Finally stepping out of my comfort zone

[Gurner]

@Gurner

Ok. Just to clarify...

If I set the resistor between V+ and the op amps non inverting input at ~800K and set The resistor between the op amps non inverting input and GND at ~1.2M then I should have the input gain stage biased just about right to send the signal directly from the input op amp's output directly to the BBDs input without the need for the 1uF DC blocking cap?

yes that's right, if I'm reading your IC's datasheet correct! (the datasheet say the input to the IC should be set at about 0.6 of VDD)

You also mentioned above that I should aim for a 4X to 8X output from the op amp gain stage. Does that still hold water with this basing arrangement? I would think so  

Yes it does, don't get bias confused with gain, regardless of the bias arrangements the gain needed works out something like....

'IC's max input level' divided by your 'guitar's maximum output signal'  


To get the best signal to noise ratio, you want sufficient gain to get the maximum signal through your IC as possible (but no too much gain that your IC input pin clips)

....for example, if the IC is expecting 4V peak to peak max & you guitar outputs 1V peak to peak max, then you'd need the opamp to have a gain of 4 or less (a bit less actually to allow a little headroom & avoid clipping) .....this can never be an exact science, because there are many variables to do with your guitar ouput -  string gauge, string material, pickup type, guitar playing style etc...this is why I recommended having  a variable gain trimpot to tweak your opamp's gain to suit.

[Govmnt_Lacky]

...this is why I recommended having  a variable gain trimpot to tweak your opamp's gain to suit.

That is the plan!  

A 100K from the inverting input to ground AND a 1M trimmer from the inverting input to the op amp output. This way, I can tweak the trimmer until I get the correct gain.

At least, that is what I interpret is a good way to adjust the output gain.

-OR-

Would it be better to have a set value between the inverting input and op amp output, then... put a trimmer between the inverting input and GND?

[Govmnt_Lacky]

One last question before I get this on the breadboard..

Is there a reason why I need to use "higher" values for the biasing resistors?

Instead of using the 1.2M/800K network to get the bias, could I use smaller values like 470K/330K? It is not EXACTLY 40/60 but it is pretty close.

[gcme93]

Both choices you've suggested draw currents in the range of uA (I believe 6uA and 15uA) rather than mA which is what we're after. We obviously avoid 100 ohm + 100 ohm because that would be drawing 60mA, which would be double what the whole pedal uses.

In terms of power there's no difference really. I might have missed something in a different aspect, but I can't really see what else would be relevant. Go for it!

George

(PS with resistor tolerances, it probably doesn't make a huge difference with the bias point precision, but theoretically 470K and 330K are more accurate to 7V)

[Govmnt_Lacky]

Both choices you've suggested draw currents in the range of uA (I believe 6uA and 15uA) rather than mA which is what we're after. We obviously avoid 100 ohm + 100 ohm because that would be drawing 60mA, which would be double what the whole pedal uses.

In terms of power there's no difference really. I might have missed something in a different aspect, but I can't really see what else would be relevant. Go for it!

George

(PS with resistor tolerances, it probably doesn't make a huge difference with the bias point precision, but theoretically 470K and 330K are more accurate to 7V)

Some of this is above my head 

So are you saying that the 800K/1.2M Vref biasing network is TOO big? Do you have a better suggestion for values?


After reading this again, I see what you are meaning! 

I also see why I need to get the output voltage of the op amp gain stage to 7.2V as this is the required input to the BBD (0.6 of 12V which is Vdd)

So essentially, keeping the values higher for the Vref resistor network will also reduce the overall current draw of the circuit... right?

If I am correct.... then why don't ALL circuits that require an op amp gain stage use higher value resistors for biasing in order to keep the current lower? Is it an availability issue in manufacturing?

[gcme93]

*Post modified to answer new question as you understand the old point *

Yepp, high values means less current is drawn.

In the scheme of things, your values don't need to go that high because the difference in current being drawn is almost nothing compared to the rest of the circuit.

If we used a ratio of 1.2K to 0.8K, the current would be 6mA which is a little too much.

However, just one multiple of ten up (12K and 8K) and the current drawn is now 0.6mA which is pretty darn low. To make it a pretty much negligible amount, 120K and 80K draws 0.06mA


I guess you're getting the point now. We want the biasing resistors to add up to about 500K or more to make sure the currents they draw are tiny. Past that all that matters is getting a suitable ratio for setting the voltage right

(in terms of manufacturers, maybe it's cheaper to use lower resistance parts? Maybe there's another tiny influence on performance?)

