Author Topic: Why I hate posting schematics...........  (Read 8555 times)

Gen. Dreedle

  • Commander, Alabama wing, Confederate Air Farce
  • Global Moderator
  • Hero Member
  • *****
    • View Profile
Why I hate posting schematics...........
« on: August 05, 2011, 03:24:29 PM »
AKA "This is not how to make a I/V stage!"

Yes, I know............I can hear y'all now............

"You don't post schematics because you are a greedy old fart, and............"

Well, yeah, that part is true, but...........you know the other reason?

"There is no other reason, you cretin!"

Wrong. There is.

Every time I do, there is one guy who "doesn't get it". At least one. Usually more. And what happens when they do?

A gazillion posts, asking the same question. Accusations I am ignoring them, or not answering them correctly, or............etc., you name it. My inbox fills up. "You didn't answer my question!" Etc.

(Still wonder why I hate you guys?)

Seriously, I don't mind helping guys out, but after a certain point, it becomes a lost cause. I know........you don't believe me.

Check out the most recent thread at Greaser's Palace. Some poor guy is trying to make a I/v converter. Six, seven pages, of questions...............extending for over a year. Poor guy is trying, but he doesn't get it. Several iterations, of the same ol' same ol'. Mostly derived from some of my older posts. Some not.

OK.............so, what is wrong with it?

Well, lots of problems.

First, I to V means you take a current, and convert it into a voltage. And what is the best way to do it?

Well, let's look at it the other way: what is the worst way to do it?

I know this will piss off some of my best buds, but tough.

Current-out DACs output a current. This is obvious. They want to see as low a Z as possible. Because it is a current, and not a voltage.

So, what is the worst thing you can do? What is the one thing that does not have a low Z input?

A voltage gain stage!!!!!!!!!

It has a high Z input. Because it is a voltage amp. Voltage amps are not supposed to load the source.

Ah, but many of you will counter you can make a voltage gain amp by sticking a low value resistor at the input, and amplify it.

Yeah, you can, but................

WHY??

What is the point?

The higher the value of the resistor, the higher the voltage. So, if you make it very high, you get a big voltage. Which means you don't need as much gain. Too bad the DAC won't like that. So, you make it as low as possible. But, now you have to amplify the hell out of it.

So, how do you decide what to make the resistor?

Easy!

Don't use a voltage gain amp! Use an amp that is designed for the job.

Normally, that means an op-amp, in the inverting configuration. Technically, that is called a transimpedance amp. I know..........it is just a voltage amp, configured to.......blah, blah..............uses feedback.......more blah..........

Just use a damn transimpedance amp, and be done with it!

Which is............in its simplest form..............a common-base amp.

I know, you audio guys don't know what a common-base is. You didn't design 70 MHz IF amps, like I used to. CB stages everywhere.

Look, it has a low input Z. Just what  you need. Doesn't need any tricks to make it low. It already is low. In the region of <10R. The only problem is you need some sort of bias network. But...........the bias network is on the base. And the base is not the input! So, the bias network is not seen at the input. (OK, not important at audio, but maybe you can see why us RF guys might like it.)

Anyway, the first thing you should do is to make the input a common-base stage.

Or.........screw it......use an op-amp..........inverting (needs feedback, to make a low Z), or non-inverting, and amplify the hell out of the signal. I don't care.

Unless you e-mail me!
"Major Danby, sir."
"Danby. D-A-N-B-Y."
"Take him out and shoot him."
"Sir?"
"I said take him out and shoot him. Can't you hear?"

Gen. Dreedle

  • Commander, Alabama wing, Confederate Air Farce
  • Global Moderator
  • Hero Member
  • *****
    • View Profile
"OK, now what?"
« Reply #1 on: August 05, 2011, 03:33:37 PM »
Ok, you have a common-base stage. It takes that current, and................

You can stick a resistor, in the collector, and convert it to a voltage, at this point.

Kinda crude. May not have great distortion numbers.

Or...........take the collector current, and shove it into another node.

"What kind of node?"

Oh..........I dunno......................say, some node, somewhere where you have two opposing current sources, fighting each other. (Idea being you can inject, or remove, current, and get the sources to do something.)

Ah, but, the caveat..........

"How do you get two opposing current sources to do something useful? Usually, it is going to bang against one rail, or the other."

YES!

"And how is that useful?"

OK, only hint:

V = I x R.
"Major Danby, sir."
"Danby. D-A-N-B-Y."
"Take him out and shoot him."
"Sir?"
"I said take him out and shoot him. Can't you hear?"

