Causes of DC offset
post-1400421
Thread Starter
Post #1 of 24

Heady

100+ Head-Fier
Joined
Jul 8, 2004
Messages
213
Reaction score
10
Joined
Jul 8, 2004
Posts
213
Likes
10
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.
 
     Share This Post       
post-1400496
Post #3 of 24

Garbz

Headphoneus Supremus
Joined
May 19, 2004
Messages
3,573
Reaction score
11
Joined
May 19, 2004
Posts
3,573
Likes
11
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.
 
     Share This Post       
post-1400525
Post #4 of 24

BrokenEnglish

500+ Head-Fier
Joined
Feb 2, 2004
Messages
748
Reaction score
10
Joined
Feb 2, 2004
Posts
748
Likes
10
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.
 
     Share This Post       
post-1400539
Post #5 of 24

00940

Headphoneus Supremus
Joined
Nov 6, 2002
Messages
4,493
Reaction score
44
Joined
Nov 6, 2002
Posts
4,493
Likes
44
output caps don't have to be bipolar. just make sure they're correctly oriented

correctly oriented meaning the positive lead amp side
 
     Share This Post       
post-1400557
Post #6 of 24

Sinbios

1000+ Head-Fier
Joined
Dec 24, 2004
Messages
1,061
Reaction score
10
Joined
Dec 24, 2004
Posts
1,061
Likes
10
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?
 
     Share This Post       
post-1400752
Post #7 of 24

amb

Member of the Trade: AMB Laboratories
Joined
Apr 1, 2004
Messages
4,933
Reaction score
39
Joined
Apr 1, 2004
Posts
4,933
Likes
39
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.
 
     Share This Post       
post-1400970
Post #8 of 24

Garbz

Headphoneus Supremus
Joined
May 19, 2004
Messages
3,573
Reaction score
11
Joined
May 19, 2004
Posts
3,573
Likes
11
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.
 
     Share This Post       
post-1401079
Post #9 of 24

Heady

100+ Head-Fier
Joined
Jul 8, 2004
Messages
213
Reaction score
10
Joined
Jul 8, 2004
Posts
213
Likes
10
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.
 
     Share This Post       
post-1401080
Post #10 of 24

Heady

100+ Head-Fier
Joined
Jul 8, 2004
Messages
213
Reaction score
10
Joined
Jul 8, 2004
Posts
213
Likes
10
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.
 
     Share This Post       
post-1401086
Post #11 of 24

nikongod

DIY-ku
Joined
Jan 24, 2005
Messages
8,882
Reaction score
120
Joined
Jan 24, 2005
Posts
8,882
Likes
120
to double check, are you measuring the offset with the input grounded? that will change things.

and, have you swaped in diferent chips?
 
     Share This Post       
post-1401122
Post #12 of 24

jasonhanjk

100+ Head-Fier
Joined
May 7, 2003
Messages
349
Reaction score
10
Joined
May 7, 2003
Posts
349
Likes
10
Try changing the problem output cap?
 
     Share This Post       
post-1401414
Post #13 of 24

Heady

100+ Head-Fier
Joined
Jul 8, 2004
Messages
213
Reaction score
10
Joined
Jul 8, 2004
Posts
213
Likes
10
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.
 
     Share This Post       
post-1401839
Post #14 of 24

PinkFloyd

Headphoneus Supremus
Joined
Jan 13, 2009
Messages
9,511
Reaction score
21
Joined
Jan 13, 2009
Posts
9,511
Likes
21
Am I reading this correctly...... an "860uF" OSCON????? I thought they only went up to like 100uF or so?

Mike.
 
     Share This Post       
post-1402311
Post #15 of 24

Heady

100+ Head-Fier
Joined
Jul 8, 2004
Messages
213
Reaction score
10
Joined
Jul 8, 2004
Posts
213
Likes
10
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
 
     Share This Post       

Users Who Are Viewing This Thread (Users: 0, Guests: 1)

Top