|   What does tube bias setting do sonically ... | jshk
Dec 3, 2002 2:47 PM | | hello to all tube fan& expert ,
As I switch from solid state amp(McIntouch 2727?) to tube amp(VTLST-150) I notice a tighter bass & clear bass sound,however, also a lost of high frequency (roll off) .My question is can the proper bias setting change that (roll off) Any suggestion or tube recommmendation are welcome (if you are familiar with the VTL tube they use >
http://www.vtl.com/pages/st150specifications.html
Many Many Thanks in advance jshk |
| re: What does tube bias setting do sonically ... | skeptic
Dec 28, 2002 7:36 AM | | Bias voltage whether for a transistor or vacuum tube amplifier adjusts the "quiescent" (no signal present) operating point of an "active" device. For most of the devices in an amplifier, preamplifier, or tuner, these voltages are fixed. But, in a class AB power amplifier, there is an adjustment to match the two or more devices so that those handling the positive going part of each cycle match as closely as possible with the those handling the negative going half. The result of adjusting this correctly is the least distortion, not a change in frequency response. In fact, a poorly adjusted bias setting results in a characteristic phenomena called crossover notch distortion becuase a pure sine wave as seen on an oscilloscope shows a notchlike anomoly where the signal crosses over from positive to negative. Some people hear this subjectively as a brighter sound but it is actually annoying and can become extremely irritating after extended listening. Early transistor amplifiers were particularly subject to this type of distortion because of the lack of familiarity and skill many engineers had with transistors and the inherent nonlinearity of bipolar power transistors in the bias region where the crossover in polarity occurs. Later more clever designs and MOSFET power transistors reduced this defect substantially.
The loss of high end in a vacuum tube power amplifier is usually due to the limitations of the output transformer IMO. Some people consider these amplifiers more musical especially when used with their screechy overly bright modern loudspeakers. The pros and cons, merits and demerits, advantages and disadvantages of each type are debated endlessly on boards like this and elsewhere with each side having their partisans, sometimes reaching almost religious fervor in their invective for their preference and against the type they don't like. In fact, neither transistors nor vacuum tubes themselves have any characteristic sound what soever, in most cases, both having dead flat frequency responses across the entire audio spectrum. Other factors including the power supply, quirks of the topology, quirks of feedback circuits, and the output transformer if there is one define the limitations and therefore characteristic sound. (It's amazing how many expensive amplifiers are badly designed.) |
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