This is a high end, 300B low power headphone amplifier with a 32 ohms output at max 6.75W. It has also 8 ohms speaker output which makes it a versatile amplifier.
The amplifier is uses all Hashimoto transformers and chokes, and it is assembled on a sturdy, high quality, 1/8″ aluminum made by Dave at LandFallSystems, and wrapped on walnut front and sides.
The amplifier has a vintage look which it is emphasized by a large Bakelite knob and an aircraft dimmable Dialight red panel indicator.
This amplifier was designed with TLC in order to provide best out of the 300B tubes. It is a single ended design and consequently the sound is pleasant, detailed and easy to listen for long hours, with a predominant second harmonic.
The power supply has a powerful 5AR4 or GZ34 rectifier, followed by a CLCLC filtering using two Hashimoto 10H chokes, and two 4 poles Mundorf HV+ high fidelity capacitors. There are 3 separate 6.3Vdc regulated supplies for the 300Bs and C3g tubes, an excellent choice for zero hum. The filament supply for the 300Bs is using super regulators from Belleson.
The audio section of the amplifier uses the C3g pentodes in triode mode loaded on a cascode CCS for minimal distortions. The C3g is used as gain stage/ driver and it is capacitor coupled to the 300B, using two excellent MiFlex KPCU copper foil Poly/Paper in oil capacitors. The final/power stage is using the 300B self-biased using Mills resistors and Mundorf Mcaps capacitors. The 300B is loaded on a specialized Hashimoto headphone transformer.
All the small parts are audio grade and tested for the best sound. I used Takman Rex, Takman Rey, Mills, AMRG, Vishay Milspec and 5W Kiwame for the supply bleeders.
The amplifier is integrated with one RCA and one XLR input, and the volume control is a ladder type TKD CP2511 potentiometer, which is very smooth and clean sounding.
The XLR is transformer coupled to the RCA, using a high quality Jensen JT_11P-1 line input transformers. There is a switch provided to allow using either the RCA or XLR input. The XLR inputs have a ground lift switch which is recommended by Jensen technical stuff to keep it on lift position.
The bottom plate has over 100 holes for natural convection and 4 custom feet using wood and vibration isolators.
The amplifier delivers 6.75 maximum on 32 ohms (14.7Vrms), 4.3W on 50 ohms, and 1.8W on 120 ohms and 0.86W on 250 ohms headphones.
The amplifier will also deliver maximum 6.75W on 8 ohms speakers. There is a switch provided to use either speakers or headphones. However, depending on the grounding scheme of the audio chain it might have no audible effect.
Specifications and measurements
The following measurements are only to give you a sense of the approximate performance. I used a computer software and interface and this comes with some artifacts and noise that are not present when the amp is connected in an audio path. One, if owns high quality audio analyzer, can take more precise measurements
- Input impedance RCA : 50kohms, unbalanced
- Input impedance XLR : 13kohms, balanced
- Frequency response: 20Hz-20kHz @ –0.5dB
- Input sensitivity: 0.44Vrms at 1.1W power output
- Max Input sensitivity: 1.12Vrms at 6.75W power output on 32 ohms
- Gain max 22.5dB (13.3x)
- AC voltage 120V, 50/60Hz
- Dimensions: 19.25″x14″x10″ (W x D x H)
- Weight: 55 lbs.
- Main fuse 3A slow blow
Price: $7000 ( not including the EML300B tubes )
Lead time: 3-4 months ( could be shorter depending on the time to procure parts).
Please email me to discuss details at : simplepleasuretubeampsRCT@gmail.com