This is another example of customisation – the customer (another friend) assisted with the design.
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
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. Each module (RIAA, tone, headphone, preamp, PSU) was built onto its own board.
Around the back
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.
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 32 Ohm to 300 Ohm.
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 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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500 KOhms
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Input Sensitivity
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850mV rms for rated power
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NFB
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10dB
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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
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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 12AX7 (ECC83)
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Tone adjustment
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±12dB at 100Hz and 10 kHz
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