OP-Amp Buffer (simple?) question

[Vallhagen]

Arrgghhh... hehe... Cant get a simple (?) task to work as i want...

Somehow inspired by this thread: http://www.madbeanpedals.com/forum/index.php?topic=2545.0, i started to play around with different components on my breadboard. At the moment hooked up like this (very simplified):

Input--> [Baxandall/"James" tonestack] --> [OP Amp buffer] --> Output.

So far it works well. The OP amp buffer is pretty much exactly this one:
(link to beavisaudio tech pages)

***

Now for my next task: I want to add some make-up gain to this circuit. Can i do that in the same OP-amp stage as i use as buffer atm? I have done a few tries, but not succeeded.

Thanx in advance. Links are welcome too:)

Cheers
/Bengt

[JakeFuzz]

Should be as easy a changing that buffing stage to an op-amp amplification stage.



Raise or lower the feedback resistor (Rf) with a trimmer to get the right amount of make up gain. This will flip the phase by 180 degrees. If you want it in phase you can do the non-inverting configuration and change R2.



When the two resistors are the same value you have unity gain. The voltage division ratio between the two sets the overall gain for the amp.

[Vallhagen]

Should be as easy a changing that buffing stage to an op-amp amplification stage.

...thats what i thought. So i guess im just tired or something, this should be easy.

I have actually tried to hook up as your second picture; if i refer to "my" picture:

*  Replaced the shorting wire between pin 6 and 2 (output and negative input) with some R=47k
*  ...and then another R=10k from pin 2 and gnd.

... that should be it, shouldnt it?  But that kills the circuit all dead.

Funny. Or strange. I must have missed something obvious:) ...

Cheers

[JakeFuzz]

Hmm that is strange. Instead of tying the 10K to ground try a bias voltage. Just split the supply voltage in half using a 100k-100k voltage divider. Before you do that get rid of the 1M resistor (Labelled R1 in your schematic) biasing the non-inverting input to half the supply. This might work although I am pretty sure the 1M-1M divider on the non-inverting input is doing the same thing.

[Vallhagen]

Thanks for your time Jake. Im still a bit confuzed why my "1st" idea doesnt work, even though ive got two solutions:

If i start over with my schematic (here i am partly copying and pasting myself):

*  Replace the shorting wire between pin 6 and 2 (output and negative input) with some R=47k
*  ...and then another R=10k from pin 2 *** via some large capacitor (i chose 4u7) *** to gnd.

...means i added a cap to ground. Here i am a bit confuzed why it is needed. But i checked a few schematics out there (viewable here on madbean: slow loris, tube screamer, orange squeezer), and yep, they all have a cap to gnd for similar stages.

Another working solution was to choose the inverting amplifier setup (your first pic). It shall be noted that the positive input shall be connected to some Vref (Input voltage/2), not gnd. In my setup, it allready was (the allready mentioned 1M resistors). Problem here is that input impedance is set by Ri (in your picture), and i want to keep that high (1M), and if i want to multiply Rf for amplification say 5-15 times, its resistance gets redicilous high (which may or may not be a problem?).

Hope i make my lines understandable:) Im still relearning... 

Cheers

[JakeFuzz]

Man that's pretty weird. I always thought that cap was there to act as a bass filter. Without it the gain is constant over the entire frequency range. I would try and use the non-inverting configuration if you can. My guess is it is something with biasing, to me though your original diagram should work. I also don't understand why the cap should make a difference except maybe it is acting as a DC blocking cap to keep DC off of the output (as the frequency goes to DC the impedance of the denominator of the voltage divider goes to infinite and the gain of the amp goes to 1). Perhaps without the cap the 4.5v on the input is being amplified to well above what the amp can deliver from the supply.

Now that I talk through it though that makes sense. The cap keeps the amp from amplifying the 4.5 volt DC bias signal (if you had a gain of 10 that would want to deliver 45 volts!). I've never had to think about it before. You learn something new every day! Keep that cap large and you can design it so the cutoff is low enough to not make a difference in sound.

