(Last Updated On: May 26, 2019)

How a Fluorescent Display became a Triode Vacuum Tube for Audio

The NuHybrid is (as you may have guessed from the name) a new type of hybrid tube headphone amplifier. This relatively little-known kit is based on the original “Millett Hybrid” headphone amplifier design from 2002, but instead of using a traditional old car-radio tube, it uses a Korg Nutube 6P1. Like the original, the NuHybrid employs a low-voltage tube stage for voltage amplification and a solid state OpAmp to drive current to the headphones.

The finished product

This article will discuss:

  • Building the NuHybrid. How to populate the printed circuit board (PCB) of this excellent and unusual tube amplifier kit.
  • Since the finished kit leaves the builder with an exposed board, we will also take a look at what’s involved in building an attractive solid wood and aluminum case to fully house the NuHybrid.
  • We will examine an intriguing new DAC (Kahdas Tone Board) that can be included in the wooden case to create an ‘all-in-one’ DAC + amplifier unit.
  • Finally, we will take a look at objective measurements, and I’ll share my opinion on how it all sounds.

Before we dive into this new tube project, and to better understand where this design comes from, let’s first take a look at the designer – Pete Millett.

Brief History of the Millett NuHybrid Headphone Amplifier

Who is Pete Millett?

Pete is a self-professed ivy-league engineering school dropout. He is an electronics engineer and has worked as a board-level hardware design engineer for more than 30 years. More recently, Pete has moved into a technical marketing engineer role for a semiconductor company. As a hobby, he’s been designing all sorts of tube audio gear for a couple of decades or so. He’s created headphone amps for Headroom and written for AudioXpress magazine.

In 2015, a new type of audio vacuum tube, the Nutube 6P1, was announced by the Korg company. Pete arranged with Korg to distribute the Nutube to the DIY community and small manufacturers through www.nutube.us.

It didn’t take long for Pete to start designing projects around the Nutube. In addition to the NuHybrid headphone amplifier, Pete offers a balanced preamp/headphone amplifier, a 50 watt hybrid class-D amplifier and a stereo buffer based on the Nutube.

The Nutube 6P1

Korg partnered with the Noritake company to develop an entirely new type of dual triode vacuum tube. In some ways, the Nutube is very similar to a conventual vacuum tube. It uses an anode grid filament structure and creates the same characteristic warm “tube sound”.

The Nutube 6P1 triode vacuum tube from korgnutube.com.
The Nutube 6P1 triode vacuum tube from korgnutube.com.

However, the Nutube does not physically resemble a traditional tube. It is built in Japan, using a process originally used for Vacuum Fluorescent Displays (VFDs). This yields a small flat box (45 mm x 16 mm) with two small rectangles (phosphor coated plates) that glow bluish-white when the tube is powered up. Technically, it is a one-pixel VFD.

A typical triode vacuum tube from Insignia HiFi.
A typical triode vacuum tube from Insignia HiFi.
Korg was established in 1963 and specializes in manufacturing electronic musical instruments such as synthesizers, tuners, digital pianos, signal processors, and digital recorders. Noritake Itron Corp., a Noritake Col. Ltd affiliated company, invented the Vacuum Fluorescent Display (VFD) in 1966. Used initially in electronic calculators in the 1970s, VFD use expanded to typewriters, home appliances, automotive panels, industrial devices, gaming machines, medical equipment and communication equipment.

Korg intends the Nutube to be used in headphone amplifiers, guitar amplifiers, guitar pedals, synthesizers, and preamplifiers. They offer an official premade evaluation board, including a Nutube 6P1 mounted on a PCB for $150, which can be used as a headphone amplifier or it may be placed in line between devices such as a source and amplifier. Korg has also recently released the HA-Kit portable headphone amplifier kit for $250 on Ebay.

Benefits of the Nutube 6P1

Improved Quality Control

The manufacturing equipment that is currently used to produce traditional vacuum tubes has often been in use more than 50 years. This leads to defective units and inconsistent characteristics between tubes. This new design uses improved manufacturing techniques.

Low Power Usage

The Nutube requires less than 2% of the electrical power of a conventional tube. It uses only 12 mW per channel so it is suitable for battery powered operation.

Low Temperature

The Nutube does not generate heat when in use.

Reliability and Long Life

The Nutube is rated for 30,000 hours of continuous use.

