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Help needed: Linkwitz-Riley frequency response curves
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Old 20th August 2019, 12:11 AM   #31
Gnobuddy is online now Gnobuddy  Canada
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Originally Posted by yannikab View Post
I trust you guys however, in that this board's demands are nowhere near 15VA.
One of the nice things about science and engineering is that you rarely have to trust someone's opinion: the answers are there in science and math, calculable and verifiable facts. As we all know, opinions can sometimes be dangerously wrong, so it's really nice to have facts to depend on.

We already had a look at the TI datasheet, and worked out power consumption of the board based on the 5532 op-amps current draw. Now lets try a different tack, this time based on thermodynamics. How much power can a little IC of that size dissipate before it heats up so much that silicon devices fail?

This table ( Package Thermal Resistance Values (Theta JA, Theta JC) for Temperature Sensors and 1-Wire Devices - Application Note - Maxim ) says that an 8-pin DIP package (the one your op-amps come in) will heat up by 110 degrees Celsius if you dissipate one watt, with the chip mounted to a two-layer copper PCB. If your PCB is in a room at, say, 20 C, that means the chips heat up to 130 C.

Silicon devices will usually fail at 150 degrees C, but that is an abusively high temperature; nobody who cares about reliability will operate them anywhere near that temperature, or anywhere near 130 C, for that matter.

But, just to make the point, let's take the ridiculous over-estimate; all five of your op-amps are boiling hot, dissipating one watt each. That makes five watts total power draw. Add another two watts for the overheated voltage regulators, you're up to 7 W total power demand.

So, just from thermal considerations, if all seven chips on your board were hot enough to cook an egg on, your board would still only need maybe 7 watts from the transformer!

Your fingertip is all you need to verify that the 5 op-amps are not, in fact, anywhere near that hot...ergo, they are in fact dissipating much less than 1 watt each. (At the specified 8 mA draw per chip, actual dissipation would be around 240 mW with +/- 15V supply rails, i.e. around one quarter of a watt.)
Quote:
Originally Posted by yannikab View Post
"...any transformer of 0.5A or more...little to be gained by using anything more than 30VA (and even that is likely to be overkill)."
I read this as saying "15 VA is plenty, but if you like ludicrous excess, by all means go up to 30 VA, even though you will gain nothing by doing so."

Good engineers are always conservative - nobody wants to be associated with a circuit that blows up regularly. I'm sure Mr. Linkwitz knows his circuit draws barely 1.2 watts. But a 15VA power supply isn't much more expensive, and offers a huge safety margin, so why not use it?

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Originally Posted by yannikab View Post
...eSim and KiCad...
I look forward to your results. I've used KiCad a few times to draw schematics, but I've never tried using it to simulate anything.

KiCad's library management process has beaten me to a standstill. I attempted to create a part of my own (an oddball vacuum tube), and create a library containing it. I found about five tutorials online on how to do this, each one contradicting the other. KiCad itself didn't correspond to any of the tutorial instructions, just to round out the frustration.


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Old 20th August 2019, 12:33 AM   #32
yannikab is offline yannikab  Greece
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Originally Posted by Gnobuddy View Post
I read this as saying "15 VA is plenty, but if you like ludicrous excess, by all means go up to 30 VA, even though you will gain nothing by doing so."

Good engineers are always conservative - nobody wants to be associated with a circuit that blows up regularly. I'm sure Mr. Linkwitz knows his circuit draws barely 1.2 watts. But a 15VA power supply isn't much more expensive, and offers a huge safety margin, so why not use it?


I look forward to your results. I've used KiCad a few times to draw schematics, but I've never tried using it to simulate anything.

KiCad's library management process has beaten me to a standstill. I attempted to create a part of my own (an oddball vacuum tube), and create a library containing it. I found about five tutorials online on how to do this, each one contradicting the other. KiCad itself didn't correspond to any of the tutorial instructions, just to round out the frustration.


-Gnobuddy
Nice analysis from a thermal standpoint.

You just interrupted (and probably stopped) my effort at making that LR schematic in KiCad. It's good cause I need some sleep. About eSim, I did not manage to run it, and personally would not recommend an installation of it on a Windows system, due to dependencies. It's a bundle of all sorts of things much like KiCad, only KiCad seems better integrated. Perhaps a Linux distribution can manage eSim's dependencies better.

