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Help needed: Linkwitz-Riley frequency response curves
Help needed: Linkwitz-Riley frequency response curves
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Old 18th August 2019, 04:36 PM   #11
PRR is offline PRR  United States
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Help needed: Linkwitz-Riley frequency response curves
Quote:
Originally Posted by yannikab View Post
...I thought just labelling my output would automagically include that label...
It isn't smart enough to know what "output" means. In the sim image I posted, I had to place the circle-arrow thingie to easily get a plot of that point.

Yes, I could also type a "command line" into PROBE to get data about an arbitrary un-labeled un-pointered point.... the syntax does not come to my fingers that easy.

(All this plotting stuff is "outside" SPICE, traditionally in a module called "PROBE". Which originally printed ASCII characters on greenbar paper. The input to SPICE was originally punched-cards, and remnants of that technology can still be found in the internal workings. Also it comes from an era when not all terminals had lower-case characters. When FORTRAN was the only good way to work formulas. Yes, SPICE was soon re-written in C, but it had to preserve many FORTRAN conventions to keep compatibility.)
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Last edited by PRR; 18th August 2019 at 04:39 PM.
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Old 18th August 2019, 04:39 PM   #12
yannikab is offline yannikab  Greece
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You 're really an old timer arn't you?
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Old 18th August 2019, 08:13 PM   #13
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by PRR View Post
...originally printed ASCII characters on greenbar paper...
A version of Spice that printed ASCII characters to a text terminal (in lieu of graph paper) was still available on Linux a few years ago. I think it's still available today.

There are a few truly Free (no cost, open source) graphical frontends too. I haven't tried using any of them, though.

If LTSpice had been open-source, it would be looking and working much better by now. As it is, it feels like a view back to the aesthetics (using the word loosely) of Windows 95. Thick-bordered windows in a fetching shade of mouse-fur grey, jagged fonts, clunky icons, et cetera.

Looks may be only skin deep, but there is one LTSpice bug that, well, bugs me every time I use it: the utterly stupid algorithm for auto-scaling the graphical output. Much of the time it comes up with utterly unusable axis limits and intervals: from 43.7 dB to 47.1 dB in steps of 432 milli dB, for instance.

But the actual simulation engine works very well, however bad the GUI. I'm thankful for that, and for the people at Linear Technology who chose to make the program a free download (no doubt with the intention of encouraging engineers to design circuits using LT parts, but it ended up benefiting a much wider community of people.)


-Gnobuddy
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Old 18th August 2019, 08:49 PM   #14
yannikab is offline yannikab  Greece
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Bugs me too. Every time I poke the running man I have to set proper axis parameters. But it's not that big a deal. My initial perception of its graphics capabilities was pretty horrendous and I was actually contemplating going down the complex math route to get my curves or use something like Matlab, Mathematica, etc.

It would be very interesting to find a strong open source version of this tool that included a rich library of components right after installation and that hopefully had a cleaner interface. I'm certain it's out there somewhere.
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Old 18th August 2019, 09:26 PM   #15
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by yannikab View Post
Every time I poke the running man I have to set proper axis parameters.
I have a tip that will make this slightly less tedious:

1) First, set the proper axis parameters by hand, as you've been doing.
2) Now left-click on the panel showing the graphical output. Some of the menu headings along the top left edge of the program will change - you will now see the item "Plot Settings".
3) Left-click on "Plot Settings", mouse down to "Save Plot Settings".
4) Give your settings file the name you want (ending in .plt), and click "Save".

Now, after you poke the running man again and LTSpice once again thoroughly screws up the axis scaling, you can repeat the process to recall your previously-saved plot settings (left-click graphical output, select "Plot Settings", and this time choose "Reload Plot Settings".)

This is still tedious, but not quite as tedious as having to reset all three y-axis parameters after every single run of the simulation engine.

Quote:
Originally Posted by yannikab View Post
It would be very interesting to find a strong open source version of this tool
A tonne of things with "spice" in the name pop up if I search my Linux distribution's package manager (see attached screenshot.) But I don't know if any of them is developed as far as LTSpice, or ideally, better.

