Op amp based DBT bias control put to test - diyAudio
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Old 15th September 2012, 06:16 AM   #1
nattawa is offline nattawa  Canada
diyAudio Member
Join Date: Apr 2010
Location: Mississauga, Ontario
Default 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.

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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?

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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.

Click the image to open in full size.

Click the image to open in full size.

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.

Click the image to open in full size.

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.
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