Does anyone know which transistors are used in the output stage of the Mark Levinson 29?
I want to check the bias settings in this amp.
Does anyone know where i can get a schematic.
Thanx
JP
I want to check the bias settings in this amp.
Does anyone know where i can get a schematic.
Thanx
JP
MJ15024/MJ15025.
But you should be able to read the TO3s on the funny heatsinks of the 29.
ML didn't make a mystery of the type of devices used.
Not on the 20, not on the 23, also not on the 27 and 29.
But you should be able to read the TO3s on the funny heatsinks of the 29.
ML didn't make a mystery of the type of devices used.
Not on the 20, not on the 23, also not on the 27 and 29.
The ML 29 is a regular class AB amplifier.
Because the heatsinks is only folded metal plating the bias is set just high enough to avoid cross-over distortion.
Put a DMM across the emitter resistor of any one of the 4 output devices and adjust between 25 and 35 mV.
Because the heatsinks is only folded metal plating the bias is set just high enough to avoid cross-over distortion.
Put a DMM across the emitter resistor of any one of the 4 output devices and adjust between 25 and 35 mV.
i have somewhere a note that you have a total bias current of the output stage arround 120mA???!!
I have the mark levison 29 schematic but the file is too much big for sending it by the forum, if you could give me your email I can send it to you, moreover I can help you for setting the bias.
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
Dudes,
it's much easier and faster to download the schematic from Jan Dupont's homepage, aka ACD
it's much easier and faster to download the schematic from Jan Dupont's homepage, aka ACD
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
unclejed613 is right, but mark levinson's amp often have more quiescent current in final bjt for running the amps with sonic performances similar to class A amps, moreover, the 29 power supplies is big for this.
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