The Elekit TU-8550 is a full-function tube preamplifier kit with a built-in MM phono equaliser — configured with 12AX7 and 12AU7 tubes throughout every signal stage, including the phono stage. This is what distinguishes it from the TU-8500: no op-amps anywhere in the signal path. The phono equaliser, the flat amplifier, and the line stage are all-tube from input to output.
Phono equaliser — CR-type RIAA
The TU-8550 uses a CR-type RIAA equaliser circuit rather than the negative feedback (NF) type. The CR approach is widely preferred for its more direct signal path and characteristically smooth, natural tonality. All time constant capacitors carry a DC bias superimposition, an additional refinement that Elekit considers beneficial to sound quality.
The phono stage is designed for MM cartridges with a standard 47kΩ input impedance. RIAA equalisation accuracy is within ±0.5dB from 20Hz to 20kHz. Gain is 29dB with a signal-to-noise ratio of 112dB (IHF-A) and maximum output of 20V rms.
Flat amplifier — volume-gain locking system
A conventional high-gain preamplifier is quiet at maximum volume but noisy at low listening levels. The TU-8550 solves this with a quadruple volume potentiometer that links gain to volume position. As volume is reduced, gain decreases to approximately 1.4×; as volume is raised, gain increases to approximately 7× (approx. 17dB). Residual noise drops to 6.5µV at moderate listening levels and 15µV at maximum. The result is a quiet, low-noise background at the listening levels that matter most.
Volume and balance potentiometers are Alps Alpine RK271 series throughout.
Flat amplifier frequency response is 5Hz–200kHz (–3dB). Maximum output is 20V rms (1kHz), with a signal-to-noise ratio of 130dB at the centre position and 126dB at maximum volume (both IHF-A).
Power supply
The B-power supply uses fast-recovery diodes for rectification and a slow-start MOSFET circuit for regulated, stabilised DC — a more consistent and lower-noise approach than tube rectification. The A-power (heater supply) is fully DC, regulated by LDO voltage regulators that eliminate hum at source.
Connectivity
PHONO IN — RCA (MM cartridge, 47kΩ)
LINE 1, 2, 3 — RCA inputs
LINE 4 — 3.5mm mini jack, front panel (streaming, phones)
PRE OUT — 2 lines, parallel RCA output
REC OUT — 1 line RCA; passes the selected input signal unmodified — useful for recording or routing to a secondary amplifier
The two parallel PRE OUT lines allow simultaneous connection to two power amplifiers. A recommended configuration is one PRE OUT to the main power amplifier and the second to a pair of SCA Seamless Subs via their low-level RCA inputs — each sub receiving a dedicated channel for true stereo low-frequency reproduction and enhanced bass imaging.
Coupling capacitor upgrade
The PCB provides generous physical space for capacitor upgrades: maximum 20mm diameter × 57mm length. Coupling capacitor choice has a pronounced effect on the character of this preamplifier — Mundorf, Duelund and Audio Note capacitors have all been used by SCA customers with strongly positive results.
Specifications
| Signal stages | ||
|---|---|---|
| Phono EQ stage | Flat amp stage | |
| Input impedance | 47kΩ (MM) | — |
| RIAA equalisation | ±0.5dB (20Hz–20kHz) | — |
| Gain | 29dB | ~17dB (~7×) |
| Max output voltage | 20V rms (1kHz) | 20V rms (1kHz) |
| Residual noise (IHF-A) | — | 6.5µV (mid) / 15µV (max) |
| S/N ratio (IHF-A) | 112dB | 130dB (mid) / 126dB (max) |
| Frequency response | — | 5Hz–200kHz (–3dB) |
| Overall | ||
| Tube complement | 12AX7 × 4, 12AU7 × 2 | |
| — Flat amp stage | 12AX7 × 2, 12AU7 × 1 | |
| — Phono EQ stage | 12AX7 × 2, 12AU7 × 1 | |
| Inputs | PHONO (RCA), LINE 1–3 (RCA), LINE 4 (3.5mm front) | |
| Outputs | PRE OUT × 2 (parallel), REC OUT × 1 | |
| Power | AC100V, 50/60Hz, 23W | |
| Dimensions | W352 × H66 × D298mm (excl. projections) | |
| Weight | Approx. 3.9kg (assembled, excl. wires) | |
Compatibility — TU-8900 and Seamless Sub
The TU-8550’s two parallel PRE OUT lines make it the natural partner for the Elekit TU-8900 power amplifier. With one PRE OUT feeding the TU-8900 and the second feeding a pair of SCA Seamless Subs via their low-level RCA inputs, each subwoofer receives a dedicated channel signal — preserving full stereo separation at low frequencies and delivering bass imaging that complements the main speaker presentation.
Standard tube complement
12AX7 × 4 (Electro-Harmonix)
12AU7 × 2 (Electro-Harmonix)
Rolling these tubes is straightforward, and the all-tube signal path means the choice of valve has an audible effect throughout — including in the phono stage.
Build options
Kit (without tubes) — $1,495
Kit (includes tubes) — $1,636
Enhanced model — $2,699 (contact SCA for details)
Build at Amp Camp — supervised weekend workshop
Built for you — see SCA Services