5687 Tube Hi-Fi Preamplifier (LED biased)
Introduction: DIY 5687 Vacuum Tube Hi-Fi Preamplifier (LED Biased)
This is a Hi-Fi line-level preamlifier project that I recently completed using the 5687 twin triode. This idea for this preamplifier started off as a transformer volume controller, evolved to a DHT preamp, and finally to a indirectly heated preamp with an audio output transformer. The 5687 tube preamplifier circuit uses LED biasing and Tamura A4714 audio output transformers.
5687 Tube Preamplifier Circuit
The preamplifier uses the double section of a 5687 tube for each channel, with each section using it's own grid stopper (220R) and it's own pair of green LED for a 4V bias. The double section is used so as to lower the effective tube impedance, resulting in better compatibility with the Tamura A4717 5k output transformers. The 5687 tubes are operated at 115V, -4V and 25mA, which is one of the more linear regions for a 5687. Initially, I tried AC for the filaments and the result was far too much hum, so DC is used for the heaters. The schematic of the preamplifier section is shown in Figure 01.
Figure 01 - 5687 Vacuum Tube Hi-Fi Preamplifier Schematic with LED Biased
Photograph 01 - 5687 Vacuum Tube Hi-Fi Preamplifier
Power Supply with RCA-83 Mercury Rectifier Tube
The power supply uses one RCA-83 mercury rectifier tube. An Omron timer is used to delay the B+ in order to allow the rectifier to warm up. The last 2 RC sections (3k6, 220uF, 3k9, 10uF) are separate for each channel, but in future I plan to remove that and use a CCS for each channel instead. The 10uF capacitor is polystyrene. Yes, those are 50H chokes!
The RCA-83 rectifier uses 5VAC for the filaments. The heaters for the 5687 tubes are provided from a full bridge of MUR860 diodes, followed by 5 x 10,000uF Elna caps. 830mA of current regulation is supplied using a LM317 regulator and a 1R5 resistor between ADJ and VOUT pins. Votlage to the tubes is about 11.5V, so I am actually running starved filaments. For more information about tube power supply design, see Matt's infomative article Power Supply Design for Vacuum Tube Amplifiers.
The schematic for the power supply is shown in Figure 02.
Figure 02 - 5687 Tube Preamplifier Power Supply Schematic
Photograph 02 - 5687 Tube Preamplifier Power Supply
DIY Enclosure Construction
The width of the chassis is determined by the width of the output transformer as that was the largest component I needed to incorporate into the chassis. Cut and drill all parts of the chassis to size, drill holes for RCA plugs and others.
Photograph 03 - Cut and Drilled Enclosure Parts
Next, I inserted the wall plugs into the painted MDF boards so that self tapping screws could be used to secure sides of the chassis. The panels were painted except the top panel which was sanded down and left as natural aluminium.
Photograph 04 - Painted Enclosure Parts
Next, I assembled the bottom to the MDF sides. Start with one side, proceed with the other and then assemble the top panel. The side panel can be screwed on followed by RCA plugs or other mountings.
Photograph 05 - Partially Finished 5687 Preamp Enclosure
The power supply portion of the enclosure was built on a Hammond chassis meant for another forgotten project. The finished preamplifier and power supply are shown below.
Photograph 06 - Finished 5687 Preamplifier and RCA-83 Power Supply