### Author Topic: PT80 delay: DC voltages of opamp question.  (Read 1994 times)

#### flo

##### PT80 delay: DC voltages of opamp question.
« on: April 21, 2008, 08:45:48 PM »
The following voltges were posted for the opamp in the PT80 delay:
http://www.diystompboxes.com/smfforum/index.php?topic=66619.0
TL072
1. 6V
2. 6V
3. 5.9V
4. 0
5. 3V
6. 6V
7. 6V
8. 12V

This what I'm measuring on my PT80 also and the thing works.
But what's bothering me is this:
If pin 5 (+input of opamp B of the TL072) is at 3V, why are pin 6 (-input) and 7 (output) then at 6V?
I expected pin 5 to be at 6V (=Vbias) and pin 6 also. I don't get it...

#### flo

##### Re: PT80 delay: DC voltages of opamp question.
« Reply #1 on: April 23, 2008, 09:36:47 AM »
How can the opamp work ok if its + and - inputs are 3V apart?
The output should have been at 0V then, so why isn't it?
The + input (pin5) should have been at 6V (Vbias) so why is it 3V (measured by me in my PT80 and by others in their PT80s)?

#### Pushtone

##### Re: PT80 delay: DC voltages of opamp question.
« Reply #2 on: April 23, 2008, 12:39:57 PM »

Perhaps the pot locations matter?  LFO? I don't have the schem up in front of me.

Why not email the circuit designer?

Or even the manufacturer. The circuit is just a modified version of the application in the data sheet.

Dave

It's time to buy a gun. That's what I've been thinking.
Maybe I can afford one, if I do a little less drinking. - Fred Eaglesmith

#### slacker

##### Re: PT80 delay: DC voltages of opamp question.
« Reply #3 on: April 23, 2008, 01:08:20 PM »
Is pin 5 the + input that is the input to the effect?

If it is then the reason it is reading 3 volts is because your DMMs impedance is affecting the reading. A lot of DMMs have an impedance of about 1Meg ohm so when you try and measure the voltage at pin 5 the impedance of the DMM forms a voltage divider with the 1M resistor and pulls the voltage down.

Here's a dodgy drawing to make it clearer, because I'm bad at explaining things

Point R 6volts ------1M---------Pin 5
|
|
1M impedance of DMM
|
|
Ground

The voltage on the pin is probably really 6 volts or as you've said the circuit wouldn't work. If you need to measure it accurately you can make a buffer out of a Jfet input opamp connect the probe to the input of that and take the output to your DMM. The buffer will have an impedance of 100s of Megs so it won't load the voltage down.
« Last Edit: April 23, 2008, 01:13:15 PM by slacker »

#### flo

##### Re: PT80 delay: DC voltages of opamp question.
« Reply #4 on: April 23, 2008, 05:23:22 PM »
Is pin 5 the + input that is the input to the effect?
Yes it is.

If it is then the reason it is reading 3 volts is because your DMMs impedance is affecting the reading.
That must be it: The impedance of the DMM.
My DMM and my oscilloscope probe both measure 3V so they probably both have a resistance around 1M.
So if I lower the 1M bias resistance that goes from Vbias to pin5 temporarily to 100k, I should measure 6V again...
And indeed! I just put a 100k resitance parallel over the 1M bias resistor and measured the voltage on pin5: 5.56V.

@slacker: Thanks a lot! The world of electronics makes sense again...