Matariki amplifier

This amplifier was built as a demo unit to show what a 15 watt EL84 amp sounds like, also as a refinement on the previous EL84 amp built for a friend. 

Included in this amp are:

  • 2-triode Phono/RIAA stage
  • Tone Controls
  • Remote control volume, standby, and input selector

This amp was built on a small chassis – same size as the previous project: aluminium, sourced from AliExpress. The front panel is 8mm thick, brushed aluminium. It required pockets being milled on the CNC from the back to accommodate the controls mounted through it.

The lights are 3mm LEDs but I decided I don’t like the bulging appearance they give when pushed through the front panel, so we laser-cut some 2mm clear acrylic into 3mm circles, so the lights on the front could be flat and flush. They press-fitted perfectly and the look was 100% what I was wanting.
 
The STBY LED is red, and the PWR led is dual-colour, it starts red at power-up and then when the HT switches on after 30 sec, it turns green, using a polarity-reverser circuit I designed based around a DPDT relay.
 
The power switch and input selector are a rotary encoder: push to toggle power, rotate to cycle through the inputs.
 

 

 

 

This is similar to the previous EL84 amp except that I found a supply of NOS Soviet triodes so I used the 6N2 and 6N1 in place of the 12AX7 and 12AU7. This necessitated changing the voltages and loads owing to the different charasterics of these tubes.

The output is exactly the same as the previous EL84 amp I built: EL84 in Class AB push-pull, ultralinear, fixed bias, with Hammond 1650E output transformers.
 
The tone control is the exactly same circuit used in the previous project.
 
The amplifier was built with the phono, tone, gain, and splitter stages on a single PCB at 160 X 100mm and holding 6 dual triodes.

 

Inside & Construction

Everything is hand built and assembled (except for the green boards which are outsourced components for the remote control)

 The power transformer is a Hammond 370FX – the 6.3volt secondary powers the heaters in the EL84 and tone control and gain/splitter stages. The heaters in the RIAA stage are powered from the 5 volt secondary, rectified with 4 schottky diodes (selected for their lower forward voltage drop) and smoothed with a 47000µF 10V capacitor. This gives 6.0VDC at the heater pins on the triodes.

The output stage uses the same fixed bias concept as the previous two EL84 amplifiers – the board running between the sockets provides the voltage trimmers and associated resistors, as well as the cathode shunt resistors. These are exposed to the top to allow fine-tuning by the user

 

Specifications

Main Amplifier

Topology
Line-level amplifier, grounded cathode gain stage, DC-coupled
cathodyne phase inverter, push-pull EL84 output in class AB using fixed-bias ultralinear topology, global negative feedback. 360V Plate Voltage.
Valve complement
Gain stage: 1 X 6N2
Phase Inverter: 1 X 6N1
Output: 4 X EL84
Power Output (measured)
15W RMS both channels driven, 1kHz continuous, resistive load
Distortion (measured)
1.5% THD at rated power, 1kHz, resistive load
Output Impedance
4Ohm 8Ohm
Input Impedance
500 KOhms
Input Sensitivity
1v rms for rated power (2.8v p-p)
NFB
10dB
Frequency Response
10Hz – 24kHz ±1dB
6Hz – 32kHz ±3dB
-6dB at 50kHz
Power consumption
230v 50Hz 90w nominal

RIAA Phono Preamp

Sensitivity
4.5mV for rated power. MM-type cartridge only, 47KOhm load impedance
Gain
+42dB at 1KHz
Freq Response
RIAA Compliant ±1dB (15Hz–20KHz)
Topology
Cascaded dual-triode 6N2 implementing RIAA filter in NFB



Tone Control  

Topology
unity-gain cathode-follower driven baxandall tone stack with NFB
Valve complement
2 X 6N2
 
 
Tone adjustment
±12dB at 100Hz and 10 kHz

Listening tests
The sound from this one is clean, detailed and very pleasing. The main amp stage is based on the well-regarded Fisher X100, with the appropriate modifications for the Russian triodes in use. 


The output stage is EL84 in fixed bias ultralinear, biased to 8.4W quiescent dissipation (70% of rated maximum)

Schematic

(click to enlarge, may need to save-as to read clearly)