[Heady]

I have built several CMOYs and 2 Pimetas. Never had problems with DC offset. I have also built a few amps using Sijosae's design of the 74HCU04 chip without problems.

However the latest one I built has DC offset of 30-50mv on one channel, the other channel is perfect. The amp has an input Polypropylene cap (1uF) and an output electrolytic cap (860uF Oscon). I am curious why I have DC offset when there is an output cap.

Would any of the more experienced DIYers like to suggest things to check? I have checked the soldering visually and with a DMM and it looks good. There doesn't seemed to be any short-circuit and the flux has been cleaned off with flux cleaner.

I am stumped. Thanks for any help.

 

[Sinbios]

I'm also having the same problem in my MINT - 130mV of offset in both channels! What are the most common causes of this?

 

[Garbz]

well the mint doesn't have an output cap so the causes are likely to be different.

Are the mint's chips getting excessively hot?

Heady if one channel is perfect then it really sounds like a wiring problem or a solder bridge. Compare the working channel to the troublesome, bit by bit.

 

[BrokenEnglish]

Quote:

Originally Posted by Heady [...]and an output electrolytic cap (860uF Oscon).[...]

just to be sure.. the oscons are bipolar electrolytics? you're dealing with AC here, so they should be...

/edit: sorry... this was a wrong statement! see 00940's reply.

 

[00940]

output caps don't have to be bipolar. just make sure they're correctly oriented

correctly oriented meaning the positive lead amp side

 

[Sinbios]

Quote:

Originally Posted by Garbz Are the mint's chips getting excessively hot?

Nope

Also, what is the offset supposed to be when the source is unplugged? Also, I've removed the pot since it was broken... what should the offset be in this case?

 

[amb]

Quote:

Originally Posted by 00940 output caps don't have to be bipolar. just make sure they're correctly oriented correctly oriented meaning the positive lead amp side

Actually this is true only if the amp has a single positive supply and the amp side is biased to 1/2 the supply voltage. If the amp has a split supply or uses virtual ground, then it's more correct to use a bipolar (non-polar) cap because the DC offset could be positive or negative relative to ground.

Heady, what amp is it that you're experiencing high DC offset with? Different amp designs have common and unique causes of this.

 

[Garbz]

In any event the DC offset should be 0mV.

If the amp has an input resistor going from the opamp to gnd the mint shouldln'r really care if there is a pot or if a source is plugged in or unplugged.

 

[Heady]

Quote:

Originally Posted by 00940 output caps don't have to be bipolar. just make sure they're correctly oriented correctly oriented meaning the positive lead amp side

Yes, I do have the Oscons correctly orientated. Thanks.

 

[Heady]

Quote:

Originally Posted by amb Actually this is true only if the amp has a single positive supply and the amp side is biased to 1/2 the supply voltage. If the amp has a split supply or uses virtual ground, then it's more correct to use a bipolar (non-polar) cap because the DC offset could be positive or negative relative to ground. Heady, what amp is it that you're experiencing high DC offset with? Different amp designs have common and unique causes of this.

I am using a 74HCU04 chip and I am using a dual supply. Very interesting info you have been giving me. Didn't know that about the power supply. Thanks.

 

[nikongod]

to double check, are you measuring the offset with the input grounded? that will change things.

and, have you swaped in diferent chips?

 

[jasonhanjk]

Try changing the problem output cap?

 

[Heady]

Quote:

Originally Posted by jasonhanjk Try changing the problem output cap?

Hi Jason, I am going to do that as soon as I figure out how to remove them. I use the caps leads as part of the point to point wiring. Not just removing the solder, unfortunately.

Nikongod, I have the input connected to a source which is on.

I am inclining towards the dual supply as amb has said, that if I used a dual supply, the cap only blocks DC in one direction. So I probably need to connect two caps in series to block DC in both directions? I have been really lucky in my other similar amps.

Thanks to all.

 

[PinkFloyd]

Am I reading this correctly...... an "860uF" OSCON????? I thought they only went up to like 100uF or so?

Mike.

 

[Heady]

Quote:

Originally Posted by PinkFloyd Am I reading this correctly...... an "860uF" OSCON????? I thought they only went up to like 100uF or so? Mike.

Sorry Mike, a typo, it was a 680uF 6.3V Oscon. I was using two AA batteries for power. Regards.

Paul