Why is the frequency response of this amplifier i made in LTSpice different from the original source?

Thread Starter

asdasd12e12

Joined Nov 24, 2021
48
So here is what i'm trying to do, same circuit same value.

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Here's what i got in LTSpice:

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The frequency response is different with the same values for the components. How?

If i dont want to center the output signal at ground can i ignore the RC circuit in the first pic (C4 and R6) or will things change much?

I also tried using some input signal to test the circuit and the max voltage i am getting isn't 1.228rms, i know i used more than the max input pressure/voltage but shouldt the output signal be capped at 1.228Vrms?

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Edit:

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crutschow

Joined Mar 14, 2008
31,115
Since your real components aren't perfect whereas the simulated components are, I expect the response differences you are seeing is due to the tolerances of the real components.

If you want 1.228Vrms out then you need to input a constant sinewave that generates the peak output just below the clipping level.
It's obviously not possible for a random sound input to give a full RMS output except for the peaks that are near clipping.
The peak output looks to be ±2.5V which is 1.76Vrms for a sinewave
 

Audioguru again

Joined Oct 21, 2019
5,413
Your circuit has a feedback capacitor to ground of only 10uF cutting the bass but the original circuit has 47uF which improves the bass levels.

Your simulation uses a mic with a Zero ohms output impedance. Use the 2k ohms of a real electret mic.
100dB is deafening. Is your mic inside a piano or drum? For such loud levels you need a 3-wire mic that has the "Linkwitz electret mic mod" which converts the Jfet in the mic from the original common source with gain and with distortion at high levels to a common drain follower with a much higher maximum undistorted level but a little less voltage gain.
 

Thread Starter

asdasd12e12

Joined Nov 24, 2021
48
Your circuit has a feedback capacitor to ground of only 10uF cutting the bass but the original circuit has 47uF which improves the bass levels.

Your simulation uses a mic with a Zero ohms output impedance. Use the 2k ohms of a real electret mic.
100dB is deafening. Is your mic inside a piano or drum? For such loud levels you need a 3-wire mic that has the "Linkwitz electret mic mod" which converts the Jfet in the mic from the original common source with gain and with distortion at high levels to a common drain follower with a much higher maximum undistorted level but a little less voltage gain.
I changed the capacitor value to 47uF and added a series 2.2kohm resistor to the input source and the output is almost the same.

I'm just trying to simulate this circuit in ltspice so i'm not really worried about de 100dB for now. What would be a realistic max SPL for some simple circuit?
 

ronsimpson

Joined Oct 7, 2019
2,523
The first circuit has a TLV6741 which is a 10mhz amplifier, while the last circuit has a TLV6001 which is a 1mhz amplifier.
The 6001 will work to neat 1mhz with a gain of 1, 100khz with a gain of 10 and only 10khz with a gain of 100. You cannot push the gain up much without hitting the gain/frequency curve.
 

Audioguru again

Joined Oct 21, 2019
5,413
I changed the capacitor value to 47uF and added a series 2.2kohm resistor to the input source and the output is almost the same.
About -3.1dB difference at 20Hz. See my simulation.

What would be a realistic max SPL for some simple circuit?
85dB continuously damages our hearing. Peaks at 92dB are very loud and will not damage our hearing.
 

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Thread Starter

asdasd12e12

Joined Nov 24, 2021
48
The first circuit has a TLV6741 which is a 10mhz amplifier, while the last circuit has a TLV6001 which is a 1mhz amplifier.
The 6001 will work to neat 1mhz with a gain of 1, 100khz with a gain of 10 and only 10khz with a gain of 100. You cannot push the gain up much without hitting the gain/frequency curve.
Then what changes should i do to make it have a similar frequency response?
 

ronsimpson

Joined Oct 7, 2019
2,523
Ple
Then what changes should i do to make it have a similar frequency response?
Please explain what is the problem. High end? Low end? Gain?

The first circuit you posted must include the response of the mic.
I think it should not be used as a mic preamp.
TI Audio TI has good resources on audio amplifiers. This link is good for choosing low noise amplifiers.
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Thread Starter

asdasd12e12

Joined Nov 24, 2021
48
Ple

Please explain what is the problem. High end? Low end? Gain?
The gain, in my simulations i got a gain around 20dB and in that paper there's a gain around -4dB. I made a new edit in this post and i would like to ask what's the difference the gain in equation (7) and in equation(8) ?
 

ronsimpson

Joined Oct 7, 2019
2,523
(7) gain = Vout/Vin. Vout is about 13x what Vin is.
(8) gain is in db. I do not understand "-4db" but the number is in db. I think it should be (math in my head) 22db.
 

Thread Starter

asdasd12e12

Joined Nov 24, 2021
48
(7) gain = Vout/Vin. Vout is about 13x what Vin is.
(8) gain is in db. I do not understand "-4db" but the number is in db. I think it should be (math in my head) 22db.
In (8) they say vout is equal Inmax * gain (from eq 7), then vout is divided by 2 volts ( i assume the standard operating voltage of the mic), with base on that i dont think the gain in (8) is the gain in (7) in db
 
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