Small Size

The Nutube is less than 30% of the size of a conventional vacuum tube. The flat shape is easier to design enclosures around than traditional tubes.

Vacuum Tube Sound

The real triode structure produces warmth and harmonics with excellent linearity.

Drawbacks of the Nutube 6P1

Cost

At $50 it is more expensive than many traditional driver tubes.

Low Transconductance

Triode tubes and derivatives (tetrodes and pentodes) are transconductance devices. This means a controlling signal of voltage is applied to the grid and the resulting amplified signal is current. This requires a low impedance source to drive the grid.

“The basic application circuit provided by Korg uses simple JFET source followers to buffer the signal in and out of the Nutube… An input buffer is used to support the relatively low impedance looking into the grid and bias source. The output buffer is needed to drive anything but high impedance loads due to the low transconductance and high plate resistance of the tube…” – Pete Millett for Audioxpress
High Microphonics

The Nutube is directly-heated, which significantly reduces power consumption and heat generation. Unfortunately this also means it is VERY easily affected by vibrations and will create an audible metallic pinging noise when moved or tapped.

“One notable side effect of the construction is that the filament wire, under tension, is a lot like a piano string—when subjected to a shock or mechanical impact, it vibrates at an audible frequency (5 kHz or so).” – Pete Millett for Audioxpress

Korg does suggest ways to try to prevent microphonic noise from external vibrations, transferred either through the mounted circuit board, or even transferred through the air around the tube. They suggest inserting some cushioning (like a sponge or soft equivalent) between the Nutube and the main circuit board, and to connect it using soft and thin wires.

Korg further recommends enclosing the Nutube in a protective box or case and using acoustic material in the enclosure around the Nutube. They go so far as to suggest placing a heavy metal plate (such as lead) on the surface of the Nutube to reduce noise. Not great for aesthetics or seeing the tube glow.

The bottom of the Nutube 6P1 from korgnutube.com.
The bottom of the Nutube 6P1 from korgnutube.com.

Technical Specifications

  • Operating Voltage: 5 – 180 V
  • Operating Temperature: -40 – 85 degrees C
  • Heater Voltage: 0.7 V
  • Heater Current: 17 mA
  • Price: $50

The NuHybrid Amplifier

Pete estimates that to build the NuHybrid amplifier, including the minimal bottom case, to cost about $115. He offers the bare NuHybrid PCB including a Nutube 6P1 on Ebay for $50, essentially giving the board away for free with the purchase of the Nutube. Pete has stated that “one goal with this design was to make it as affordable as possible.”

The Nuhybrid PCB from pmillett.com.
The Nuhybrid PCB from pmillett.com.

The kit is intended to be an easy project for someone to complete with minimal electronics experience. To this end, Pete offers a comprehensive assembly instruction manual and a premade Bill of Materials (BOM) to make ordering the right parts as easy as possible. The current parts list total is $66.51 on Mouser. A detailed BOM is a huge benefit for first-time builders and a great way to get your feet wet in DIY.

One of the best things about DIY is that you can often ask questions directly to the designer and, in some cases, assist with the development of a project. Pete is a regular contributor to the Head-Fi NuHybrid discussion and frequently answers questions and provides assistance.

NuHybrid Circuit

The dimension of the NuHybrid PCB is 5.5” x 2.7”. Power input is 24V DC and current draw is approximately 80 mA.

The Nuhybrid PCB and Nutube fresh from Ebay!
The Nuhybrid PCB and Nutube fresh from Ebay!

The input signal comes from RCA jacks (located on the rear of the PCB) and travels through the volume pot. Because of the location of the volume pot in the circuit, the input impedance is equal to the impedance of the volume pot (the default value is 10K). The signal is capacitively coupled into a pair of emitter followers using 2N3094 NPN transistors.

The NuHybrid is Class A2, meaning that the control grid is biased slightly positive, so the grid draws a bit of current when driven. The output of the tube buffer is capacitor coupled using an 10uF capacitor in the Nutube grids. Positive grid bias of 0-3.3V is adjustable using trim pots. The 700 mV, 17mA per triode filament power and the positive grid bias comes from a 3.3V linear regulator.

The Nutube plates are coupled directly to a pair of OpAmp buffers (OPA551). The OpAmp output is capacitively coupled to the headphone jack through electrolytic capacitors (bypassed with small film caps).

