Mike Keith
Well-known member
No kidding, you missed my point..
Again are we designing UHF or Audio amps???
This is my last comment on this...
Again are we designing UHF or Audio amps???
This is my last comment on this...
Linear to Log POT using resistor
- Thread starter khstudio
- Start date
Help Support GroupDIY Audio Forum:
[quote author="Mike Keith"]so your saying the value of a pot changes with frequency?[/quote]
And there's the misconception: You're confusing the term "impedance" (which doesn't have to change with frequency) and the term "reactance" (which does by definition).
Impedance has a reactive component, and a resistive component - both of which can be (very, very close to) zero. If the reactive component is zero, the resistive component is all there is, and impedance = resistance.
Peace,
Al.
And there's the misconception: You're confusing the term "impedance" (which doesn't have to change with frequency) and the term "reactance" (which does by definition).
Impedance has a reactive component, and a resistive component - both of which can be (very, very close to) zero. If the reactive component is zero, the resistive component is all there is, and impedance = resistance.
Peace,
Al.
Viitalahde
Well-known member
Mike, this thing has nothing to do with with frequency ranges, think about it.
It's of course pure terminology, but you just can't define a "spot" where an amount of Xl & Xc changes resistance to impedance. It's just easier to talk about impedance here.
If the specs say "output Z: 1k from 20Hz-20kHz", we know it's pretty much pure resistive impedance. If it's 1k at 20Hz, 10k at 2Khz and 100k at 20Khz we're heavily on the inductive side of impedance.
Of course this is a matter of loading, too.
An output with a small sized coupling capacitor might be linear from 20Hz-20khz to a 100k load, but it sure is not going to be that when pushing to a 100ohm load.
When impedance is impedance in that example?
It's of course pure terminology, but you just can't define a "spot" where an amount of Xl & Xc changes resistance to impedance. It's just easier to talk about impedance here.
If the specs say "output Z: 1k from 20Hz-20kHz", we know it's pretty much pure resistive impedance. If it's 1k at 20Hz, 10k at 2Khz and 100k at 20Khz we're heavily on the inductive side of impedance.
Of course this is a matter of loading, too.
An output with a small sized coupling capacitor might be linear from 20Hz-20khz to a 100k load, but it sure is not going to be that when pushing to a 100ohm load.
When impedance is impedance in that example?
Mike Keith
Well-known member
OK I lied about this being my last comment.
My point was and still is that in the Audio band that tiny infinitesimal amount would be considered to be non existant, I know how some lay persons think, they will freak out about the inductance of the pot they are using and look for one with NONE...
I know the difference between impedance and resistance.....
I can rattle off formulas all day too, does that help someone with NO electronic knowledge? If that is the case...
My point was and still is that in the Audio band that tiny infinitesimal amount would be considered to be non existant, I know how some lay persons think, they will freak out about the inductance of the pot they are using and look for one with NONE...
I know the difference between impedance and resistance.....
I can rattle off formulas all day too, does that help someone with NO electronic knowledge? If that is the case...
Mike Keith
Well-known member
[quote author="Viitalahde"]Mike, this thing has nothing to do with with frequency ranges, think about it.
It's of course pure terminology, but you just can't define a "spot" where an amount of Xl & Xc changes resistance to impedance. It's just easier to talk about impedance here.
If the specs say "output Z: 1k from 20Hz-20kHz", we know it's pretty much pure resistive impedance. If it's 1k at 20Hz, 10k at 2Khz and 100k at 20Khz we're heavily on the inductive side of impedance.
Of course this is a matter of loading, too.
An output with a small sized coupling capacitor might be linear from 20Hz-20khz to a 100k load, but it sure is not going to be that when pushing to a 100ohm load.
When impedance is impedance in that example?[/quote]
This is not even close to the original topic, sure a caps capacitance reactance changes with frequency...thats why I use a 470uf cap in the output of the Mic Pre I build, so the reactance will be LOW at low frequencies, it will not be zero...
It's of course pure terminology, but you just can't define a "spot" where an amount of Xl & Xc changes resistance to impedance. It's just easier to talk about impedance here.
If the specs say "output Z: 1k from 20Hz-20kHz", we know it's pretty much pure resistive impedance. If it's 1k at 20Hz, 10k at 2Khz and 100k at 20Khz we're heavily on the inductive side of impedance.
Of course this is a matter of loading, too.
