VBITNGC Tube/IC hybrid poweramp

Written by Walter Geeraerts 2006.9.19

Introduction
The VBITNGC is a Tube/IC Hybrid Amplifier designed by Franz Gysi a fellow DIYAudio.com member. You can find the project antetype below:
T-network: the better feedback solution?
Projects for next hollidays (the world smallest VBITNGC??)
To make this project more accessible for other people I designed a single layer circuit board based on this schematic.

STEP BY STEP Assembly

Stuffing the board

As always start with the smallest parts first. In this particular case that’s J1, a small jumper wire that connects IC1 and C17 of the delay circuit to ground. After that you can proceed with all the other parts. I do suggest that you save R104 and R107 until last. The reason therefore is that they need to be placed on the solder side of the board. In case of R104 it’s very simple, there is just no room for it on the component side unless you want to cram it underneath the pins of the LM3875 chip. For R107 it’s possible that one hole will be blocked depending on the size of capacitor C101. To come back to R104 for a moment; you will notice that there are two positions foreseen on the board. In the normal position the resistor is placed between pin 7 of the LM3875 and the small power supply ground plane. In the alternative position the resistor is placed between pin 7 of the LM3875 and a separate GND connection. In case of the latter R104 can be connected to the small signal ground plane right in front of the tube base. This option maybe can improve the sound quality of the amp?

GND Connections

The image below shows how all the ground planes are connected to the High Current ground plane, this is done by placing some wires at the solder side of the board. Best is to, I know it’s a bit of a design flaw, drill out the holes to a diameter of 1,5mm so wires up to 1,5mm² (16AWG) can be used.

Power Supply Connections

This image shows you how we connect the power supply’s. Whether you do this on the solder side or the component side is entirely your choice. To avoid problems or malfunctioning of the amplifier I suggest that you measure the voltages and correct polarity of each individual power supply before placing these wires (better save than sorry).

The connections to be made are as follows:

HC+ of power supply to HC+ of amplifier circuits (left and right)

HC- of power supply to HC- of amplifier circuits (left and right)

LC+ of power supply to LC+ of tube buffer circuit (pin 6 of tube socket)

LC- of power supply to LC- of tube buffer circuit (connection between the two R102’s)

Transformer Connections

For this amplifier we need three different voltages. The high current stage can go up to 36VDC rails, the low current stage needs ±60VDC rails and finally the time delay circuit and tube heater which require 6,3VDC. I strongly suggest that one uses 100V capacitors in the low current stage, they will fit without any problem and have some margin over the 63V types. I used a 2x22VAC, wired in series, Talema transformer and measured (no load) rails of +/- 71.5VDC! Another important thing a want to emphasize, make sure that you twist all the wires that carry AC voltages to avoid hum.

1 2 3

For the HC current stage one can use a transformer with dual secondaries up to 2x25V. Connect one secondary to AC1-AC1 and the other one to AC2-AC2 (see fig 1) For the LC stage a transformer can be used in the range from 18V to 22V. First wire the two secondary windings in series. Next connect the two remaining wires to the 45VAC holes on the board (see fig 2). Finally the transformer for the time delay circuit and tube heater, this will be connected to the 7VAC holes on the board. I used a small 5VA transformer with a 6V secondary. Because of the 6V I used an LT1085 low drop voltage regulator, if you want to use an LM317 or equivalent because the LT’s are to expensive you will need a transformer with a higher secondary voltage. The LM’s need a 3V difference between the in- and output so the 6V won’t cut it.

VBITNGC PCB is available. The below button is for 2PCB order including shipping.

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undefined, November 14, 2006 8:38 AM