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Old 24th April 2013, 07:50 AM   #111
Bonsai is offline Bonsai  Taiwan
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Quote:
Originally Posted by dmccoach View Post
Thank you all - I can't find a schematic for the (older, or original not the new "Super") Eastern Electric Minimax DAC but I will continue to look. The LME9940 is a "common" (DIY, smart? effective? is another issue) swap (with the brown-dog adapters) for the stock I/V conversion stage and output. The stock opamps are I/V 2 x NE5532. After this opamps, there're 2 outputs stages, one based on a single tube (12AU7) and other SS with the opamp NE5534.

Scanning the net reveals many- many - comments about them (LME49990) running hot (in all kinds of audio circuits. It is *apparently* common. The specs allow for a range up to about 185F, but I didn't understand why my right and left channels seem to run at different temps. It could be my laser temp. reader is not consistently hitting the hot-spot on the U2 and U7 due to the angle at which I have to take a reading.

OPERATING RATINGS(1)
Temperature Range
TMIN ≤ TA ≤ TMAX 40C ≤ TA ≤ 85C
Supply Voltage Range 5V ≤ VS ≤ 18V

But, the LME4990 run so hot (occasional reading up to 140F on U1) that I might worry about putting the Minimax lid back on top and driving ambient up inside the unit.
If this is running as a 'summer' on a current output DAC, then what you may be seeing is stray capacitance between the summing node and GND that's causing it to break into oscillation. Also, keep in mind that any noise coupled between the actual summing point (i.e. where the resistors meet) and the inverting input will be amplified by the full loop gain of the opamp - and in the case of those LME devices, this could be over 100 dB at a few kHz
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