Hi,
i am planning to do a 5.1 hometheater.i am attaching my schematics for 5 channels.please verify my schematic.also if you can please explain to me the kind of feedback used in the topology.
expecting your valuble suggestions
Hello Vishnu,
U3C, R19 and C9 comprise a DC servo/filter loop to remove any DC content at the speaker terminals. R19 and C9 form a low pass filter and integrator that apply the average DC value of the output to the non-inverting (reference) input of U3. Keep in mind that a large, continuous application of a non-symmetrical input signal can cause the DC value to shift in that direction (but this is usually not a big issue).
These DC servo loops are often used around audio power amplifiers since they can have high DC offsets, and multiplied by the closed-loop gain can result in several hundred mV to volts across the speaker terminals. The loop is used to null out this DC offset.
This way the audio signal chain can still be DC coupled (for "tight bass") in the audio band, but the loop time constant is set to sub-audio frequencies as to not interfere with low bass frequencies. The loop will slowly correct any DC offset on the speaker terminals without affecting the audio.
The LM3876 has a max spec of 15mV, and at a gain of 20, that could be 300mV of DC across the speaker coils (0.3VX8ohms=38mA=11mW). That is just from the amplifier - if the preceding stages add offset, then the offset at the speaker terminals could be into the volts - and now you are talking some serious power across the voice coil.
However - if you do use this circuit, you really should have a protection relay on the speaker output. At power up (and power-down), the voltage on the input signal and the charge on C9 is largely indeterminate - and can cause large "thumps" that can damage the speaker (or just be annoying at minimum). The output could "wander" several volts at turn on until the loop settles.
The relay should engage when the detected DC output is below a threshold (usually 50mV). Most of the speaker relays on consumer/pro equipment independently monitor the DC content of the speaker output and are not just a simple time delay. If the DC content is too high (at any time), the relay opens.
The LM3876's internal "anti-thump" circuit will not protect you against this since it is caused by the external circuitry and loop time constant.
If this is all too much for you...then you can simplify things and just AC couple the LM3876 (Datasheet Figure 1). This puts the power amp in a DC gain of one, but an ac gain of 20.
Eliminate the servo loop (C9, R19, U3C) and connect U3D pin 14 to R13 and place a 22uF cap between R14 and ground.
The input network you have between J5 and U3D breaks any DC coupling - so the tight DC coupling and fancy servo loop around the 3876 circuit is largely nullified (pardon the pun).
Also, R20, a 10 ohm resistor, will attenuate your output quite a bit if you use a 4-8 ohm speaker...you may want to make that a 0 ohm...
Paul Grohe
Integrated Signal Chain Applications, SVA
Thank you very much for you great suggestions paul,
so that feed back is actually a DC servo feedback right.will this make the offset to zero,i mean will i have to adjust the value of R19 and C9 for that....is there any equation for that?
yes i understand what you are saying of the need for protection let me try to add a circuit for the protection of speakers..
ohh Then most of the amps do have an output resistor in parallel with an inductor.i think i missed the inductor there.i will add and post my circuit over here again
Good day sir!
May I ask some questions? I'm having a trouble in understanding the servo-loop test method. My problem is how to test an op-amp particularly the Output Voltage Swing using servo-loop test method for work bench and ATE set-up. I'm clueless about this thing. Please help me. Thank you in advance.