Review: Hagerman Audio Labs Tuba Headphone Amplifier

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Quality over Quantity.

Hagerman Audio Labs is a one-man company, located in Honolulu, Hawaii, and dedicated to the design and creation of high-quality audio products. It is the brainchild and creative output of Jim Hagerman, a veteran in the analog electronics industry. Jim designs his products from scratch, ensuring that each is innovative, unique, and built to last.

Hagerman Audio Labs Tuba Headphone Amplifier

“The Tuba is Hagerman Audio Lab’s first headphone amplifier (in ten years) and it introduces Jim to a brand new group of audiophiles. These days, it seems the majority of headphone users have moved on to digital sources and, as such, may not be aware of Jim’s lengthy expertise in analog audio reproduction. It’s really nice to know that there are still pros like him producing new designs.”

What We Like 😍
  • Delicate and refined sound.
  • Works well with very efficient headphones and IEMs.
  • Includes high-quality Mullard EL84 tubes.
  • Hand-built and designed by a tube amplification expert.
  • 10-year warranty, 30-day trial period, and free worldwide shipping!
What We Don't Like 🤢
  • Serious, no-nonsense looks may not be for everyone.
  • Crossfeed circuit, although subtle, cannot be turned off.
  • Combined power/volume potentiometer feels light and has ‘play’ in the movement.
  • Not enough power to drive inefficient planar magnetic headphones to loud volumes.

Tuba Specifications

  • 10k/50k ohm input impedance
  • 16/32 ohm output impedance (LO/HI)
  • 10Hz to 150kHz bandwidth (-3dB)
  • 400mW output power @ 32 ohms
  • 6Vrms output voltage
  • 0.25% distortion @ 0dBV 400Hz
  • 6 x 10 x 3.5 inches
  • 15Vdc @ 1.35A external power supply
  • EL84 x 2

Hagerman Audio Labs built its reputation in audiophile circles with the truly impressive Trumpet phono stage. A world class design that was a Stereophile Class A Recommended Component for years. Jim started with a clean slate and designed the chassis, circuits, and everything else.

“I’ve done a lot of great things since, but my favourite is still the original Trumpet phono stage. It is my ‘piece de resistance.’ I broke a lot of new ground with it, did things nobody else had done, and came up with a revolutionary machine that really stood out from the crowd.” – Jim

The Tuba headphone amplifier ($670) was created from this pedigree. It’s the first headphone amplifier that Hagerman Audio has produced since 2008’s Castanet (HA-10), based on a 6H30 tube, and ‘The Ripper’ before that. Since then, Jim’s been primarily offering phono preamps, burn-in tools, guitar pedals, and guitar amps.

The Tuba’s austere back panel.
The Tuba’s austere back panel.

All products are handcrafted by Jim, come with a 10-year warranty, 30-day trial period, and free shipping world-wide. Those are impressive numbers and clearly illustrate Jim’s confidence in his work.

Before we take an in-depth look at the Tuba, let’s get to know the man behind the designs, Jim Hagerman.

Who is Jim Hagerman?

“I take a path of elegance. That is, I find a circuit or topology change that goes around the barrier, rather than trying to push through it. It’s working smarter, not harder.” – Jim
Jim in his lab from Stereophile.
Jim in his lab from Stereophile.

Jim grew up and went to college in Minnesota. He spent eight years in Boston and then went to San Diego. He worked for a number of technical companies including Digital Equipment, Hughes Aircraft, and Nokia. This is where he learned the world of analog electronics design.

“For me, it helps in the audio world, especially doing radio frequency stuff. When you’re up in the megahertz and gigahertz, it’s a different world. Very analog. Everything—position, trace length, RF, digital, it’s all very critical. That’s what I bring to the audio world…” – Jim

In the early 1990s, Jim was considering guitar amplification for his own business. He saw an opportunity in tube amplification due to many of the older technicians retiring, and few young technicians taking up the torch. Jim started reading everything available and studied for years before taking the plunge. Somewhere along the way he got diverted into phono stages and came up with the Inverse-RIAA Filter kit.