Output RCA jacks are included for use as a preamp. The output RCA jacks are deactivated when a headphone plug is inserted in the jack. Both headphone and RCA outputs are muted for approximately 10 seconds upon start-up to eliminate any ‘thump’ noise from capacitor charging.

Assembling the NuHybrid

The NuHybrid is only available as an unfinished kit. You must purchase the bare PCB, Nutube, internal components, power supply, case, etc. and assemble it yourself.

To populate the NuHybrid PCB, you will need a few basic tools:

  • Soldering iron
  • Solder – Tin/lead 63/37 is recommended. The PCB has a tin/lead hot air solder leveled (HASL) coating.
  • Wire cutters
  • Fine tipped needle nose pliers
  • Tiny flathead screwdriver
  • Digital Multimeter (DMM)

Experience is not necessary to populate the PCB, as this is designed to be a basic kit to assemble, however a little soldering practice would not hurt. With some care and careful following of instructions, this project is entirely appropriate for a first-time builder.

Resistors are the first component to be soldered to the board.
Resistors are the first component to be soldered to the board.

Assembly of the components on the PCB can happen in any order, however the instructions organize things to install the lowest profile components first and work up through the taller parts. Parts are numbered from the lower left corner when the PCB is positioned so that the writing is the proper orientation.

If you are new to electronics, some components are polarized (such as capacitors and diodes) and MUST be installed in the proper (marked) orientation! Other components such as resistors can be installed in either orientation, although for neatness, I recommend picking a standard and sticking with it.
Small capacitors, diodes, and OpAmp sockets are added to the board.
Small capacitors, diodes, and OpAmp sockets are added to the board.

While the leads of components can be bent with your fingers, I find that more attractive results are obtained if I estimate the distance between mounting holes by eye, and then bend the leads to perfect 90 degree angles (using needle nose pliers) prior to insertion.

In general, to populate a PCB with components, each component is inserted in the board from the top through the appropriate holes. The component is pulled down to be flush with the top of the board and the leads (tinned metal wires extending from the component) are gently bent outwards on the back of the PCB to hold it in place. This temporarily holds the component in place, and readies it for soldering.

The soldering iron is momentarily touched to heat the solder pad on the board and the lead evenly and at the same time. Solder is applied to the hot pad and lead, which liquifies and creates a small, neat mound. If done properly, the solder cools to a cone shape and hardens to encompass the pad and hold the lead securely. The excess lead beyond the solder is then trimmed off using wire cutters.

This is just a quick overview of the soldering process, and there are many excellent in-depth tutorials available if you are new to soldering or want to learn more.

A closeup of the populated board.
A closeup of the populated board.

Repeat until all components are attached, triple-checking before applying solder for all components (capacitors and diodes) that have to be oriented properly for polarity.

The instructions walk the builder through installing (in order):

  • Resistors
  • Diodes
  • IC Sockets (for holding OpAmps)
  • Film Capacitors
  • Electrolytic capacitors
  • Bias Trim Pots
  • Transistors
  • Voltage Regulator
  • Test Points
  • Relay
  • Inductor
  • Volume Control
  • Headphone Jack
  • Power Switch
  • Power Jack
  • Input and Output RCA Jacks
  • Power LED
  • Nutube
  • OpAmps

Power On Test

It may seem like a long assembly list, but populating the board is an easy to finish project in a single afternoon or evening (about 2-3 hours depending on the builder’s skill). Once the board is fully assembled, and DOUBLE-CHECKED AGAIN, it’s time to plug it in and flip the switch. The green power LED should immediately light up and the Nutube should start to glow bluish-white.

Using a Digital Multimeter (DMM) measure the DC voltage between the test points. The voltage should be adjusted to 11V using a tiny flathead screwdriver on the trim pots. The 11V is Pete’s recommended value, and any voltage setting you choose will not harm the amplifier, but will influence the sound.

Find your sacrificial…err, I mean… test headphones. Plug the headphones and a source in and give it a listen.

First listening test. It worked!
First listening test. It worked!

If there are any issues encountered, it’s almost certainly a problem with the soldering of one or more components. Since you carefully quadruple-checked the orientation and positioning of all components, an open joint or shorted leads are the most likely candidates. Almost all electronic troubleshooting recommends reflowing and redoing all solder joints carefully, one by one. Joints should look like tiny volcanoes, not blobs or balls.

