TL072 preamp for active A/B switcher

[ragingben]

Hey everyone,

I'm attemping to make an active A/B box (2 bass to one amp), which uses... http://www.rason.org/Projects/opamps/design3.gif

...preamp circuit but with a TL072, instead of the 741. I'm going to have this twice (other than the voltage divider that creates the reference voltage) using both sides of the op amp (one for A and one for B). I have checked and the voltage divider is working nicely, from a 9v supply I have 9v at the Vcc pin and around 4.8v at the non-inverting input 1. No matter what I do I can't seem to get the circuit working correctly though, and it's starting to drive me crazy (it has been a few days)! Can anyone verify that it looks correct? I have replaced the 100k resistor which goes from the output to the inverting input with a 10k, and the 1k which is between the audio input and the inverting input with a 2.2k, as I wanted around 5x gain. The 5x gain should be like a 5x volume boost shouldn't it? It looks so simple I can't imagine what I'm doing wrong!

When I say I can't get it working correctly my signal seems to go distorted (but not louder, and the inverting input has 4.8v on it) (I'm using a drum machine as a test source at a moderate output volume).

I'm doing this on a breadboard, but heres a vero layout I designed, it may point out where I'm going wrong, who knows!



Any help is much, much appreciated

[PRR]

DC voltages on the IC. (Hint: one +9V, one zero, and six near 4.5V.... are any way-wrong?)

Stepping back: 1K or 2K input resistors are a HEAVY load on guitar, and maybe on some drum-boxes. You really want a non-inverting configuration.

[ragingben]

Cool, non-inverting Is something I can do, I only went inverting because this one was. Is this not a great schematic to follow then?

Does this look OK as a non-inverting config (providing the Vr was running to the non inverting input?



Yeah, they are all around 4.6v, that would be pins 1,2,3,5,6,7, and 4 is at 0v and 8 is at 9v.

EDIT: Sorry, didn't see the chematic until after I posted, thanks for that! Just to clarify Vcc is 9v input? I never understand what Vcc is! If I replace the gain pot with a 10k resistor and put a pot at the output acting as a voltage divider would that be a reasonable way of controlling the volume? I only say this as I'm having a toggle that switches between active and passive, and that way one pot could work for both active and passive modes...

[ragingben]

Just want to say - many thanks she is working a beaut!

[DavenPaget]

Meh , like Paul said , 1K is a mad load . It just shaves off everything , probably good for your MP3 or sorts , but no good for a guitar .

[PRR]

> Just to clarify Vcc is 9v input? I never understand what Vcc is!

You are right to question it.

IRE standards 1956. V_c is collector DC voltage. Double the subscript, V_cc, to indicate collector supply voltage.

There's no naked collector here. V_cc is technically incorrect. However just before chip opamps became common, most circuits use NPN transistors. "V_cc" was the positive supply voltage. This use carried-over into transistor+chip use, then (wrongly) into chip-only use.