Gen. Dreedle

  • Commander, Alabama wing, Confederate Air Farce
  • Global Moderator
  • Hero Member
  • *****
    • View Profile
So, how do you keep the output at 0 V...........
« Reply #2 on: August 05, 2011, 03:57:24 PM »
"I dunno............a servo?"

Yeah..........and how do you effect a servo?

"I dunno."

OK, you know how to make an integrator, which is the first part, and then..........

Most of you will make it a second-order integrator, which is the worst thing you can do........

"Yeah, but if I make it first-order, I need an inverting stage."

So?

Decent op-amps, for servos, are not hard to come up with. Much better to spend a few more shekels, than screw it up, trying to save a few pennies.

"But, how do I hook it up? Where do I put it? How is it going to work?"

Same way you are doing the first stage.

Except:

I = V/R

Or, you can be a putz, and stick it in the input, of your voltage gain stage. I don't care. Just don't bug me!
"Major Danby, sir."
"Danby. D-A-N-B-Y."
"Take him out and shoot him."
"Sir?"
"I said take him out and shoot him. Can't you hear?"

Gen. Dreedle

  • Commander, Alabama wing, Confederate Air Farce
  • Global Moderator
  • Hero Member
  • *****
    • View Profile
So, let's review:
« Reply #3 on: August 05, 2011, 04:26:20 PM »
The whole point of making a I/V stage, and servo'ing it...........

The goal is to move current around. Once it is far enough removed from the DAC, where you have lots of headroom, and equal swing, you now make it a voltage.

I/V.....servo..........same idea.

Which reminds me.............

A lot of you guys are going to use a DAC, with differential outputs.

Know why they are differential?

To get 3 dB more S/N, with a crappy single +5V rail.

And I know what 99% of you are going to do..............

Differential output.............means we need something with a differential input.

No, you don't.

Yep, almost every damn one of you can not think of anything else to do, other than make a differential input stage.........and that means diff-amp, and that means...........

Voltage gain stage!

Yep, back where we started from.

OK, look.............stick the output currents into that  common-base stage, and having done so, come up with a way to make one set of fighting current sources control another set of fighting current sources.

No, I am not going to tell you. This is supposed to be an exercise in using your brain.

Hint:

One of my former "students" has a pretty good idea how this works. Over at The Pub. (Yes, you are going to have to dig it up.)

Yeah, I know..........it is a differential input. So what? Ditch the diff input, and replace with common-base.

No more hints.

Use your brain, instead of wanting us to lead you by the hand. In the long run, it will serve  you better.
"Major Danby, sir."
"Danby. D-A-N-B-Y."
"Take him out and shoot him."
"Sir?"
"I said take him out and shoot him. Can't you hear?"

Gen. Dreedle

  • Commander, Alabama wing, Confederate Air Farce
  • Global Moderator
  • Hero Member
  • *****
    • View Profile
"Yeah, but what if................
« Reply #4 on: August 05, 2011, 04:31:32 PM »
............We have a DAC, with voltage output?"

Same circuit works. You just have to stick the resistor, that would normally go from the output to ground.........

And make it go from output to common-base input.

"But how do we set the gain?"

Hint:

Use same load resistor, on the input. Gain is now a ratio, of two passive parts.

Ok, now I really mean no more hints!

(After you "get it", you can post your thanks, here. Wait.....no, you can't. Unless you figure out how to register!)
"Major Danby, sir."
"Danby. D-A-N-B-Y."
"Take him out and shoot him."
"Sir?"
"I said take him out and shoot him. Can't you hear?"

MystereoN

  • Sr. Member
  • ****
  • The Brave die once, a coward dies a thousand times
    • View Profile
Re: Why I hate posting schematics...........
« Reply #5 on: August 06, 2011, 10:13:55 AM »
Shouldn't be easier to just post LINK, pointing out Figure 3 and 4?

"A picture is worth a thousand words"  ;)
Never argue with a fool,
they will drag you down to their level,
then beat you with their experience!!!

Gen. Dreedle

  • Commander, Alabama wing, Confederate Air Farce
  • Global Moderator
  • Hero Member
  • *****
    • View Profile
Re: Why I hate posting schematics...........
« Reply #6 on: August 06, 2011, 11:06:20 AM »
That link didn't exist, back in '88 (or was it '89?), when I started designing I/Vs that way. One of the first, if not the first, commercial CD players with such a stage. No idea it existed...........never needed it to exist. Just sat down, one day, started doodling, and voila! Sounded good, and that was good enough.

Besides, folks come here for the thousand words. Smart ass.

BTW, guess who used to work with Goudreau?
"Major Danby, sir."
"Danby. D-A-N-B-Y."
"Take him out and shoot him."
"Sir?"
"I said take him out and shoot him. Can't you hear?"

