Please send me the mark levison 29 schematic also.
My email address is askchan@netvigator.com
best regard
My email address is askchan@netvigator.com
best regard
attach a DMM across one of the .22/5W resistors (one of them appears to be R67 or R87) and adjust R72 for about 5mV.
quickie way to adjust for proper bias: (be sure you know exactly what you are doing here before continuing)
1)put shorting plug on input
2) attach an oscilloscope to the output
3) attach a 10 ohm 20W resistor to the output hot
4) attach the other end of the resistor to another amplifier
5) input a 1khz sine wave to the other amp, and adjust the other amp for 10V p-p output
6) observe crossover spikes on the oscope with the oscope set for 1mV/division
7) adjust bias until the spikes just barely disappear
at first glance, one would think "but i'm DRIVING the output stage, and that's a no-no", but you're doing this through a 10 ohm load, very similar to bridging an amp. plus the fact that you're only applying 10V, just enough to get a bit past the crossover region. the amp's output impedance is higher in the crossover region (when underbiased), which is why you get the spikes. once the bias is adjusted properly, the output impedance becomes constant, and the spikes go away. a side benefit of this is that you now have a "direct reading" method of measuring the output impedance of an amp, since this setup reads out 1mV/milliohm on the scope, so a 5mv wave on the scope would mean that your amp has a 5 milliohm output impedance. also, before starting, you want to be sure that your amp used as the signal source also has it's bias adjusted properly
before performing this test on an ML amp, or any other high value equipment, i recommend that you learn and get used to the procedure on an amp you can afford to make your mistakes on, since ANY method of adjusting bias can go wrong, resulting in leakage of magic blue smoke out of several components
quickie way to adjust for proper bias: (be sure you know exactly what you are doing here before continuing)
1)put shorting plug on input
2) attach an oscilloscope to the output
3) attach a 10 ohm 20W resistor to the output hot
4) attach the other end of the resistor to another amplifier
5) input a 1khz sine wave to the other amp, and adjust the other amp for 10V p-p output
6) observe crossover spikes on the oscope with the oscope set for 1mV/division
7) adjust bias until the spikes just barely disappear
at first glance, one would think "but i'm DRIVING the output stage, and that's a no-no", but you're doing this through a 10 ohm load, very similar to bridging an amp. plus the fact that you're only applying 10V, just enough to get a bit past the crossover region. the amp's output impedance is higher in the crossover region (when underbiased), which is why you get the spikes. once the bias is adjusted properly, the output impedance becomes constant, and the spikes go away. a side benefit of this is that you now have a "direct reading" method of measuring the output impedance of an amp, since this setup reads out 1mV/milliohm on the scope, so a 5mv wave on the scope would mean that your amp has a 5 milliohm output impedance. also, before starting, you want to be sure that your amp used as the signal source also has it's bias adjusted properly
before performing this test on an ML amp, or any other high value equipment, i recommend that you learn and get used to the procedure on an amp you can afford to make your mistakes on, since ANY method of adjusting bias can go wrong, resulting in leakage of magic blue smoke out of several components
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