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Old 26th December 2019
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Radardoug's Avatar

Summit TLA 100A opamp failure

So I've had a Summit TLA 100A on the bench for service. Apparently the bright spark who designed this was not that bright. There is a Bal/Unbal switch on the back panel. In the Unbal position, one of the John Hardy copy opamps is shorted to ground via a 75 ohm resistor. Now Johns opamps are specced for working into 75 ohms continuously, but apparently the Summit copies are not. So the opamp dies from heat exhaustion. Now the idea of shorting an output to set up an operating mode is just plain dumb. What needs to happen is that the output pin on the XLR should be grounded, and the opamp disconnected from the output pin. Fortunately this mod is possible. Follow these steps.

1/ Remove the brown wire from the Bal/Unbal switch from point D on the pcb.
2/ Cut the track from the bypass relay that connects to trace D. Cut close to
the relay so you only cut the relay connection off.
3/ Connect brown wire to this disconnected relay contact.
4/ Connect a wire from the unused contact on the Bal/Unbal switch to D on the pcb.

I've attached a couple of pictures.

Note that when the opamp dies, the whole output is affected, because the bright spark connected the two opamps in series, and death of one affects the other. It also puts full rail d.c. on your output! Well done bright spark! Feel free to justify your design decisions if you read this.

As a side note, you can get replacement opamps of higher quality and cheaper from the great John Hardy, still alive and doing good work. Please support him rather than Summit, who ripped him off.
Attached Thumbnails
Summit TLA 100A opamp failure-summit100_1.jpg   Summit TLA 100A opamp failure-summit100_2.jpg