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
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.
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.
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.
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.