Pinball / Standout Op Amp Voltage Question

[brazdj]

Hello!
I have the Pinball all done on a breadboard and my op amp voltages do not quite match those that are posted. The only pins of concern are the non-inverting pins downstream of a 2M2 resistor, so: Vb -> 2M2 -> Pin 4. I am getting a voltage of ~ 1.7 whereas the Pinball pdf states a voltage of ~ 3.54.

Granted I do not have all of the components for a true Pinball. I am skipping to a true bypass configuration so I exclude most of the Power & Switching components. I do include the 1N5817, the Inductor, the zener diode, both 0.1 uf caps, and the 150 uf cap. So my voltage on Pin 8 of the op amp is ~ 8.9 V (coming from a ~ 9.4 V DC One Spot). My Vb voltage is therefore ~ 4.45 V.

How is the current on the PCB being limited such that the 2M2 resistor only drops 0.46 Volts?

Thank you for your time,
Daniel

[madbean]

The likely explanation is that your multimeter is creating a voltage divider with the 2M2 resistor which causes you to read the true voltage lower than expected. IOW, the act of measuring causes the result to change. You could stick a 10M to ground at the circuit input (all the VFE circuits have this) to see if that gets you a better reading but I don't think it will. I wouldn't be concerned about it unless you actually hear the typical result of a mis-biased opamp. That would be a ratty, "farty" sound on notes.

[brazdj]

Thank you very much Madbean.

I did try the 10 M resistor to ground and got no discernible difference in my measurement. However! I did that 10 M test on a separate board (recreated the power section and voltage dividers separately to test away from pinball circuit). I will add the 10 M to my Pinball later on to see if that impacts my sound.

I had been testing the circuit with input from an MP3, not my guitar. So with the MP3, the sound was distorting quite a bit.

Hopefully the 10 M to ground on the input will help in the actual circuit. If not then I will just plug in different resistors to get that ~0.5 Voltage drop.

Will let the message board know my findings once complete.

[brazdj]

Oh!, Quick Aside on the posted audio-section Schematic:

This won't affect people who build the PCB, but for others the 10K to ground just upstream of the "1K -> output" should be 100K to ground.

[brazdj]

Gulp...  When I plugged in my guitar it worked beautifully... The voltage on those pin 4s is still ~ 1.7 V. I did add the 10M pull down resistor.

Thank you for your patience.