Help with understanding voltages on op amp stages

[caspercody]

I have been working on my Sansamp Classic to get it to work. And gone as far as removing all switches, and most of the input buffer. When this was all together, I had the controls cranked and barely got any noise. If I hit the strings hard I got noise but sounded shorted, gated sounding, and barely audible. When I turn the volume control I get a crackle sound (crackle not okay). Now with switches removed, and input buffer still the same problems.
I am still getting almost 1 volt lower on my first stage of IC1 (see below). Does anyone have any voltage readings for this project? Should these pins all be close to the same voltage? What components affect the voltage for that first stage?



Q1
Drain 7.93v
Gate 4.183v
Source 0v

IC1
pin 1 4.381v
Pin 2 4.58v
Pin 3 3.67v
Pin 4 0v
pin 5 4.34v
Pin 6 4.38v
Pin 7 4.41v
Pin 8 7.93v

IC2
Pin 1 4.386v
Pin 2 4.386v
Pin 3 4.357v
Pin 4 7.91v
Pin 5 4.373v
Pin 6 4.383v
Pin 7 4.382v
Pin 8 4.36v
Pin 9 4.24v
Pin 10 4.25v
Pin 11 0v
Pin 12 4.354v
Pin 13 4.383v
Pin 14 4.359v

Thanks
Rob

[caspercody]

MAKE SURE YOU HAVE YOUR JFETS IN THE RIGHT DIRECTION!!! This pedal calls for a 2SK117, and I can not get them. So I used a J201. Printed out the data sheets, and thought they just needed to be reversed. Nope, bend one leg to the side, and the middle out.

Never give up, I spent the last two nights trying to get this to work. Tonight using a audio probe (again) it just didn't seem right. Sound going into the gate but nothing out of the source. Than I looked at the data sheets (again). And finally saw it.

Now I need to solder all those switches back in. Think I will have a beer first...

[caspercody]

I did get this back all together, and now it works.

But I am still curious about the voltage on the first stage of IC1. Pin 3 is still less voltage than pin 2, or pin 1 and the distortion does not seem as much as in the demos I hear. I read in a article on Geofex that the voltages should be within mvolts of each pin. Right now I am reading:

Pin 1 4.2v
Pin 2 4.2v
Pin 3 3.8v

[caspercody]

No one?

To make this question simple, on a op amp stage should the voltage of all three pins be really close? I have the following voltages:

Pin 1 4.2v
Pin 2 4.2v
Pin 3 3.8v


Is pin 3 to low? This is a TL072 chip.

[PRR]

> should the voltage of all three pins be really close?

In audio: often but not always.

Is there a schematic?

[anchovie]

Seeing a lower value on pin 3 in this opamp configuration is a common phenomena with cheap multimeters. I believe its something to do with the size of the Vref resistor and the internal resistor in the multimeter.

[R.G.]

... and a cigar to anchovie.

The impedance at the + input of a good opamp is quite high - megohms if not gigaohms. (note to the checkers out there... no, not all opamps are this good - it's a textbook example). The bias resistor is also usually quite high - 1/2 to several megohms. So if your meter has an impedance of only 1M, it can pull more current than the opamp. The way this works out is that while you measure the + input, the other inputs follow it. When you remove the meter to shift to the output or (-) pin, the load on the + input goes away and they all revert to normal. Some voltages are so high impedance that you can't measure them with normal (read that as "inexpensive") equipment with any accuracy.

The fact is that in a properly functioning opamp being used as an amplifier, all three pins (+, -, and output) are within millivolts of each other. The millivolts is the input offset rating from the datasheet times the DC gain of the stage. Generally this is under 15mV. Some high precision opamps have the offset well under a millivolt.

If the opamp circuit is such that the DC gain is one, and the output is within, say, 50mV of the Vref, the opamp is doing what it should be - at DC at least.

[caspercody]

Thanks for the replies!

The schematic is in the layout gallery. Just type in Sansamp, and there will be 12 results. One of which is the schematic. I have not installed the 330K on VR1 (if that would make a difference).

I use a Fluke DMM.

Would this lower voltage have a effect on the distortion of this pedal?

[R.G.]

Would this lower voltage have a effect on the distortion of this pedal?

Probably not. What's really wrong appears at first glance to be your JFET voltages. The voltage of the gate of an Nchannel JFET must be lower than the source for it to work properly. You report gate higher than source. That would, if the pinout and voltage tests are accurate, account for your symptoms.

[caspercody]

Thanks RG!!!

I re checked the voltages (because I forgot those I listed where when I had the Jfet gate and source backwards) and these are the correct voltages:

Drain 7.6v
Gate 3.87v
Source 5.135v

So the gate is less than the source. But thanks for that information RG, may come in handy later on knowing that.

But my IC pins are still off a little:

Pin 1 4.22v
Pin 2 4.3v  (and voltage climbs up when I leave the meter on)
Pin 3.7v  (and voltage lowers when I leave the meter on)


The rest of the voltages look okay. Should I be looking more into the variance in the voltages on the first three pins? Or when do they fluctuate?


This does work, but does not seem to be as distorted as I hear in some clips.

Thanks
Rob

[andymac1962]

Part of the Sansamp sound is the IC they use.  TLC2262 or TLC2272.  Use this for IC1, rather than TL072 to get better distorted sound.
IC2 just forms frequency shapers so TL074 is fine here.
Glad you got it to work.
I built a sansamp GT2 from ROG & it was ok, but is was so much better sticking a TLC2262 or two in there in hte sections where the distortion comes from.....
Also, TLC2262 are only rated for 16V absolute maximum supply voltage, so dont use them if you have 2x 9V batteries in series,   and if you are using a 12V plugpack, stick a voltage regulator on the power supply, cause those plugpacks (wallwarts) can have higher than 16V output at light load....  dont want to fry your chips.

Cheers
Andy

[caspercody]

Thanks!! Next time I order some more equipment, I will order one up.

[tcio]

I apologize if this is out of the scope of this thread but I wasn't sure where to post this. There are literally TONS of Op amp threads and although I have spent a great deal of time searching I really don't know exactly what to type in for the search so came up empty handed so far. I am trying to put a part of an amp circuit into a foot pedal and in this specific circuit the opamp takes both a positive 15 volts and a negative 15 volts. I have tried a couple of negative generator circuits but couldn't get equal voltages out of both the pos & neg plus I would prefer less parts and to simplify the circuit. Is there any way around this? Is there a way to get a circuit like this (+/-v) to work by substituting that negative voltage section with something else and still get the Op amp to function properly? I am using the NE5532 chip.

[PRR]

You typically bias the inputs halfway between the supply rails.

When you have + and - rails, you "simply" refer inputs to the common zero volts.

When you have one rail, you can "often" rig a half-voltage supply, reference your inputs to that.

"There are literally TONS of Op amp" applications, "part of an amp circuit" doesn't narrow that down, so there's no instant answer here.