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Front and rear. The REF75 is a tube power amplifier with a solid-state power supply. The unit is very large and seemingly very solid. The enclosure is not particularly rigid but attempts to look it with a thick front panel with large analog power/bias meters recalling those of measuring equipment from the 70s. That’s also how studio amplifiers looked then. The meters are manufactured by Hoyt Electrical Instrumentation, a 1904 US company whose founders manufactured meters as early as 1890! Those meters are located symmetrically around a large mechanical power switch in the center accompanied by a green LED. Under each meter is a knob in the firm's typical shape. These knobs switch between power output and bias monitoring for each output tube. Bias is adjusted manually using an included plastic screw driver. On both sides of the front panel are large handles like those from classic studio and laboratory equipment.
The rear is sparsely populated. It sports two balanced XLR inputs at the top located quite far from each other. Below are gold-plated speaker terminals with separate taps for 4Ω and 8Ω. Those are rather small and only accept spades or bare wire, not bananas. In the center is a small toggle to turn off meter illumination, two 12V trigger sockets and a 20A IEC power inlet.
Interior. The interior is spacious with lots of free space around the tubes. The electronic circuit mounts on a single PCB located near the front. The audio signal from the XLR inputs runs via long cables to the front. These cables are three twisted lengths of stranded copper. They are long and vibrate hard. Moreover they run right between the two power tubes. I’d rather they were not there at all. The input buffer and gain stage are built on JFETs followed by a single Sovtek 6H30 dual triode per channel with rubber dampers to minimize microphonics. The 6H30 drives the output tubes.
The output stage consists of two Tung-Sol KT120 beam tetrodes per channel working in class AB push-pull. This is a combination of ultralinear and Audio Research’s patented ‘partially cathode-coupled’ topology claimed to be superior to conventional pentode or triode operation. The driver tubes sport gold-plated pins whilst the output tetrodes run with standard pins. The tubes arrive in a separate foam-lined box. To install them means opening the amp by removing the bent top and side plates. The bias needs to be adjusted manually but the adjustment points are conveniently accessed from the front. The bias adjustment range is shown on the meters.
The input JFETs are direct-coupled. Thus the only coupling capacitors in the signal path are between driver and output tubes. They look very solid. The manufacturer claims that these capacitors are a brand-new design developed specifically for their 40th anniversary preamplifier.
The solid-state power supply is located in the center between the gain circuits. It’s divided into three sections with separate secondary windings for JFETs, anode and filament voltage and another for bias voltage (-75V). What primarily draws our attention is a large capacitor bank for the anode voltage filtration. It consists of 12 Nichicon capacitors arranged in neat equal rows. Before them sits a very complex voltage regulator system for the input JFETs with many capacitors of different types and from different manufacturers. It’s worth noting that all rectifier diodes are bypassed with polypropylene capacitors to minimize diode switching noise.
The transformers mount at the rear with the mains transformer in the center flanked by an output transformer on either side. All iron doesn't mount directly to the bottom plate but instead to a large channel bar which additionally stiffens the rear enclosure section. The output transformers are small but as was once explained to me by Eva Manley, what matters most for push-pull is the quality of the transformer steel sheet and the precision of the windings. The enclosure is made of bent sheet metal and its top corners are reinforced with additional bars to form a sort of cage. The top panel forms a whole with the sides and is bent sheet metal with lots of small cooling slots. The unit sits on five rubber feet, four in the corners and the fifth in the center.
Technical specs from the maker Output power: 75 watts
Frequency response: at full power 7Hz - 60kHz (-3 dB), at 1W -0.7Hz - 75kHz
Input sensitivity: 1.4V (25dB to 8Ω)
Input impedance: 300
Absolute phase: not reversed (pin 2 = hot)
Negative feedback: approximately 15dB
Rise time: 10V/ms
Hum and noise: < 0.06 mV RMS (-112dB)
Dimensions: 483 x 222 x 495 (WxHxD)
Weight: 21.3 kg
