MU-4 is stereo integrated amplifier where famous, appreciated and easy accessible KT-88 tubes are used in the output stage. Electro-Harmonix tubes version is presented on attached picture. Output tubes work in Push-Pull and fixed bias mode with moderate 8dB negative feedback. It yealds 50 Watts of power per channel and vivid, very dynamic sound. Biases of output tubes pair in each channel are maintained in exact symmetry by specialized servo. As a result harmonic distortion, noise and hum are effectively eliminated. From the user point of view regulation comes down to bias setup only for one tube per channel. Via tiny switch on the rear side of amplifier volume bar can be easly changed to precise miliammeter, which helps much to setup bias level recommended for particular tube type. For KT-88 it is simple middle of a scale. This way the main disadvantage of fixed bias circuit that is necessity of recurrent bias verification and trimming was effectively reduced. The use of external or – what is often practice – build in the front pannel ammeters was also avoided. In our case it would be unacceptable for our visual design concept. Output transformers work in ultralinear mode for harmonic distorsion reduction.
In the voltage amplifier section 12BH7 i 6N1P double triodes were used. Huge 400W toroidal transformer guarantees high margin of spare power supply for amplifier. Anode voltage is applied after 30 seconds warm up time from power on the aplifier. Speakers are switched on after additional 15 seconds. It extends tubes lifetime and protects speakers against damage. Preamplifier is made of low noise op amps and supports three sources described on a front panel as PH (PHONO), CD and AUX. Noisless and reliable reed relays are used for input switching. Headphone output is also available. And the last but not least – true jewel – solid, chrome coated remote controller with comfortable big buttons made of SWAROVSKI crystals. Active input, volume level (including MUTE) can be changed remotely. All those functions (except MUTE) are also available in amplifier through set of tiny buttons hidden underside of front panel. Like in all of our products case elements and transformers was produced by polish manufacturers and manually assembled. Thus we can say with pride: „handcrafted in Poland”.