Hello,
I have built a circuit, which consists of 2x Parallel LM3886 Amplifier IC's, I have done this using the application note "AN-1192 Overture™ Series High Power Solutions". My circuit specifically follows the circuit on page 21, which uses the non-inverting DC servo amplifier schematic. I have constructed 2 of these circuits and tied them together in parallel, which effectively is identical to the top half of the bridge circuit on page 20.
I have followed all of the application note and used all components recommended (some identical to) that of the app note. I am using a bench PSU with +/- 30Vdc to power my circuit as it's currently in the development stage and have the circuit set-up with a gain of 11.
My understanding is the DC servo circuit is there to effectively achieve zero DC offset at each amplifier output. When measuring the DC offsets of each circuit (inputs grounded), on IC1 there is 0V DC offset, but on IC2 there is 1mV DC offset.
Q1. Is a difference of as low as 1mV in DC offset between my circuits (even with the identical DC servo circuit in each feedback loop) enough to cause current flow problems between the outputs of my amplifiers?
Observation 1: When I don't drive the input, leaving it open (with no input signal) and power the circuit with no load attached to the output of the amplifiers, apart from the small resistors recommended in the application note, the current draw from the 'idle' circuit is around ~110mA per rail, which is OK. The power supply sits happily in idle.
Observation 2: When I drive the input with a 1kHz sine wave at 1V RMS and power the circuit again with no load attached to the output of the amplifiers, apart from the small resistors recommended in the application note, the circuit sits happily with the desired output (approx. 11V RMS). The current draw when the circuit is turned on is the same as the 'idle' situation above from both power supply rails. But, this lasts for a 5-10 seconds until current starts leaking somewhere and the current draw on each power rail starts to increase linearly with time, approximately 10mA every 3-4 seconds, the current draw keeps increasing steadily and does not stop!
Observation 3. If I add any load, the circuit doesn't like it, the power supplies draw a lot of current instantly on turn-on, probably the amps fighting again.
Q2. Does anyone know what would cause the current consumption to increase linearly with time as I am seeing in Observation 2 (with no load!) ?
Sorry if there is lack of information, this is my first post on TI, I just wanted to explain this problem happening with my circuit, and to see if anyone has an answer as to why they think something like this would happen?
Thank you in advance for any answers.
Nick
