Guvnor / inspired distortion with a weird voltage reading

[nonoxxx]

Hello I made a distortion circuit wich is something between a rat and a guvnor,

Everything is fine and sound fine except the voltage reading at the 5th pin of the opamp (approximately 3v instead of 4.5v).

The circuit is  the guvnor tag board on tagboard's blog (with other values and minus the tonestack replaced with the rat's filter)  and i get the 4.5v before the 1M resistor , but 3V after at the opamp's pin.

I would like to know if it's something normal for theGuvnor circuit?

Thanks

[antonis]

Everything is fine and sound fine

So you simply are curious...

What's your multimeter impedance..??
(time for me to be curious..)

[nonoxxx]

Thanks Antonis, but I have no idea how I can know the impedance of my multimiter lol

[nonoxxx]

So if I understand correctly the input impedance of the multimeters should do a voltage divider with the 1M resistor,

I know that on this kind of circuits but with a  10 K ( like on the tubescreamer) I dont't notice voltage change with my multimeter (or it's a really little change)

[antonis]

Quite right nonoxxx..
(it is actually set in parallel with the resistor you measure but you can consider it voltage dividing in the sense of Thevenin equivalent resistance, which is the parallel combination of the 2 impedances..)

When an expected voltage across 1M resistor is 4.5V and measured voltage is 3.0V "something" loads 1M resistor..
(that "something" should exibit an impedance of 2M2..)

For a measurent on a 10k resistor there also should be some domination but much less than the one on the 1M resistor..
(about 100 times lower..)

If you have a precision power supply, you can easily determine your multimeter's impedance by taking 2 - 3 measurements on some high value 1% resistors..
(1M, 4M7, 10M..)

[nonoxxx]

Thank you , SO I think all is Ok with this circuit .

Another question is why  the value of these resistance in the input change a lot from on circuit to another , for what I know it's called a 'pull up' resistor , but why it's  10 k on a tube screamer and why 1 meg on other circuits ?

[antonis]

I'm afraid I can't follow you.. 
(on which specific resistor do you refer ...??)

We usually call "pull-up" a resistor connected to positive supply (for connecting there following circuit when some previous output is disconnected)..

If you refer on non-inverting input bias resistor, 10k is used on TS 'cause signal comes from an input buffer of very low output impedance..

1M resistor is used in cases of "direct" coupled signals to non-inverting input..
(where signal source impedance could be high enough to form a considerable voltage dividing effect with bias resistor..)

[nonoxxx]

Antonis, yes I refer to the input bias resistor, it's 10 k on a TS and 1Meg on a Guvnor, and for me it is supposed to do the same thing ( non inverting opamp input)

[nonoxxx]

How I understand :
Buffered signal -> 10k (low impedance)
Unbuffered signal -> 1M (high impedance )

[nonoxxx]

So for the signal this resistor value  need to be higher than the signal impedance

[antonis]

It simply is voltage dividing..

A 10k signal source feeding a 10k input "loses" 50% of its initial voltage..
Same source feeding a 1M input "loses" 1% of its initial voltage..

If we also deal with series/shunt capasitors (coupling/decoupling) bias resistor values are also set in accordance with desired cut-off frequency..

<So for the signal this resistor value  need to be higher than the signal impedance>
Partially true..

If we deal with pure resistive items (which rarely is the case..) we don't mind a lot as far as we can amplify the "rest" amount of the signal to our initial desired level..
(a signal of 1V can be set to 10V by an amplification factor of 10 - same results by an amplification factor of 20 for a signal which has lost 50% of its initial amplitude..)

I presume we have to approach impedance (mis)match issues..

[PRR]

> I have no idea how I can know the impedance of my multimiter

You already have data. A point that should be 4.5V drops to 3V when the meter is looking at it.

OR: you use your Meter Brand/Model to look-up the specs.

As you have not given a clue, I looked-up two basic meters and extracted their specs as example of how to find information. I doubt either of these is yours.

[nonoxxx]

Thanks , I got a velleman and I 'd take a look at the specs but the impedance is missing.

So Now I know why, so I can calculate the impedance .

[PRR]

> I got a velleman

Velleman makes a dozen DMMs. Most, not all, have specs. But if you don't want to say which model you have, that's OK.

[nonoxxx]

I was not at home , it's a velleman DVM831 , on the Velleman's website in the manual the impedance is omitted

[FiveseveN]

Missing in the manual and manufacturer's site but still listed in some shops (1 MOhm): https://www.ljudia.se/produkter/tillbehor-bygg-sjalv/instrument/velleman-dvm831 . That is the most common value.
A schematic of exactly what you've built would be useful in troubleshooting. See the debugging thread: http://www.diystompboxes.com/smfforum/index.php?topic=29816.0

[nonoxxx]

In fact there was no poblem with my build , I was just curious, I was just surprised  by the voltage

Be curious is a good thing because Antonis answers help me to understand more in depth impedance's things

[antonis]

I was just curious, I was just surprised  by the voltage

Glad to hear your curiosity doesn't include mains voltage surprises..

[nonoxxx]

I try to don't mess with 220 V

[PRR]

> 4.5v before the 1M resistor , but 3V after at the opamp's pin.

This clue tells you the meter is like a 2Meg load.

1Meg with 2Meg makes a 3:2 divider. 4.5V in makes 3V out. Only while poking the meter at it; the 4.5V at other pins when not poking the 1Meg bias tells us it is likely 4.5V when you are not looking right at it.

Remember that in future. Poking 10K nodes is little error, but as you get toward 1Meg nodes the meter loading is significant.

That testing says 2Meg and a store says 1Meg means they changed the meter guts without changing the model number, or the shop didn't actually test it.