Issue with signal follower circuit and source input impedance

[BeatleManiac]

Hello all!

I'm designing a color organ to animate LEDs around the perimeter of my bass amp.  I want to just "listen" to the source guitar signal and use the value to change various animation parameters.  It works fine with the practice bass amp I'm using to test the enclosure, but not with my gigging amp.

Here are the specifics:

Inputs:
(2) 1/4" guitar jacks that are paralleled. I want to pass signal even if my circuit is off. Guitar cable ground to my enclosure ground, signal to a 1uf Poly cap to an op amp configured as a buffer, i.e no gain.  From there the output of the op amp sends the signal on to the rest of the circuit.

I connect my guitar cable to one of the input jacks and use another guitar cable to go from my effect to my amp.

My practice amp is a 5W Roland micro cube bass amp with an input impedance of 1MOhm.  Connecting my enclosure to that amp allows me to see the signal in my circuit.  I can use the gain knob on my enclosure to adjust the input level to the effect.

My gigging amp is a 250W TC Electronic BG250-115 bass amp with an input impedance of 500kOhm, half that of the Roland.  When I connect to the big amp I can hear the guitar signal fine in the amp with no noticeable coloration from my effect.  However, I see no signal in the effect, even when the gain knob is all the way up.

Any suggestions on what I can do to fix this issue?  I'm still a noob when it comes to electronics but I've learned a lot over the past year and a half developing this project.  Any help would be appreciated.

Thanks,

Steve Hurd
Floyds Knobs, IN

[Marco]

Guitar cable ground to my enclosure ground

It has to be connected to your circuit ground too.

signal to a 1uf Poly cap to an op amp configured as a buffer, i.e no gain.  From there the output of the op amp sends the signal on to the rest of the circuit.

What kind of opamp? How did you set the DC bias of the buffer?

[BeatleManiac]

It is a plastic enclosure.  Signal ground=circuit ground.

I didn't think I needed to set the dc bias since I'm passing the signal through a cap.  Wont the cap block any dc signal?

Op Amp: M89742, LF353N

Steve Hurd

[Marco]

Wont the cap block any dc signal?

If you don't set the DC bias, it will be set by leakage currents. Through the capacitor and into the opamp input. Or in other words, very hard to predict.

Given that you want it to be biased at ~1/2 your supply voltage, hard to predict is not what you should be looking for. Bias the positive input of the opamp with a high value resistor divider between the supply rails.

PS. I am assuming it's a non inverting buffer and you are using a single ended power supply.

[BeatleManiac]

I'm using +- 7v for the supply rails.  I'm at work, so I can't confirm the inversion config, but it's whatever the recommended config is for a buffer.

Since I have +- supply rails, can I just bias the op amp with a ground reference?

Steve Hurd

[Marco]

Since I have +- supply rails, can I just bias the op amp with a ground reference?

Yeah, just a single high value resistor to ground.

[BeatleManiac]

I'll try that when I get home.  Thanks!

Steve Hurd

[BeatleManiac]

I am using it in the non-inverting configuration.  What is the difference, if any, between putting the grounded resistor on the positive input vs the negative input?  Is there any concern about having a direct ground connection (through the resistor) to the input?  ie noise or coloring the source signal.

I had to redesign one of my boards which also happened to have the op amp on it so I'm just now getting back to this.

Thanks!

Steve Hurd

[Marco]

The negative input is connected to the output, it's not floating. The positive input when connected only by a coupling capacitor would be floating and have no DC bias (it will of course still have a DC voltage, but it's generally going to go to the rail making the circuit useless). The bias resistor forms a high pass filter together with the coupling capacitor, so the cut off should be low enough. But with a 1 Meg resistor and a 1 uF capacitor it's 0.15 Hz, so don't worry about it.

[BeatleManiac]

Excellent!  Thank you!