02-EL84 Amplifier I

Project 2 - an EL84 Amplifier build commissioned by a friend.

The design requirement for this amplifier was to provide an integrated amp with RIAA Phono input, and the capability to switch between two sets of speaker outputs.

Once again, a custom-wound power transformer was used, suffering similar aesthetic challenges to the previous project.

The chassis selected was about 5mm too narrow to fit the power transformer and the output transformers across the back in the usual configuration. So it had to be non-symmetrical

The front three small-signal valves form the RIAA Phono stage, the rear two are the line-level voltage gain and phase splitter.

The Bias test points sit between the EL84 output valves, with recessed trimmers to adjust, and test points to measure the voltage across the 10Ohm cathode shunt.

The Slovak-made JJ EL84 output valves on this one have quite a pleasing amount of light-leakage from the filaments and cathodes. Unlike the Russian Electro-Harmonix small-signal valves which are hard to see any filament glow from at all

Inside, the amp is crowded. Point to point wiring inside a tight working space.

Polyethylene Film capacitors are used for inter-stage coupling, and also in the RIAA stage, which as at top left in this photo. The power supply board sits under the output transformers, The power supply board has my usual 555 timer-based circuit to delay the B+ turn-on be 30sec giving the valves plenty of time to warm up first. By use of this circuit, combined with heater elevation for the small-signal valves, I can get away with avoiding the diode on the cathode follower in the RIAA stage, which is DC-coupled to the previous gain stage.

6 PCBs inside this case, including 5 home-made ones

Some specs and tech details

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. 370V 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	50 KOhms
Input Sensitivity	300mV rms for rated power
Frequency Response	6Hz – 55kHz ±3dB
Power consumption	230v 50Hz 190w nominal

RIAA Phono Preamp

Topology	Phono-level amplifier, cascaded grounded cathode gain stage, DC-coupled cathode follower, RIAA equalisation
Sensitivity	4.5mV for rated power. MM-type cartridge only, 47KOhm load impedance
Valve complement	Gain stage: 2 X 12AX7 (ECC83) Cathode Follower: 1 X 12AU7 (ECC82)

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)

The RIAA stage works well. The Cathode follower is needed to drive the volume control which represents a 50K load across the input. The noise floor is low, hum is non-existent, and distortion does not occur even on the loudest passages. 

This is the circuit. You'll need to click it to see full size and download / print / zoom / scroll, or whatever.

You'll probably need to download this to see it clearly

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