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Old 6th December 2012, 08:44 AM   #1
Bonsai is offline Bonsai  Europe
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Default Simple Feed Forward

Heres a simple feed forward (F/F) concept that reduces distortion by about 15dB at 20kHz. Ive shown it used on a CFA topology amp here but dont see that it would not bring similar benefits to VFA as well.

Distortion sim results for 20kHz at various output powers into 8 Ohms:-

Output Power 8 Ohms No F/F With F/F
6W 21ppm 21ppm
75W 67ppm 14.6ppm
225W 186ppm 30ppm
272W 237ppm 43ppm
300W 338ppm 72ppm

At 3 Ohm output load (640W 62V pk output) the distortion is 46ppm with F/F and without 355ppm an improvement of 17.75 dB

At 300W out, the output is swinging to within about 2V of the rails i.e. just entering clipping.

So, you can see it only does the job at high powers, exactly where its needed. Because the opamp is running in its small signal region the voltage at Vb is about 2Vpk to pk - you don't really need a blindingly fast device for the job.
The sim just used an LT1057 high speed JFET input op-amp that was available in the LTSpice library. A more thoroughly worked design would use something better and probably run the F/F amp in class A for that last touch of refinement, but this needs more investigation.
The diodes between the opamp output and inverting input just ensure when the amp is overdriven, there is feedback around it. This ensures a fast recovery as the power amp comes out of saturation (which on a CFA is very fast) with no overhang caused by the F/F control amp. When the amp clips with the current F/F control amp circuit, a large DC offset (3-4 volts) develops I still need to do some work on that bit to sort it out. Under notmal operating conditions, provided there are no DC offsets on the input, the F/F control amp will also act to remove output offsets.
Please note, this is a conceptual circuit, so not all the bugs are ironed out.
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Old 7th December 2012, 09:58 AM   #2
Bonsai is offline Bonsai  Europe
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Moderators, I tried to delete this post because I reposted it over in the correct forum (solid state).

Please delete this one.

Thank you
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