He wrote a paper “On Reference RIAA Networks” for an audio publication in 1995. Around that time, Jim moved to Hawaii and created the VacuTrace, a unique piece of laboratory test equipment that converts an analog oscilloscope into a full-featured vacuum tube curve tracer. This was his stepping stone to creating the Trumpet phono stage, and as they say, the rest is history.

Jim and his Trumpet in Hawaii from Stereophile.
Jim and his Trumpet in Hawaii from Stereophile.
You may have noticed that Jim uses instrument names for his creations: Trumpet, Tuba, Bugle, Cornet, Piccolo, etc. This likely harkens back to Jim’s childhood where he grew up playing trumpet in the school jazz band and orchestra.

Somewhat surprisingly, Jim doesn’t consider himself an audiophile like most of his customers. Digital Rights Management led Jim to give up on digital in 2008, and although proudly an “analog only” kind of guy (no digital sources need apply) he doesn’t have a huge vinyl collection.

“I’m a designer. An inventor/designer/creative type. I’m not a musician, and I’m not really an audiophile. I don’t collect. I do it mostly as a business. I do it to try and understand what the customer likes and dislikes. But technically I’m not an audiophile myself. My advantage is: I’m a designer—a product designer and circuit designer, so I was able to bring my skills into this arena.” – Jim

This design philosophy is perhaps the most interesting part of the products produced by Hagerman Audio Labs. Jim believes that topology is the key, not just the parts. He firmly believes that a better topology with inexpensive parts will outperform an existing basic design built with the most expensive components.

A look at Jim’s notes as he reworked the Tuba design (from Facebook).
A look at Jim’s notes as he reworked the Tuba design (from Facebook).
“My motivations come from trying to move things forward in a way that hasn’t been done before. I ask myself, what can I do to break the mould? How can I do this better? Innovation is about thinking outside the box; breaking rules, not following them.” – Jim

Tuba Aesthetics

Enter the Tuba headphone amplifier. It is the first of a new series of products built into a custom metal chassis, somewhat more upscale than previous budget units (Cornet 3, Bugle 2, Piccolo 2) in terms of appearance. The design is reminiscent of retro laboratory or industrial equipment. It’s a heavy dark grey painted aluminum case with minimal white lettering and a pointed plastic knob.

The no-nonsense design aesthetic of the Tuba.
The no-nonsense design aesthetic of the Tuba.

The look is aggressively understated. No polished wood, no polished aluminum, and a bare minimum of switches and controls, means that the Tuba isn’t for those looking to visually impress. After reading about Jim’s design philosophy, I’m sure that doesn’t come as a surprise.

“The new products are intended to be giant-killers in terms of performance, as I put the money into circuit topology and components, not eye-candy.” – Jim (In a personal email exchange)

While certainly not unattractive, the Tuba’s looks underwhelmed me at first. I’m unabashedly a fan of beautiful wooden enclosures and acres of polished aluminum, so a chassis that felt more like an ancient steel PC computer case, rather than a Mac, took me a while to get used to.

I’m happy to report it’s grown on me. I can appreciate the no-nonsense design and focus on sound quality. As the saying goes: it’s what’s inside that counts. The Tuba is a solid machine, built to last generations, and able to compete with far more expensive units.

“My goal for each product is to set a new benchmark in terms of performance-per-dollar. In that sense, sonics are of utmost importance.” – Jim
The combination power / volume control, high impedance headphone output and yellow indicator LED.
The combination power / volume control, high impedance headphone output and yellow indicator LED.

Really, my only trifling complaint is with the ‘feel’ of the merged volume and power knob. The knob is depressed to power the Tuba on and off. The combination of the light plastic knob and the small potentiometer inside yields an impression of play (slop) and lightness. This is a departure from the weight and gravitas imparted by a solid aluminum knob that turns like syrup, usually the norm on higher-end equipment.