[Govmnt_Lacky]

I just want to say a great big...

THANK YA VERY MUCH!!!
to all who have helped me understand what the heck I am looking at!!

Without the shared knowledge on this site it would take me 10x longer to understand!!!

More to come.... HOPEFULLY  

[Gurner]

Re the high value input resistors, it's not just to do with aiming for less current draw, but if you're plugging in a raw guitar signal, you need to keep the input impedance of your circuit high (else you'll experience treble rolloff on your guitar's signal).....that's why it's best to stick with resistances in the order of 1M.

[gcme93]

Re the high value input resistors, it's not just to do with aiming for less current draw, but if you're plugging in a raw guitar signal, you need to keep the input impedance of your circuit high (else you'll experience treble rolloff on your guitar's signal).....that's why it's best to stick with resistances in the order of 1M.

+1

Ahh of course! I knew I was forgetting something

[Govmnt_Lacky]

Re the high value input resistors, it's not just to do with aiming for less current draw, but if you're plugging in a raw guitar signal, you need to keep the input impedance of your circuit high (else you'll experience treble rolloff on your guitar's signal).....that's why it's best to stick with resistances in the order of 1M.

Are you speaking of the Vref resistor network -OR- the resistors that set the op amp's gain?

I would like to keep the current draw down as not to limit my powering options. I will start with the 100K/1M trimmer for the gain and go from there.

I dont mind using the 800K/1.2M Vref network however, I just wish I could find a more common value than 800K  I guess I will make due.

[Gurner]

Are you speaking of the Vref resistor network -OR- the resistors that set the op amp's gain?

I dont mind using the 800K/1.2M Vref network however, I just wish I could find a more common value than 800K   I guess I will make due.

It's the vref resistor network I was referring to....don't get too hung up on the exact values....for example, in the earlier random example I gave (on a whim, I probably picked hard to source resistor values!) 750k top resistor & 1.2M lower resistor should be close enough    ......you are ideally shooting for a 40:60 ratio (top resistor:bottom resistor in your vref resistor chain)   & where the resistor values are common & easily sourcable ....therefore if you use a 1M lower resistor, the upper resistor needs to be (1,000,000/60) * 40 = 666k    or if you use a lower resistor of 1.5M then the top resistor needs to be (1,5000,000/60) * 40 = 1M

[Govmnt_Lacky]

UPDATE:

Got this on the breadboard and it looks like I will need to tear it all down and start again 

After applying power to take some measurements, my regulated PS was showing a current draw of about 600mA 

It slowly crept down to 200mA and I could detect the ever so slight odor of burning failure!  I believe it was coming from the 4013 clock chip.

I am thinking that I may have hooked something up incorrectly OR some of the components were touching and they were not supposed to. I will need to re-build it and give myself a bit more space.

More to follow 

[Ronan]

There is a quick check for breadboards drawing to much current - before applying power, measure the resistance between power (+9V) and ground. If you have say 100 ohms resistance, then the current will be at least V=IR, 9=I x 100, I= 90mA. Most chips draw a few mA each, bias resistors much less, its a good guess that most analogue circuits will draw less than say 30mA. So if you measure less than 300 ohms, then double check your circuit. Most circuits I have built measure well over 1K from power to ground. After I cooked a battery, I always do this check.

[digi2t]

There is a quick check for breadboards drawing to much current - before applying power, measure the resistance between power (+9V) and ground. If you have say 100 ohms resistance, then the current will be at least V=IR, 9=I x 100, I= 90mA. Most chips draw a few mA each, bias resistors much less, its a good guess that most analogue circuits will draw less than say 30mA. So if you measure less than 300 ohms, then double check your circuit. Most circuits I have built measure well over 1K from power to ground. After I cooked a battery, I always do this check.

Shit... just printed that, and pinned it to the wall over my bench. Thanks for the tip Ian. That's about as KISS as KISS can be.

[Ronan]

I forgot to mention that it may take a minute or two for the reading to settle, as the filter cap/s charge up.

[Govmnt_Lacky]

To be honest, I have put this on the back burner for now 

Between XP ALLs, Ludwigs, Electric Mistresses, and a few more... I have to clear some serious room before "experimenting!"