AMP9-Basic output DC offset voltage
The AI power up instructions make no mention of this, but powering up an AMP9-Basic with the unmute jumper in place defeats the output DC offset null comparator.
Your DC offset voltage will be around 100mV on each channel.
This is quite high and not really acceptable.
You can use the unmute jumper to null the DC offset voltage to below 10mV.
What this means is you power the amp up with the unmute jumper in place (J2, pins 3&4). Once the amp is powered on (with the awake jumper in place) remove the unmute jumper to null the outputs correctly.
I've added a detailed power on procedure as a comment on the AMP9-Basic AI page here:
http://www.41hz.com/forums/content.p...ic#new_comment
Additionally you can change the values of R7 and/or C5 to increase the unmute delay if you are powering on the amp without the unmute jumper in place as per Mr. Busy (FFF) post here:
http://www.41hz.com/forums/showthrea...0148#post30148
From the TAA4100A Datasheet:
AUTOMATIC OUTPUT OFFSET VOLTAGE CORRECTION
The TAA4100A contains an automatic DC calibration routine that reduces the output offset voltage to a
maximum of 25mV when the device is active (mute off). Please note that the DC calibration is done on
the transition of MUTEB from low to high. The entire calibration sequence takes approximately 6mS after
the MUTEB pin is pulled high.
<EDIT 18-Aug-2010>
Keeping R7 stock & replacing C5 with a 10uF capacitor increases the unmute delay to about 2 seconds.
Powering up the amp unmuted, it takes about 2 seconds to see DC offset on the outputs.
Powering up the amp muted and then removing the mute jumper, it takes about 2 seconds to see DC offset on the outputs.
The big difference is even with this additional delay, the DC offset voltage is still much higher when powering up the amp unmuted, than when unmuting with the jumper.
In the first instance the offset is 15mV, in the second instance it is 2mV.
Conclusion,
Increasing the unmute delay timer circuit decreases the offset from 100mV to 15mv, when the amp is powered up unmuted. Powering up the amp muted and unmuting it, reduces the output DC offset voltage to ~ 2mV, regardless of any increase in the unmute delay timer.
[Note, on my AMP9-B, channel 4 DC offset is much higher than the other 3 channels - 115mV, 53mV, 15mV as per above test procedure]
