OpAmp pre-amp : LTSpice simulations problems, tips

[ericgibbs]

Hi.
Ref your asc file, I used my LM324 model.

The AC plot solid line is the 'gain' response of the amplifier and the dotted line is the 'phase' response of the amp.

E

 

[Willen]

My advice to you as a newcomer to LTS , is not to 'bloat' your folders with unwanted models, just add models as required as you learn.

I tried by picking two files and placing LTs directory and adding '.include TL072.301', shown here But got same error message (failed.GIF of above) so I loaded whole mega pack to the LTS at once, to get it working.

Are you a member of the Yahoo/LTspice user group.? It is free to join, it has many tutorials and models.

I yahoo group is not cellphone browser friendly (Java Operamini). I am totally depended to cellphone browser as a internet connection. Connecting cellphone to computer as modem is deadly charges cost.

Could you post an image of your LTSpice folder tree.??

I already had posted my LTS's root folders. Maybe you missed my #17 post. here again.

Hi.
Ref your asc file, I used my LM324 model.

The AC plot solid line is the 'gain' response of the amplifier and the dotted line is the 'phase' response of the amp.

Does the LM324 performing good around 1MHz? Plot seems not so bad with LM324 but people used to say it's horrible OpAmp for audio. Maybe I need to learn something about dB measurement to understand at least which is normal dB level and which is extremely low.

 

[ericgibbs]

hi,
Got your Folder Tree
Use your Windows Search to find the matching SUB and ASY file Names

 

[Willen]

hi,
Got your Folder Tree
Use your Windows Search to find the matching SUB and ASY file Names

Oh yes, then the relation between two files is located to the internal codes inside, instead of file name.

I am about to ask two remaining thing, you did in your simulation-

1) Some models just need to place in sub and sym folder then it starts to work normally (like I used LMXXXX and TL431 regulators). But why some models need to use .include function?

2) How you created a jumpers? You marked V+ in red color at +9V Voltage source and marked just same V+ in OpAmp supply instead of wires to the +9V. Seems easy trick.

(Asking basic trick because I printed a basic tutorial of LTS and it's very basic and related too describe its every features instead of guidance to do it. I tried to read Help file of LTC and it's very advanced.)

 

[ericgibbs]

1) Some models just need to place in sub and sym folder then it starts to work normally (like I used LMXXXX and TL431 regulators). But why some models need to use .include function?

hi,
I use the include when the model is not in the LTS lib/sub
Extract from Help:
This directive includes the named file as if that file had been typed into the netlist instead of the .include command. This is useful for including libraries of models or subcircuits.

2) How you created a jumpers? You marked V+ in red color at +9V Voltage source and marked just same V+ in OpAmp supply instead of wires to the +9V. Seems easy trick.

Press down key F4 on your keyboard and create a Net Name or Label then place the label on the circuit point.

E

 

[AnalogKid]

The circuit was fixed to mix Microphone plus music and amplified with 'pre-emphasis' for a FM transmitter project.

I thought the US pre-emphasis time constant was 75 us. Yours is 10.2 us.

ak

 

[Willen]

I thought the US pre-emphasis time constant was 75 us. Yours is 10.2 us.
ak

Hi, many engineers said that the north America use 75uS and Europe, Australia and other newly introduced countries to FM etc use 50uS. I am South-East Asian so I guessed we use 50uS. I didn't hear about 10.2uS pre-emphasis till now. I know nothing too.

 

[AnalogKid]

For 50 us change the capacitor to 33 nF.

ak

 

[eTech]

Hi etech,
100% agreement on that point when sharing, but IMO would not recommend it as a regular method to the OP, as it quickly becomes a littered folder.

Whatever works for the user...placing them outside Program files directory works for me..

Most users I interact with use the LTSpice example of having a sub folder in the Program Files/LTS, whatever works for the user is OK.

Eric

Agreed...whatever works for the user.
But I stand by my recommendation NOT to place user files in "Program Files", not just for LTspice, but for any program.
If using newer OS's, the "Program Files" folder is a privileged area of the file system. It requires administrative permissions to access.
If a users system is such that one must elevate permissions to place files there then revert back to lowered permissions...then I'm sorta ok with that. If the user files are kept out of privileged areas, it becomes an non-issue.

eT

 

[audioguru]

The datasheet of the LM324 shows that with a supply from 10V to 15V its open loop gain at 20kHz is only 30dB which is about 32 times. Then its horrible crossover distortion is not reduced much. It has crossover distortion because its output transistors operate at a very low idle current for the opamp to be "low power". The open loop gain of a TL07x opamp at 20kHz is about 50dB which is about 320 times. It does not have any crossover distortion.

The low current in the output transistors of an LM324 causes them to have a poor slew rate where they cannot swing high frequencies enough. At an output of 20Vpp it has trouble above only 2kHz. A TL07x with an output of 20Vpp works well up to 180kHz.

The LM324 has a high noise level (rumble and hiss). The TL07x is low noise for audio.
The LM358 is as dual opamp version of the LM324.

 

[Willen]

...<pre-emphasis calculation>...

For 50 us change the capacitor to 33 nF.
ak

Hi,
The pre-emphasis schematic was taken from **broken link removed**, a very intelligent designer. And modified some other parts for my purpose by the help of audioguru. I had learned a poor idea to calculate a pre-emphasis created by a series resistor with bypass capacitor. But Here both R and C in series and you are saying 33nF cap should be placed to get 50uS. Designer was saying 1.5k and 4.7nF will give around 50uS. It would be nice if I get some knowledge about pre-emphasis calculation.

 

[audioguru]

At ultrasonic frequencies the reactance of the 4.7nF pre-emphasis capacitor is a dead short so the gain is 1+ (10k/1.5k)= 7.7 times (about 17dB).
When the reactance of the 4.7nF capacitor equals the resistor value of 1.5k ohms then the gain is reduced 3dB with a slope of 6dB per octave below it. At lower frequencies the gain is reduced until it is 1 where it stops decreasing. So the cutoff frequency is 1 divided by (pi x 4.7nF x 1.5k ohms)= 22.7kHz where the gain is -3dB at 14dB, and at 11.35kHz the gain is about 11dB, and at 5.7kHz the gain is about 5dB, and at about 2.9kHz the gain is 1.

EDIT again. Another way of calculating the pre-emphasis of the 4.7nF capacitor in series with the 1.5k resistor and when the opamp has a 10k negative feedback resistor is that the low frequency gain is 1 when the reactance of the capacitor is high, then when the reactance of the capacitor equals the value of the feedback resistor (10k) the gain is increased to 3dB and increases at 6dB per octave above until the reactance equals the 1.5k resistor when the gain is 3db from maximum gain of about 17dB.