An output with a small sized coupling capacitor might be linear from 20Hz-20khz to a 100k load, but it sure is not going to be that when pushing to a 100ohm load.
When impedance is impedance in that example?[/quote]
This is not even close to the original topic, sure a caps capacitance reactance changes with frequency...thats why I use a 470uf cap in the output of the Mic Pre I build, so the reactance will be LOW at low frequencies, it will not be zero...
khstudio
Well-known member
[quote author="alk509"][quote author="khstudio"]18 or 20k rev log[/quote]
You can turn a B pot into an A pot, but you can't turn anything into a C pot. Like Jakob said, this comes up every once in a while, mainly fueled by the faulty "Secret Lives of Pots" article by R. G. Keen...
[quote author="khstudio"]I can only seem to find 10 & 25k.[/quote]
Oh, c'mon - just use the 25K and be happy!
Peace,
Al.[/quote]
I was told NOT to use a value higher than the feedback resistor.
Some of you guys should realize that not everybody around here is an expert... I may be pretty good but I have a lot to learn & am still a newbee :green:
Kevin
You can turn a B pot into an A pot, but you can't turn anything into a C pot. Like Jakob said, this comes up every once in a while, mainly fueled by the faulty "Secret Lives of Pots" article by R. G. Keen...
[quote author="khstudio"]I can only seem to find 10 & 25k.[/quote]
Oh, c'mon - just use the 25K and be happy!
Peace,
Al.[/quote]
I was told NOT to use a value higher than the feedback resistor.
Some of you guys should realize that not everybody around here is an expert... I may be pretty good but I have a lot to learn & am still a newbee :green:
Kevin
skipwave
Well-known member
This is a case where everyone is correct, in turns, but there is still a debate going on. I'm a newb, as well, but it makes sense to me how this point could be important to getting the predicted behavior from a circuit.
[quote author="gyraf"]The scheme works for converting a lin pot to a log-like potential divider - but you sacrifice the constant-impedance load of a real log pot. [/quote]
Although I don't understand how the "constant-impedance" term applies :?: , I think that if the pot value contributes to the impedance at a point in a circuit, like say between the plate of a triode and the grid of another, then strapping a resistor across it would complicate determining the frequency of the filter formed. It would make an impedance-dependant circuit behave differently from design.
[quote author="gyraf"]The scheme works for converting a lin pot to a log-like potential divider - but you sacrifice the constant-impedance load of a real log pot. [/quote]
Although I don't understand how the "constant-impedance" term applies :?: , I think that if the pot value contributes to the impedance at a point in a circuit, like say between the plate of a triode and the grid of another, then strapping a resistor across it would complicate determining the frequency of the filter formed. It would make an impedance-dependant circuit behave differently from design.
[quote author="khstudio"]Some of you guys should realize that not everybody around here is an expert...[/quote]
EDIT: Pink unicorns, Hello Kitty, and a big bowl of ice cream with a cherry on the top.
[quote author="skipwave"]I don't understand how the "constant-impedance" term applies[/quote]
When the pot is set up as a potential divider (as log pots often are), the source sees it as a roughly* constant impedance - no matter where the wiper is, the source will always see the pot as a resistor of whatever value the pot is.
Peace,
Al.
* Of course, this is an over-simplified view, since it doesn't take into account whatever you load the pot with, but you get the picture.
EDIT: Pink unicorns, Hello Kitty, and a big bowl of ice cream with a cherry on the top.
[quote author="skipwave"]I don't understand how the "constant-impedance" term applies[/quote]
When the pot is set up as a potential divider (as log pots often are), the source sees it as a roughly* constant impedance - no matter where the wiper is, the source will always see the pot as a resistor of whatever value the pot is.
Peace,
Al.
* Of course, this is an over-simplified view, since it doesn't take into account whatever you load the pot with, but you get the picture.
khstudio
Well-known member
[quote author="alk509"][quote author="khstudio"]Some of you guys should realize that not everybody around here is an expert...[/quote]
OH, JESUS F. CHRIST! ALL I DID WAS TELL YOU TO USE THE PART YOU HAVE! WELCOME TO THE INTERNET, PUNK! YOU DIRTY, ROTTEN, PIECE OF SHIT MOTHERFUCKER! SUCK MY.....
:razz:
(Just so we're clear, the above is a friendly joke. I swear! :grin: <-see? Have a :sam: on me!)
Peace,
Al.
[/quote]
:shock:
I don't get you're joke.