I tried to keep this section as brief as possible as Pete supplies all the necessary details in his excellent instruction manual.
The fully assembled stock kit using all the components listed in the BOM from pmillet.com.
The fully assembled stock kit using all the components listed in the BOM from pmillet.com.

Making a Decorative Wood and Aluminum Case

Pete’s BOM calls for mounting the amplifier as a bare board seated in a small, plastic half-case with no top, sides, front or back. Since all voltages used are low, there is no danger in using it this way and it does nicely show off the Nutube. However, if you’ve read any of my DIY School articles, it should come as no surprise that I would be building a more attractive enclosure for my NuHybrid.

Testing the size of the case pieces.
Testing the size of the case pieces.

I ended up building a wooden case using birch hardwood cutoffs (less than $10) from my local hardware building supply store. While I was there, I found a sheet of metal grating in an attractive Union Jack pattern. (It was $20 but it is 2’x3’ and will serve for many future projects as well). Just what is needed to view the Nutube and to allow any heat to escape.

While the Nutube does not generate heat, the linear regulator on the board does get hot.

Front and back panels were manufactured from 2” wide x 1/8” thick aluminum stock from another hardware store $12).

Tools Used for the Case Build:

  • Table saw
  • Miter saw
  • Band saw (for cutting the aluminum stock)
  • Router table and various bits (including T-molding bit)
  • Random orbit hand sander
  • Drill and various bits
  • Metal file
  • Dremel
  • Screwdrivers
  • Tin snips
  • Cabinet wood glue
  • Sandpaper
  • Hot glue
  • Stain and polyurethane spray

Case Assembly

I used a table saw to cut the ½” birch to a uniform 5 ½” depth. I mitered each of the four pieces using my trusty old sliding compound miter saw to create a box (top, bottom and sides) 10 ½” wide x 3 ¼” high. The height was dictated by the aluminum stock width of 2”. Two small wooden pieces were added to the inner sides to give something for the front and rear aluminum plates to screw into. All the pieces were simply glued together using cabinet glue and clamped until they dried..

Boards for the case routed, cut, glued and finally clamped.
Boards for the case routed, cut, glued and finally clamped.
The box I made is larger than what would be needed if simply enclosing the NuHybrid and using the recommended external power supply from the BOM. I happened to have a 24V power supply board on hand (a leftover from a previous project) and decided to incorporate it into the same enclosure as the NuHybrid.

Using a small router table, I created the front profile and the rear slot to fit the aluminum plates. I also used the router to create the cutout for the top grate and the channel for the grate to slide into. I used a T-molding router bit to align a channel for the grate, centered in the wooden top piece.

The union jack grill slides in a slot that goes through the back of the case.
The union jack grill slides in a slot that goes through the back of the case.

After a disaster involving trying to do the top cutout in one piece, I settled for having to glue in a couple back pieces to hold the grate. This required, of course, completely having to make a new top piece since the first was wrecked spectacularly. Not my original intent, but it turned out fine.

Once I was happy with the fit and finish (take your time sanding, working up to 220 grit, it pays off in the end) I applied a few coats of burgundy stain. Finally 4 coats of gloss polyurethane was sprayed to seal and protect it. Although that may all sound like a complex process, just follow the directions on the cans. Wipe the stain on with a clean rag (I used blue paper shop towels) and once dry, rattle-can spray a few light coats of polyurethane; it is pretty hard to mess up.

Burgundy Varathane stain.
Burgundy Varathane stain.
More details on wood finishing techniques can be found in my Building a Bottlehead Crack article if interested.

Front and Rear Aluminum Plates

I centered the Nutube board vertically in the case and carefully measured and marked the front aluminum panel. Once marked, I center punched the holes and drilled the appropriate openings for screws, switch, power LED, headphone jack and the volume pot. I sanded the front and back surfaces of the panels to ensure that there were no stray metal shavings.

I found a very low-profile nut in my parts bin (from another potentiometer) that fit the thread pattern on the headphone jack. It would be very nice if the headphone jack came with a nut, but it does not. I used my scavenged nut to tighten the NuHybrid PCB to the front plate. A thin heatsink from a high intensity LED made a perfect decorative washer around the headphone jack under the nut.

When you are into DIY and like to tinker, don’t throw any potentially useful bits away. You just never know when those bits will find a purpose.
Measurements before the front aluminum plate is drilled.
Measurements before the front aluminum plate is drilled.