Tuba Design

The Tuba is designed around EL84 tubes used in a single-ended, zero-feedback, parafeed triode arrangement. A matching output transformer (OPT) offers low distortion and high signal headroom, and supports high or low impedance headphones with dual outputs. Two different taps on the OPT secondary offer two output impedance levels:

  1. 32 ohm for High impedance headphones,
  2. 16 ohm for Low impedance headphones.
The warm tube glow of the Mullard EL84 tubes.
The warm tube glow of the Mullard EL84 tubes.

Jim is not a fan of output transformer-less (OTL) cathode follower circuits, as they have a trade-off between output impedance and stability. This lead to his use of an output transformer in the Tuba design.

Although a transformer increases expense, Jim felt it was more than offset by a more natural and non-fatiguing sound signature. This was further enhanced with a parafeed design, where the tube is loaded by an inductor (or choke) and the output transformer is coupled to the tube with a capacitor. Removing DC current from the output transformer improves its bass response and linearity.

Inside the Tuba. Note that the volume potentiometer is not installed in this picture (far left).
Inside the Tuba. Note that the volume potentiometer is not installed in this picture (far left).
“…I’m not trying to make it sound like anything… I went with single-ended class A tube amplification as the added warmth is not such a drawback. You can crank this thing to ear-splitting volumes and not get fatigued. That’s when you know you’ve got it right. Tons of headroom, no feedback.” – Jim (In a personal email exchange)

The EL84 tube was selected with the end user in mind. The EL84 is readily available and NOS are plentiful, allowing for an easy sonic upgrade. It is worth noting that the included Mullard EL84 tubes are widely considered good sounding tubes, and are certainly far from bottom of the barrel. The EL84 tube is often described as having a tight tonal characteristic, with a strong, warm midrange focus and rich, dynamic harmonics.

The top of the board, standoffs, and tube sockets.
The top of the board, standoffs, and tube sockets.

After a 30 second warm up delay, an internal switching power supply boosts the 15Vdc input by a factor of ten for B+. Jim implemented a special open-loop (no feedback) circuit that improves sonics, and the relatively low B+ (“only” 140V) decreases build cost and improves tube life-span.

“B+ is created using a high frequency switching circuit (as opposed to one switching at 120Hz, right in the middle of the audio band) that provides a very clean and low noise result.” – Jim (In a personal email exchange)

Jim views applying negative feedback as a crutch in amplifier design. Negative feedback trades gain for higher linearity (reduced distortion), but can lead to instability or more audible harshness if not properly implemented.

A look at the internal circuitry.
A look at the internal circuitry.
Jim now offers decorative solid walnut wooden panels as a $50 add-on that attach to the sides of the Tuba. In my opinion, they significantly improve the aesthetics of the Tuba, both with the visual warmth of wood and the slightly wider stance with the panels in place. I liked the idea so much that I created oiled cherry wood panels for my own.

The Tuba, although not obvious in the literature, utilizes a crossfeed circuit designed by Jim. This circuit cannot be disabled. Beyond anything else, this may impact who purchases the Tuba.


The current version of the Tuba comes with crossfeed disabled unless requested by the customer.

Crossfeed, like equalization, is a controversial topic for headphone enthusiasts. It tends to divide along two opposing viewpoints (seriously though, these days what doesn’t). On one hand are the Purists, who believe any fine tuning to the sound isn’t ‘how the music was intended to be heard.’ On the other hand are the Tweakers, who believe loudness buttons, equalizers, and crossfeed are viable options to improve the music they listen to.

Crossfeed has to do with the differences between listening to music from speakers versus headphones. When we listen to stereo speakers, both ears hear sounds from both speakers. Music coming from the right speaker reaches the right ear, but also reaches the left ear, albeit somewhat quieter and delayed. It is this delay and attenuation that our brain uses to determine direction.

Illustration of how our ears hear music from speakers from Eilex.
Illustration of how our ears hear music from speakers from Eilex.

