music | simplepleasuretubeamps

This project started when I found a pair of LL2765/75mA amorphous core waiting to be used. I’ve built a 300B HP amp using these types of transformers. I needed something different. I needed an HP amp that was more versatile and could be used as a high gain preamp and low cost tubes, the power being preferable DHTs. The DHP wired triode 4P1L was the proper tube for this implementation. I’ve been successful building several PSE4P1L amplifiers and tried them on headphones. The result was a very pleasant (2nd harmonic predominant), quick and clean sound. 4P1L in triode mode has quite a bit of input capacitance and 2 in parallel will give the driver about 400pF to deal with. Therefore, the driver must be a powerful one and considering the gain I needed, I chose and tested the 6H30PI and the 6H6P. These are inexpensive tubes and a very good match to the 4P1L. I initially considered a dual chassis HP amp. However, I was confident I can make it quiet in one enclosure, and I did. Despite my efforts to make it as compact as possible, the amp/preamp enclosure is 19.25×15.5×11” ( WXDXH) and weights 45 lbs.

The power supply section takes about 2/3 of the real estate. It has an audio grade 200VA Antek toroidal that powers the high voltage and the DC filament regulator for the driver. There are 2 separate supplies for each of the 2x4P1L filaments and it is regulated by the highly praised Coleman filament regulators. The high voltage is tube rectified, using TV dampers 6CJ3, and CLCLC filtered using Lundahl LL2742 chokes in CMMR configuration for the highest common mode ripple rejection. This power supply provides DC all around and makes this HP amp/preamp very quiet.

The signal section of the amp is very simple. The driver is loaded on a CCS and it is capacitor coupled to the power stage. The power stage is loaded on the Lundahl LL2765AM/75mA and it is self-biased as well as the driver stage. I implemented it with high quality and appropriate type of parts and materials.

The chassis is made of 5/4” American cherry hardwood stained with General finishes Antique walnut and coated with 5 coats of General finishes oil based topcoat. The top plate is a sturdy 1/8” brushed aluminum that holds all the equipment besides the IEC which is installed on the back of the wood chassis.

The exquisite sound headphone amp is provided with a high Quality Neutrik phone jack. The volume pot is a TKD CP2511 and a 1.5” NOS bakelite knob. There are few features on the top plate:

On the left side is the mains On/Off switch
In the middle there is a 500mA NOS Hoyt panel meter. This measures the high voltage supply current which should be around 170mA
On the right side there are 2 sets of RCAs. One in the back is the input and the other the output when used as a preamplifier.
On each side of the 4P1L tubes there is a set of tip jack insulated test points. Measuring the voltage (either side) you will basically read the total plate current of the corresponding channel. The voltage in mV corresponds to the current in mA.

The spec measurements of this device showed the following:

Maximum output power is 3W on 32 ohms headphones, 0.56W on 120 ohms and 0.22W on 300 ohms
The input sensitivity is about 0.9Vrms (6H30PI driver) and 0.6Vrms (6H6P driver) for full power output.
Output impedance of this HP amp/preamp is around 7 ohms
The frequency response is 20Hz-20kHz +/- 0.5dB
The THD is 0.55% at 1W on 32 ohms, 0.81% at 2W on 32 ohms
The overall gain is 19dB when using the 6H30PI driver, and 22dB when using the 6H6P driver
The max output voltage is 8Vrms
Power consumption 130W

This is a “one of a kind” preamplifier. It uses the 26, an early DHT, capable of a pure sound . Who ever is familiar with the 26 sound can say that it excels in musicality and emotion.

At the demand of the client, the preamplifier was carefully designed in one enclosure. Despite the challenges of a single enclosure DHT preamp, the 26 device is as quiet and as little microphonic as possible. 26 tube is known for picking up noise/hum from proximity to the AC radiating area. I highly recommend a dual enclosure scenario, with a separate power supply. However, if used with low power tube amps that have low to medium overall gain, this preamp will perform great.

The preamp has 2 distinct zones (looking from above): on the right hand side is the power supply zone and on the left hand side is the main signal path zone.

The power supply area is packed with separate supplies for each filament of the 26 tube, the regulated DC for the remote control unit and the high voltage unit. Each 26 tube filament has a raw DC supply using 40,000uF of filtering capacitance per channel, and a much praised Coleman filament regulator. The high voltage supply is using a type 80 tube as rectifier and a CLCLC filter, followed by a special Mosfet shunt regulator that brings the ripple down to uV levels.

The signal path of this preamp is as short as it can be, and free of coupling or decoupling capacitors. That was possible by:

Using filament bias circuit, where the filament is in series with the low value (10 ohms) bias resistor. This resistor doesn’t need to be decoupled. The filament current is adjusted by the Coleman regulator (which is a CCS-constant current source).
Using a line output transformer by Lundahl LL2763/7mA

The line output transformers are made by Lundahl in Sweden, and they have 4 balanced secondary windings. There are 3 4PDT switches on the back panel, aligned with the left and right RCAs. If they are all toggled upwards, the gain of the preamp is 12-13dB, and if they are downwards the gain of the preamp is 6-7dB. The high gain position implies a 2:1 output ratio and the output impedance of the preamp is 2200 ohms. The low gain position implies a 4:1 output ratio and the output impedance of the preamp is 600 ohms.

The input of the preamp is DC coupled using a motorized Alps Blue Velvet remote control by Bent Audio. This is a high quality, super quiet remote.

The preamp is provided with 3 inputs switched by a high quality Seiden selector switch made in Japan.

The enclosure is a very sturdy 1/8” thick aluminum chassis by Landfallsystems.

The top plate is custom CNCed and anodized at Front Panel Express. The sides and front are wrapped with ¾” American cheery hardwood that was stained with General Finishes Antique walnut gel stain and coated with 4 layers of oil based urethane.

The preamp will sound smooth, clean and clear sound with 0.05% THD second harmonic predominant at 2Vrms output.