Kicad - an open-source schematic-editor and PCB design program - now includes some circuit simulation capabilities too, I think. And they're probably based on the SPICE engine.

Kicad's origins go back to the 1990s, but researchers at CERN began using it and improving it about five or six years ago, and progress has been rapid since then.

If you find an open-source SPICE variant that is as good or better than LTSpice, please do let me know!

-Gnobuddy
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Old 18th August 2019, 09:34 PM   #16
yannikab is offline yannikab  Greece
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Thanks for the plot settings tip.

From your screenshot, ngspice + ngspiceui may be a strong contender. ngspice is cross platform but apparently command line based, so the question mark is how good ngspiceui is.

I did a quick search earlier and found a tool called qucs that seems to be good, although it's not spice based (I think).

And a sweet quick link: gplEDA Homepage
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Old 18th August 2019, 09:49 PM   #17
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by yannikab View Post
I was actually contemplating going down the complex math route to get my curves or use something like Matlab, Mathematica, etc.
You could certainly do this with a relatively straightforward circuit such as your Linkwitz-Riley filters. But, depending on your electronics interests, sooner or later you will encounter a circuit too complex to break down into a complex-number polynomial fraction.

And when that happens, one starts to feel a deep gratitude towards Laurence Nagel and the Regents of UC Berkeley.

The attached screenshot shows one of my LTSpice simulations. A dozen carefully tuned damped sinewave generators generate a waveform approximating what you get when you pluck a guitar string; this waveform is scaled by a simulated op-amp, then fed to a simulated two-stage vacuum tube guitar preamp, and the resulting output waveform is computed. Finally, the output waveform is also turned into a WAV file that I can listen to.

What I found from this and similar experiments is that the DC bias levels in the triode stages fluctuate during the duration of a single guitar note; those fluctuations in bias translate to fluctuating amounts of distortion that add subtlety and richness to the guitar tone. The envelope of the note is altered too, rounding off the harsh initial attack.

You do not get these effects when you use an almost-perfect device like an op-amp to amplify the guitar signal. You also do not get them from the majority of guitar distortion pedals on the market, 99% of which use an op-amp and a couple of clipping diodes to generate unvarying, unsubtle, boring distortion.

I cannot imagine how much DIY code it would have taken to do something like this without SPICE - certainly I would never have taken on such a project!


-Gnobuddy
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Old 18th August 2019, 09:52 PM   #18
yannikab is offline yannikab  Greece
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This forum could use a Like function. :-)
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Old 19th August 2019, 03:54 AM   #19
PRR is offline PRR  United States
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Help needed: Linkwitz-Riley frequency response curves
Quote:
Originally Posted by Gnobuddy View Post
...A tonne of things with "spice" in the name pop up if I search my Linux distribution's package manager...
Many of them not "our" SPICE.

A lot seem to be "Simple Protocol for Independent Computing Environments", "allows users to view a computing "desktop" environment ... from anywhere on the Internet".

Easyspice and ngspice seem to be e-sims. NGspice is fairly well known. It harks back to planning your sim on paper and punching cards/lines (no schematic editor), although third party interfaces are available. Their Resources Page is worth a look.
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Old 19th August 2019, 04:13 PM   #20
yannikab is offline yannikab  Greece
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I focused on ngspice last night, going through their resources pages. It seems to be a mature simulation engine and the good thing is that it is integrated in more complete software packages that provide luxuries like schematic editing. I started downloading eSim and KiCad last night but transfer was interrupted due to a battery outage. I 'll try to build the same circuit for the LR crossover in both of them, see how well they work (or not).

Speaking of the LR crossover, I decided to snatch a cheap implementation of it for further experimentation. Sadly, the good people in the far east that sell it seem to not know what they are selling:

lr.png

My specific query is about powering the unit. Their product description states:

"8. AC adapter power supply range Dual 14 to dual 19V"

Now, I can not understand if this means a regular center-tapped-secondary transformer (+-15V AC) or simply +-15V DC.

Can you guys tell by the looks of this board which case it is? Because honestly I do not expect a reliable answer from the seller. If the power input is +-15V AC there should be additional components for DC conversion to feed the opamps.
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