In addition, reflections within the ear canal and from the environment (floor, walls, ceiling, and furniture of various shapes, materials, and sizes) further color the sound and provide more directional information to our brains. Additionally complicating things is that we don’t stay still; we continually move our heads. This non-stop movement, of course, changes angles, reflections, and the delay of the sound waves we’re hearing.

None of the above apply when we listen to music through headphones. Sounds from the right driver only reach the right ear and vice versa.

When listening through headphones, there are no environmental reflections, nor movement to help determine spatial direction and placement.

Illustration of how our ears hear music from headphones from Eilex.
Illustration of how our ears hear music from headphones from Eilex.

Without the natural hearing cues, our brain tends to interpret the music unnaturally and originating from within our head. This can cause stress and listening fatigue of different degrees for different people.

Crossfeed implementation

The purpose of crossfeed in a headphone amplifier or source is to more closely approximate the experience of stereo speaker listening. Because our brain’s ability to determine direction is somewhat frequency dependent, a crossfeed circuit should subtly mix low frequencies to both ears (less directional) while retaining high frequency separation (more directional), all the while slightly delaying the crossfeed signal to the other ear.

Comparison of crossfeed amplitude and delay with frequency from Meier Audio.
Comparison of crossfeed amplitude and delay with frequency from Meier Audio.
“I experimented with several crossfeed circuits, but they seemed a bit strong. I wanted something more transparent and natural, just enough to tame the soundstage without actually impacting separation, so I developed a circuit of my own tuning that is both subtle yet effective.” – Jim (In a personal email exchange)

When done properly, crossfeed can make a more cohesive soundstage and reduce listening fatigue. The end result should provide a more natural and realistic sound and listening experience, and not be an overt gimmick.

Glowing front LEDs.
Glowing front LEDs.

The crossfeed argument

Anti-crossfeed purists will often report that they find crossfeed blurs the music, making it less defined, with less focus, and a lack of desired crispness. Of course, whether you like crossfeed or not has much to do with your personal listening preferences and expectations, as well as how the circuit is executed. Some implementations sound better than others.

Purist viewpoint

  • That’s the way the music was recorded and intended to be heard.
  • Crossfeed is inaccurate and alters the history of the music.
  • A good recording has microphone placement to represent the soundstage and proper reproduction without crossfeed duplicates this soundstage.
  • Crossfeed blurs the sounds across the soundstage and muddies the stereo image.
  • Crossfeed meshes instruments together so that separation and details are lost.

Tweaker viewpoint

  • Most music is stereophonic and not made or mixed for headphones.
  • Crossfeed makes stereo recordings more accurate for headphones.
  • What good is ‘pure sound’ if it fatigues or bothers the listener?
  • It is unnatural for the brain to hear sounds through just one ear.

Testing crossfeed

The effectiveness of crossfeed has much to do with the type of music you are listening to. Older recordings (often of the classic rock variety) sometimes used very separated channels within songs. Extreme examples placed vocals on one side and guitars or drums in the other with little to no bleed over.

Technological advancement in recording and playback moved music from predominantly mono to stereo through the 1960s. This led to much experimentation and some extreme examples of stereo panning in recordings of the time. Albums originally released in mono (such as the Beatles’ early albums) were occasionally re-released in the new ‘stereophonic’ format.

While this practice was more commonly found in 60s rock, some feel that the large orchestral or choral music benefits from crossfeed the most.

“I originally left it out, but then started playing my Beatles collection set (I only do vinyl). Damn, that one-ear stereo effect really bothered me!” – Jim (In a personal email exchange)
The Beatles recording Getting Better in March 1967 from Beatles Radio.
The Beatles recording Getting Better in March 1967 from Beatles Radio.

Extremely wide stereo separation seems to be less common within modern recordings. Crossfeed is much less noticeable with recordings that are equally balanced between both channels. However, crossfeed implementation should still benefit the headphone listener with improved spatial cues and reduced listener fatigue regardless of the recording.

