Op amplifier

[Edwin Fitzpatrick]

I am building a Guitar amplifier pre. stage with a TL071 op amp, Split power supply. My question is..... In the feed back circuit (gain) there is a electrolytic capacitor to ground. What should the polarity of this capacitor be ?
Fitz

 

[(*steve*)]

A circuit diagram would be an enormous help right about now.

 

[Edwin Fitzpatrick]

OP AMP

Hi Steve,
Here is circuit,
Fitz

 

[CDRIVE]

It's not there.

Chris

 

[Edwin Fitzpatrick]

OP AMP

Hi Steve,
I guess the Scan did not work. Any way, It is a standard op amp circuit, ref. theTL071,
a 68K resistor from pin 6 to pin 2 ,Pin 2 through a 4.7 K resistor, through a 1 mfd
electrolytic capacitor to ground.
Fitz

 

[CDRIVE]

It's easy to post attachments. We all do it all the time.

Chris

 

[(*steve*)]

Edwin, when you're replying, do the following:

1) click on "Go Advanced"
2) Click on the "attachments" tool (it's the paper clip)
3) use one of the browse buttons to browse to and select the file
4) click on "upload"
5) wait...
6) check for the file appearing in "current attachments", if so you're gold. Otherwise maybe the file was too large.
7) close the attachment dialog
8) press "preview post" (do you see a thumbnail of your image?)
9) complete your post
10) press "submit reply"

There's more steps there than you absolutely need.

The most common problem is trying to upload a file that is too large.

 

[Raven Luni]

If you're working from a schematic where the polarity is not shown, are you sure its an electrolytic?

Anyway, if its on the output then the polarity shouldnt matter since its passing AC but it's conventional to have the negative side connected to ground in these cases. Be sure to use (non polarised) coupling capacitors to block any DC signal components - that way you avoid potentially harmful output such as a constant signal of the opposite polarity.

 

[CDRIVE]

<snip>
Anyway, if its on the output then the polarity shouldnt matter since its passing AC <snip>

Sorry, but that's not true. A single ended OpAmp, NPN Collector and N-Ch FET Drain never go negative. The input side of the cap sees signal perturbations no lower than zero volts to some value approaching the positive rail. The output side of the capacitor delivers an AC voltage-current above and below zero. Thus true AC. When I use polarized caps on the output of the aforementioned devices I always insure that the positive end of the cap is the input side of the cap. Of course the opposite it true for PNPs, P-Ch FETs, etc that use a negative supply rail.


Chris

 

[KrisBlueNZ]

The electrolytic is in series with the resistor that forms the bottom of the voltage divider that is the feedback loop. It is there to provide low-frequency rolloff to the amplifier's response.

The basic stage gain is about 15.5. (Calculated from 1 + (68k / 4k7).)

If there will be significant low-frequency content in the input signal, the electrolytic will develop significant voltage across it. In that case you should use a non-polarised electrolytic. In general though, the voltage across the electrolytic is fairly small and a standard electrolytic can be used, connected with either polarity.

 

[BobK]

The - input will pretty much follow the + input, so there would only be a few millivots (same as the input) across the capacitor to ground. So it should be safe.

Bob