Just to clarify who said what (cause I think it's very important) the 30VA was not Siegfried Linkwitz's recommendation, but Rod Elliot's. He's from Australia and designs/builds neat stuff. By the way, I was glad to see Linkwitz is still around, unlike Butterworth for example. Even has his own website online (linkwitzlab) where he explains his own creations.

But returning to the recommendation, that was not intended for the LR board itself, but a separate power supply board ESP makes which can power their LR board with +-15VDC. He calls it "preamplifier power supply", so I am guessing the 30VA recommendation for it is for covering various types of preamplifier boards that involve gain (unlike the LR crossover) therefore more power draw.

Last edited by yannikab; 20th August 2019 at 12:40 AM.
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Old 20th August 2019, 12:47 AM   #33
yannikab is offline yannikab  Greece
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Damn, just found out Siegfried Linkwitz passed away less than a year ago, after all. RIP.
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Old 20th August 2019, 12:52 AM   #34
Gnobuddy is online now Gnobuddy  Canada
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Originally Posted by yannikab View Post
...probably stopped...my effort at making that LR schematic in KiCad.
Oh, don't let me stop you! The schematic-drawing side of KiCad seemed quite nice, as long as you're using parts provided in the built-in libraries. I only encountered major headaches trying to make my own part.
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would not recommend an installation of it on a Windows system
The only thing I recommend installing on a Windows system is Linux, making sure you completely erase Windows in the process.
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Originally Posted by yannikab View Post
...Rod Elliot's.
I'm familiar with his website. Lots of neat designs there.
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Originally Posted by yannikab View Post
He calls it "preamplifier power supply", so I am guessing the 30VA recommendation for it is for covering various types of preamplifier boards
Gotcha.

Times have changed, and next time I need an op-amp power supply, I plan to use one of these: RAC10-12DK/277 Recom Power | Power Supplies - Board Mount | DigiKey

It's a tiny little brick that you feed 120V (or 240V) into at one end (two pins), and DC voltage comes out the other: three pins carrying +12V, 0V, and -12V respectively. Most op-amps will run very happily on +/- 12V instead of +/- 15V.

There is a +/- 15V DC version of the same module, but for some reason, it costs twice as much. There's lower demand for it, I expect.

I will add an external stage of RC filtering to reduce ripple. The switching frequency is high, so it doesn't take a lot of RC filtering to dramatically reduce the remaining ripple.


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Old 20th August 2019, 05:24 AM   #35
PRR is offline PRR  United States
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> labelling my output would automagically include that label

I'm not an LTSpice user, but am again poking at it.

The "label" is human-arbitrary, SPICE (generally) would not presume to understand humans.

I *now* see that LTSpice's labels have an option for None, In, Out, or Bi-Di. Apparently this only affects the *drawing*, whether the arrow points in or out. Other than drawing rendering, it seems to have no effect. Used well, it may help humans see/think more clearly. OTOH, the tutorial I was following said to call a label "in" and make it Type "output". That was meant to connect a source to a load over an un-drawn "line" called "in".

Yeah, I didn't get how to plot until I hovered the cursor around and saw it changed to a "funny pencil" (meter/scope probe). I like my old sim which had a probe "part" I could leave on the drawing run to run.

I now know that both old-PSpice and new LTSpice give me a kink in the back. Even more than text-editing, circuit-editing strains me.

And what is with LTSpice's F9 for undo??? WTF uses F9 for undo? (I undo a lot.) Tip: most keystrokes can be re-assigned in a menu. Ctrl-Z (undo in 99% of apps) happens to conflict with some Zoom command so I nuked the conflicter (I think to Ctrl-A).

Last edited by PRR; 20th August 2019 at 05:31 AM.
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Old 20th August 2019, 06:41 AM   #36
Gnobuddy is online now Gnobuddy  Canada
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...I like my old sim which had a probe "part" I could leave on the drawing run to run.
I've noticed that if I do the Plot Settings -> Save Plot Settings thing, after that LTSpice will redraw my graphs every time I click on the running man icon, without my having to probe the output(s). I'm not sure if this happens every single time I do the "Save Plot Settings" thing, but it certainly happens often.

