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51 votes
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What's the point in a preamp?

In audio gear, it is useful to do most of the signal manipulation at a standard level, known as "line level". This includes mixing, equalization, compression, etc. Some signal sources (microphones, ...
Dave Tweed's user avatar
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26 votes
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Why does this op amp stop working after some time?

The input bias currents are charging up C4 and C5. Add resistors between the inputs and GND.
Transistor's user avatar
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20 votes

What's the point in a preamp?

Quick and dirty answer: Buffering is one reason. Interconnects between things can have a lot of capacitance and require a lot (comparatively) of current to drive. Noise immunity is another. Think ...
Hearth's user avatar
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15 votes

Why does this op amp stop working after some time?

Op-amps require a DC path on each input to supply the tiny current drawn by the inputs. Without it, the DC voltage of the input is undefined. In practice, that means that the input will drift either ...
Davide Andrea's user avatar
15 votes

Why does this op amp stop working after some time?

You're missing the input bias resistors on the positive inputs. The bias current, around -20nA for the LM358, on the input pin will charge up the input capacitor and after some time will cause the op-...
qrk's user avatar
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14 votes

What's the point in a preamp?

SUMMARY: feedback from Power Amp into the Input stage of a Preamp will cause ECHOS that degrade instrumentation settling into your ADC, or musically your ears will not enjoy. So we physically separate ...
analogsystemsrf's user avatar
12 votes

What's the point in a preamp?

To minimize the noise factor, which is the SNR of the output divided by the SNR of the input. An ideal amplifier should keep the SNR constant, since the input noise is amplified by the same amount as ...
user110971's user avatar
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8 votes
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Common Emitter Amplifier Problem (Incredibly lower Gain than input, and Distorted)

To start out, you need something that can both push and pull on your speaker. This "something" is called the output stage. (Which implies that there is more than one stage, by the way.) Broadly ...
jonk's user avatar
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8 votes

What is the purpose of these passives in a microphone pre-amplifier circuit?

The first group are bias for the microphone capsule. the top 2K reduces supply noise into the capacitor, and the bottom 2k provides bias current The second group set the bias voltage for the op-amp ...
Jasen  Слава Україні's user avatar
7 votes
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What is the best preamplifier circuit for electret condenser microphone?

Add a resistor in series with the 100 uF capacitor across R3 as shown below: - Without that resistor there is nothing that defines the AC gain of the amplifier and the effect of the 100 uF without ...
Andy aka's user avatar
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7 votes

What is simpliest possible guitar active preamp design

Instead of using a split battery, I think a virtual ground at 1/2 the supply would be better. That way you are essentially running from a single power supply. That leaves the possibility open, for ...
Olin Lathrop's user avatar
7 votes

What is the purpose of these passives in a microphone pre-amplifier circuit?

I refer to this datasheet. Starting with the microphone and its power/biasing: simulate this circuit – Schematic created using CircuitLab The microphone drops a few tenths of a volt (I assume 0....
Simon Fitch's user avatar
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6 votes

How does a preamplifier improve the Signal-to-Noise Ratio (SNR)?

There's two things to consider with a preamp which explains why it improves SNR. The first, and most important, is that the pre-amp is closer to the source (such as a microphone). There's fewer ...
Cort Ammon's user avatar
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6 votes

Vintage Oscilloscope Preamp Whitepaper

Perhaps C.R. Battjes "AFTR Class Notes" Tektronix 1977. It describes a cascode "ft doubler" circuit. Referenced in Wideband Amplifiers (Starříč & Margan).
Spehro Pefhany's user avatar
6 votes

What's the point in a preamp?

In addition to what was already said, with guitar amplifiers often the intended usage scenario is intentionally introducing some distortion by overdriving the amplifier. If there was only one gain ...
rackandboneman's user avatar
6 votes
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Designing 2-stage BJT CE class-A voltage pre-amplifier with specified goals and without global NFB

THD There are several sources for increasing THD. Among them is variations due to the Early Effect and signal-related gain error. But the signal-related gain error is the main problem here, I think. ...
jonk's user avatar
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6 votes

Why is there a winding on a capacitor?

Those few turns look to be a "gimmick" capacitor. Should behave as small (a few pF) capacitor between plate and grid. Provides some degree of negative feedback at high frequencies. Given the ...
Denis's user avatar
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6 votes
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How do I design preamplifier and amplifier (for a speaker) circuits from scratch?

Can anyone please suggest me some thing how to design the Pre-Amplifier and the Amplifier circuit for a speaker from scratch Start with a specification of what you intend to achieve in terms of ...
Andy aka's user avatar
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6 votes
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Transistored pre-amplifier

Discussion This is a single-ended circuit and it's assumed that the signal input shares ground with the amplifier. In a case like this, I'd want to know a few things: does the source need power (...
jonk's user avatar
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5 votes

How does a preamplifier improve the Signal-to-Noise Ratio (SNR)?

Is the noise not amplified the same amount as the signal? From a different but related perspective than Cort Ammon's answer, in a low-noise multi-stage design, one puts most of the overall gain in ...
Alfred Centauri's user avatar
5 votes
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Op-Amp output voltage exceeds the power rail voltage (in simulation)

If it is SPICE's AC analysis it's not surprising. In AC analysis, SPICE figures out the small-signal linearized model of the system, then applies the input voltage to that model, assuming it is ...
TimWescott's user avatar
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5 votes
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Op amp introduces phase shift at high audio frequencies

I haven't found a real data sheet on this part, and judging from the gain BW product (using mHz for MHz), I suspect this is an audiophool part. Without going in to too much detail, higher gains will ...
qrk's user avatar
  • 10.2k
5 votes

Voltage Buffer Vs Impedance matching

I am working on low low-frequency circuit, so will impedance matching really be an issue? When we are dealing with signals whose maximum useful frequency has a wavelength significantly greater than ...
Andy aka's user avatar
  • 464k
4 votes

How to test the input impedance of a Microphone Preamp?

Assuming a preamp has negligible reactive impedance: Apply oscillating voltage signal of known amplitude. Measure current. Divide. Done! If trying to do things right: do the same over a range of ...
Marcus Müller's user avatar
4 votes
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Best microphone picking up sound within a small room?

You need to pay attention to the following items: High sensitivity - measured in dB, and always negative. You want a value closer to 0. That is to say that -38 is better than -44. Sensitivity is ...
JRE's user avatar
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4 votes
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Is signal from electret microphone capsule (with FET) inverted?

As others have stated, in general, in many situations not only does not matter as absolute phase is impossible to guarantee with audio over more than a short distance. However, if you you wish to ...
Kevin White's user avatar
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4 votes

Troubleshooting simple JFET preamplifier design

You need a resistor from gate to GND to setup the dc operating condition of this FET. A typical value is 10 MOhm.
Jens's user avatar
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