TL071 Buffer Question

[Dave Eason]

Hi Guys,

Right, I have a bit of an issue with something.  I'm using a TL071 as an input buffer to my wireless project, which then goes into an NE571 compressor circuit.  Likewise in the receiver, the NE571 expander feeds an output buffer.

Due to the way the NE571 is set up, the built in op-amp is set up as an inverting amplifier, where a variable gain cell acts as a feedback resistor, altering the gain, depending on the input signal.  Due to this,  I had to make the buffers unity-gain inverting amplifiers (in fact, the feedback resistor on the input buffer will be a trimmer, so you can reduce the gain if the pickups are too hot). 

My problem is that when I use unity gain inverting amps, with R1 and R2 both being 2K2 (as its all I have here at the moment), the signal is a hell of a lot weaker and quieter than when I just make the TL071 into a non-inverting voltage follower type circuit.

Surely there should be no difference?  If I run the signal throught eh voltage follower, into the compressor, then into the expander then throught the output buffer into my amp, it sounds fine, like the circuit was just a "guitar cable", maybe with a tiny bit of harmonic distortion.  But its out of phase of course.

Its all on bread boards and Im going to the lab on friday to do some proper tests and set ups with the scope and function generator.

Any ideas?!

Dave

[Dave Eason]

bump

[David]

Read the "Basic Buffers" article by Jack Orman on his site www.muzique.com.  That article was a revelation to me!

[Dave Eason]

Cheers!

Man there're some great technical notes there, I never looked at before!  He says use large resistors in the inverting buffer situation; essentially as R1 represents the input impedance of the circuit.  In theory, say with a 1kHz sine through it,  the size of R1 and R2 wouldn't be critical;  the math works out as long as they are the same.  It's strange!  I'm using a MAX1044 as a power supply, creating V+ and V- out of a DC source, so I don't need to bias the signal or the op amp I'm guessing.

[Dave Eason]

D'oh..

Stupidity Kicks in..

It's probably quiet for the fact that my input impedance was only approx 1K, I guess that's pretty low for guitar pickups, so it was getting loaded down loads, and thus quieter, yeah?

[Jeremy]

Yeah, the impedance your guitar saw was 2k2. 

The output impedance of a guitar can be pretty hefty; perhaps you should consider putting a normal non-inverting buffer (source follower or emitter follower or non-inverting op-amp buffer) at the front of both your transmitter and your receiver. 

After all, if the compressor and the expander both invert your signal, won't it be right side up after it goes through them both anyway?  Sure, the signal will be upside-down while it's flying through the air, but it will come out alright at the amp.

But perhaps I don't understand your circuit completely.

[Satch12879]

There's an unofficial commandment that Dave(?) Manley coined long, long ago:

"Thou shall not invert the signal."

That being said, indeed loading, as is often, is the culprit.

[Dave Eason]

cheers guys, yeah 2K2 s'what I meant.

I was expecting it to be inverted once, then back again, then fly through the air, then become inverted and then inverted again giving the original phase.  But I guess you're right Jeremy, if it's inverted once, then transmitted, then received and inverted again, it will indeed be the right phase at the output.  But I've got an inverted amp so I can reduce the gain if necassary; obviously this can't be acheived with a non inverting set up.