Classic 25 Description
|The following information originates from the Classic 25 Construction and Operating Manual.
Our experiences during the development of line level pre-amplifiers emphasised the importance of handling these high level signals with the same care that we would apply when dealing with the much lower level signals from a phono cartridge. It was vital that the selection, attenuation and following driver stages were designed to minimise compression of the signal whilst still allowing coupling with unrestricted bandwidth to the power stages of the amplifier.
The volume and balance controls as well as the gold plated selector switches were chosen after careful sonic evaluation and are coupled directly to a high linearity series regulated push pull gain stage. This in turn feeds our proprietry floating para-phase phase splitter configuration with carefully selected component values to minimise amplitude distortion. The resulting signals are further amplified and buffered by a driver stage before being fed to the output stage. The EL34 (6CA7) output stage is operated in the CLASSIC ultra linear push pull configuration with the valves biased in pure class A operating mode. The resulting high voltage signal is impedance matched to 4 or 8 ohm loudspeaker outputs by our in house designed high quality output transformers.
A fundamental of good amplifier design is the power supply and care has been taken to ensure the supply is kept constant under varying signal conditions thereby always allowing the amplifier to retain its smooth composed sound quality. This has been achieved by the use of our in house designed mains transformer in conjunction with high quality smoothing capacitors which ensure a very stable low impedance supply regardless of circuit demands. The supplies to the input valves are further heavily decoupled to allow them to operate without being influenced by the output stage. The input and phase splitter valves are DC heated to reduce noise and ensure stable thermal operation when dealing with the lower level input signals.
The Classic 25 uses selected polyester signal coupling capacitors and close tolerance metalfilm resistors throughout the audio circuit. The PCB is of a heavy duty type which has been carefully designed to minimise interstage interference and provide good ground return paths.
This amplifier offers the facility to accept up to 4 inputs as well as a tape loop and therefore represents an excellent choice for audio enthusiasts seeking improved performance from the high quality line source components that are currently available.
If a tape recorder is connected to TAPE OUT it is possible to monitor what is going onto the tape by setting the TAPE.SOURCE switch to TAPE. This allows direct comparison between the source signal and what is being recorded. This may be useful for setting correct recording levels.
The speaker output terminals are located on the rear panel, two red and one black for each channel. The two red binding posts correspond to the two secondary windings on the output transformer, l.e. 4 and 8 ohms. The positive conductor in each speaker cable should be connected to the terminal which most closely matches the impedance of the speakers. The negative conductor is connected to the relevant black terminal.
N.B. Most sensibly designed loudspeakers have a minimal Impedance of 4- 8 ohms. Comparison of the results obtained using the 4 or 8 ohm terminals is recommended. Use whichever suits the loudspeakers.
The Classic Stereo 25 reaches optimum performance within 20 to 30 minutes from switch on.
Allow any valve amplifier to cool down before moving it. The valves are rated for about 6000 hours use. As a guide, if the amplifier is used for 4 hours each day the valves should last for approximately 4 years.
We sincereIy hope you enjoy many years of pleasure from your Classic Stereo 25 amplifier. Please keep this manual safely for future reference.
CLASSIC STEREO 25 SPECIFICATIONS