OH, JESUS F. CHRIST! ALL I DID WAS TELL YOU TO USE THE PART YOU HAVE! WELCOME TO THE INTERNET, PUNK! YOU DIRTY, ROTTEN, PIECE OF SHIT MOTHERFUCKER! SUCK MY.....
:razz:
(Just so we're clear, the above is a friendly joke. I swear! :grin: <-see? Have a :sam: on me!)
Peace,
Al.
[/quote]
:shock:
I don't get you're joke.
khstudio
Well-known member
I was told that a higher value pot would make gains too low & would make the op-amp unstable?
Also, after testing my circuit (API 312 style), 10k might even be good. The gain is pretty low @ 10k with a sm58 but not sure about a u87 yet til I get the phantom hooked up.
Also, after testing my circuit (API 312 style), 10k might even be good. The gain is pretty low @ 10k with a sm58 but not sure about a u87 yet til I get the phantom hooked up.
Brian Roth
Well-known member
Ummmmmm...I am staying away from the nuances of tiny inductances and capacitances inside of a 20K-ish pot at audio frequencies, and the attendant cussing, etc. in this thread <g>
I have in my hand a copy of a "vintage" API 312 app note, probably from the 1970's. No dates found, but it has the Huntington, NY address, which should be a clue to someone who can cross reference API mailing addresses vs. years.
In a chart for selecting the gain resistor between pins 9/10, they clearly show that an OPEN circuit (ie, infinity Ohms) is acceptable for the least amount of gain.
That makes perfect sense since the gain adjust R is in the shunt portion of the negative feedback loop. With infinity there, the 2520 is operating at unity gain, since the circuit is for an non-inverting opamp.
The app note shows a 20K resistor between the output of the 2520 and the inverting input, so a 20K pot "WFO" on the shunt leg will result in 6 dB of gain from the 2520.
If there is any interest, I'll try and scan/post the app note on my website.
As for off-the-shelf pots with reverse log tapers, Digikey is now stocking PEC hot molded carbon pots in a variety of values and tapers. I would suggest a 25K rev. log for this application.
http://dkc3.digikey.com/PDF/T061/1283.pdf
Bri
I have in my hand a copy of a "vintage" API 312 app note, probably from the 1970's. No dates found, but it has the Huntington, NY address, which should be a clue to someone who can cross reference API mailing addresses vs. years.
In a chart for selecting the gain resistor between pins 9/10, they clearly show that an OPEN circuit (ie, infinity Ohms) is acceptable for the least amount of gain.
That makes perfect sense since the gain adjust R is in the shunt portion of the negative feedback loop. With infinity there, the 2520 is operating at unity gain, since the circuit is for an non-inverting opamp.
The app note shows a 20K resistor between the output of the 2520 and the inverting input, so a 20K pot "WFO" on the shunt leg will result in 6 dB of gain from the 2520.
If there is any interest, I'll try and scan/post the app note on my website.
As for off-the-shelf pots with reverse log tapers, Digikey is now stocking PEC hot molded carbon pots in a variety of values and tapers. I would suggest a 25K rev. log for this application.
http://dkc3.digikey.com/PDF/T061/1283.pdf
Bri
[quote author="skipwave"]
Although I don't understand how the "constant-impedance" term applies :?: , [/quote]
I think that Jakob is saying that a real log pot's terminal-to-terminal resistance is constant across it; it's presents a constant load to what is driving it, regardless of wiper position. When you slug the pot with a resistor from wiper to ground this resistance (impedance :twisted
varies, with the resistor being in parallel at the top of the pot travel and completely removed when the wiper is at ground.
I saw those in Digikey too, Brian. 29 bucks for a dual, but they have them! Now all I want is a center-tap...
Although I don't understand how the "constant-impedance" term applies :?: , [/quote]
I think that Jakob is saying that a real log pot's terminal-to-terminal resistance is constant across it; it's presents a constant load to what is driving it, regardless of wiper position. When you slug the pot with a resistor from wiper to ground this resistance (impedance :twisted
varies, with the resistor being in parallel at the top of the pot travel and completely removed when the wiper is at ground.I saw those in Digikey too, Brian. 29 bucks for a dual, but they have them! Now all I want is a center-tap...
Brian Roth
Well-known member
Crusty... a dual? Well, I guess you'll need a dual reverse log to adjust two channels via one knob, but then you have the issue of "how closely do they track?"
