SPOTFIRE Fusion 35 Hybrid Tube/Class-D Amplifier Kitset (35 Watts) -
Why so Long Coming
Every time this amplifier seems ready to go, I think of something to make it better. A classic engineering trap!
What strikes me most is just how well these generic, low-cost TPA3255 Class-D amplifier modules perform, at a price that seems to defy logic. But as someone who enjoys the sound of single-ended tube amplifiers such as the SE5, I sometimes find their sound a little “clinical”.
The Fusion35 project is about tailoring that sound to my own taste: introducing a modest amount of predominantly second-harmonic distortion to give some of the character that many tube enthusiasts enjoy.
The latest addition is a relay that bypasses the tube voltage-gain stage. You can now choose between two configurations: the Class-D module driven through a 12AU7 preamplifier, attenuator and cathode follower, or through the cathode follower alone. The attenuator compensates for the preamplifier’s gain, allowing comparison at similar listening levels.
The LEDs beneath the tubes change between red and orange to indicate the selected configuration. It’s a real “Fusion” of tube circuitry and modern solid-state technology.
The volume control now sits after the preamplifier stage. For a given input signal, this keeps the preamplifier’s contribution of second harmonic consistent as you adjust the volume.
There has also been plenty of performance testing. In my measurements, the Class-D module alone produces about 0.007% third harmonic, with an even lower second-harmonic component. With the 12AU7 cathode follower connected, second harmonic becomes dominant, measuring about 0.04% at 5 W output. Switching in the additional preamplifier stage raises this to about 0.3-1% depending on the input signal voltage, adding more of that classic tube character.
The detailed distortion and intermodulation measurements of my prototype are in the click-box on this page.
Being a radio amateur, I have also paid particular attention to switching-frequency signals reaching the speaker cables, where they can cause radio interference. An additional high-frequency filter in each speaker output provides approximately 18 dB of extra attenuation at 450 kHz, with greater attenuation at higher harmonics of the switching frequency. If this isn't a problem for you, you could just leave those components out.
(Common-mode emission was already extreamly low, so the folter is a differential-mode filter. Although differential-mode emission from balanced lines such as speaker cables would normally be low, in places that the balanced cable bransches-out, such as inside speaker boxes, the radio emissions could be significant).
I’m now using the Fusion35 as my everyday amplifier, choosing the full preamp configuration. To my ears, it matches my best tube amplifier — and betters it for bass.
Read on…
The SpotFire Fusion 35 blends a valve input stage with a high‑efficiency Class‑D output to deliver 35 W per channel.
A relay switching circuit changes the circuit configuration, and the sound, merging the character of classic tubes with the precision and efficiency of a Class-D output stage.
35 watts per channel (70 watts into 4 ohms), delivers a powerful musical performance while allowing you to adjust the flavour of the sound to suit your taste.
Whether you prefer the clean accuracy of solid-state, or the glowing vintage tone of tubes, the Fusion 35 amplifier adapts to your music, your system, and your mood.
Hybrid Tube/Class-D Design – the best of both worlds. Tubes in the input stages create the even-order harmonic structure that is characteristic of the famous single-ended tube amplifier. While a modern Class-D output stage provides 35 Watts RMS output, easily capable of driving small average efficiency speakers.
Compact Footprint – big sound in a small, stylish enclosure. The heaviest and most expensive tube amplifier components - the output transformers - are not used in a Class-D design. This makes the Fusion 35 amplifier surprisingly small and lightweight compared to a tube-only design of the same power output.
Low-Noise Performance: The unique power supply design, DC-powered heaters, and earth-loop breaker resistors, virtually eliminate hum. Perfect for both lower and higher efficiency speakers.
DIY Made Easy: Choose from solderless kits or full DIY builds with premium components. The Fusion 35 valve amplifier kit is straightforward to assemble but, due to the variety of different component types and wire connections, it is a little more challenging that the SE5 amplifier.
To make the process hassle-free, you can order the solderless kit, which comes with pre-assembled and tested PC boards. Alternatively, if you prefer building everything from scratch, you can order the full kit of parts and solder each component onto the PC boards yourself.
High Quality Components. All SPOTFIRE amplifiers feature high-quality parts, such as polypropylene coupling capacitors, low ESR conductive polymer hybrid aluminium electrolytic cathode-bypass capacitors, and gold pin tube sockets. The PC Boards are extra thick with 2-oz copper tracks, boosting durability and heat transfer.
The Class-D amplifier module comes pre-assembled and is rated to 300 Watts RMS. In this design it's loafing along at 35 Watts and runs very cool and unstressed.
Functionality. The front panel has a large solid metal volume knob, and toggle switches for turning power on or off and choosing inputs. The top facing panel has controls for sound/configuration and balance. The rear panel has input connectors, speaker connectors, and a system earth connector. You can switch between two sets of RCA inputs, and a Bluetooth input for streaming from a phone or computer, using the input selector toggle switch. A 3.5mm input jack is also provided on the RCA 2 input.
On my latest prototype I've included an option for sensitivity selection. If you are using a DAC or a line amplifier with an output signal about 1V RMS, then you can use the lower sensitivity setting which will make the gain (volume) the same between both preamplifier configurations. You can then seamlessly switch between the two preamp configurations to compare their difference in sound.
There is no headphone socket on this design, but headphones can be connected externally.
Safe Internal Power: No exposed mains wiring. Fully enclosed, inverter-based HV power supply. Mains power does not enter the chassis. The mains power supply complies with all Australian safety standards and regulations. (The internal circuitry contains potentially dangerous voltages up to 280V DC).
Small Footprint: The SPOTFIRE Fusion 35 doesn't take much valuable bench space. It is an exact match to the SpotFire PR5 phono preamp. Dimensions are 185mm wide, 215mm deep, and 165mm high.
Cost: The full kitset with an assembled PC board is $tba
