4P1L PSE Power Amplifier for audio enthusiasts
Built around my favorite vacuum tube (that won’t break your bank) it sounds as good as the most praised DHTs. 4P1L is a better than original equivalent/replica of the RL2 German direct heated pentode. In triode mode, pure class A will deliver 1.75 clean watts. This amp uses 2 tubes in parallel and delivers 3.5 clean watts on 8 ohms speakers. The driver is the well known 6E5P which is able to properly drive the 400pF input capacitance of the final stage. The design was considering no use of coupling/decoupling caps in the signal path. This was accomplished by having the power stage in filament bias mode where the filament current goes through the filament and a low value resistor (16.6 ohms /100W ) which has no need to be decoupled. There are not many power tubes that can run in filament bias because of the bias requirements will lead to way too much power dissipated on the bias resistor and therefore heatsinking issues. The driver is using a quasi self bias circuit based on Schottky diodes and therefore no need to decouple. The driver is transformer coupled to the power stage using a Lundahl LL1692A interstage transformer. The power stage is loaded on a Lundahl LL1664/80mA 3k/8ohms.
In order to make this amp as compact as possible ( and I would say that it is not as compact as many of you wish to be ) and considering the complexity of the power supply, I used Antek audio grade toroidal transformers. The main Antek is a 300W transformer, and it is used to supply the HT and filaments of the rectifiers ( 2x 6CJ3), and also few other boards for the following purposes : one for the regulated filament voltage of the drivers, one for the supply needed by the DC mA meters, and the third one for the speaker delay circuit. The second Antek is a 200W dual 22V secondary transformer, and it is used to provide the filament bias circuit voltage. The filament bias circuit has a raw supply with 40,000uF on each channel, followed by the well praised Coleman filament regulator, adjusted for 20V on the bias resistor ( or 1.2A constant filament current ). The high voltage has TV dampers as rectifiers and a CLC filter ( 50uF-LL1673/10H-470uF. B+ of 320V supplies for 2 CCS circuits on each driver and through a Maida Neurochrome regulator to the final stage.
The amplifier is perfectly quiet and that’s the result of a careful power supply design.
I chose all components for the best sound while keeping the cost under control. This amp is optimized and if anyone is interested to order it, just know that I will not use other “upgraded” components.
The volume control uses a very high quality TKD CP2511 50k potentiometer, and therefore the amp can be used as an integrated with single input.
The amplifier runs warm and some parts are higher temperature than others. The highest temperature ( besides the vacuum tubes ) of 130F is on the Arctic heatsinks used for each bias resistor ( each has to continuously dissipate 24W ).
The amplifier is built in my style with all the equipment attached to the top plate ( a CNC from Front Panel Express) which sits on a heavy hardwood case. This particular case is made of 5/4″ thick genuine mahogany. The amplifier weights 56 lbs and measures 20″x15″x11″ (wxdxh).
Putting aside the size, weight and power consumption, this is an extraordinary good sounding single ended amplifier. I am using it in my working space with small bass-reflex Alpair 12P Gen2 speakers. This is a great setup for a near field, low listening levels ( max low 90dBs ). This amp might not be perfect for everyone. However, if you have a small to medium room and you listen to levels that won’t hurt your hearing then this amp is as audio pure as possible.
Please contact me at firstname.lastname@example.org for pricing and lead time.