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Old 19th April 2013, 01:04 AM   #1
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Default TC40H004 substitute for AB amp

Although this Toshiba chip is available, at a price, I wonder if a modern chip could be used instead. It's used as a pre-amp for the photodiode signals from a Sony CD player head. Quite a common series of chips in older machines I believe.

It's a hex inverter, and describes itself as C2MOS. What is shown of the circuit in the datasheet looks pretty standard, with an input resistor followed by a simple push-pull CMOS output.

A standard 74HC04, also a hex inverter, switches much more sharply than the Toshiba chip. Worse, when biased with feedback at around Vdd/2, the HC draws 12mA per inverter, when it's not oscillating, which adds up to more than the chip or the circuit can handle. The Toshiba chip draws just 1.7mA per stage and, with much less class A gain, is less inclined to oscillate.

74C04 looks a bit closer but not much. I thought maybe there would be a 4000 series equivalent but a 4004 is something else altogether. The hex inverter is 4069. From what info I can find there are buffered and unbuffered versions, with the 4069UB seemingly most suitable for analogue use.

But I can't find out what current it draws when idly biased half on. It also has a few dB more gain in class A, and so compresses sooner.

Worth trying do you think? Better ideas, maybe?
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Old 13th May 2013, 01:22 AM   #2
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Hmm...the 74HCU04 looks about right. The "U", for unbuffered, means it's suitable for analogue, I gather. I'll give it a go.
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