Buff <-> Boost Query

[samhay]

^thanks for correcting my misinformation  Tongue I've done this with tl072s and NE5532s without it affecting the operation of the other opamp in the package.

No worries - I suspect the worse that will generally happen is that you will use more current than necessary.

[PRR]

> Does the size of the 10uF cap between the 100k resistor and ground in any way affect the voltage gain?

We usually set gain with resistors.

A cap is "like a resistor" except it is open to DC and short for high frequencies.

So we compare the cap to the reistor it works with.

In antonis' first plan, we have 100K against 10uFd. The 10uFd is (nearly) open at DC, but gets close to 100K impedance around 0.17Hz. At 0.34Hz it will be 50K. At 1Hz it will be about 17K. At 10Hz less than 2K. At 80Hz (bottom of guitar) about 200 Ohms.

So what is the difference 100K or 100.2K? "No difference" to the ear, barely detectable on a sharp audio voltmeter. And from there on up, negligible difference.

And this could be 100uFd or 1uFd and still insignificant difference from low audio all the way to infinity.

My proposed 2K 10uFd has the cap equal to resistance about 8hz. By 80Hz, "insignificant".
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> Shouldn't be "safer" a value of 10k..??

If you wish. But all the general-purpose opamps (741, 1558, TL072....) show 2K loading on their spec-sheets. And a guitar pickup "can" have a noise-resistance as low as 5K (though usually higher with resonance and volume knob). 10K isn't wrong; a lot better than some gear. 2K is sure to be quieter than your guitar's self-hiss. (Also quieter than many GB opamps....
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> Edit: Something like this should work:
> some op-amps (perhaps like the TL072) will struggle with the 1k load of R6.

Yes, though it might be drawn more clearly.

The 1K is usually fine, because when you are at that end of the pot the gain through the opamp is very low. In this case, about 1.5V. Sure you can find signals to over-drive a gain of 1.5, but not most guitars.

Also most opamps are specced to drive 2K with a 30V supply (15V either way), meaning they can deliver 5mA cleanly. If we re-think that down to 9V supply, 4.5V each way, maybe 3V peak swing, it *seems* we could do 3V/5mA= 0.6K load. Actually the current may fade a bit at the lower voltage; also the very popular TL07x amps are mostly big resistors and don't scale as current sources.

1K is "fine" in most low-voltage non-precision work.

[induction]

I wouldn't ground the unused op-amp's inputs. Most op-amps (at least those that aren't single supply/rail-to-rail) won't like this very much, so leave the non-inverting input floating, or tie it to half supply. Or even better, use the second op-amp to buffer your reference voltage.

thanks for correcting my misinformation  I've done this with tl072s and NE5532s without it affecting the operation of the other opamp in the package.

Grounding the non-inverting input doesn't interfere with the other op-amp, it just gobbles up supply current. Read more here.

[antonis]

> Shouldn't be "safer" a value of 10k..??

If you wish. But all the general-purpose opamps (741, 1558, TL072....) show 2K loading on their spec-sheets. And a guitar pickup "can" have a noise-resistance as low as 5K (though usually higher with resonance and volume knob). 10K isn't wrong; a lot better than some gear. 2K is sure to be quieter than your guitar's self-hiss. (Also quieter than many GB opamps....

Despite of your avatar's look, you're a quite winsome guy...
(thanks a lot for putting yourshelf out to do "elementary" lessons to brain confused guys like me....)

[antonis]

I'm not  familiarized with Op Amps topology but I've seen many drawings that use low value resistors for creating the biasing voltage (propably to avoid noise interference) but they use a big value resistor for biasing..

http://www.muzique.com/lab/imp.htm

Definately I'm missing something.. 

I know the basics about impendance but I thought that it was concerning of signal "loss" and not of noise interference..

Even if we consider the biasing supply as a noise source with an impendance eual to the voltage divider resistors, this impendance has to face the OpAmp's input resistance which is MUCH greater, so we have an insignificant noise "loss"...
Making the values of the voltage divider resistors lower, we simply lower the ratio of noise loss (unwanted).. 

Or is there something which deal with the noise "amount" proportionaly to the resistance value..??

[induction]

Impedance is simply voltage divided by current. It's just like resistance, except it is frequency-dependent, and accounts for phase by using complex numbers (ie. numbers with real and imaginary components). The concept is used for many, many (dare I say "most") things in electronics, not just "keep input impedance high and output impedance low."

