Hi All,
I have a Rotel RA-971 mk2, when I measure it's DC offset on the speakers outputs I get 16.8mv for the left channel and 37mv on the right channel but on both the left and right channels my Fluke shows a "-" reading like the reading is in reverse polarity.
1. What is the reason for the reverse polarity reading?
2. How can I "balance" the DC offset readings between the left and right channels?
on the pcb there are only 2 bias adjustment pots
Thanks
I have a Rotel RA-971 mk2, when I measure it's DC offset on the speakers outputs I get 16.8mv for the left channel and 37mv on the right channel but on both the left and right channels my Fluke shows a "-" reading like the reading is in reverse polarity.
1. What is the reason for the reverse polarity reading?
2. How can I "balance" the DC offset readings between the left and right channels?
on the pcb there are only 2 bias adjustment pots
Thanks
These readings are fine, and there's no need to worry about them.
Polarity of the small output offset voltages has no significance.
Adjustments you see are for biasing the output stage, not for offset adjustment.
The circuit is self adjusting and has no offset adjustment (due to unity gain at DC),
like many other amplifiers.
Polarity of the small output offset voltages has no significance.
Adjustments you see are for biasing the output stage, not for offset adjustment.
The circuit is self adjusting and has no offset adjustment (due to unity gain at DC),
like many other amplifiers.
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Thanks.
I know that the pots are for bias adjustment, not DC offset, I was just wondering if the DC offset can be balanced?
Another question, in the srvice manual it's mentioned that the bias should be adjusted to 8mv, how high it's safe to go with the bias? is 15mv safe?
I know that the pots are for bias adjustment, not DC offset, I was just wondering if the DC offset can be balanced?
Another question, in the srvice manual it's mentioned that the bias should be adjusted to 8mv, how high it's safe to go with the bias? is 15mv safe?
The existing DC output offset voltages reflect the circuit design and internal components,
and cannot be easily improved. Small offsets such as these are not at all important, and
do not affect the performance of the amplifier.
The output stage bias should not be increased. Your suggestion would double the idle dissipation
in the output transistors, and could cause the amplifier components to run hotter and possibly fail.
and cannot be easily improved. Small offsets such as these are not at all important, and
do not affect the performance of the amplifier.
The output stage bias should not be increased. Your suggestion would double the idle dissipation
in the output transistors, and could cause the amplifier components to run hotter and possibly fail.
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My idea is to "force" the amplifier to "stay" longer in class A(correct me if I'm wrong), the amplifier is working with higher bias (about 13.5mv across the test points) for 30 mins now and it's not a lot hoter than before (if at all), I can still put my hand on the heatsink without get any burns😛
Of course you can turn the pots higher, but there's no way to know
how well this particular amplifier will tolerate it, either in the short term
or the long term. Even for just one watt class A operation, the bias current
would have to be set much, much higher (around 0.5A per polarity).
There are two paralleled output transistors per polarity in the output stage.
However, the design idle current of your amplifier is 2 x 0.008V/0.22R = 0.072A.
There's no guarantee that the junctions of the output transistors are at safe temperatures,
even if the heat sink does not burn you.
how well this particular amplifier will tolerate it, either in the short term
or the long term. Even for just one watt class A operation, the bias current
would have to be set much, much higher (around 0.5A per polarity).
There are two paralleled output transistors per polarity in the output stage.
However, the design idle current of your amplifier is 2 x 0.008V/0.22R = 0.072A.
There's no guarantee that the junctions of the output transistors are at safe temperatures,
even if the heat sink does not burn you.
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Around 7 times higher idle current per transistor, for 1W class A at 8 ohms.
So about 56mV bias voltage per transistor, instead of 8mV. This is not a suggestion, though.
BTW, there is for each output stage design, an optimum bias current for minimum distortion.
This is where the output impedance is the most linear. Higher bias current will then actually
increase distortion. This has to do with something called output stage transconductance doubling.
So about 56mV bias voltage per transistor, instead of 8mV. This is not a suggestion, though.
BTW, there is for each output stage design, an optimum bias current for minimum distortion.
This is where the output impedance is the most linear. Higher bias current will then actually
increase distortion. This has to do with something called output stage transconductance doubling.
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