My Signal generator has DC on it's output ;(

JJman

Member
Messages
994
I noticed that my signal generator has 4.5VDC on it's outputs. :NUTS It a cheap DIY kit "Elenco Elec Model FG-500." This was messing with the gain of the 1st triode when using it into my amp on the workbench.

So I checked Mouser for a new chip since it's outputs comes right off the chip. I found the 2206CP:

http://www.exar.com/Files/Documents/XR2206_104_020808.pdf

Good thing I read the PDF for it. Page 10 says it places DC on the outputs = to half the power (9v battery.) So it's supposed to be there.:worried

I used a large value cap to block the DC from going into the 1st triode but it's takes some time to charge.

Should I just put a small cap in the generator? What value would be adequate? I've got some "chicklets" laying around. Maybe 0.05u or so.

I guess they didn't want to spend the extra 5cents on the caps in the kit? Isn't DC a universal no-no on the inputs of audio?
 

skipm45

Member
Messages
292
It might be a bit tricky to correct.
Typically it's difficult for oscillators of this type to swing through zero unless athey are powered from a dual voltage supply. Some op amps have a built in charge pump to generate the (-) voltage required, but many just use a single supply amp and don't care about the DC offset.

If all else fails, try this oscillator. I think it works very well.:
http://www.proaudiosuppliers.com/apex-aao1.html

HTH

Skip
www.skipzcircuits.com
 

Ronsonic

Member
Messages
3,303
Yikes - +4.5VDC on the grid of a tube is not going to be good. Most home stereo stuff and PA type gear has an input cap.

The smallest cap you can put in there is determined by the lowest resistance and lowest frequency you are going to drive. C= 1/ (2*pi*F*R) You can also add a load resistor after the cap, something like a 1K, so that the load is always pretty much constant and use that in the calculation.

Either calculate or just grab the biggest film cap you own and stick it in there. If all you've got are .05s parallel a couple for .1. That will at least work in a guitar amp or pedal.
 

JJman

Member
Messages
994
I use a resistor in line when I'm working on my fuzz pedal since it loads the signal. (Those bastard transistors :mad:)

Since I'm usually using it for tube amps I'll try my 0.1u polyester film 50v from my Radio Shack stash. Nice and small so it will fit in there easy. Should be interesting to see if it will still put out a measurable 1hz signal thru the cap.
 

JJman

Member
Messages
994
Put it in. Also put a 4.7meg resistor in there across the output so the blocking cap charges itself. Otherwise it was waiting for a load to charge, like thru my DMM or scope (set to DC coupling.) Now I know it will never have DC on it's output. Worked fine and the signal range is not affected.:BEER
 




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