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To understand
JE Audio’s balanced power amplifiers VM60 (and VS70) better, I asked designer John Lam for further clarification. "We employ the Dual Balanced Feedback Topology (DBFT) as shown in these basic block diagrams. Full specifications of DBFT can be found in US patent 7304535. The key advantages of DBFT are:
1/ Two pairs of feedback loops are made available.
2/ There are many possible combinations for adjusting the two pairs of feedback loops to obtain the desirable amplifying conditions. Hence an optimum combination can be employed for setting the two feedback loops. In conventional balanced amplifier using just one pair of feedback loops, a designer has only one way to apply feedback for fine-tuning the power amplifier's performance. However it is very often found that either too much or too little feedback is used in such conventional balanced amplifiers.
"With DBFT however there are a number of ways to adjust the two pairs of feedback loops. Thus DBFT makes possible the design of a balanced amplifier with as many desirable features as needed for power amplifier application.
There is one shortcoming though. While DBFT amplifies balanced signals perfectly, it cannot correct unbalanced DC biasing induced by mismatched vacuum tubes or tube ageing. Since the first 3 triodes are direct-coupled, the DC plate voltage of one device will determine the DC biasing condition of the other. Then again the DC plate voltage of that part will determine the DC biasing condition of the following. In other words, a slight mismatch between the two input stage triodes will create a large DC level difference at the plates of the other triodes. As a result the balanced condition cannot be maintained.
"To overcome this shortcoming, we employ a patented DC self-biased differential amplifier. Full specifications of this DC self-biased differential amplifier can be found in US patent 7733172. To put it in the simplest way, the DC self-biased differential amplifier detects the DC levels at its input (i.e. the grid voltages). If the two DC voltages are different, it corrects them by readjusting the cathode voltages. As a result the DC voltages of grid to cathode and hence DC voltages of the plates are very closely maintained. By implementing the newly invented DC self-biased differential amplifier for DBFT, the balanced amplifier can truly amplify balanced signals whilst maintaining balanced DC biasing conditions at the same time."
