modified 8 years ago

Very High Gain Single Stage Preamp

01:24:49

A gain of over 1350V/V! Of course, this has a bad THD of ~3.5%. It’s THD can be improved by a negative feedback such as a resistor in series with the capacitor, but some of the gain will be lost as well.

published 8 years ago

rectum_wrecker

8 years ago

Useless

hurz

8 years ago

You do have negative feedback already. Its the 628kOhm resistor which is working against the 33uF capacitor. Put an additional resistor in series e.g. 10k and you get a nice feedback loop. Sure the overall gain will drop, but the THD as well! Many users do forget a feedback resistor at its amplifier inputs and connect directly a capacitor to the base input transistor, bad design.

jason9

8 years ago

I imply that it doesn’t have any negative feedback because the 628kOhm resistor only sets the bias, and because of the capacitor it doesn’t function as a negative feedback at high frequencies like that from audio. A 10kOhm resistor in series with the capacitor would be an example of a negative feedback that I suggested for use if the THD is to be improved.

hurz

8 years ago

628k without 10k are negative feedback for DC which sets you the operating point.

jason9

8 years ago

With your suggested 10kOhm resistor in series with the capacitor this amplifier has a gain of 50V/V (100mV input for full power) and a THD of 0.05% for a second harmonic distortion of 2mV at full power. This seems pretty good to me, the only drawback being the lack of gain, but two of these in series would have a gain of 2500V/V and a THD of 0.1% which to me seems very good.

jason9

8 years ago

Yeah, the 628kOhm is negative feedback for DC but not for AC which is what this is actually amplifying and therefore the only relevant feedback type.

jason9

8 years ago

I’ll change the description to clarify.

hurz

8 years ago

DC feedback is a VERY relevant part of an amplifier to have a proper DC operating point and not talking about temeratur effects. Build it test it and see what it does for you.

rectum_wrecker

8 years ago

Shit.

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