FCFIFRAGA%CIUC2.UC.RCCN.PT@CUNYVM.CUNY.EDU (FRANCISCO AMARAL F FRAGA) (09/27/90)
>Now the question is: has anybody used their class A amp in >conjunction with a variac? Does it have any unwanted audible >side-effects? Also, a recent poster noted that he couldn't tell >the difference when his amp had preheated half-an-hour versus a >whole day. Has anybody else performed a similar and would like >to comment? Please DON'T use a variac to simulate a idle mode on any amplifier without having the manufacturer advice first. It can heavily damage the amplifier. The main power dissipation in amplifiers comes from the bias current for the output devices. In order to reduce heat and power consumption, some amplifiers have provision to reduce the bias current, or even switching off the power for the output devices. Just to mention a few names some Audio Research and Beard valve power amplifiers had the possibility of even operating with reduced current and Roland Research amplifiers shut off the power devices in idle mode. If you reduce the mains voltage to your amplifier some very nasty things can happen : electrolytic capacitors, semiconductors and valves could be reverse polarized, due to the absence of current from non biased active devices capacitors can become over polarized exceeding their rated limits. I remember reading in an old Philips tube book that operating filaments under or over the specified voltage would affect tube reliability. Something I hate in amplifiers is preheating. Some designs only sound good when you are needing to switch them off. I had an Electrocompaniet power amplifier (quite nice sound) and one of the reasons why I exchanged was the fact that it needed about two hours heating before being audible. Some Krell amplifiers need about six hours before reaching their best, Roland Research amplifiers sound good after one hour heating if left on the idle mode. Quad amplifiers, specially the 405, also benefits from some preheating. (These are some amplifiers I am used to listen to in some friends systems.) Tube amplifiers need some preheating too, but usually less time then solid state ones,(perhaps it is due to the absence of dissipating fins which take a longer time to warm up?), and do not sound so badly just after being switched on. -------------------------------------------------------- Francisco A. F. Fraga Dept. Physics Univ. Coimbra 3000 Coimbra PORTUGAL tel. 351-39-34668 fax 351-39-29158 FCFIFRAGA@CIUC2.UC.RCCN.PT