SOLVED A Breadboarder Needs Help - Dist 250

[BuddytheReow]

Hey guys,

I hate to ask your help, but I've been trying for a few days to get a simple Non-inverting opamp to work. This is the Distortion 250 without diode clipping. I used to build this one in my sleep 6 months ago, but now nothing. I guess I've been too busy soldering and designing transistor circuits for Chuck's contest, LOL. Anyway, I've breadboarded it and taken it apart at least 4 times now and for the life of me I can't get it to work . I either get no signal, a very "starved" sound, or unity gain. The gain knob doesn't work. Here is a picture of the breadboard. I hope you all can see it clearly. I do have signal going into the opamp so that part's good. Here's also the schematic. Please read to the end. There are lots of pictures.



The top part is the voltage divider and R5. All 1M (I checked before installing and they are within tolerance). The weird part is when I check the voltage right at the divider I get about 2.5 volts. Seem kind of odd, right?

The bottom left is the input section. A 10n cap into a 1k resistor with a 1m resistor going to ground. The red jumper then goes into the non-inverting opamp (pin 3).

The bottom middle is the feedback loop. Jumpered from the - Input to a 1m resistor and then going to the output with the black jumper. To complete the feedback loop I've got a 47n cap to the gain pot (1m) on lugs 2 and 3. Lug 3 goes to the 4.7k resistor. I know the order is backwards but I don't think that makes a difference?

The output goes to a 1u electro cap and then to my output signal.





At first I thought it was the breadboard and checked continutity on all the rows/columns (a real pain). Then I built this again on a different breadboard with no luck. Here are my IC voltages:

1 0
2 1.92
3 1.11
4 0
5 0
6 1.97
7 9
8 0



As you can see I have it chip powered correctly (pin 7 is 9v and pin 4 is 0). I am wondering about the other voltages here.

 

[BuddytheReow]

I should also mention too that I've changed chips. I've tried with a tl072, 4558, and a 741. They have different pinouts but function the same way.

 

[fig]

move your input wire one spot

 

[benny_profane]

Is R3 going to ground?

 

[Coda]

move your input wire one spot

What he said.

 

[BuddytheReow]

Is R3 going to ground?

Yes, it goes to ground. The order is backwards (pot, resistor) but I don't think that matters?

 

[BuddytheReow]

move your input wire one spot

No, that’s good. Bad camera angle. Here’s a different angle of the board

 

[benny_profane]

Yes, it goes to ground. The order is backwards (pot, resistor) but I don't think that matters?

It shouldn’t as long as the GAIN control is in the circuit before ground.

 

[benny_profane]

Where’s C1? Also, verify all the power wiring.

 

[BuddytheReow]

Where’s C1? Also, verify all the power wiring.

Isn't C1 just for tone filtering? I don't have it right now

 

[benny_profane]

Are the GAIN pins 1 and 2 connected, or just connecting the signal to lug 2? If GAIN isn’t working, there may be an open circuit there.

 

[fig]

Have you probed it? yank the output wire from the breadboard end and touch along the audio path.

Also, there are extra OUTs on the protoboard...

 

[Chuck D. Bones]

The bias is definitely wrong. Pin 6 should be at 4.5V or close to it. The protoboard has it's own +4.5V ref, so try using that instead of your voltage divider. When I build voltage dividers to make Vref, I never go over 100K and I make sure I size the resistors for the amount of current they have to carry. In your circuit, the current is low enough that 100K will be fine.

4558 is a dual 741, so they should behave exactly the same. 1M is a bit large to use for feedback and input bias on 741 & 4558. But it should be fine with a FET-input opamp like TL072. When you plug in a dual opamp, you re-wire it, correct?

The schematic looks ok, so I refer you to Bill's Law. It's a simple circuit, so maybe the next thing to try is take it all apart and rebuild it.

 

[BuddytheReow]

The bias is definitely wrong. Pin 6 should be at 4.5V or close to it. The protoboard has it's own +4.5V ref, so try using that instead of your voltage divider. When I build voltage dividers to make Vref, I never go over 100K and I make sure I size the resistors for the amount of current they have to carry. In your circuit, the current is low enough that 100K will be fine.

4558 is a dual 741, so they should behave exactly the same. 1M is a bit large to use for feedback and input bias on 741 & 4558. But it should be fine with a FET-input opamp like TL072. When you plug in a dual opamp, you re-wire it, correct?

The schematic looks ok, so I refer you to Bill's Law. It's a simple circuit, so maybe the next thing to try is take it all apart and rebuild it.

I’ll look into this tomorrow. Totally forgot about the other power inputs. Yes I rewire it when using the dual opamp. Gotta spend some time with the wife tonight

 

[Chuck D. Bones]

Hopefully that goes better than this project

間違った 穴

 

[BuddytheReow]

OK. I am either very dumb or very stubborn. Let me explain what I did. If you want the short version, I got it to finally work.

First, I took everything apart and started from scratch. @Chuck D. Bones pushed me in the right direction: the pin voltages weren't what they were supposed to be even though the chip was powered correctly. He mentions a tl072 above (a dual opamp, but we only need one of them) so I put that in there. Looking around the net early this morning I stumbled upon my OWN thread "Buddy's Breadboard and Circuit Design Notes" the project from Beavis audio. This has the schematic and a breadboard layout. Keep reading past the pic. I decided to use the protoboard's 5v input to replicate the voltage divider section since that was giving me problems earlier(still wondering why that was).


So, I used this as a guide (literally for breadboarding) and noticed something different than the way I've been breadboard this over the past week. In the feedback loop (that's the part going from pin 1 back to pin 2 in the bottom portion of the schematic) there are actually 2 separate breadboard "loops": the blue and orange jumpers going to R6/C5 and the separate green jumper going from pin 2 to the bottom right of the layout to include C4, gain pot, and R5.

This is interesting because the way I've been doing it is closer to how the schematic reads, meaning I had only 1 "loop" and had the gain pot "leg" attached to the resistor. THIS MUST BE THE KEY DIFFERENCE TO MAKE THIS WORK! For the life of me I don't understand why this needs to be its own separate section from the opamp's - input. If someone could please chime in with a little bit of theory that would be awesome. I will post my breadboard pic all spread out across the board to semi-mimick the Beavis Audio layout. I will try this again, but using a more condensed layout to help with general circuit building. I am a tinkerer, remember? LOL

 

[BuddytheReow]

Here’s my breadboard, Beavis Audio style

 

[BuddytheReow]

If you can breadboard a circuit a different way and still get it to work then you’re good to go. I’ll marked this one as solved. Thanks everyone for helping!

 

[fig]

So you went with a rocket engineer's advice over mine?

 

[BuddytheReow]

So you went with a rocket engineer's advice over mine?

Actually, no! I did take your advice about the multiple OUT plugs on the protoboard and used it to my advantage. I keep forgetting that the protoboard is more than just a test box….