I have a technics se-a900s power amp and su-c800u preamp, at normal volume the amp gets HOT, took the cover off and felt the heatsink and it was too hot to touch, measured 60c on the heatsick is that normal with technics mos class AA? And it there something I can do?
It could be normal, 60°C is not extremely hot. What's the ambient temperature? Does it stand freely?
The fan should engage at high power output. Does that work?
The fan should engage at high power output. Does that work?
ClassAA is actually ClassAB, just little bias to lower the crossover distortion. Should not be hot if volume is down.
DC settings.
This amp has one chip on input for both channels, providing voltage gain, followed by discrete output stage as power pack with bjt drivers and mosfet outputs. Running at +/_ 50 volts.
There is complete service manual at hifiengine. I would just check if all the expected voltages at checkpoints are as in manual.
This amp has one chip on input for both channels, providing voltage gain, followed by discrete output stage as power pack with bjt drivers and mosfet outputs. Running at +/_ 50 volts.
There is complete service manual at hifiengine. I would just check if all the expected voltages at checkpoints are as in manual.
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@Kim666,
the “Class AA” seal has absolutely nothing to do with synchro bias, sliding bias or super A. Unfortunately, Adason is somewhat mistaken here. The technology of the “class AA” is more similar to the circuit principle presented by Dr. Sandman.
There are two amplifiers per channel, the so-called voltage amplifier and the current amplifier. Both work via a low-impedance bridge circuit to the load, both see the load, the loudspeaker. The trick is the following, the combination of a dynamic level shifter, a negative feedback and local, as well as a total global negative feedback, lead to an elegant bootstrap effect. Incidentally, this technique can also lead to a negative internal resistance, but this is only a side note.
The entire power amplifier therefore contains two amplifiers. The voltage amplifier virtually sees the load (the LS), but at the same time it also controls the voltage curve at this load. Its input voltage must follow exactly (proportionally increased by the amplification factor or k+1) due to an elementary principle, the principle of the vanishing input variable.
This part is a relatively standard EF push-pull output stage, with very low and constant power dissipation, its rails usually follow the level, its BIAS is constant and is usually set via a stink-normal VBE multiplier as bias voltage. It lies between 10mA and 50mA, depending on your taste.
This part is a relatively standard EF push-pull output stage, with very low and constant power dissipation, its rails usually follow the level, its BIAS is constant and is usually set via a stink-normal VBE multiplier as bias voltage. It lies between 10mA and 50mA, depending on your taste. Note that this small output stage does not have to provide the necessary load current, which is why it works 100% in operating class A. The entire negative feedback system ensures that the complex load of the loudspeaker is virtually transformed, it appears to the “voltage amplifier” to be n times higher impedance than it actually is. Depending on the explanation model, this last sentence may or may not be correct. But if it serves the simple understanding and is helpful to legitimize the class A stamp, a more exact (also mathematical derivation) is irrelevant. This also applies to Quad's “current dumper”, which can absolutely only be explained mathematically.
So be it!
The actual supplier of the required current is the current amplifier, it is a typical class B type, according to Douglas Self's diction, pay attention (because there is always confusion here), B is actually AB, depending on the perspective and definition adopted.
The current amplifier is also a normal push-pull output stage of the EF type. However, this amplifier is the worker, the slave. Its master is the small class A voltage amplifier, which controls everything. The slave is embedded in the bridge, a feedback system that is apparently more similar to the "Current Dumper" than the "Dr. Sandman network". In other words, in plain language, the combination of positive and negative feedback as well as the level shifters must be set up precisely, through their dimensioning, so that a) the bootstrap works and b) the entire system does not oscillate and c) all possible dynamic compensation currents remain extremely small. Dynamic means that this misery is frequency-dependent.
In short: the Matsushita company has made real high-end commercially successful and affordable for every customer. Class AA has no switching on and off or crossover distortion. Of course, only in practical terms. Theoretically, it is 100% distortion-free and completely independent of the load.
Now to your problem: If the heat sink reaches a temperature of 60°C in normal use at normal room volume, then internal compensating currents flow and under certain circumstances one or both channels oscillate.
No one on this forum will be able to diagnose the cause of this remotely. If so, then it is a fluke.
After three decades of flawless work, such a complex circuit system is bound to have weaknesses. My urgent advice is the following: find a specialist workshop that understands this technology 100% and have the wonderful amplifier professionally repaired there.