[Vallhagen]

Hmm. I think you nailed it in your last paragraph. And if that is correct, i dont have to choose much of gain in the voltage divider to hit the roof. If R2=R1 (your 2nd picture) the setup shall give a gain of 2 (as Gain=1+R2/R1); want to deliver 9V, which obviously IS the roof.

Maybe it should work in a system feeded with +/- DC and 0V as "middle" ground? I guess i have to play around a bit more with this.

This actually required more brainwork than i thought in the first place I dont complain though, im learnin too...

There "must" be a way of - at least theoretically - hookin up a gained OP amp stage without frequency loss...?

Cheers

[JakeFuzz]

Yep I am certain that cap is there for that reason now. You could move to a bipolar supply and keep the input voltages referenced from zero. That is the only way I can think to get around the DC bias necessary on the input signal to keep the amp at the operating point. I guess you could somehow choose the operating point and gain low enough to where the output isn't clipping the supply; say for an op amp gain of two you set the bias point of the input to 2.25 volts and now your output is still riding on a 4.5V DC offset. Now your input signal cannot have a negative peak large than -2.25V or the amp will clip. I would stick with the large capacitor though, at some point you will move into a frequency range where there will be no noticeable difference in sound.

I just built up a Klon buffer BTW. Super handy to have in front of a medium to large sized pedal board. I didn't know I was getting so much loss in high end until I plugged this thing in. I like that idea of an active tone stack right before too, very cool 

[Vallhagen]

Yep I am certain that cap is there for that reason now.

[...]

I just built up a Klon buffer BTW. Super handy to have in front of a medium to large sized pedal board. I didn't know I was getting so much loss in high end until I plugged this thing in. I like that idea of an active tone stack right before too, very cool 

Good. Then we hopefully have solved at least a bit of this task:) ... And i guess i will keep the large cap to gnd. Hmm; i spent a few minutes with a Tone Stack Calculator to get flat response ... How do i calculate / simulate the frequency response of the added cap in the amp stage? Is it just

Fc=1/(2*Pi*RC)   ? ... and which R do i use? R1 in your picture?

If so, I can tell that a voltage divider in the kOhm territory, and cap about or over 1 uF, gives a cutoff frequency far beyond hearability:) ... So the cap to ground seems like an audibly "safe" solution. Thats  good news, even in theory:)

***

Yea the Klon buffer seems to have a few more components than my choice above. And it shall be fairly well renowned if i get things right:) ? Perf board design?

Im not sure where im on my way with this project (besides just experimenting for fun/learn/play/listen), but in the back of my head grows the idea of some general solution to implement in future boxes partly designed by myself.

[JakeFuzz]

The gain of the amp is going to be a ratio of the impedances in the feedback (the feedback resistor) and the resistor/cap to ground. The cap adds a frequency dependent (complex) part to the impedance of the cap/resistor combo. If you keep the frequency as a variable and solve for the generalized gain equation you could generate a gain vs. frequency plot and find the knee frequency. I am not quite sure if that passive filter equation works in this case. I would have to go through and add up the reactance and resistances to get the equation.

The Klon buffer is my new favorite toy. It honestly doesn't sound like anything just plugged straight in. But driving a long chain of pedals brings the high end sparkle back. Yeah I found one of the many Vero layouts out there and just built one. It was super small and fit easily inside a 1590A.

You should post your design here when you get it finished. I am sure lots of people would like to have and active EQ/Buffer unit.

[Vallhagen]

I am not quite sure if that passive filter equation works in this case. I would have to go through and add up the reactance and resistances to get the equation.

Damm, too long time i did some serious math  ... But i went back to the books (or rather web calcultors) and made a Z(R1, C) and gain plot 5 Hz - 100 Hz. I think i did the calculus correct.

I skip the details (i can type em down, if you or someone else want to view), but it sure looks very much "allright". If the idealized gain should be 5,7; the frequency dependent gain is 5,3 at 8 Hz, 5,6 at 16 Hz ... 5,67 at 27 Hz... and reach 5.70 at 53 Hz.

... in other words, all negligible as well as the verrrrry small phaseshift (the imaginary part of the complex math).

***

I must try that Klon thing too. Sounds interesting:)

Cheers