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.
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.
Around the back
Inside
Everything is hand built and assembled (except for the green boards which are outsourced components for the remote control)
The main board containing the phono, tone, gain and phase splitter stages
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
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Topology
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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. |
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Valve complement
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Gain stage: 1 X 6N2
Phase Inverter: 1 X 6N1 Output: 4 X EL84 |
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Power Output (measured)
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15W RMS both channels driven, 1kHz continuous, resistive load
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Distortion (measured)
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1.5% THD at rated power, 1kHz, resistive load
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Output Impedance
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4Ohm 8Ohm
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Input Impedance
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500 KOhms
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Input Sensitivity
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1v rms for rated power (2.8v p-p)
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NFB
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10dB
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Frequency Response
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10Hz – 24kHz ±1dB
6Hz – 32kHz ±3dB
-6dB at 50kHz
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Power consumption
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230v 50Hz 90w nominal
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RIAA Phono Preamp
| Sensitivity |
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| Gain |
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| Freq Response |
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| Topology |
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Tone Control
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Topology
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unity-gain cathode-follower driven baxandall tone stack with NFB
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Valve complement
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2 X 6N2
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Tone adjustment
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±12dB at 100Hz and 10 kHz
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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)
