In the past famous valve amplifiers were designed for 800 Ohm loudspeakers without using output transformers. Especially the Dutch Philips produced such Output-Transformer-Less (OTL) amplifiers. Nowadays again these amplifiers are in the center of the attention, as proven by the contest of the 2022 European Triode Festival (ETF) in the Netherlands.
My TubeSociety (TS) students got triggered by the ETF-challenge to make their own OTL-design. For them I designed the VDV-800-5, converting impedances from 800 Ohm into 5 Ohm and vice versa. In TS we use this transformer to test our new OTL valve amplifiers using standard low impedance loudspeakers. However, this transformer also matches old 800 Ohm loudspeakers when driven by modern low impedance amplifiers.
In this design special attention is paid to lowest leakage inductance, to maintain the 800 to 5 conversion without high frequency loss. The net result is a remarkably clean and wide frequency range.
Primary impedance: 800 Ohm
Secondary impedance: 5 Ohm
Power: 15 Watt
Low frequency range: below 10 Hz
High frequency range: above 200 kHz
Dimensions: 91 mm diameter; 30 mm height
Weight: 1 kg