Dear All,

We have 15 channel class AB type amplifier on single PCB. All these channels generated with same schematic with top sheet and room layout. 14 Channel works fine with calculated gain but 1 channel has a low gain according to others. We have try many things but we could not identify problem. Except Input and output of the channels, all the remaining traces are the same, we have exluce traces and made it through directly input components and very short cable with output but nothing changed. Do you have any idea for the layout problem? Please find schematic and signal at requested gain (bad signal) as attached.

 

What are the Voltages at Q2 Collector and Emmiter, and R8,9 junction?

 

What are the Voltages at Q2 Collector and Emmiter, and R8,9 junction?

Voltages are; 24V@R8 & R9 Junction, 25.2V@Q2collector, 22.8V@Q2emitter

 

Then i would suggest swapping the op amplifier.

 

Is the gain to the junction of R8 and R9 also low or does it match the other channels?

 

Where are you measuring the output...........secondary of T1?

Why is the frequency of one output........about double the frequency of the other?

 

Is the gain to the junction of R8 and R9 also low or does it match the other channels?

They are also low and different from other channels

 

Where are you measuring the output...........secondary of T1?

Why is the frequency of one output........about double the frequency of the other?

Secondary of T1 is connected to Capacitive and resistive loads, and yes I am measuring at that point.
Frequency is the same but since its shape is not sine oscilloscope could not get the right frequency measurement.

 

They are also low and different from other channels

Is it still low if you disconnect the transformer from the output?

 

Is it still low if you disconnect the transformer from the output?

No, it is same with the other channels. By disconnecitng transformer also I am disconnecting resistive and capacitive load.

 

As was said in the other forum, your problem is either a parasitic or a layout problem. Without a layout drawing, there is no way that we can help you.

 

As was said in the other forum, your problem is either a parasitic or a layout problem. Without a layout drawing, there is no way that we can help you.

Please find attached

 

We have solved the oscillation problem. The reason was "Ferroresonance" related with low output capacitance. When we changed 220nF to 10uF and add serial 5 ohm resistance oscillation has gone. But the problematic channel still insists on low gain, now it is not oscillates but distorts. I am suspicious about layout but, I could not figure out how.

Please find Analog Inputs, Transformer Outputs and Opamp -IN and OUT connections below. Transfomer OUT goes under the OUT pin of opamp but differen layer, is it problem? What measures should we take for opamp layout?
Green traces-Analog Inputs
Light Blue traces-Transformer outputs
Yellow Rectangle-Problematic Channel

 

We have solved the oscillation problem. The reason was "Ferroresonance" related with low output capacitance. When we changed 220nF to 10uF and add serial 5 ohm resistance oscillation has gone. But the problematic channel still insists on low gain, now it is not oscillates but distorts. I am suspicious about layout but, I could not figure out how.

Please find Analog Inputs, Transformer Outputs and Opamp -IN and OUT connections below. Transfomer OUT goes under the OUT pin of opamp but differen layer, is it problem? What measures should we take for opamp layout?
Green traces-Analog Inputs
Light Blue traces-Transformer outputs
Yellow Rectangle-Problematic Channel
View attachment 125104

With no power on board Measure input and output resistance of good channel then compare to channel in question.