MystereoN

  • Sr. Member
  • ****
  • The Brave die once, a coward dies a thousand times
    • View Profile
Re: Why I hate posting schematics...........
« Reply #7 on: August 06, 2011, 02:59:55 PM »
Quote
Besides, folks come here for the thousand words. Smart ass.

I know. Especially I like rantings section  :)
For the same reason I wrote "A picture is worth a thousand words" italicized and colored.

BTW, guess who used to work with Goudreau?
[/quote]

You?
Never argue with a fool,
they will drag you down to their level,
then beat you with their experience!!!

Gen. Dreedle

  • Commander, Alabama wing, Confederate Air Farce
  • Global Moderator
  • Hero Member
  • *****
    • View Profile
Re: Why I hate posting schematics...........
« Reply #8 on: August 06, 2011, 03:19:15 PM »
I never worked at Digital Switch. (That is the e-mail address, for Pete, on that site.) It was started, down the street, from where I worked. Some guys I worked with were part of the start-up, in positions of authority.

Guess again.
"Major Danby, sir."
"Danby. D-A-N-B-Y."
"Take him out and shoot him."
"Sir?"
"I said take him out and shoot him. Can't you hear?"

MystereoN

  • Sr. Member
  • ****
  • The Brave die once, a coward dies a thousand times
    • View Profile
Re: Why I hate posting schematics...........
« Reply #9 on: August 06, 2011, 04:31:30 PM »
Without knowing whole background and people it is like winning a lottery jackpot.

It is the same case if I ask you what was I doing back in 1989. Care to guess?

Maybe Phred?
Never argue with a fool,
they will drag you down to their level,
then beat you with their experience!!!

seagreen

  • Hero Member
  • *****
    • View Profile
Re: Why I hate posting schematics...........
« Reply #10 on: August 06, 2011, 06:35:14 PM »
Without knowing whole background and people it is like winning a lottery jackpot.

It is the same case if I ask you what was I doing back in 1989. Care to guess?

Maybe Phred?
Ding!
No double blind testing was used in the making of this post.

Gen. Dreedle

  • Commander, Alabama wing, Confederate Air Farce
  • Global Moderator
  • Hero Member
  • *****
    • View Profile
Re: Why I hate posting schematics...........
« Reply #11 on: August 07, 2011, 08:17:41 AM »
See, it wasn't a trick question. It just seemed like one.

So..........back in '89, you were playing with your "GI Joe", in the backyard. Or whatever your cultural equivalent is.
"Major Danby, sir."
"Danby. D-A-N-B-Y."
"Take him out and shoot him."
"Sir?"
"I said take him out and shoot him. Can't you hear?"

MystereoN

  • Sr. Member
  • ****
  • The Brave die once, a coward dies a thousand times
    • View Profile
Re: Why I hate posting schematics...........
« Reply #12 on: August 07, 2011, 04:19:45 PM »
Quote
See, it wasn't a trick question. It just seemed like one.

Yes, but without knowing people it is difficult to guess. Pure luck this time.

Quote
So..........back in '89, you were playing with your "GI Joe", in the backyard. Or whatever your cultural equivalent is.

I would like to play with my "GI Joe" if I could. Hate to dissapoint you, but there was very little free time inside stinking commie military prison. I was playing too much with my tongue :)
Never argue with a fool,
they will drag you down to their level,
then beat you with their experience!!!

Gen. Dreedle

  • Commander, Alabama wing, Confederate Air Farce
  • Global Moderator
  • Hero Member
  • *****
    • View Profile
Re: Why I hate posting schematics...........
« Reply #13 on: August 07, 2011, 05:22:45 PM »
Ouch. No wonder you are such a troublemaker. And intolerant of fascists, like the PTB, at The Pub.

You should write a book. I can be the editor!
"Major Danby, sir."
"Danby. D-A-N-B-Y."
"Take him out and shoot him."
"Sir?"
"I said take him out and shoot him. Can't you hear?"

MystereoN

  • Sr. Member
  • ****
  • The Brave die once, a coward dies a thousand times
    • View Profile
Re: Why I hate posting schematics...........
« Reply #14 on: August 08, 2011, 06:04:56 AM »
not really a troublemaker, just don't like to be controled by "Big Brother".
But "The Biggest Brother" is already knocking on the back door  :-\
http://www.infowars.com/china-calls-for-international-oversight-of-the-us-dollar-suggests-single-global-currency-replace-it/
Never argue with a fool,
they will drag you down to their level,
then beat you with their experience!!!