inverting opamp questions from beginner designer

Started by darron, December 27, 2007, 08:48:47 AM

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darron

hey there. i'm trying my (limited) skills at designing some effects for myself. i made a passive tremolo before http://www.diystompboxes.com/smfforum/index.php?topic=61732.0 and am now trying to make it an active tremolo.

well, i've accomplished the task so i didn't have any problems on the way. the tone quality is questionable now. i've done it with two inverting opamps in the style of the tremulus lune, using the tonepad schematic http://www.tonepad.com/project.asp?id=42. it's an inverting opamp in unity gain feeding straight into another inverting opamp with gain varying from around one to zero. i've used my own LFO with LED/LDR combo, which is working great. the problem is that the tone is badly dulled down. it seems to have something to do with my incorrect use of referenced ground points. i'm using a jrc4558d opamp instead of the tl072, and i also tried playing with a opa2134pa. i'd want to use the opa2134pa in the end if it works. the pinout for all of these dual opamps are the same.

my main interest would be if somebody could point me in a practical direction for using inverting opamps with audio? there isn't really much out there. everything i read says to plug the non-inverting (+) input of the opamps into ground, whereas the tremulus lune sends it to the halved voltage reference. if i do that on one amp of the dual opamp, it works but sounds dull. if i do it on both i get no noise coming out. if i don't do it at all and leave it floating the signal goes through clear and not dulled down but the attenuation doesn't work anymore and the signal goes right around and doesn't get altered by the ratio of the resistances. i've also tried singing them to ground and the halved voltage through capacitors like in a non-inverting opamp. also, i've tried connecting it with a 2.2m resistor with similar effects to a direct link.

other than the LFO and the opamp, my design is the same as the tremulus lune for the very simple opamp part. i assume that if i used the correct opamp then the issue would go away. i doubt this dulling is common with the tremulus lune after searching and reading that everyone loves the tone as a clean boost with the depth all the way down. i ommited the 1M pull down resistors at the beginning and end as it's just on breadboard right now.

hope it's not too complicated and that someone has a simple-ish solution for me?

much appreciated (:

darron
Blood, Sweat & Flux. Pedals made with lasers and real wires!

PerroGrande

When you use an inverting configuration with a single supply, you need to provide a reference point to the op-amp.  The examples you see where the non-inverting input is grounded are using a split (positive and negative) supply.

Another thing to keep in mind with inverting configurations is that the value of the resistor in series with the inverting input is effectively the impedance of the unit.  If you have an inverting op-amp first *and* the value of the series resistor is low, your circuit could suck tone, which could explain the dull sound you are hearing.

Here is a basic example:



This inverting configuration is using the pinout of a single op-amp (as opposed to a dual amp), but the concept is the same.  The non-inverting input is tied to the 1/2 supply reference through a resistor (the value is approximately the value of the series and feedback resistor in parallel).  The stage will have a gain of 2 as determined by the ratio of the resistors.  The input impedance of this stage will be 100K -- the value of the series resistance.


darron

hey PerroGrande! thanks for the example, i was a bit shocked to think that you found a perfect example, then realized that you would have had to have made it. thanks for the effort (:

i see what you mean. the value of my series and feedback resistors are so low that they are less than 1K, in a matter of ohms. maybe i made my LED/LDR too good. i suppose that this might be the reasons that the tremulus lune uses no resistance at all on the second gain stage and connects the non-inverting input straight to the half-supply reference. still, people like the tone of it.

for a fix, do you think that if i got my series resistor in approximately the 100k or so range and the feedback resistor to match to make unity, that would make a large improvement? i'd try it right away but i'd have to go to the shop to buy a different LDR. if i used 2x 100k resistors then i'd hook up the reference with approx. 50k (thanks for the tip of using the resistance of the two in parallel, i didn't know that). i don't suppose a buffer will help. the chip i want to use has jfet inputs also.

any thoughts on why the tremulus lune doesn't do this with either of the opamps in ic1? also, people don't complain of a dulling effect, they say the contrary.

you've taught me some very good foundation to work with in the future. thank you very much.

darron
Blood, Sweat & Flux. Pedals made with lasers and real wires!

Gus

Google, ASK etc.
Look in the schematic section at circuits like the opamp muff fuzz.
  Read some books

gez

A great book to learn about op-amps is the following:

Operational Amplifier User's Handbook by R. A. Penfold (Babani Publishing)

"The first part of this book covers standard operational amplifier based "building blocks'' (integrator, precision rectifier, function generator, amplifiers, etc), and considers the ways in which modern devices can be used to give superior performance in each one. The second part describes a number of practical circuits that exploit modern operational amplifiers, including high slew- rate, ultra low noise, and low input offset devices. A range of interesting projects are described."

Easy to understand and dirt cheap (even cheaper if secondhand on Amazon/ebay).

