Classique – 6BM8/ECL82 SE – 3W – White Paper

This is the white paper for the Classique – 6BM8/ECL82 SE – Stereo Audio Integrated Amplifier Kit.

The Design

6BM8/ECL82 SE Classique Kit - pictured in a custom-made elm wood chassis and stocked mounting plate.
6BM8/ECL82 SE Classique Kit – pictured in a custom-made elm wood chassis and stocked mounting plate.

In many ways, the beauty of this design is in its simplicity. The single end topology, using two amplification stages of the valve produces a unique and very pleasing output. The design of the audio amplification stages is very much based on an old and tested design provided by Philips datasheet for the 6BM8/ECL82 valve.

The placement of parts and wiring, using the kit mounting plate was done in such a way with the aims of:

  • Minimizing the audio signal path
  • Single star grounding scheme
  • Separation of audio and power stages

The overall size of the amplifier is also small compared to the “normal” high-street separate system. The aluminium chassis (the top plate) is only 300mm x 260mm (which is about 2/3 of the width of some other amplifiers).

Power Supplies

There are two power supply PCB’s included in the kit. A main power supply and a heater filament power supply.

Main Power Supply

Image of fully populated main power supply PCB (left) and valve heater filament power supply PCB (right).
Image of fully populated main power supply PCB (left) and valve heater filament power supply PCB (right).

The two amplifier PCB’s are powered by the main power supply board. At the heart of the power supply board there are four Silicon Carbide Schottky diodes which forms a full wave bridge rectifier. This rectifier circuit provides the starting point for the DC voltage to the system.

These Silicon Carbide Schottky diodes in the form of 2-Pin TO-220 diodes provide a much higher breakdown field strength and greater thermal conductivity coupled with a significant reduction in power-loss at high switching frequencies. Rated to be working at maximum of 175 °C they should be able to handle the task with no problem.

The DC is then piped through two sets of RC filters which lower and smoothe the DC current. Two long lasting and high quality Cornell-Dubilier 100 microFarads electrolytic capacitors are used in the RC filters. The resistors are made up by four 7 watts wirewound high power dissipation Silicone coated resistors. Rated at 7 watts per resistor, these resistors should be able to handle the 80mA current through it without a flinch!

We have also included a bleeder resistor at the end of the main power supply board. This tiny but very important resistor is here for safety reason as it makes sure that the two large capacitors are discharged at a safe rate if the  board is powered down (or powered up without any load). We have seen some designs where a bleeder is not included – not safe.

Valve Heater DC Power Supply

Heater filament ripple voltage of 640mV (peak to peak)
Heater filament ripple voltage of 640mV (peak to peak)

The heater filaments are powered by an unregulated DC power supply. Each filament runs at around 6.3VDC and draws 780mA.

Direct current is used in order to ease the process of building by minimizing risks of hum.  The power supply for the filament contains a total of 30 000 microFarad of smoothing capacitor value. As an alternative, alternate current can also be used (if desired) by connecting the filaments directly to the 6.3VAC pins on the secondary winding. A tight twist of the cables (use two per valve filament) and a carefully routing of the cables around the edge should keep the hum to a very acceptable level.

This power supply board is pretty minimal with only 4 diodes and 2 smoothing capacitors.

The rectifier diodes used are of high quality, STMicroelectronics STPS3L60Q Schottky Diodes. These diodes where chosen as their average voltage drop is spec at 0.53 volts (needed because we are trying to get 6.3 VDC from 6.3 VAC!!).

The smoothing capacitors are the Nichicon Aluminium Electrolytic Capacitor from the 5101-4 VY Series which are miniature sized, low impedance and high reliability aluminium electrolytic capacitors.

Audio Amplification

Classique – 6BM8/ECL82 SE
Classique – 6BM8/ECL82 SE

The B+ supply is aimed to be around 250VDC running at 40mA per channel.

On the triode stage, the anode resistor is 220kOhm and the cathode (plate) resistor is 2.2kOhm. The plate voltage is aimed to be around 100V. At this stage, the supply voltage is aimed to be around 230VDC. On this setting the distortion value should be low at 1.6%. The cathode is not bypassed. This will lower the output level but reduces issues with the oscillation and keeps the system stable.

There is a feedback line coming from the positive (hot) secondary winding of the output transformer. The feedback is being fed to the triode cathode through a 33K resistor with a 330 pico Farad capacitor in parallel. The combination of the resistor and the capacitor determines how much feedback is received as well as the low pass frequency.

In between the triode and pentode sections, we have a 0.15 microFarad decoupling capacitor. Being in the middle of the two amplification stages, this capacitor plays a vital role in the circuit. We have chosen the Vishay 1822 Series capacitor as this has been our go-to capacitor for a number of years.

The pentode section has a load of 5K ohm in the form output transformer. Using a supply of 250V and cathode resistor of 560 ohm the system is outputting around 3 watts per channel.

PCB’s specifications:

  • Double layered 1.6mm thick with 1 oz copper layer
  • FR-4 TG130 board material
  • Tracks routed according to the IPC-9592 minimum clearance standard

The Transformers

There are three transformers included in the kit:

  • 1 x power transformer
  • 2 x audio output transformers
Transformers from our trusted supplier
Transformers from our trusted supplier

These transformers are handmade in the United Kingdom by one of the long established traditional transformers suppliers – Primary Windings. We have done extensive tests on a number of transformers and have sourced the best in terms of cost to performance value. The transformers were constructed using high grade grain orientated M6 0.35mm laminations, for lower loss and double insulated. They also contain separate primary and secondary windings for better safety.

Aluminium Chassis / Mounting Plate (optional)

A laser cut 3mm aluminium mounting plate with all holes pre-drilled and cut can be ordered (optional) as part of the kit from the chassis/enclosure page.

The idea of the kit mounting plate is to provide users a reference design as the first step in building the amplifier. Users will be able to set up the amplifier without having to purchase anything extra (OK, you need purchase cables and a fuse). The design and placements of parts on the plate will address any problem regarding noise / hum.

The kit however does not include the chassis/plinth. You will need to provide somewhere to place the mounting plate to start of with (or purchase a chassis from our shop!).

The mounting plate was also designed to be as simple as possible to make it versatile. It is nothing more than a piece of plate with four mounting holes at the corners. A compatible chassis will just have to make sure that these four holes can be bolted to a body.

Screws / Nuts / Connectors

All screws / nuts / bolts and spacers / stands are included so that all components can be mounted onto the mounting plate.

Stereo Volume Potentiometer

A high quality Japanese made Alps RK27. The potentiometer is a very important component and it lies in the beginning of the audio signal path therefor we have chosen a really high quality device to be part of this kit. The RK27 is normally included in a high-end stereo system and is widely regarded by a lot in the audio community.

Chassis / Plinth

We have a number of different chassis or plinth which are compatible with this kit. You can choose one from the chassis/enclosure page and include that in your order!

Audio Analysis

In this section we will present you some numbers and graphs, which might or might not mean anything to you. The graphs are really to show how the amplifier behave in a semi-controlled environment. Some people value these scientific numbers in deciding if an audio device is worth the costs whilst some people use their ears and listening experience.

Square Wave

Square wave test - input of 701mV (RMS) into 8 ohm output.
Square wave test – input of 701mV (RMS) into 8 ohm output.

Sine Waves

The parameters:

  • Input 0.8V (RMS) of sine wave
  • Output loaded with 8 ohm fixed load

Frequency Analysis

Measured Frequency Response - 6BM8/ECL82 Single Ended Stereo Amplifier
Measured Frequency Response – 6BM8/ECL82 Single Ended Stereo Amplifier