? about appliying Vref technique to +/- supply opamp

[onboard]

Heres a schem from the LF353 app notes that I have on the breadboard.


After alot of searching and reading on virtual grounding to use a single V supply, my guess would have been to supply Vref at pin 3 (IC1 + input), pin 4 (IC1 -V)  and keep pin 5 (IC2 + input) grounded.

That didn't work at all, so I kept moving things around until the circuit ran. (nicely too, I had already hooked it up with a +/- 18V supply to have something to compare to)

My question is, why does it work with the changes I highlighted? It seems almost backwards to what I though I understood, so I'm probably mixed up about the concept. If someone could explain this example, I'ld probably get the larger picture.

[aron]

I believe you need to ground pin 4, and give both pin 3 and pin 5 VREF. This is a dual op amp, so both inputs need to be biased.

[onboard]

When I hit pin3 with a Vref tapped off the same point that supplies pin5 the signal disappears. (canceled out because it creates a loop?) And when I give pin3 Vref from a seperate voltage divider, there's no change in output at all...

Wierd.

[spongebob]

Hm, that's really strange...

The first opamp there is just a buffer with a gain of 1, and the resistor from pin 3 to ground (or vref in this case) sets the input impedance of the circuit, if you want to run your guitar directly into this then increase it to 1 Meg. If you leave out this resistor, the DC level in the first opamp is not defined (there's a cap at in- and output)!

Can you measure voltages at each pin and post them? This usually tells alot...

You also might want to add an output cap after the last opamp to get rid of the 4.5V DC offset.

[onboard]

V=7.83 (time for a fresh battery...)

Pin voltages as per the schem, with pin 3 getting Vref
1) 3.86
2) 3.82
3) 3.88
4) 00.5mV, I never have read 0.00V at ground on this bb
5) 3.89
6) 3.89
7) 3.89
7.78

Hmmm, I'll go look at R.G.'s opamp debug rules, seems like something's funny with 5, 6, and 7 reading the same...

edit- uhhh, I forgot 8 parenthesis is "cool"...kinda goofy

[spongebob]

Voltages look ok to me, everything biased at V/2.

Pin voltages as per the schem, with pin 3 getting Vref

How did you apply Vref? Through a resistor?

[Jason Stout]

How did you divide your voltage to get Vref?

[onboard]

I used the ole' two 10k resistors in series between +V and ground, and tapped the junction, with a +10 uf to ground (+ towards supply) for tidiness. The meter reads a nice 1/2 supply.

As an aside, the circuit sounds great fed with a Discman, but when I added a 1M to ground at the signal input and tried a guitar, all I get is barely audible blatty distortion.

This would be a nice tone control to tweak and apply to guitar or bass. The range is a strong +/- 20dB.

[Jason Stout]

As an aside, the circuit sounds great fed with a Discman, but when I added a 1M to ground at the signal input and tried a guitar, all I get is barely audible blatty distortion.  

I think you just found your problem. The circuit works fine with the cd player and it should work fine with guitar, so double check the wiring at circuit input (guitar to effect, shield to ground) - I'll bet your problem is there.

[onboard]

:oops:  

Uhhhh, I don't belive I just did that.

Really.

The "barely audible blatty distortion" was because my headphones were still plugged into my practice amp which kills the speaker but I was sitting there like a shmendrick staring at it cranked going "Why does that sound so tiny and crunchy?"

Probably just blew up my headphones...good thing I wasn't wearing them.

Anyway, yeah, it sounds great.

edit - Plenty o' headroom with two batteries reading around 16.5v. I think the opamp is good to 18v

[Jason Stout]

Çongrats!! ........Don't feel bad, the headphone thing happened to me once.

[aron]

COOL! It's solved then!

If you have the time, why don't you post a revised schematic when you are finished?

[onboard]

Ya sure, you betcha! Right now I'm tweaking values to try and tune it in a little closer to guitar/bass range. Kinda like a blind squirrel looking for a nut, though. Soon as I get something halfway decent, I'll post a revised schem in a new thread.

I already had it running at 18V through a 150W bass combo with a P-J...dang. +20dB pumps.

The only drawback I can see is the way they controls are so strongly interactive. More parametric than shelving. Could be a good thing though! With stock values, that midrange hump peaks right at 1kHz, with +10dB at 300Hz and 3kHz  :wink: