Monday, March 28, 2016

Alum Rock Technology 304 TL stereo amplifier


The 304TL stereo amplifier shown on this site is optimized for use with 8 to 16 ohm high-efficiency speakers and has the following features.

A classic audio channel, in which half of a 6SN7GT dual triode drives a 304TL through an interstage transformer designed specifically for the 6SN7. With the 6SN7's 7,700 ohms plate resistance, the unloaded output of this interstage transformer is typically flat +/- 1/2 dB from 20 to 90,000 Hz. The amplifier uses no feedback.

All transformers are manufactured to the highest standards of quality, using typical military-standard procedures. Visible transformers are styled with end-bells and have similar appearance to the Stancor, Triad, Thordarson or UTC units of the past. However, the output and the plate transformer end-bells are potted. (Detailed photo of an unpainted output transformer.) Transformers and inductors mounted below chassis use heavy machined aluminum frames and specially designed heavy-duty terminal strips.

Direct current 10 V @ 26 A filament supply with choke-input filter.

1,000 VDC @ 300 mA high voltage power supply with:

866A mercury vapor rectifiers.
Choke-input filter, three LC sections, and oil-filled capacitors.

125 uF capacitor in third section. This capacitor stores 62.5 joules.

Autotransformer control of both filament and plate voltage. This allows the user to warm the filaments slowly and to operate the amplifier at reduced plate voltage and dissipation if desired. A mechanical interlock between the autotransformers allows the knobs to be rotated only in the correct sequence. Operation is fail-safe in the event of a power interruption.

Front-mounted bias controls and plate current meters. This, together with control over the plate voltage, allows the operator to choose the quiescent operating point of the 304TLs.

12 gauge steel chassis, with extensive use of machined aluminum plates and mountings.

3/8 inch thick borosilicate (Pyrex) glass cover.

Gain control and three inputs via pairs of XLR connectors, wired for unbalanced input in the Ampex convention. (Pin 2 is signal ground, pin 3 is signal.)

Convection cooling and quiet operation, with attention paid to audible hum which can be mechanically generated by power components.

Frequency response +/- 1/2 dB from 21 to 20,000 Hz.

Output and damping capabilities:

50 Vpp maximum.
3 ohms effective source impedance.
30 W/channel at 500 Hz and 10 W/channel at 20 Hz into 8 ohms at the 5% total harmonic distortion level.

Voltage gain (no load) 10.

Hum and noise less than 2 mVrms.

Heat dissipation approximately 300 W plus 304TL plate dissipation, which is determined by the quiescent operation point chosen and is 300 W maximum.

Case temperature measurements:

At 100 mA plate current: 38° C temperature rise above ambient (24° C) at the hottest spot, which is the portion of the left side panel directly opposite the left 304TL.

At 150 mA: 41° C temperature rise at the same hottest spot. Typical case temperature rise at this current (at places such as the front of the chassis) is approximately 16° C.

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