Opamp causing Garbled sound, why??

[R Spencer]

Hey guys,

First post but I've read hundreds, including several massive threads started by Deadastronaut (Hey Rob!)

I have a problem. I'm a newbie so I don't have a lot of ideas on how to fix this.  I have read the troubleshooting post but that was not much help.

I built a preamp. It works great.  This is the link to that schematic. http://www.jer00n.nl/2010/07/28/clean-boost-for-guitar-or-bass/

I decided to add tone controls so I connected the output of the preamp circuit to the input of this circuit. http://sound.westhost.com/project28.htm.  Funny thing happened, the sound comes through but at a much lower volume and all garbled.  I made an audio probe and went backwards through the circuit until I found the change in sound. I noticed that the signal going into the initial opamp (I assume just a buffer) is fine and sounds normal.  However on the output, number 6 pin of a TL071, the garbled sound begins.  The rest of the circuit I think is working correctly as the tone controls seem to be working on the garbled sound. I think that if I figure out the buffer problem everything will be ok.

FYI, I used all TL071 opamps as that is all I have.  I did switch out the buffer opamp to see if that cured the problem but it stayed the same. I then added 10nF and 100nF bypass caps to both the 9v pos and neg inputs of the opamp to see if that would help but the problem remained the same.

I am a true newbie, this is only the 3rd circuit I've built on a breadboard so any and all suggestions will help.  I don't know if it is a polarity problem, bias issue, wrong opamp for the job... or what.

Thanks in advance,

Randy

[wilrecar77]

If you are using a unipolar 9v supply (like a guitar pedal), try switching the ground coming off of the 100k on that first opamp to a 4.5v reference supply. It is likely a biasing issue. The westhost circuit probably assumes a bipolar power supply, and if this is for a guitar pedal you are likely using a unipolar (+9v and a 0v ground) supply. The opamp's positive input needs to be biased by a reference voltage to half of your power supply.

In a bipolar power supply situation, this "half" of the power supply IS ground; 0v would be halfway in between +9v and -9v. (if we were using a bipolar 9v supply).

Since (if) you are using a 9v unipolar supply, a 4.5v voltage reference is equivalent to the 0v in the bipolar situation. All voltages are relative.

If you tie the 100k on the first opamp to 4.5v instead of ground and it works when you audio probe the output of the first opamp, then you should replace all of the ground symbols on that schematic with vref (4.5v). I *think* that should work, but I'm a bit fuzzy on the whole simulated inductor thing. Keep the opamp power supply ground at real ground. You can create the vref with a voltage divider, though if your preamp uses opamps and it works, use its vref.

For example, if you place two 10k resistors in series from +9v to ground, the junction of the two resistors will be 4.5v. To make it more stable, you can place a big capacitor from the junction of the resistors to ground.

If you need your vref to be even more stable (those simulated inductors might be picky, I really don't know), you can buffer the 4.5v reference of the resistors with an opamp. Tie the 4.5v straight to the positive input, and connect the output to the negative input. Put a smallish cap from the output of the vref buffer opamp to ground and that should be a stable vref.

[R Spencer]

Thanks Patrick, I thought that it might be something like that.  Very helpful explanation of how to make a voltage divider.  The pedal is powered by a 9v battery so your response is a very likely solution to the problem.

It's late night here so I'll have to try that out over the weekend. I work all day tomorrow so it may be late before I can get to it.

FYI, I corrected the link to the preamp.  Here it is again just in case you wanted to look at that. http://www.jer00n.nl/2010/07/28/clean-boost-for-guitar-or-bass/

Thanks,

Randy