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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Transistor Preamp
Transistor Preamp
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Old 16th September 2019, 03:01 PM   #381
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by sgrossklass View Post
...the operating point of this circuit is not conducive to low distortion.
Interestingly enough, because of the exponential transfer function of a BJT, THD is essentially independent of operating point, and depends only on the amplitude of the signal appearing across base and emitter.

IIRC, THD for a BJT common-emitter amplifier is around 2% for a 1mV (peak) input signal, regardless of operating point, and goes up rapidly as the signal amplitude increases. So there is actually no such thing as an operating point conducive to low distortion for a single BJT common-emitter stage!

(As long as the output is kept away from clipping, of course. In practice, biasing for adequate output headroom is more important than trying to bias for lower distortion.)

This characteristic (distortion % independent of operating point) holds true as long as the Ic/Vbe characteristic remains exponential. In practice, that holds true over a very wide current range - some four or five orders of magnitude, until the current gets big enough for bulk and ohmic resistances in the device to begin to dominate.

But the amazing thing about transistor amps is that BJT-based audio amps are a wonderful example of making a silk purse out of a sow's ear. The device itself has tons of harmonic distortion - but it also has such high transconductance that engineers came up with relatively simple circuits with extremely high voltage gain, allowing lots of negative feedback, which cleans up the distortion so much that the end result is far lower distortion than you could ever get out of a much more linear vacuum triode.

Now we can go buy a cheap op-amp with a gain of 100,000 - 1000,000 for pennies, apply feedback around it, and end up with distortion thousands of times lower than the ability of the human ear to hear it. Essentially perfect amps, for pennies. Amazing!

Sadly, available open-loop voltage gain for the simple common-emitter amplifier posted to this thread (in the Hack-A-Week link) is defined by the DC voltage across the collector resistor, at about 35 * (voltage across Rc). The circuit shown, run on 3 VDC, will have an open-loop voltage gain of barely about 50 times.

On the plus side, this is about as much gain as you can get out of a 12AX7 vacuum triode with a 300 V DC power supply! On the minus side, this isn't enough voltage gain to apply much negative feedback, and so the Achilles heel of the bipolar junction transistor - its high nonlinearity - is exposed.


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Old 17th September 2019, 02:02 AM   #382
PRR is offline PRR  United States
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Originally Posted by keilau View Post
It is ok for a low fi application such as boasting an cell phone output.
R1 makes NO sense unless it is for an electret capsule (naked or in a headset).

At moderate speech levels, a few mV, the THD will be acceptable.

In a hi-fi system, the juice coming out of R1 may annoy the source, and hundreds of mV signal will make it a "fuzz box".
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Old 17th September 2019, 03:24 PM   #383
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Most time we do not need any "pre-amp", may be;-)
Leave it, unless you want to do disco or be the loudest in the village;-)
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Old 23rd September 2019, 04:39 PM   #384
Bigun is offline Bigun  Canada
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Transistor Preamp
One reason I can think of for wanting an active pre-amp is to use a circuit that makes a linear pot behave like a log pot. Linear pots are known to have far better ch-ch tracking. The only example I'm aware of is the Shindo Claret all-tube pre-amp but I'm sure there are SS examples.
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Old 23rd September 2019, 05:05 PM   #385
Mark Johnson is offline Mark Johnson  United States
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The Self preamp in Linear Audio uses the "Baxandall Volume Control" circuit which includes a linear (not logarithmic) potentiometer. If you download the documentation from this page , you can see the full schematic.

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Old 23rd September 2019, 05:28 PM   #386
Mark Tillotson is offline Mark Tillotson
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That circuit is designed for low source impedances only, and I believe doesn't go to fully mute.
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Old 23rd September 2019, 07:24 PM   #387
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by Mark Tillotson View Post
That circuit is designed for low source impedances only, and I believe doesn't go to fully mute.
I agree about the low input impedance. It does look to me as though voltage gain goes to nominally zero, though, when the pot is fully CCW. That setting results in a nominally zero-ohms feedback resistor from output to inverting input.


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Old 25th September 2019, 01:00 PM   #388
Mark Tillotson is offline Mark Tillotson
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5 ohms output impedance though, rather the sub-ohm you'd get with a single opamp output amp. That allows a small voltage to be injected at -60dB or so.


Rather than parallel that opamp, its output should have been augmented after the feedback point.
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