Or wait for a total failure!
All the best,
HBt.
the “Class AA” seal has absolutely nothing to do with synchro bias, sliding bias or super A. Unfortunately, Adason is somewhat mistaken here. The technology of the “class AA” is more similar to the circuit principle presented by Dr. Sandman.
There are two amplifiers per channel, the so-called voltage amplifier and the current amplifier. Both work via a low-impedance bridge circuit to the load, both see the load, the loudspeaker. The trick is the following, the combination of a dynamic level shifter, a negative feedback and local, as well as a total global negative feedback, lead to an elegant bootstrap effect. Incidentally, this technique can also lead to a negative internal resistance, but this is only a side note.
The entire power amplifier therefore contains two amplifiers. The voltage amplifier virtually sees the load (the LS), but at the same time it also controls the voltage curve at this load. Its input voltage must follow exactly (proportionally increased by the amplification factor or k+1) due to an elementary principle, the principle of the vanishing input variable.
This part is a relatively standard EF push-pull output stage, with very low and constant power dissipation, its rails usually follow the level, its BIAS is constant and is usually set via a stink-normal VBE multiplier as bias voltage. It lies between 10mA and 50mA, depending on your taste.
This part is a relatively standard EF push-pull output stage, with very low and constant power dissipation, its rails usually follow the level, its BIAS is constant and is usually set via a stink-normal VBE multiplier as bias voltage. It lies between 10mA and 50mA, depending on your taste. Note that this small output stage does not have to provide the necessary load current, which is why it works 100% in operating class A. The entire negative feedback system ensures that the complex load of the loudspeaker is virtually transformed, it appears to the “voltage amplifier” to be n times higher impedance than it actually is. Depending on the explanation model, this last sentence may or may not be correct. But if it serves the simple understanding and is helpful to legitimize the class A stamp, a more exact (also mathematical derivation) is irrelevant. This also applies to Quad's “current dumper”, which can absolutely only be explained mathematically.
So be it!
The actual supplier of the required current is the current amplifier, it is a typical class B type, according to Douglas Self's diction, pay attention (because there is always confusion here), B is actually AB, depending on the perspective and definition adopted.
The current amplifier is also a normal push-pull output stage of the EF type. However, this amplifier is the worker, the slave. Its master is the small class A voltage amplifier, which controls everything. The slave is embedded in the bridge, a feedback system that is apparently more similar to the "Current Dumper" than the "Dr. Sandman network". In other words, in plain language, the combination of positive and negative feedback as well as the level shifters must be set up precisely, through their dimensioning, so that a) the bootstrap works and b) the entire system does not oscillate and c) all possible dynamic compensation currents remain extremely small. Dynamic means that this misery is frequency-dependent.
In short: the Matsushita company has made real high-end commercially successful and affordable for every customer. Class AA has no switching on and off or crossover distortion. Of course, only in practical terms. Theoretically, it is 100% distortion-free and completely independent of the load.
Now to your problem: If the heat sink reaches a temperature of 60°C in normal use at normal room volume, then internal compensating currents flow and under certain circumstances one or both channels oscillate.
No one on this forum will be able to diagnose the cause of this remotely. If so, then it is a fluke.
After three decades of flawless work, such a complex circuit system is bound to have weaknesses. My urgent advice is the following: find a specialist workshop that understands this technology 100% and have the wonderful amplifier professionally repaired there.
Or wait for a total failure!
All the best,
HBt.
That's absolutely correct.
The AN7062N IC basically contains the famous input circuit of the SE-A100...
followed by the output stage, which is in the RSN6000B V-MOS source follower in combination with 2SC3311airta & 2SA1309 airta BJT's.
That's the voltage amplifier. The current amplifier is in the SVI3205B hybrid block and is definitely not a V-MOS, but a BJT EF blaster - but, I'm not really sure whether there isn't a MOS-FET source follower power amplifier in there.
If that is the case (I think it was!), then this hybrid block will unfortunately have a constant power dissipation of about 2 * 10W, but that will never heat the heatsink to 60°C.
HBt.
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Sorry that i was wrong sbout sliding bias. Have to read about it more. I used to have Technics receiver, long time ago, with classAA output, pretty close to top of the line, with great spectrum analyzer, parametric eq, digital delay and so on. My brother is still using it.
Here is power output.
Here is power output.
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