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Old 2nd November 2011, 01:48 AM   #1
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Default Proper way to balance PA150?

I have built quite a lot of 1875, 3875, and 3886 amplifiers for various things, but now i am building a "PA150" (Shine 7 type) for driving the woofers on a biamp speaker project.

anyways, is there a "proper" way to balance the output of each chip?
I am not sure if i should wait to install the output resistor on each one, and try and get them as close as possible then connect the resistors
or scope the difference between the outputs with the resistors installed and tweak them.

what about later down the road, after everything has had a while to settle in, i dont want to be disconnecting and reconnecting the output resistors all the time.

I think it would be a lot easier with 2 amps than 3!
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Old 2nd November 2011, 09:57 AM   #2
bcmbob is offline bcmbob  United States
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Take a look at this Post. There are several posts on the subject in this thread.



BPA300 Round 2



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Old 2nd November 2011, 04:06 PM   #3
AndrewT is offline AndrewT  Scotland
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I think (I have never built a BA) that one must remove the offset of each individual chipamp before adding in the coupling resistors.

One end of these resistors could be attached to a copper wire post to make it much easier to disconnect & reconnect of necessary.

I fear for the idea of setting output offset when fully warmed up.
I further fear for changes in output offset when Tc/Tj changes either due to weather or to current operating condition.

I have stated my preference for setting up a DC servo on a simple single channel amplifier. I'll resume here: disable the Servo. Set the offset cold to zero mVdc. Check again when completely cold and adjust if necessary.
Allow to warm up, monitor and plot the changing output offset.
Cool down. Enable servo. Starting from cold, monitor and plot the output offset.
Compare before and after.

Can you see the similarity here?
If the offsets were to be quite different at cold start up, the paralleled chipamps may not tolerate that imbalance and go wrong every time one starts up the amp from cold.
No matter how good the warm or hot setting up of offset is, the cold performance will determine the start up condition..
I think the output offsets must be set to be near identical and as near zero as possible when cold. This allows balanced start up.
If the offsets of each amp were monitored and plotted, one could compare the trend of each to it's partners. If they differ significantly reject the worst. Alternatively servo the warm up offset adjustment.
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Last edited by AndrewT; 2nd November 2011 at 04:08 PM.
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Old 2nd November 2011, 04:56 PM   #4
bcmbob is offline bcmbob  United States
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Andrew, I built two BPA-150 amps that are working but I never could get the DC offset lower than 11/17 with a bunch of interactive adjustments. Your described method is something I would like to try. Could you provide a diagram and/or link to a servo unit appropriate for the process?
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Old 2nd November 2011, 05:55 PM   #5
AndrewT is offline AndrewT  Scotland
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the servo method is in the datasheet or ap note.
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Old 2nd November 2011, 07:17 PM   #6
bcmbob is offline bcmbob  United States
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You lost me. Sorry to be so dense, but datasheet or ap note for what item?
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Old 2nd November 2011, 07:20 PM   #7
AndrewT is offline AndrewT  Scotland
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PA 3886 servo is in the National literature for PA applications. I can't recall if it's a datasheet or an ap note.
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Old 2nd November 2011, 08:13 PM   #8
bcmbob is offline bcmbob  United States
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Thanks !
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