This was a project for a friend with a unique requirement: The amplifier needed to have the option to connect two sets of speakers. This amplifier was conceived as an addition to his “man cave” which contained a formidable collection of speakers acquired from auctions and secondhand shops. So the need was to connect more than one pair and be able to switch between them.
Also required was a phono stage, since he had a large collection of vinyl and a wonderful vintage Audio-Research turntable from the late ’60s.
The amp was built on a small chassis to fit into the available space. The switch on the front is the speaker selector, the power switch is at the back.
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 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
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, 10dB global negative feedback. 360V Plate Voltage. |
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Valve complement
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Gain stage: 1 X 12AX7 (ECC83)
Phase Inverter: 1 X 12AU7 (ECC82) 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% 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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50 KOhms
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Input Sensitivity
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800mV rms for ratedpower
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Frequency Response
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6Hz – 55kHz ±3dB
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Power consumption
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230v 50Hz 190w
nominal |
RIAA Phono Preamp
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Topology
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Phono-level amplifier, cascaded grounded cathode gain stage,
DC-coupled cathode follower, RIAA equalisation |
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Sensitivity
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4.5mV for rated power. MM-type cartridge only, 47KOhm load impedance
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Valve complement
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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.
