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Chip Amps Amplifiers based on integrated circuits

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Old 6th April 2008, 05:13 PM   #11
CJ900RR is offline CJ900RR  Sweden
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What do you say PA and AndrewT. Is the Zobel ok now?
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Old 6th April 2008, 05:19 PM   #12
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From what I understand, since the board uses the dc servos, wouldn't that eliminate the need for the zobel anyway? My understanding was that both were to prevent oscillation in the circuits.
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Old 6th April 2008, 06:26 PM   #13
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zobels and servos have two entirely different functions.

The zobel gives the amplifier a proper load at higher frequencies to prevent oscillations which might damage it.

The servo replaces the capacitor in the feedback loop normally used to imit DC-gain to unity. Thus working at very low frequencies.

Otherwise amplifier gain * DC input offset might give several tens of volts DC on the output which can give severe problem with parallelled chips if not using large output resistors.

It is possible to trim output offset by injecting a small current into one of the inputs of the amplifier but offset drifts with time and temperature... the servo is always adjusting DC-offset to within the limits of the OPs own offset which is neglible in this application.
Also, the cost of low offset op good enough for this application is not very large (a few at most for the amplifier) and neither does they demand a lot of PCB real estate.

Large output resistors reduces efficiency, creates a heat source on the PCB and is also negative as the Q-value of the loudspeaker is reduced (less "coupling" or "control") , it is the same effect as having a very long loudspeaker cable.

so both zobel and DC-servo is essential for the amplifier to operate safely.
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listen! Can you hear that ...
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Old 7th April 2008, 05:57 PM   #14
CJ900RR is offline CJ900RR  Sweden
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Well one channel is up and running since 1 hour. Measured DC-offset with input to ground: 1.7mV, 0.4mV and 0.7mV. The servo-circuits does its job. No hum hizz or other bad sounds. Has been playing on a volume-level of 60% ca and the LM3886 does not even get hot. Supply voltage is about +/-32V. No Zobel is present what so ever (oh im a daredevil )

Two images, yet again sorry for bad mobile-shoots.

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Old 7th April 2008, 06:04 PM   #15
CJ900RR is offline CJ900RR  Sweden
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One more, and yes I know main wiring is dangerus...

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Old 8th May 2008, 08:39 AM   #16
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I am interested in buying also.
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Old 10th May 2008, 12:00 AM   #17
giufo79 is offline giufo79  Italy
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Me too, at least 4 pieces
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Old 10th May 2008, 09:40 AM   #18
CJ900RR is offline CJ900RR  Sweden
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Well Im going to place a order soon. Im going to Romania in buisness monday to friday so next weekend I can tell you more.
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Old 10th May 2008, 03:43 PM   #19
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Hello Calle

This is a nice board layout and I am interested in buying some (depending on the price).

What happened with your other design (using 6 LM3886)?

Cheers!
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Old 4th June 2008, 10:12 PM   #20
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Just checking...
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