I used plastic standoffs, from a TV wall-mount, under the NuHybrid board and (without the top grill in place) drilled pilot holes and screwed the PCB down into the bottom of the case. Since the NuHybrid is centered vertically on the front plate, it means there is almost ¾” of empty space underneath it. This space lead me to another upgrade that I’ll get to in the next section.

The power supply was mounted on shorter plastic standoffs immediately beside the NuHybrid board. Rather than installing the NuHybrid rear power input, I simply attached two wires directly to the power supply 24V output. In addition, I did not use, nor install the default RCA jacks on the NuHybrid board. Separate insulated RCA jacks were mounted to the rear aluminum plate, along with the power cord input for the power supply board.

The first rear plate. Soon to be replaced.
The first rear plate. Soon to be replaced.
For all non-circular holes required in the aluminum plates (such as the power cord input) I first drilled through the plates and then enlarged and smoothed the openings using a Dremel and metal file. The final ‘brushed’ finish was obtained with a random orbit sander and 180 grit sandpaper.
Conductive copper tape was applied to every interior surface before the boards were mounted permanently.
Conductive copper tape was applied to every interior surface before the boards were mounted permanently.

To finish up, I took everything out and completely covered the inside of the wooden case with conductive copper tapein an effort to minimize interference (EMI) and unwanted external noise from cell phones, wifi, etc. As it turned out, it is pretty much immune to cell phone noise – a big plus. Finally, the top grill was cut to fit using tin snips, slid into place from the back and secured semi-permanently with a couple dabs of hot glue in the rear slot.

Adding a DAC – The WesionTek Kahdas Tone Board

All that extra space behind and under the NuHybrid board got me to thinking about options. I’d recently read about this tiny and inexpensive DAC board that was measuring better than DACs at many times the price.

The Kahdas Tone Board measured at 110 dB SINAD (signal above noise and distortion) from Audio Science Review.
The Kahdas Tone Board measured at 110 dB SINAD (signal above noise and distortion) from Audio Science Review.

The Kahdas Tone Board is an odd duck; it is entirely without a case, as it was originally intended to be inserted directly on top of a tiny computer board. However, due to the outstanding sound quality measurement results, it has been quickly gathering popularity in the DIY community.

The “Generic Edition” of the tiny Khadas Tone Board.
The “Generic Edition” of the tiny Khadas Tone Board.

And (I rationalized to myself) wouldn’t it be nice to have all-in-one functionality in a single case that enclosed the DAC, amp and power supply?

Technical Specifications

  • DAC: ES9038Q2M, 32 Bit Stereo
  • THD+N: <= 0.000337% @ 1 KHz
  • SNR: >= 120 dB @ 10 Hz – 20 KHz
  • Sample Rate: PCM: 384 KHz @ 32 Bit, DSD: 256 @ 1 bit
  • Jitter: Reference Clock: < 3 ps, Data Clock: < 30 ps
  • Audio Formats: APE2/FLAC/OGG/WAV/ACC/ALAC/MP3/WMA
  • USB-C Port: Power Supply(5V/500mA) & USB 2.0 Data
  • S/PDIF: In: Up to 192KHz
  • RCA: Stereo Output
  • Dimensions: 82.0 x 74.5 x 17.0 mm
  • Price: $100

Installing the Kahdas Tone Board

After taking a few measurements, I determined that the Tone Board would fit perfectly behind and a bit underneath the NuHybrid PCB. Of course this meant making another aluminum rear panel to fit. Holes had to be cut for the USB C input, digital input RCA jack, and two stereo output RCA jacks for the Tone Board. Tightening the nuts on the three threaded RCA jacks is more than sufficient to hold the Tone Board securely, so I did not attempt to attach it to the wooden case itself.

The Tone Board is attached to the rear plate and slides partially under the NuHybrid PCB.
The Tone Board is attached to the rear plate and slides partially under the NuHybrid PCB.

I used the fixed spacing of the Tone Board RCA jacks to orient and align the NuHybrid’s input and output jacks on the rear panel. It ended up all being a reasonably tight fit, nonetheless it all went in there so well, you’d swear it was the plan all along.

