Help Coupling 2 cicuits

liquidair

Joined Oct 1, 2009
142
How's did it go yesterday? Get it working?

I don't see how you'd use an optocoupler or even why in this case. The fundamental problem when combining the two circuits is that the transistor is using ground as it's positive voltage reference and the opamp circuit is using is using +9V (or +4.5V with respect to ground). Ground, V+ and V- are different things to the circuits, but they need to be the same things to work together.

I tried to draw a schematic for you to better explain what is going on and why it's not working but I couldn't draw anything that seemed like it would help (and besides, I'd prolly get flamed for the "errors" in the schematic).

Let's see if I can explain. Transistor circuits generally use a single supply, so 9V and GND in this case. When used as an amplifier, they need a positive base bias voltage but note, this bias voltage is not generally 1/2 the supply like it is in opamps. The bias voltage essentially sets the water level, or the midpoint for our signal to swing up and down.

In your circuit, you grounded the + terminal of the battery and the emitter was tied to the - terminal. Since you said you are getting a signal out with gain, I assume that you have a bias voltage of some sort, eventhough it may not look like the picture in the link i posted.

Opamps can use a single or dual supply, in your case, since you didn't show a ground connection on the power pins, you are making the 9V look like a dual supply to the opamp or +/-4.5V with respect to ground. Again, since you said the circuit works on its own, I assume the supply is referenced somehow to ground (this would be two equal high value resistors, one from + to gnd, one from - to gnd (this is what Ron was talking about).

So our transistor's collector is referenced to gnd, it's emitter is referenced to -9V and it's base must be some - value between (check with a DMM). Our opamp's + pin is at +4.5V, it's - pin is at -4.5V and I assume it's pin3 input is at 0V or gnd.

When we combine the two circuits, our opamp gets it's + pin grounded (by the transistor's supply connection but our voltage reference is also grounded, which means we bypassed the reference resistor from V+ to gnd, which means our input to the opamp is now at V+ or ground in this case. Because the opamp no longer has a half supply reference, it won't work. Does that make sense?

So again, your options are:

If your would like to use the transistor circuit, then the opamp needs to be run as a single supply. For convention's sake, rewire the transistor to a positive supply and ground the negative portion of the supply. The opamp will then have a +9V + and a GND -, so you need to generate a half supply voltage for pin3 with 2 simple resistors (or see the recent post about the guitar preamp).

If you want to keep the opamp as a dual supply config, then use a second opamp in place of the transistor.

Hope this helps!
 

Thread Starter

DCent

Joined Sep 21, 2012
20
How's did it go yesterday? Get it working?
I did some shopping yesterday but got back home too late to try anything. I'm going with changing the first circuit to an opamp boost. If there's any weirdness I'll then apply the voltage splitter resistors to the second circuit and see if that works (the schematic I found for the boost already has them.)

I think I'm understanding the problem and solution here. Thanks for the assistance/explanation.

In my research I found a product called "virtual batteries" which allows for positive grounded FX pedals to be used off the same daisy chain supply with neg ground pedals. I don't know how it accomplishes this (couldn't find a schematic) but it could potentially address my initial problem. My circuit isn't dependent on the positive ground and I believe you've come up with a nicer solution but some older guitar pedals depend on that setup for their tonal properties. Would be nice to have these interface with traditional setups using a hardwired version of the "virtual battery." Might be able to come up with something really interesting.

...

Also on the optocopler, do you know if those half-wave rectify signals? I'm guessing they would.
 

liquidair

Joined Oct 1, 2009
142
The virtual batteries thing looks interesting.

As far as optocouplers go, I'm not sure what you mean. I haven't really got into them until just a few days ago where I'm building a my first digital device, so I'm using them to control relays without having to have a digital control lines crossing over my audio lines potentially adding noise to the circuit.

Can you let me know what device you are looking at? And how were you thinking of hooking it up to solve your problem?
 

Thread Starter

DCent

Joined Sep 21, 2012
20
As far as optocouplers go, I'm not sure what you mean.

Can you let me know what device you are looking at? And how were you thinking of hooking it up to solve your problem?
On the optocouplers, I thought it might be able to break up the part that was making the short happen. The transistor worked, but it caused a lot of noise, so maybe optocouplers could do it better. I'm going with changing the first part though, to remove the short.

I didn't have a specific part in mind, and the application thing was just general... making positive grounded fuzz guitar pedals (which sound different than the neg ground counterparts) that interface effortlessly with traditional setups.

The half-wave rectify thing... I tried making my own crude optocoupler before I knew such things existed (basically an led modulator beaming into an led attached to a amplifier) ala this: http://www.youtube.com/watch?v=AI1LGcb9oGI

I was thinking I could get some interesting noise generation (static and pops) by messing with light interference. Didn't quite work as intended, but I noticed the output sound wave was rectified, which makes sense.

So, I have another idea using half-wave rectification to build a distortion circuit and if the optocouplers work in that one, it's pretty much built... on paper anyway.
 

liquidair

Joined Oct 1, 2009
142
Thank you for that video, pretty cool stuff. I never even thought about that.

As far as using an optocoupler (look at LDR's) in that way, you do have the halfwave rectification as you say. You might be able to use a precision circuit of some sort to get full wave rectification, but I don't think it would be suitable for a pedal. I also doubt the audio quality would be that good (which may or may not be an advantage, in a pedal that could get cool).

Do you have any specific pedal that uses the positive ground setup? I can't imagine how you'd get a potentially better tone, but it would be interesting to take a look. I haven't looked at guitar pedals for a long time.
 
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