EL84 Amplifier with extras

This project was commissioned by an overseas customer. The requirements list for this one was complex:

  • Amplifier with remote control volume
  • Tone controls
  • Headphone output
  • Phono input
  • Small chassis suitable for international shipping

For the main amplifier, the same topology and transformers as the previous EL84 amplifier was used. However this one has a higher tube count because of all the extras.

Also, this amplifier was constructed entirely on printed circuit boards, to enable easy re-use of the modules in subsequent projects.

Around the back

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 12AX7 (ECC83)
Phase Inverter: 1 X 12AU7 (ECC82)
Output: 4 X EL84
Power Output (measured)
15W RMS both channels driven, 1kHz continuous, resistive load
Distortion (measured)
1% THD at rated power, 1kHz, resistive load
Output Impedance
4Ohm 8Ohm
Input Impedance
500 KOhms
Input Sensitivity
850mV rms for rated power
NFB
10dB
Frequency Response
6Hz – 55kHz ±3dB
Power consumption
230v 50Hz 190w nominal

RIAA Phono Preamp

Sensitivity
4.5mV for rated power. MM-type cartridge only, 47KOhm load impedance
Gain
+46dB at 1KHz
Freq Response
RIAA Compliant ±1dB
Topology
Cascaded Dual triode ECC83 / 12AX7 implementing RIAA filter in NFB



Tone Control

Topology
unity-gain cathode-follower driven baxandall tone stack with NFB
Valve complement
2 X 12AX7 (ECC83)
Tone adjustment
±12dB at 100Hz and 10 kHz