I did not internally connect the Tone Board to the NuHybrid, so this means an RCA patch cord is needed to be a jumper from the Tone Board stereo RCA outputs to the NuHybrid RCA inputs. This was done purposely so I could run the Tone Board outputs to my input switch box and use it as the DAC for all my desktop headphone amplifiers.
The redesigned back plate to accommodate the Tone Board. Visible from left to right: power input, NuHybrid input (upper red/white), NuHybrid output (lower red/white), Khadas Tone Board stereo output (red/white, grey nut), Khadas Tone Board S/PDIF input (black, grey nut), Khadas Tone Board USB C input. The RCA cable pictured functions as a jumper from the Tone Board output to the NuHybrid input.
The redesigned back plate to accommodate the Tone Board. Visible from left to right: power input, NuHybrid input (upper red/white), NuHybrid output (lower red/white), Khadas Tone Board stereo output (red/white, grey nut), Khadas Tone Board S/PDIF input (black, grey nut), Khadas Tone Board USB C input. The RCA cable pictured functions as a jumper from the Tone Board output to the NuHybrid input.

To finish it off, I soldered wires to a couple of unused 5V DC pads on the Tone Board PCB and installed a yellow LED on the front panel to the right of the volume control. I aligned and spaced it so it was an match to the NuHybrid power LED – which helped to visually balance the front panel.

Through some trial and error, I found the right combination of resistors (specifics will depend on what LED you choose) to sufficiently dim the yellow LED to match the green NuHybrid power LED. The yellow LED illuminates any time the Tone Board is plugged in via USB and receives power.

I have found the Tone Board to live up to its reputation. It looks, feels and sounds excellent; far beyond its modest (approximately) $100 price tag.

NuHybrid Upgrades

Pete recommends a few potential upgrade options when assembling the NuHybrid PCB:

Capacitors

The default capacitors are all high quality Nichicon Fine Gold audio electrolytic capacitors and Wima polyethylene film caps. The electrolytic capacitors in the positions C3 and C6 are directly in the audio signal (as well as the input coupling film caps in C20 and C21), so these are the most likely candidates for upgrade if desired.

It may be difficult to find better capacitors that will fit on the PCB, but depending on if you mount the NuHybrid in a custom case, it may be possible to mount components to the bottom of the board.

For a more detailed discussion of the variety of capacitors available, check out the Head-Fi thread.

OpAmps

As with most OpAmp rolling, there are many available options (and opinions on them) to replace the OPA551. The OpAmp must support a 24V power supply, 8 port socket and be unity gain stable.

Head-Fi builders report success with the boutique SparkOS Labs singles and Burson OpAmps. Other compatible options should include: AD797, AD825, AD8065, ADA4610, NE5534, OPA134, OPA604, or OPA627.

Power Supply

A linear power supply can be used in place of the suggested switching power supply, “…assuming it has reasonable filtering and/or is regulated so that it doesn’t have a lot of 120Hz output ripple. 120Hz ripple will probably cause more audible artifacts than the switching regulator ripple.” – Pete Millett on Head-Fi

RCA Connectors

The stock RCA jacks are basic non-gold plated versions. Off board mounting (like I did) will require separate insulated connectors. There are lots of options, but I went with basic gold plated jacks.

Transistors

It is possible to swap in a JFET (2SK170 or LSK170) in place of the 2N3904 transistors, however a gate stopper resistor (1K in series with the gate terminal) is needed to prevent oscillation.

Volume Control Potentiometer

“Unfortunately, all small (and cheap) volume controls suck. They have lots of channel mismatch, and they are noisy or get noisier with time. The pot I used here is not bad, but it is noisy at the bottom end of travel, which bugs me.” – Pete Millett

I used the stock BOM pot in my own build and I agree with his assessment. It is indeed noisy (crackling sounds) and somewhat unbalanced (left channel louder) at the very lowest settings, but this is below any usable listening level for me, so has minimal impact in practice.

If you are willing to pay the price for an upgrade, Pete did design the board to also accept a couple common high-quality potentiometers, the Alps Blue Velvet ($20) or the TKD 2CP-601 ($40). The original is a 10k pot, selected because there should be less noise pickup with a lower input resistance, but Pete states that replacing it with 50k or 100k is fine.

Some builders on Head-Fi have reported noise issues with upgraded volume potentiometers that were solved by running a separate grounding wire to the pot shaft.
The glow through the top grill. The blue on the left is a white LED on the Tone Board shining up through the blue Nutube PCB. The Nutube is in the middle and the green power board LED is on the right.
The glow through the top grill. The blue on the left is a white LED on the Tone Board shining up through the blue Nutube PCB. The Nutube is in the middle and the green power board LED is on the right.