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Originally Posted by PRR View Post
Even more than text-editing, circuit-editing strains me.
I feel the same. And for some reason, schematics drawn with LTSpice cause much more eye / mental strain than, say, schematics drawn with KiCad. Something to do with the jagged lines and sharp corners and crudely drawn symbols, I suspect.


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Old 20th August 2019, 07:37 PM   #37
PRR is offline PRR  United States
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> Plot Settings -> Save Plot Settings thing

Thanks for the tip! (Can't find all this stuff in one day...)
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Old 21st August 2019, 09:14 AM   #38
Gnobuddy is online now Gnobuddy  Canada
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Thanks for the tip! (Can't find all this stuff in one day...)
You're quite welcome!

I've spent a lot more than a day tinkering with LTSpice, and there are still a tonne of things I don't know about its capabilities.


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Old 21st August 2019, 08:06 PM   #39
PRR is offline PRR  United States
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I've worked in PSpice (and priors) for 30+ years and still find little tricks.

I noted something in LTSpice which can be useful. After a DC (.op) run, if you hover parts on the schematic, the power dissipation appears in the bottom of the window. (At least resistors and batteries; don't have a tube/transistor on the bench yet.)

This would almost be useful for a question which came up elsewhere: what resistors to use for a HIGH power switched attenuator. He had plan and values but not power ratings. The problem can be solved with DC: put 28.28VDC in, that's same-as 100W @ 8r. But the resistors are in two switched strings, 8 steps, any one of the resistors "could" be the hot one (inspection will suggest the worst case but I guessed wrong a few tries), so 8*18=144 values to compute. A spreadsheet would do it in an instant but setting-up would a tedious chore. (I advised building with all 10W, spread out, feed 24VDC, nose/finger check for hot or cold parts, watching that output voltage is nominal (nothing blew yet).)

Do you use pot? Does your pot blow-up if setting is zero or 1? (Divide by zero error.) There is an elegant fix. Maybe everybody knows but me.
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Old 21st August 2019, 08:52 PM   #40
Gnobuddy is online now Gnobuddy  Canada
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Originally Posted by PRR View Post
After a DC (.op) run, if you hover parts on the schematic, the power dissipation appears in the bottom of the window.
I've used that trick too. A very useful tip.

There is also a way to manually enter an equation to plot, and that equation can, for instance, be (some current) * (voltage1 - voltage2), so you can plot power dissipation in a device against time if you want, even if that device has no grounded terminals.

A while ago I was trying to design a variable guitar amp speaker attenuator, but the input impedance needed to be reasonably constant at around 8 ohms. The question was how to calculate that input impedance across the audio frequency range (the attenuator included reactive elements as well as resistors.)

In LTSpice, I found I could insert a 1 milli-ohm resistor in series with the input to the attenuator. That makes the current into the input port easily accessible within LTSpice. With that available, I could then plot (input voltage / input current). Voila, input impedance as a function of frequency!

More on how to enter your own equations to plot in this tutorial: https://www.digikey.com/eewiki/displ...ered+Functions

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Do you use pot? Does your pot blow-up if setting is zero or 1?
(Divide by zero error.) There is an elegant fix.
The pot model wasn't included when I downloaded LTSpice (apparently it was included in some older versions), so I rolled my own, see attached image.

The good: I was able to figure out the equations to make a standard audio-taper log pot, as well as a linear pot. They're in the attached image.

The bad: doing it this way doesn't look like a pot on the schematic, just two ordinary resistors. Understandably, this confuses people.

The ugly: I was able to prevent the equations blowing up at zero pot rotation by adding 1 milli-ohm to each equation. It works, and the error is negligible when the pot resistance is much greater than 1 milli ohm. But this kludge is far from elegant.

The ridiculous: I attempted to follow a couple of tutorials on how to create your own pot model (which means it looks like a pot, and uses the equations you type in.) I failed two or three times, then decided it wasn't worth spending more time on.

I also downloaded the old LTSpice pot model from somewhere and tried to incorporate that into my version of LTSpice. The results were erratic and unreliable, so after more wasted time, I gave up on that as well.

These days I feel that what's left of my life is too short to waste hours struggling with uncoperative software. So if I find a workaround, I take it, and move on.


-Gnobuddy
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File Type: png LTSpice_Potentiometer_Simulation.png (52.1 KB, 50 views)
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