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nattawa 15th September 2012 06:16 AM

Op amp based DBT bias control put to test
A while back I posted in the Bob Cordell Power Amp Book thread an op-amp based bias control schematic for a Diamond Buffer Triple (DBT) output stage utilizing the On-semi ThermalTrak output devices. Encouraged by the positive response I got around and put this design to bench test.

I rigged up a simple OTL amp based on two separate aluminum plates that serve as heatsinks, one big one for the TT transistors, the other one smaller for the pre-driver and driver transistors.

The op-amp is On-semi NCS2001. It's designed to operate off single cell Ni-MH battery swinging rail to rail, and is guaranteed to work on a 0.9V supply voltage. Can you find it in the picture?

In my original circuit I was assuming the bias voltage needed for a DBT output would be in the range of 1.1-1.4V, so I had the op-amp directly take supply on the bias voltage. The test result indicated that may not be a good idea. I found that the pre-driver transistors have a higher Vbe drop than do the driver transistors by 0.166V combined which jacks up the bias voltage by that much. At measured heatsink temperature of 70C I read a 0.975V bias voltage between the base of the pre-driver transistors, only 75 mV into the "green". So in the final circuit the op-amp gets power at the output of the pre-driver, comfortably at about 2.3V.

The bias voltage tracks temperature quite well. The power is turned on at 22C and I started taking readings at 24C. The voltage was taken between the emitter of the output transistors.

I'm now working on a power amp PCB design based on Fig. 3.14 of "The Book" featuring a DBT with max. 5 pairs of TT output transistors, TMC, solid state speaker relay, and I'll put this bias control in it.

Bonsai 12th August 2015 12:39 PM

Nattawa, you could also add a 1N4148 each side of the spreader and take your opamp supply off of that. That will open up a whole lot of other possibilities for you wrt opamps.

nattawa 12th August 2015 02:53 PM

Exactly, Bonsai. Bob gave that recommendation as well. The added diodes eat slightly into the voltage swing but that usually is not a problem. We indeed have a vast pool of op-am we can choose from at 3V and up supply voltage level.

I have later put off the DBT OPS and went with "normal" 3EF, as I found the DBT possess some peaking in frequency response in simulation that was on the other hand not so much of a concern to me in a normal 3EF. I ran out of room in a compact PCB layout as well, as a DBT has a lot more components.

Bonsai 12th August 2015 03:07 PM

Looking forward to seeing your results!

Bravick 12th August 2015 03:10 PM


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