| BlairHatcher |
Hello can someone please explain the difference between an audio taper & a linear taper pot. By audio taper do they mean logrithmic.
Also are the bourns series of potentiometers ok to use with gainclone .
Regards
Blair |
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| Nuuk |
Many argue that a law-faked linear pot will out-perform a log pot.
I prefer a stepped attenuator first and a law-faked linear (cermet or carbon, not plastic film) second. ;) |
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| JojoD818 |
| quote: | Originally posted by Nuuk
Many argue that a law-faked linear pot will out-perform a log pot.
I prefer a stepped attenuator first and a law-faked linear (cermet or carbon, not plastic film) second. ;) |
Nuuk,
Please show me how to fake the law of a linear pot.
Thanks,
JojoD |
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| Peter Daniel |
As this thread is in a chip amp forum, I presume people are talking about using linear pots (law faking) with chip amps.
For some reason, I could never make it work properly at the input of NI amp. There was simple not enough control (in certain volume ranges) and it was not comparable with a good log pot. I'm talking here about control range and not sound quality. |
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| carlosfm |
| quote: | Originally posted by Peter Daniel
I'm talking here about control range and not sound quality. |
Yes, I also don't like it because of that.
It's not a pure log pot, range is a little strange. |
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| AudioFreak |
| quote: | Originally posted by carlosfm
Yes, I also don't like it because of that.
It's not a pure log pot, range is a little strange. |
so called log pots aren't log these days either, they're just dual taper. |
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| Stocker |
| quote: | Originally posted by carlosfm
Yes, I also don't like it because of that.
It's not a pure log pot, range is a little strange. |
Did you try changing parallel resistors, to alter the "curve" of the taper? |
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| Peter Daniel |
| quote: | Originally posted by AudioFreak
so called log pots aren't log these days either, they're just dual taper. |
I wouldn't be so sure about that, at least this is not a case with better quality pots.
Attached is the chart for all the standard taper types available with Nobel potentiometer. As you can see there are two different linear tapers available and 3 different log tapers.
Those characteristics are usually very accurate, and with mono pots (31 detents) I have no problem matching them to 0.5 dB (for two separate pots, on any given detent position). |
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| Mad_K |
| quote: | Originally posted by Peter Daniel
As this thread is in a chip amp forum, I presume people are talking about using linear pots (law faking) with chip amps.
For some reason, I could never make it work properly at the input of NI amp. There was simple not enough control (in certain volume ranges) and it was not comparable with a good log pot. I'm talking here about control range and not sound quality. |
I find the control range to be excellent; the channel tracking is also much better. I use 100K pot with 22K resistor. |
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| Peter Daniel |
| quote: | Originally posted by Mad_K
I find the control range to be excellent; the channel tracking is also much better. I use 100K pot with 22K resistor. |
So, since my NI amp is already using 22k shunt resistor (input to ground) all I need to do is to add 100k linear pot in front of the amp (to take advantage of fake law principle). Is that correct?
Because if it is, it doesn't even come close to what a log pot offers (with regards to control range). |
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| Mad_K |
| That's right. Shure you didn't use a 100K log? |
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| homer09 |
Ok im using BrianGT's NI kit. What is the purpose of the 22k shunt resistor R2 in the circuit?
If im using a log pot, then should this resistor be removed? Because from what i understand about fake lawing, you must place a resistor between input and ground pins, which is the exactly where the 22k shunt resistor is right now. So if i dont remove this resistor and i use a log pot, im I not fake-lawing a log pot? (which sounds ridiculous to me)
SO im very confused here. If someone could just clear up which of the following schematics are correct?
In my first schematic, this is the way i think a log pot should be used to control volume.
In my second schematic, this is the way i think a linear pot should be fake lawed to provide similar behaviour as schematic 1. |
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| homer09 |
also, what is the role of R1? is it necessary?
here is schematic 2: |
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| homer09 |
| Anyone? Confirm or or correct? |
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| thomas997 |
| quote: | Originally posted by homer09
also, what is the role of R1? is it necessary?
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I think Peter Daniel has explained these a few times.
R1 was something to do with decreasing DC voltage coming from the pot (?).
I believe R2 isnt necessary if your using a pot, just for if your using say the output from a preamp or decoder. |
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| homer09 |
thanks thomas997.
i sat down for a couple of hours yesterday, and figured out what all the resistors do. this was pretty hard considering im no electronics expert. anyway, i made a spreadsheet that compares different fake lawing resistor values and came to the conclusion that the 1/20 ratio is the best.
as long as i maintain the same ratio between pot and fake law, i get the same volume curve. the only thing that changes is the input impedence range. which range should i tweak my values to? what is the safe, optimal range? i know that too low an impedence will overwork your source (right?) but what is the effect of too high an impedence? |
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| Peter Daniel |
If you are using 250k pot (or any pot that is more than 25k), R2 in the range of 22k, or similar, is neccessary to reduce input shunt resistance, otherwise the DC offset will be rather high (300mV or so).
You might use R1 or not, it's not really critical, with a pot I'm usually not using it. |
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| blyloveranger |
| quote: | Originally posted by Peter Daniel
You might use R1 or not, it's not really critical, with a pot I'm usually not using it. |
When you are referring to R1 here, do you mean the R1 on the first schematic shown is not needed, i.e. the one with a 25k log pot? Also in another thread someone asked
| quote: | | What if one does not have any input cap or input resistor, just a pot and a shunt resistor? any problems there? |
and the response was
| quote: | | Yes, there is a potential for problems. The DC impedance, looking back from the opamp input (which is a source of leakage current) to the audio source, will be all over the place. That will affect the DC offset, because the leakage current will induce a different voltage depending on the position (resistance) of the pot. Even though this voltage is small, it is amplified by the gain of the opamp. |
So I am trying to figure out whether a cap and a shunt resistor is needed, if I am using a pot? Since by this quote it seems that he feels that they are important. Is it one of those things where I am just going to need to build it to see?
Patrick |
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| homer09 |
From what ive read, it seems that having an input resistor is "safer" for your amp/source/speakers. But its not really necessary if you dont have any major dc offset problems without it. Peter i believe suggests to not use it if possible because he designed a minimalist GC, so one less component in the signal path could only help (if there arent other consequences of course).
if you are using a pot, then there is some resistance in the signal path anyway (except at max volume of course) so you should not need the input resistor. If you are building a true amp (no volume control) than an input resistor is probably necessary.
any corrections welcome from the more experienced :smash:
What i would like to know is what causes DC offset to increase or decrease when you change volume. How is DC offset related to input resistance? maybe with answers to those questions, i will be able to decide what value of linear pot to use in the fake lawing setup. And if the input resistor is still necessary in that case. |
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| edjosh23 |
Would putting a greater value resistor is R2 decrease dc offset, if one cannot find what is causeing the problem, which is of course better?
Thanks,
Josh |
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