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Old 24th April 2013, 07:50 AM   #111
Bonsai is offline Bonsai  Taiwan
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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.

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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Old 21st July 2014, 09:08 PM   #112
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Great post about Op Amp "Singing" (Oscillation)

Its more years than I care to remember - however, advise about Op Amp compensation
can often be obtained from application notes or even data sheets themselves.

The NE5532 is an Op Amp that uses bi polar transistors
Many of the Other Op Amps mentioned here have jFET's on the input.

Zin of an Op Amp with a jFET input stage would be significantly different.
Rin of a jFET Op Amp would be in the order of 10^12 or 10^13 ohms
and Cin would be in the order of 10 pF

Its important to remember that at the x1 setting, a scope probe adds about 15pf // 1 M ohm.
So be careful not to disturb the very circuit you are trying to measure.

Using the x10 setting on the probe will help.
However, a wire added to the scope probe will act like a receiving antenna.
If its place near the circuit it should pick up any "singing"
Mooly was bang on showing oscillation was hiding in the blurred trace.

One other comment - the chassis of a scope is earthed.
So a scope can only measure voltages with respect to 0 v.

Last edited by Uunderhill; 21st July 2014 at 09:13 PM.
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Old 21st July 2014, 09:24 PM   #113
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To be clear, the AD797 is a bi polar Op Amp too.

AD797 datasheet, Pinout ,application circuits Ultralow Distortion Ultralow Noise Op Amp
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Old 22nd July 2014, 05:53 PM   #114
Mooly is offline Mooly  United Kingdom
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Good point on the scope leads introducing extra capacitive loading which can in some cases tip a circuit over the edge. Worst offenders are the simple "coax and crop clip" type leads that are surprisingly common. If in doubt then it can pay to solder a lowish value resistor to the point of interest to provide isolation and then probe the free end of that.
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Old 5th August 2014, 05:28 AM   #115
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Originally Posted by Bonsai View Post
Here is a low cost oscillation sniffer. You can build it into an old marker pen case. It usefully covers c. 100KHz to a few MHz - just where there are usually problems in audio.


Do you have a higher resolution of the schematic?
Something must be wrong with my PDF viewer because
at 300 percent I still can't read it.

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Old 5th August 2014, 05:44 AM   #116
Bonsai is offline Bonsai  Taiwan
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I'll try again when I get home Sync. I am travelling on the road this week.
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