Q: How is the Dual Balanced Feedback Topology (DBFT) being used in this power amplifier different from the conventional design?
A: A fully balanced amplifier will cancel out the noises and distortions picked up by the audio cables caused by electromagnetic interferences. The DBFT is a patented design that allows a vacuum tube amplifier to be operated in a fully balanced condition, from the input stage to the output transformer. The dual feedback pairs operate to improve the linearity and balancing of every stage in the power amplifier by using only small amount of feedback. These cannot be achieved by using a single pair of feedback found in the conventional design.
Q: Is the power amplifier VM60 phase non-inverting?
A: Yes. The output signals at the loudspeaker binding posts (+VE and –VE) are in phase with respect to the input signals at the XLR receptacle. The pin configuration of the XLR receptacle is given as follows: pin-1 is ground; pin-2 is +VE and pin-3 is –VE signals.
Q: What is the break-in time for a pair of new VM60 amplifiers?
A: The VM60 has been gone through a mild break-in at the factory. When you power it up the first time, it should sound reasonably well. But it may take about 50 to 100 hours before it completely breaks in. However, it is not advised to turn on the amplifier continuously all day. It is recommended to turn it off for one hour after it is operated for 3 to 4 hours.
Q: Do I need to bias the tubes of the amplifier?
A: It is not needed for the VM60. The VM60 power amplifier is auto-biased so that optimum operation condition for the power tubes can be achieved. This will reduce net DC current flowing to the output transformer and, therefore, prevent the transformer from early saturation and improve low frequency response.
Q: Can I substitute a tube in VM60 by a same type but different brand?
A: When substituting KT88s, do not just substitute one KT88 because four KT88s are matched tubes. If you want to get the best results, when substituting the KT88s replace all four pieces by four matched ones. For the small signal tube 12AX7, it is recommended that you only use the same tube purchased from JE AUDIO. Because the 2 x 12AX7 and 1 x 6DJ8 tubes are connected in a direct coupled fashion so as to minimize phase shift and to enhance low frequency performance. The first 12AX7 is a critical tube in setting up the right operating voltages for the second 12AX7 and 6DJ8 tubes. JE AUDIO only selects the best 12AX7 tubes after over at least 50 hours of burn-in and extensive testing in search of good balancing.
Q: Can I swap the same type of tubes within the VM60?
A: It is best not to swap the four KT88 by positions. Since they are matched tubes, changing the positions will not enhance the operation or sound quality. It is not recommended to swap the two 12AX7. The two 12AX7 have been tested so that they work best in the current positions in terms of noise level and operating voltages.