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Old 7th May 2008, 05:36 AM   #131
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Another new wrinkle, along with a further set of tweaks - in the circuit below, the sonics (as well as THD) can be tuned by adjusting a single resistor, R18. If it's low, the H2 component is significant, but the THD is also higher - similar to the classic JLH1969. By increasing R18, it's possible to progressively lower the THD, but at the cost of decreasing the H2/H3 ratio.

In principle, it's possible to have a selector switch (or maybe a CD4066 quad-bilateral switch) to select the desired sonics from the front panel - say Classic JLH, Normal or Low-Distortion; or maybe even a log-potentiometer.
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Old 7th May 2008, 05:38 AM   #132
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Here's the simulated 16W THD20 FFT at ~ -60 dB THD (R18 ~= 4.7k):
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Old 7th May 2008, 05:39 AM   #133
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Here's the simulated 16W THD20 FFT at ~ -80 dB THD (R18 ~= 47k):
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Old 7th May 2008, 05:40 AM   #134
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Here's the simulated 16W THD20 FFT at ~ -100 dB THD (R18 ~= 470k):
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Old 24th May 2008, 06:10 AM   #135
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An idea for input bootstrapping on the original 4 transistor JLH topology follows - this is only a concept for simulation, although it could be built to deliver >2.5W into 4 ohms with a single +12v rail.

The main idea here is to increase the input impedance sufficiently to allow the use of a small input coupling capacitor, which could be high quality, e.g. styroflex or silver mica.
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Old 24th May 2008, 06:12 AM   #136
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And here's the THD20 FFT at 2.5W into 4 ohms.
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