Description

The F4 hybrid amp is based on Nelson Pass’s First Watt F4 unity gain buffer amp and a vacuum tube gain stage (Preamplifier) integrated. All the gain of this amplifier comes from the tube stage and it is about 18dB (X8).

F4 was designed by Nelson Pass. As many people have started, I thought I was building it for myself as all the reviews and description were convincing. I am a tube guy and I thought that I am going to bring a little solid state spice in my audio life. This amp sounds excellent as described by Nelson Pass. I built the F4 with extreme love and care. I’ve selected the best parts for it. The enclosure is very good quality, Italian made, HIFI2000 Standard 4U amp classic bought at DIY Audio store. The F4 clone PCBs are very good quality and came from the same store. Coupling capacitors are Elna RFS Silmic II and PS capacitors are 10mF/35V Nichicon KG Gold Tune ( total of 12, 6 for each supply). All resistors are excellent sounding Takman REY, except for the 0.47 ohms/5W which are Ohmite Audio Gold wirewound. Rectifier bridges are XFRED 68A/600V. Power transformer  is Antek 2218 (200VA/18 V dual). Main switch is anti-vandal (mod-smart.com) 250vac/5A purple ring LED illumination 12V. Power MOSFET were carefully selected and paired, and were attached to the main heatsinks using mica insulators and ceramic thermal compound. The amplifier was tuned-up to the manual specifications ( .43A per power MOSFET and very low DC offset on the outputs ( around 1mV)) and runs warm( you will have no problem touching the heat sinks).

The preamp stage is a classic common cathode using the direct heated pentode 4P1L triode strapped. The stage is using filament bias, consequently eliminating the need for cathode decoupling capacitor. The filament bias is supplied by excellent regulator made by Rod Coleman. The tube’s load is a cascode CCS using DN2540N5 mosfets. The anode current is set to 30mA (4P1L likes to run hot at a minimum 20mA plate current).

The preamp stage is a classic common cathode using the direct heated pentode 4P1L triode strapped. The stage is using filament bias, consequently eliminating the need for cathode decoupling capacitor. The filament bias is supplied by excellent regulator made by Rod Coleman. The tube’s load is a cascode CCS using DN2540N5 mosfets. The anode current is set to 30mA (4P1L likes to run hot at a minimum 20mA plate current).

The preamplifier part was built on a 1/8″ FR4 board. The preamp is powered by three transformers ( toroidal for HT and two dual bobbin for filaments).  Coleman filament regulators are used. The preamp is coupled to F4 by a quality Dynamicap E 1uF capacitor. All components were selected with great care. Alps stereo potentiometer is used for volume control.

 

 

Specifications

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 a audio path. One, if possesses a higher quality audio analyzer, can take more precise measurements.

 

 

 

 

 

 

 

Frequency response : 8.8Hz – 38kHz @-1dB

Input sensitivity : 1Vrms for output of 9W on 8 ohms

Voltage Gain: 8.5 times or 18.5 dB

Preamp Headroom: x4.25 or 12dB at 1Vrms input (9W output)

Max Input Signal : 4.25Vrms ( a signal higher than 4.25Vrms will produce higher distortions)

Schematics

 

Final thoughts:

The amp sounds better than I thought. The F4 current buffer brings good dynamics and transparency. It reveals the sound-print of the preamp/gain stage.  Choosing the 4P1L was not at all random. This amazing DHT pentode when run in triode mode presents perfect linearity and a pleasant distribution of the harmonics. In addition it has low plate resistance, being able to easily drive the F4.

This hybrid represents a very good compromise between price and sound quality.

However, I looking forward to balance the cost towards the preamp. It was difficult to fit all the components into the same enclosure. Next year I am planning to design an upgraded version of the preamp.