Bri
Bri
[quote author="Brian Roth"]Crusty... a dual? Well, I guess you'll need a dual reverse log to adjust two channels via one knob, but then you have the issue of "how closely do they track?"
Bri[/quote]
I was thinking for a frequency control on something like the Sontec (a bridged "T" in the feedback loop of an amplifier). A log gives a decent sweep.
Bri[/quote]
I was thinking for a frequency control on something like the Sontec (a bridged "T" in the feedback loop of an amplifier). A log gives a decent sweep.
Brian Roth
Well-known member
Aha...a different application vs. the 312 originally posted...now I understand <g>.
Bri
Bri
khstudio
Well-known member
Brian,
I'd love to see the notes & Thanks for the info & advise :thumb:
The 2520 might be OK, like it says in the notes you have but I'm using these Yamaha NE80200's in my circuit. If I'm correct the gains go down with more resistance? so going past 20k "MAY BE" too low a level for my opamps:?
If I'm wrong PLEASE correct me.
In the thread I started about my pre's, I got a lot of great advise & this was one of them - "That some opamps can be unstable @ too low of a GAIN & it COULD cause damage to the opamp.
IIRC, your saying it's OK to use a 25k?
I'm sorry to beat you guys up but I'm building about 10 pre's & the pots aren't cheap 10 X about $10 = $100. That's a lot of $$$ for the wrong parts.
Thanks again,
Kevin
I'd love to see the notes & Thanks for the info & advise :thumb:
The 2520 might be OK, like it says in the notes you have but I'm using these Yamaha NE80200's in my circuit. If I'm correct the gains go down with more resistance? so going past 20k "MAY BE" too low a level for my opamps:?
If I'm wrong PLEASE correct me.
In the thread I started about my pre's, I got a lot of great advise & this was one of them - "That some opamps can be unstable @ too low of a GAIN & it COULD cause damage to the opamp.
IIRC, your saying it's OK to use a 25k?
I'm sorry to beat you guys up but I'm building about 10 pre's & the pots aren't cheap 10 X about $10 = $100. That's a lot of $$$ for the wrong parts.
Thanks again,
Kevin
Mike Keith
Well-known member
I have built many Pres with far different values than API ect uses with great results, put them in the hands of TOP engineers and they loved them...
The Op Amp used has alot to do with how stable it is...
Most if not all IC's are not very stable, thats why you slug them down with the feedback cap.
The Op Amp used has alot to do with how stable it is...
Most if not all IC's are not very stable, thats why you slug them down with the feedback cap.
[quote author="khstudio"]:shock:
I don't get you're joke.[/quote]
Oh, man... I dug myself into an even deeper hole...
The point I was so cleverly trying to make is that people's comments on the internet lack all the subtleties of face-to-face speech, so we sometimes get offended by things that weren't meant to offend at all. Even when someone is all out calling you names, they may still be laughing their asses off typing it, thinking you'll get the joke!
So take everything everybody says on the internet with a grain of salt, and remember Hanlon's razor: "Never attribute to malice that which can be adequately explained by stupidity".
:thumb:
Peace,
Al.
I don't get you're joke.[/quote]
Oh, man... I dug myself into an even deeper hole...
The point I was so cleverly trying to make is that people's comments on the internet lack all the subtleties of face-to-face speech, so we sometimes get offended by things that weren't meant to offend at all. Even when someone is all out calling you names, they may still be laughing their asses off typing it, thinking you'll get the joke!
So take everything everybody says on the internet with a grain of salt, and remember Hanlon's razor: "Never attribute to malice that which can be adequately explained by stupidity".
:thumb:
Peace,
Al.
rafafredd
Well-known member
can you scan and post it, Brian?
Thanks!
Brian Roth
Well-known member
khstudio...I have no info on the opamps that you mentioned, so I can't comment on their stability at low gains.
The "garden variety" NE5534 requires a compensation cap at low gains.
As for the API "app note", I'll scan and post a "copy of my copy" (my "original" is a photocopy that API handed out at an AES show back in the 1970's) in a day or two when I am in the mood to muck with my scanner <g>.
Bri
The "garden variety" NE5534 requires a compensation cap at low gains.
As for the API "app note", I'll scan and post a "copy of my copy" (my "original" is a photocopy that API handed out at an AES show back in the 1970's) in a day or two when I am in the mood to muck with my scanner <g>.
Bri
Similar threads