NuHybrid Performance and Measurements

My personal NuHybrid build follows the BOM with the small differences of power supply, RCA jacks, case, and volume knob. None of these changes should dramatically impact the sound quality from the standard build and as such, this performance discussion should be applicable to anyone building a stock NuHybrid.

Power

The gain is unmodified from the Nutube stage (about 6x), which proves to be a bit too much for sensitive IEMs. Paired with the KZ ZS10 Pro (24 Ohm, 111 dB) there is audible hiss with the volume control turned all the way down. In this position the music also audibly bleeds through. As a result, I do not recommend the stock NuHybrid for pairing with highly efficient IEMs.

The drive capability is limited by the output buffer. Pete rates the power output of the stock NuHybrid as approximately 6.5V RMS into 30 or 150 ohms, or 1.3 watts into 30 ohms.

Using the OPA551, the NuHybrid is more than powerful enough to easily drive any of my full sized headphones from the efficient Meze 99 Classics (32 Ohm, 103 dB), all the way up to the power intensive Beyerdynamic T1 (600 Ohm, 104 dB). The T1 can get to louder than comfortable listening volume at approximately ½ on the NuHybrid’s volume control. That means it has plenty of power on hand for any of my full-sized headphone needs.

The almost completed case (pre Tone Board LED) and Beyerdynamic T1 (version 1) headphones.
The almost completed case (pre Tone Board LED) and Beyerdynamic T1 (version 1) headphones.

Frequency Response

The frequency response is impressively flat and with less than 1dB of mismatch between the two halves of the Nutube, which is better matching than the volume control. Pete describes the overall frequency response of the NuHybrid as:

’…the LF response is -3dB well below 20Hz, and the HF response drops well over 40kHz…”

In personal listening, the above seems accurate. The amplifier appears to have no issues reproducing either end of the audible spectrum, with no audible bumps or dropouts with any frequencies. Overall, it’s a remarkably pleasant and even sounding amplifier.

Harmonics

Tube amps are not theoretical “ideal” amplifiers that simply apply voltage gain. With a tube amplifier, it is important to understand that harmonic distortion is part of the intended sound signature. Harmonic distortion may be defined as the presence of frequencies in the output signal that are not present in the input signal. These frequencies may be even or odd.

Single-ended circuits are asymmetrical, so they tend to create more even harmonics, while push-pull circuits are symmetrical, so they tend to create more odd harmonics.

Harmonics may be seen at 2nd, 3rd, 4th, etc levels. Higher-order harmonics tend to be more audible (and thus less desirable) because they are further from the original frequency. 7th order harmonics and above are considered very undesirable as they tend to be harsh sounding.

Even harmonics are typically considered more pleasant-sounding than odd harmonics. The desirability of low-level, even harmonics is why 2nd order harmonics are often touted as “what you want” with a tube amplifier.

At first thought, that seems bad! Imagine… making up signals that weren’t there originally. But harmonics themselves aren’t necessarily a bad thing. In the real world, all musical instruments, even our voices, have harmonics. These are sometimes called overtones. In fact, certain harmonics may even be pleasing to the listener and make listening more pleasurable. – FiveFish Audio Blog

Of course, personal preference and the music listened to, may lead one to prefer the sweet, tube-like characteristics of even harmonics, or the more-detailed, brighter sound of odd harmonics. Lower-order harmonics tend to mask higher-order harmonics, but higher-order harmonics tends to be more audible, and as we noted above, less pleasant sounding.

So a typical “good sounding” tube amp will try to minimize total distortion, and make sure the higher-level harmonics are at a smaller amplitude than lower-level harmonics (typically in a stair-step pattern downwards).

Harmonics are measured using a Fast Fourier Transform (FFT) graph which displays a signal both in the time domain and the frequency domain. The harmonics of a signal’s frequency appear at higher frequencies than fundamental frequency on the FFT graph.

NuHybrid FFT using the OAP551 from pmillet.com.
NuHybrid FFT using the OAP551 from pmillet.com.

The NuHybrid output displays traditional single-ended triode amplifier characteristics with strong 2nd order harmonics and subsequently smaller higher-level harmonics. As we discussed above, these measurements are consistent with that “tube sound” that some folks (such as myself) enjoy.