The point of noiseless biasing is that power supply ripple (AC) sees a short circuit (low impedance through the cap) to ground at the Vref node. This allows power supply noise to bleed to ground and not enter the op-amp input. The high resistor value between Vref and the op-amp input is necessary because the guitar signal is also attached to the op-amp input, and you don't want to bleed your signal to ground through the cap and low-value resistors as well. (You will lose high frequencies preferentially if you do this.)

[knutolai]

Quote from: knutolai on Yesterday at 03:57:29 PM
Quote
I wouldn't ground the unused op-amp's inputs. Most op-amps (at least those that aren't single supply/rail-to-rail) won't like this very much, so leave the non-inverting input floating, or tie it to half supply. Or even better, use the second op-amp to buffer your reference voltage.

thanks for correcting my misinformation  Tongue I've done this with tl072s and NE5532s without it affecting the operation of the other opamp in the package.

Grounding the non-inverting input doesn't interfere with the other op-amp, it just gobbles up supply current. Read more here.

good stuff. Thanks!

[antonis]

The high resistor value between Vref and the op-amp input is necessary because the guitar signal is also attached to the op-amp input, and you don't want to bleed your signal to ground through the cap and low-value resistors as well.

Yeaaappp...
(so simple, but..  )

Thanx induction for clearing up..!! :beer: beer:

[ashcat_lt]

The high resistor value between Vref and the op-amp input is necessary because the guitar signal is also attached to the op-amp input, and you don't want to bleed your signal to ground through the cap and low-value resistors as well.

Yeaaappp...
(so simple, but..  )

Thanx induction for clearing up..!! :beer: beer:

Yep.  I said that in reply #15...

[PRR]

> consider the biasing supply as a noise source with an impendance eual to the voltage divider resistors, this impendance has to face the OpAmp's input resistance which is MUCH greater

It's too complicated to cover without face-2-face around a blackboard.

For *hiss*, the resistance level shouldn't matter, because we put a *capacitor* on there. A mere 10uFd gets us down below 1K Ohms all across the audio band, lower than opamp or system hiss.

The cap also knocks-down much of any crap coming from the power supply. (Even batteries hiss; great power supplies may be quiet but shared with other pedals there can be junk on the line.)

[antonis]

Edit: Something like this should work:



Will need some tweaking - e.g. some op-amps (perhaps like the TL072) will struggle with the 1k load of R6.

Thanx Sam..

But shouldn't be a capacitor between lug 2 and Vb for low pass filter..??

Or between pin7and GND..??

[PRR]

Why do you want a low-pass? (It was not mentioned before.)

A cap at the pot wiper turns it from "gain" to "treble" (with no contol of LF gain).

A cap at IC1B pin 7 just loads the Vref. No effect on audio, except some chips go crazy with a large capacitor on their output.

C2 R4 *is* a low-pass, at 35KHz. That's fine for general buff/boost use. If you *also* want this buff/boost to trim the highs (why?), then make C2 bigger. 10nFd will shave the top of guitar audibly. 100nFd will make it super-mellow.

However the effect of C2 happens most at high gain (boost), not when working as a unity-gain buffer.

[antonis]

Why do you want a low-pass? (It was not mentioned before.)
A cap at the pot wiper turns it from "gain" to "treble" (with no contol of LF gain).
A cap at IC1B pin 7 just loads the Vref. No effect on audio, except some chips go crazy with a large capacitor on their output.
C2 R4 *is* a low-pass, at 35KHz. That's fine for general buff/boost use. If you *also* want this buff/boost to trim the highs (why?), then make C2 bigger. 10nFd will shave the top of guitar audibly. 100nFd will make it super-mellow.
However the effect of C2 happens most at high gain (boost), not when working as a unity-gain buffer.

My bad..!!
I meant high-pass to cut-off the low frequencies at about 80Hz..
So, combined with the low-pass (C2R4) form a band-pass filter..
(perhaps lowering the cut-off frequency of C2R4 to 23kHz by a 1n5 cap..)

P.S.1
I haven't find a suitable fret table (or calculating formula) for the audible guitar high frequency to help me design the band-pass filter for a "clean" guitar signal..
(OK.. I know that fundamental guitar frequencies range from 82Hz to 1.174kHz (open low E to 22th fret high E) but what about harmonics..??)

P.S.2
I've seen a cap at IC1B pin 7 at some shematics - like Blues Driver 2 with opamp Vref - and I thought that it serves as an extra voltage regulator..)