M3 amplifier and low op amp supplied voltage
Jun 12, 2009 at 10:39 PM Thread Starter Post #1 of 7


New Head-Fier
Apr 27, 2009
I am currently building a M3 amplifier with a sigma11 power supply (at 24v) and i'm having a problem with the supplied voltage (Vop+ - Vop-) to the op amp. I get around 3.86V which seems low... considering that it should be near 24- 2v = 22v

also according to the schematic posted on The M³ Stereo Headphone Amplifier the voltage across the TLE is the voltage applied to the op amp, however when I measure the voltage across the pins at the extremes of the chip (which I believe should be (Vop+ - Vop-)) I get 3.46V. It is still low... but why would there be an increase in voltage from the TLE to the op amp?? shouldn't they have the same voltage?

The Q5 BJT's have a base-emitter voltage magnitude of approximately 0.6V, I don't see any cold joint, the sigma11 power supply is working fine at 24v.

Does anyone know what could possibly be the problem..? Could it be a damaged TLE?

Any help is appreciated

Jun 12, 2009 at 10:47 PM Post #2 of 7
Not to sound derogatory, because I've done this too, but you did check the pins you were measuring with the diagram right?

Spent a good half an hour one time wondering why the wrong pins had the right voltage
Jun 13, 2009 at 1:55 AM Post #3 of 7
well actually i was comparing the traces on the board with the schematic to determine where each pin was connected but i agree that the diagram is much easier to follow.. and yeah i was wrong about the Vop+ and Vop- of the TLE, the upper pin is Vop+, the middle pin is Vop- and the lower pin is the virtual ground. So i was measuring Vop+ instead of (Vop+ - Vop-) earlier.

However that was not the main problem, I noticed that the voltages of pin 4 and 7 under the PCB were not the same as the same pins on the op amp adapter, so i resoldered the opamp and now the TLE voltages (Vop+/-) are the same as the op-amp applied voltage.

Now Vop+ is 4.7V relative to the virtual ground and Vop- is -4.7V, which makes 9.4V and is still too low. At least now the voltages are symmetric...
Jun 13, 2009 at 4:02 AM Post #4 of 7
Be sure you're measuring the correct opamp pins... The opamp's pin numbers go counterclockwize (looking at it from the top):

[left]8765 ++++ ) | ++++ 1234[/left]

If you're not getting the correct voltages at pins 7 and 4 relative to ground (but they are evenly split), then one or both of Q5+/Q5- is probably blown.
Jun 15, 2009 at 1:49 AM Post #5 of 7
you were right amb, finally the problem was a defective Q5+. thank you for your prompt support!

the sound coming out of this thing is quite amazing by the way, thanks for sharing your design too!
Jun 15, 2009 at 2:05 AM Post #6 of 7
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