04-EL84 Amplifier II

Project 4 - an EL84 Amplifier build commissioned by a friend.

This amplifier was requested by a friend who had a long list of requirements:

  • Phono/RIAA stage
  • Headphone output
  • Tone Controls
  • Remote control volume

And this all needed to be in a small chassis owing to the fact it is being shipped to Europe.

The preamp and output are exactly the same as the previous EL84 amp I build: 12AX7 gainstage, 12AU7 phase splitter, running into a push-pull pair of EL84 with 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 each module on a separate circuit board, to facilitate a modular approach to building future projects. The circuit diagram shows the boards according to colour (click to enlarge/download)

Each module (RIAA, tone, headphone, preamp, PSU) was built onto its own board.

RIAA board (left) and tone control board (right). The tube sockets are on the copper side, to allow mounting through the chassis. The sockets are flanged, and the boards mounted to the chassis with standoffs, so that removing the tubes does not risk ripping copper off the boards.

PSU board

The power transformer is a Hammond 370FX which in hindsight does not represent a good choice for this amplifier. Previous projects have utilised custom-made mains transformers, but in the dual interest of cost and aesthetics, this amplifier used an off-the-shelf part.

The problem with this transformer is that it runs hot. Despite the total load being lower than its maximums, it runs hot and will continue to run hot even if not all of the boards are powered up.

If using Hammond mains transformers in future, I will be applying a generous de-rating margin.

Space in the chassis was very tight. Many variations on the theme were tried before deciding on the design.

Front view. From left: power switch, input selector, mode selector, tone controls, volume control, remote sensor cover, headphone socket.

The recessed bias adjust trimmers and exposed test points are discreetly positioned between the EL84 outout valves 

Rear panel

The headphone stage is a White Cathode Follower push-pull topology utilising the ECC99 and coupled to the load by 470µF capacitor, no transformers are used. This stage will cleanly power headphones from 32Ohm to 300 Ohm.

-- Test results on the headphone stage are pending and will be reported here when available. --

Some specs and details

Main Amplifier

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
Frequency Response
6Hz – 55kHz ±3dB
Power consumption
230v 50Hz 190w nominal

RIAA Phono Preamp

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

Tone Control

unity-gain cathode-follower driven baxandall tone stack with NFB
Valve complement
2 X 12AX7 (ECC83)

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 12AX7 gain and 12AU7 phase splitter, though this design runs that 12AU7 closer to its 5mA sweet spot for linearity from a 300V B+ than the Fisher does.

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

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