Denon TU-500

Meant to be the center of the rig, the FM-only TU-500 has meters to validate the output level, a balance knob and even a NULL balance function.
Said meters can also be used as FM signal strength and output balance indicators... And, naturally, the TU-500 sports multipath and oscilloscope outputs.

The reason why everybody remembers the TU-500 is its scale display : a partly visible 12cm drum which not only allows to keep more front space for other controls but also to multiply the overall length of the scale - here at 28,3cm.

Inside is, 5-gang varicap front-end : the input first goes through a single tuning circuit (one gang) then to dual-gate Mos-Fet amplification, then to three gangs before local oscillator mixing and amplified via, again, dual gate Mos-Fets.

The modified Clapp oscillator circuit renders transistor functions insensitive to changes in the electrical supply voltage and temp changes ; the oscillator frequency is stabilized because the oscillator output is fed to the second gate of the dual gate FET.

The normal, main IF section is made of 2 three differential amplifier stage DC ICs ; PLL in the MPX stage ; 3-stage LPF filters for the 19Khz and 38Khz carriers ; 2-stage DC amplification for the low-impedance audio outputs.


The TU-500 would have several siblings until 1979, all of which featured more or less the same functions : the TU-300 and TU-400, receiving an AM section were on the lower end while the TU-355 probably was a rebadge/rehash with updated looks, alongwith the TU-501 and TU-701.

The TU-850, with less functions, certainly was in the same league as the original.