Plenty of text books cover the basics, but the Penfold books tend to be more 'user friendly' and more practical.
"They always say there's nothing new under the sun.  I think that that's a big copout..."  Wayne Shorter

PerroGrande

Hi Darron,

You certainly don't want to use ultra-low values in the inverting stage for the reason of impedance that I mentioned before (you also don't want to use ultra-large resistors, either, for reasons of noise, among others).

So -- it doesn't matter if your op-amp has FET inputs, if you use very low resistances, you're going to have issues with impedance.  I would certainly recommend raising the resistances to something more reasonable. 

The inverting input tied directly to 4.5 volts does not change the tone.  The resistor I included was to quell errors due to the bias currents drawn by op-amps with bipolar inputs.  FET op-amps generally can go without this resistor all together.

If your LDR resistance is dropping very low, forcing you to use low value resistors, you can do some tricks with Ohm's law to help control your LDR.  Adding series and parallel resistances can be used to limit the effective minimum and maximum resistance that your LDR presents to the op-amp circuit.   For example -- say that you create a crazy LDR that goes from 20 Meg resistance to 500 Ohms resistance.  This is a very wide range and probably impossible to directly control op-amp gain.  However, if you put a 47K resistor in series with the LDR and a 47K resistor in parallel with the LDR, now you have a different range. 

By Ohm's law, when the LDR is low (500 ohms), it will dominate the 47K resistor in parallel with it producing an effective resistance of just under 500 ohms.  When the LDR is high, the 47K will dominate.  So now the LDR looks like a variable resistance of between 500 and 47K ohms.  But... by virtue of the series resistance (which adds), the effective resistance is now between about 47.5K and 94K --  a much smaller and controllable range (it might be too small for your application, but I was just picking values to make a point).   You can also fiddle with how much light the LDR cell gets by messing with the LED side of things -- i.e. limiting the brightness of the LED.

darron

#6
Quote from: Gus on December 27, 2007, 10:45:18 AM
Google, ASK etc.
Look in the schematic section at circuits like the opamp muff fuzz.
  Read some books

hi. i didn't realise that the muff fuzz was an inverting opamp. thanks. it's harder to find inverting schematics for audio because people want to keep thinghs in phase.
in terms of reading some books, i took a look over brian wamplers book with little help. i went over craig anderton's which actually has a circuit that only inverts the phase, but it's for a split supply so i couldn't make anything useful out of it for my single supply approach. from everything that i've googled, there's little help. i've found some really good links to opamp stuff but they are mostly for non-inverting:
http://colomar.com/Shavano/intro_opamp.html
http://www.generalguitargadgets.com/richardo/distortion/index.html



Gez, I'll take a look for that book. Extra knowledge would be really good for me, and you think that it takes a practical approach so it sounds perfect. I wanted to stay away from opamps but they just seem too useful, and easy(er) (:





PerroGrande, thanks heaps for your help. I'm sure that you're advice will have fixed everything. I'm just going to try a different LDR though so that I can keep the wide sweep, and get a high quality  sweep also instead of limiting the LED.


thanks again guys.

darron
Blood, Sweat & Flux. Pedals made with lasers and real wires!

gez

Quote from: darron on December 27, 2007, 06:48:29 PM


Gez, I'll take a look for that book. Extra knowledge would be really good for me, and you think that it takes a practical approach so it sounds perfect. I wanted to stay away from opamps but they just seem too useful, and easy(er) (:

It's practical in that it gives a lot of useful advice, without going into too much detail: Penfold is an excellent educator and is good as explaining the big picture.  It's an old book, so a little outdated, but IIR (years ago I got it out of the local library and copied stuff into a notebook) a lot of it is still relevant; plus it covers the basic building blocks and has schematics.  I'm sure you'll find it useful.  Amazon should have copies (it only costs around £4 new).
"They always say there's nothing new under the sun.  I think that that's a big copout..."  Wayne Shorter

darron

Thanks heaps PerroGrande. I ended up taking your advise. Since the light resistance of the LDR is only about 200 ohms, I put a 56k resistor in series and a 56k in for the feedback control. I hooked the half voltage up with a 33K resistor as it's the best that I have for now. I used a 33k on the first inverting stage also as it's in a similar range of around 56k on series and feedback. There's no capacitor between stages. It sounds MUCH brighter now. :D

Would you think a jfet buffer might be a good idea for before or after the circuit? Am I working with reasonable values or resistance as I can easilly change them all? I now have a really cool sounding tremolo that I'm extremely happy with.

Thanks for all of your help. It's really made a massive difference.
Blood, Sweat & Flux. Pedals made with lasers and real wires!

PerroGrande

Always glad to help, Darron -- good to hear you are more happy with your tremolo.

If you like the sound, I'd be a little reluctant to change anything.  The values you have are certainly reasonable choices, and you have lots of room to adjust them if you choose.

However, that said, you might find some benefit from a simple JFET buffer ahead of your circuit.  The 33K input resistor is a bit on the low side for a guitar circuit and could suck some tone.  A buffer would allow you to keep your values the same and eliminate any and all impedance issues.  A jfet buffer is also an easy build -- so if you don't like what it does, you're not out a bunch of time or expense.