These measurements were done at the recommended 11 volt bias voltage, which was selected to yield a minimum overall Total Harmonic Distortion (THD). Lower voltages will increase distortion.

The bias pots can be adjusted to change or tune the harmonics as desired. Setting the bias voltage slightly lower (8-9 V rather than 11 V) will introduce more 2nd order harmonics, which tends to smooth any harsh higher frequencies. For all my listening and observations, I stuck with the recommended 11 V.

The NuHybrid is not designed to be a minimal THD amplifier. At 1V RMS, THD is approximately 0.6%. Clipping (5% THD) occurs at 6V.

Custom DIY Grado clones and the wooden encased NuHybrid look good together.
Custom DIY Grado clones and the wooden encased NuHybrid look good together.

Sound Quality

The NuHybrid conveys the impression of exceptional detail and transparency, with the expected warm harmonics of tube amplification. The sound is smooth across the entire audible spectrum and manages to sound lush and fun. It may not be to your tastes, but for a tube amp fan like myself, the NuHybrid does not disappoint.

The NuHybrid presents a quiet background with most anything but the most sensitive of headphones. However, keep in mind that it is not designed to be a very low distortion amplifier. It performs much as you would expect a traditional single-ended triode tube amp to perform, with audible 2nd order harmonics.

Drawbacks

While the Nutube adds great flavor to the sound, NuHybrid isn’t without the compromises inherent to the choice of using the Nutube 6P1 in the circuit. It does exhibit noticeable ringing when powered on (for about 10 seconds) following the muting circuit engaging.

The sound is audible through all the headphones I tried (but reduced with less sensitive headphones) before slowly fading away. Moving the case, or even just adjusting the volume control causes slight ringing microphonics as well. If you are a frequent volume adjuster, or sensitive to the tube ringing sound, the NuHybrid may not be for you.

Unfortunately, this ringing is just the normal microphonic behavior of the Nutube and not something that can be ‘fixed’. Pete originally thought the power on ‘ping’ was due to the activation of the muting relay, but later amended this to be caused by the flipping of the power switch, which has a firm ‘click’ when engaged.

Conclusion

I am extremely satisfied with how this project turned out. The NuHybrid amplifier sounds great, albeit with the caveat of having too much gain to pair well with highly efficient IEMs. It has power to spare for demanding headphones such as the Beyerdynamic T1 and Fostex T50RP mk3. Regardless of the unique design of the Nutube, it produces a traditional tube sound, just housed in a brand new shape.

A good look through the grill at the color and pattern of the glowing Nutube.
A good look through the grill at the color and pattern of the glowing Nutube.

Of course, that pleasing tube sound comes with a significant compromise, which may be a deal breaker for some. While the NuHybrid’s ringing sound is very similar in timbre to traditional tube ringing, the Nutube is far more microphonic than any conventional tube I’ve ever encountered. I’m used to having to strike or tap a tube amplifier to elicit a pinging noise. However with the NuHybrid, flicking the power switch, or even adjusting the volume, will set the tube ringing.

If you can live with, and perhaps embrace these characteristics (as I have), the NuHybrid is a terrific sounding, powerful, and unique tube amplifier. Tube amplification is subjective; tubes will never objectively measure perfectly. Distortion and harmonics are part of the charm. But if tube sound appeals to you, the NuHybrid doesn’t disappoint. I highly recommend the NuHybrid to anyone interested in DIY, tube amplification, or simply for anyone who desires an excellent sounding desktop headphone amp with plenty of power for full sized headphones.

If I may make a recommendation, I feel the quality of the NuHybrid kit warrants a better case than the open-board design from the BOM. You don’t have to go all out in creating a case from scratch, as there is a readily available 3D printed option. In addition, there are many interesting alternative case ideas in the Head-Fi thread.

Also worth noting, if you are in the market for a new DAC, the Kahdas Tone Board really is deserving of consideration. This surprisingly well-made device is performing well beyond its modest price point. While it certainly doesn’t need to be included in the same enclosure as the NuHybrid, its tiny size makes the Tone Board a tempting addition to any DIY project.

As with all DIY projects, you gain the personal satisfaction of creating something exclusive and ending up with something exactly how you want it. It’s just that much simpler when you pick a project like the NuHybrid that has excellent support, detailed instructions, a comprehensive BOM, and an active forum for discussion or questions. It makes it so easy!