Music to test crossfeed

  • Elanor Rigby – The Beatles
  • The Doors (self-titled album) – The Doors
  • You Lost that Loving Feeling – The Righteous Brothers
  • The Sound of Silence – Simon and Garfunkel

Crossfeed in the Tuba

The crossfeed circuit in the Tuba is very, very subtle. Honestly, if Jim didn’t tell me, I sincerely doubt that I would have noticed it on my own. To test, I simultaneously connected the Tuba and a Bottlehead Crack (no crossfeed) and played the music tracks above. Switching back and forth quickly between them, I compared the hard panned sections of the songs.

Eventually, I ended up plugging one ear and listening solely through the other to distinguish the difference in sound. Like I said, subtle. Don’t let the fact that you can’t disable crossfeed talk you out of this amplifier.

Low and High impedance headphone outputs.
Low and High impedance headphone outputs.
For those interested in experiencing music recorded for headphone listening, check out binaural recordings. These recordings are made using two microphones (often positioned in a mannequin’s ears) to create a three dimensional sound for the listener. Conventional stereo recordings do not factor in ear spacing or the physical interference of the head. Binaural recordings only sound proper through headphones because the natural crosstalk of speaker playback interferes with the sound. There are few mainstream recordings, but this list captures a number of good options.

Tuba Sound

As expected, the Tuba is a very well-mannered and non-fatiguing amp to listen to. If you are familiar with triode tube amplification, the Tuba displays the expected warmth of sound. This warmth is primarily due to harmonics or overtones, the sounds between the notes. Although all amplifying devices create harmonic distortion, tubes, especially when used in single-ended amplification, produce a harmonic profile that many consider pleasant.

Tube amps are not theoretically “ideal” amplifiers that simply apply voltage gain. With a tube amplifier, it is important to understand that harmonic distortion is often 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 and high or low.

Even and odd harmonics from The Tube Store.
Even and odd harmonics from The Tube Store.

Second order (low-level, even) harmonics are often considered “what you want with a tube amplifier”. They tend to add a pleasant sounding, organic warmth to the sound. Throughout my time with the Tuba, I found it to be very laid back and non-aggressive. It’s an easy amp to like and to listen to for extended periods.

I should mention that one of the moderators of a headphone enthusiast Facebook group (that I administer) purchased a Tuba and has reported a loss of very low frequency reproduction after his Tuba warms up with Beyerdynamics DT1990 headphones. He returned the amplifier to Jim for diagnosis and no problems were found. I was unable to reproduce this with my headphones (with either of the two Tuba amplifiers I have on hand) although none of my headphones are as capable of sub bass as the DT1990. We used the opening of Sweet Jane by the Cowboy Junkies as test material.

Tuba Power

The Tuba’s relatively low power reinforces the impression of refinement and quality over quantity. This is a bit of a concern as headphone pairing becomes important. I’m used to OTL tube amps that require high-impedance headphones only, so I was excited to spend time with a tube amplifier that should be less picky about headphone pairings.

Sure enough, the Tuba sounds great with very sensitive in-ear monitors (IEMs) like the KZ ZS10 Pro. It has a very low noise floor and doesn’t add any unwanted hiss to playback, something I can’t say about most of the other desktop amps that I use.

It’s the other end of the spectrum that is more of an issue. Jim mainly used the Sennheiser HD600 and Grado SR125 for development, two very different headphone types with contrasting loading characteristics. So I tried my own HD650 and SR325 headphones (SR325 using the Low output and HD650 using the High output).

I can happily report that the Grado SR325 has never sounded better with any other amplifier I’ve tried. The laid-back sound of the amp paired with the brighter Grado signature is a terrific combo. It made me remember why I fell in love with the Grado house sound the first time.

The HD650 also worked well. However, normal listening levels require about ½ on the volume knob. Cranking up to ⅔ or ¾ is necessary if you really want to boogie (I’d forgotten how good that first Digital Underground album really is).

Sennheiser HD650 headphones plugged in. Bottlehead Crack in the background.
Sennheiser HD650 headphones plugged in. Bottlehead Crack in the background.

