by 3.5" wide piece of maple.


A little bit of routing and a few 45 degree cuts on a miter saw and we have a frame. Note the inside of the frame is rabbeted so the front and rear plate can sit flush. The 5/8" pieces in the middle are for screws to hold the plates in place. This is a photo after an initial coat of Minwax Pecan colored stain and seal combo. Yes I know it's sacrilege for anyone who works with wood regularly, but it's quite and easy.

Frame to hold PCB inside

As wood doesn't protect from errant EMI/RFI, I've added some copper shielding tape which will be electrically connected to the front and rear aluminum plates to get 100% coverage.


Copper EMI/RFI Tape for shielding

The board is quick and easy to populate and should take even the most OCD solder slinger less than 3 hours. Below is an image of the PCB. The checkered areas indicate where double-sided tape is applied to the board to hold the NuTube in place.

NuHybrid unpopulated PCB

Below you'll see my PCB about 90% populated. I changed a couple of the parts in the BOM. The BOM calls for Wima polyester, but you can switch them out with Polypropylene for about $2.50 more and they fit without issue. I also went with Elna Silmics in the power supply. The Silmics sport a 7.5mm lead spacing versus 5 on the Nichicons, so they have to be pressed into place otherwise they'd stand crooked. They also encroach a bit on other areas of the PCB, so the Wimas needed to be mounted on the bottom. I also went with Takman carbon film resistors in most areas. The 1/4 watt variety are only 36 or so cents a pop at Sonic Craft, so it won't break the bank to use them.

Note the large-and-in-charge Silmics in the power supply section  

I picked up two aluminum plates 1/8" thick for the front and back. I usually get them pre-cut from eBay but will also use a miter saw with a metal blade if the size I need isn't available. The back of the unit needs holes for four RCAs and 24v DC. Although I doubt the unit gets very hot due to the low power requirements, I drilled a hole pattern at the top for heat to escape, as with wood it will trap it in otherwise. The aluminum is spritzed with Hammertone paint in a copper finish.




1/8" thick aluminum plate with holes for RCA, DC

Attaching the unit to the front panel are the 1/4" inlet nut and the TKD potentiometer nut. I went with the nicer TKD potentiometer as suggested by Pete. $40 is a bit painful but I'm tired of using Alps Blue Velvets for every build. The front LED, switch and coupling capacitors (Russian Silver Mica) are connected with fly leads (22ga. silver clad teflon solid core wire I had left over from a Bottlehead build). The wiring to the RCAs is Cardas 24ga. stranded in teflon. Note the space between the two mica caps in order to reach the adjustment pots. The RCAs are Switchcraft units I had on hand. The switch is much higher rated than it needs to be, but I have tons of surplus switches and like the big old school ones. If you're looking for low cost and high quality, check out Apex Jr.








Attaching the panels gives the nearly finished product...



Vent holes probably not needed, but look nice


The last touches were an engraved front plate and changing the pilot light to a funkier looking one.


All done and sounds great!

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.