By contrast, my modified Bottlehead Crack requires ⅓ – ½ of its useable range to achieve the same volume. The Crack, shod with Tung-Sol tubes, also sounds more alive and engaging through the Sennheisers. Admittedly the HD650 and Crack pairing is the very definition of synergy, and seems to bring out the best in both partners.

The 600-Ohm Beyerdynamic T1 are just that much harder for the Tuba to drive. Adequate volume can be achieved, however it requires most of the volume control.

How about the planar Fostex T50RP mk 3 (and Alpha Dog variant)? Fuhgeddaboudit. There just isn’t enough power on hand to drive them properly.

So take a look at your headphone collection before you consider a Tuba. IEMs and efficient full-size cans? Absolutely. Inefficient planar magnetic monsters? Not so much.


Jim Hagerman at Hagerman Audio Labs is an easy man to respect. He brings an old fashioned sense of innovation to design, construction, and sales that appear focussed on quality rather than profit. Jim found his love of music reproduction early–reportedly at 16 with Klipschorns and Supertramp’s Crime of the Century–and throughout his career, he has genuinely added to the pursuit of high-end audio.

It is easy to make something complicated and expensive. I desire simplicity and elegance, which are much more of a challenge, requiring innovation and outside-the-box thinking.” – Jim (In a personal email exchange)

Jim started by offering kits to enthusiasts, and still is one of the few to publish his schematics online. This illustrates his commitment, pride, and confidence in his designs, even though it opens him up to intellectual property theft and cheap knock-offs.

The Tuba is Hagerman Audio Lab’s first headphone amplifier (in ten years) and it introduces Jim to a brand new group of audiophiles. These days, it seems the majority of headphone users have moved on to digital sources and, as such, may not be aware of Jim’s lengthy expertise in analog audio reproduction. It’s really nice to know that there are still pros like him producing new designs.

It definitely makes a statement.
It definitely makes a statement.

The Tuba is a unique product. It really doesn’t remind me of anything else currently available. The combination of tube warmth, polite sound, highly-efficient headphone compatibility, and crossfeed will make it ideal for some, but not for all. It certainly brings something different to the table for someone who currently owns an OTL tube amplifier.

Like its namesake, the Tuba is not a jack of all trades. However, if the no-nonsense design appeals to you, and if your headphone collection is a good match, it is well worth the audition. Remember that free shipping, a 30-day trial period, and a 10-year warranty means you have little to lose. You might just find exactly what you are looking for.

💬 Conversation: 4 comments

  1. Very nice cover of this amplifier. I was considering it until I read that is not powerful enough to drive old orthos, which I happen to like and collect.

    Any suggestions on what other tube amplifier would work for these? I am especially concerned about impedance pairing, since these are fairly low impedance and low efficiency.


  2. Great review Trav. I think this amp might fit my collection perfectly. My headphones are mostly very efficient and I tend to listen at fairly low volume.
    Not cheap though, landed in Canada.

  3. I use a Hagerman Audio Labs Castanet (the headphone amp the Tuba is based on)
    and it powers all of my headphones quite well (Sennheisser HD 600, HD 650; AKG K240 Sextett; HiFiman HE4XX; Fostex T50 RP MK3; AudioQuest Night Hawk Carbon;
    Grado SR60; NAD HP50 VISO; Meze 73 Classics; Superlux HD 681 EVO.

    The Castanet (HA 10) is the nicest sounding headphone amplifier I have ever
    used, and I imagine that the Tuba only improves on it.

    Add a 10 year transferable warranty and a price of just $649 (The Castanet would
    cost closer to $2000 today) and you have one of the best values in headphone
    amplifiers available.

    Jim Hagerman is also great to deal with!

  4. Hmm this is interesting, the sound signature you just described exactly matches my thoughts on my Elekit TU-HP03 hybrid tube amp. Try checking them out if can, especially when you’re also a fan tube sound. Very good pair with my Sony Ex1000 iems. Makes them less fatiguing, which is what they’re known for, and also adds a natural sense of space which really reminds me of my speaker setup.

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