Checking offset

dudeinaroom

Jut wondering, when checking the D.C. offset of an amplifier would it be best to short the inputs of just have nothing connected for checking it. I do have a couple of shorting caps I could use ( the kind that come with a preamp for unused inputs). Was kind of thinking it would be as then there wouldn't be any stray signals from the likes of EMI or RF presented to the input. Any thoughts anyone?

Thanks for taking a look.

heiney9

Maybe start here:

Good tutorial

http://ns1.audiokarma.org/forums/showthread.php?t=5634

H9

dudeinaroom

Thanks for the link H9. Will give it a look when I get a few.

dudeinaroom

Thanks for the link H9. Will give it a look when I get a few.

heiney9

What amp are you checking? If the output is capacitor coupled and not direct coupled, measuing DC offest is a waste of time. Or if you have an output that corrects for DC offset like servo's, etc.

H9

FTGV

heiney9 wrote: »

If the output is capacitor coupled and not direct coupled, measuing DC offest is a waste of time.

Absolutely not the case,the input cap(not output) is there to block DC from entering from the source and has no affect on the inherent offset within the gain stages that follow it.Offset measurements can be useful(indicating a fault) regardless of AC or DC coulping or the presence of a servo.

dudeinaroom

wow...20 min apart double post.....lol. It's a Parasound HCA 1000A. I just hooked up my DMM and am watching it go from .1 to 1.4 mv, I'm going to wait about ten min. and see if it settles.

heiney9

Hmmmm.......I read not to bother. Of course if the blocking cap is leaking then that;s an issue or if the servo control is malfunctioning. In general unless it's DC coupled, DC offset should be low, hence the waste of time comment.

Perhaps I am interpreting some one else's POV incorrectly. If the output stage of the amp is capacitor coupled (not the input) then the cap should be doing it;s job blocking the DC at the output. No? Perhaps I have things backwards or misread the OP's original post.

FTGV

dudeinaroom wrote: »

wow...20 min apart double post.....lol. It's a Parasound HCA 1000A. I just hooked up my DMM and am watching it go from .1 to 1.4 mv, I'm going to wait about ten min. and see if it settles.

There will be some minor flucuations over time and temp but with those low numbers you will be well within the safe zone.

heiney9

My guess is those readings are so low this is a capacitor coupled amp or has active correction circuitry.

FTGV

heiney9 wrote: »

. Of course if the blocking cap is leaking then that;s an issue...

Only if the source is passing DC,the leaky input cap will have no effect on the offset generated within the amp,(ie differential input stage)

If the output stage of the amp is capacitor coupled (not the input) then the cap should be doing it;s job blocking the DC at the output.

Wow really brock I figured you would know that coupling caps are used at the inputs,not outputs.There are very rare exceptions of SS amps with output coupling caps the only one I,m personally aware of is the original Zen because of it being single eneded with a single polarity power supply.SE tube amps ofcourse having output transformers so can't pass DC.

heiney9

I see Fred, I know what it means, I wrote it wrong, sort of turned it on it's end. I realize the cap is placed at the input for DC filtering of the output stage. I got the image confused in my head.

Perhaps I should just pay attention to my work and not try to do two things at once like confusing people on the forum :redface:

heiney9

It's always been my understanding that if the output is DC coupled, then DC offset is a non-issue unless the cap is failing. Maybe that's not entirely correct.

FTGV

heiney9 wrote: »

It's always been my understanding that if the output is DC coupled, then DC offset is a non-issue unless the cap is failing.

That can be the case with a preamp,which many have capcaitively coupled outputs.However the only amps requiring a cap at their output,s are single ended /single polarity types and these require rather large caps roughly equivilent to big PS filter caps (ie.several thousand micro farads.)

dudeinaroom

Okay the fluctuating reading were because I just placed the probes in the hole for bare wires and was not holding them in place. As per the link I waited ten min. jut to let things warm up and settle. Super nice readings! using the 200 mv scale I ended up with 1.0 on the left and 1.4 on the right. Was reading the left as - then I was like duh....flip the probes around. He he. I love the mids and highs on this amp, lows are plenty detailed, could use just a little more butt shaking energy though. I've been meaning to check it for a little over a year as I felt the clipping indicators came on too quick for my likings. No I don't run it with them blinking like a light show, crap the only time I have ever seen them light is when I first got it. Right now I can crank the pre to full and never see a flicker. Gotta love running passive. When running active, if I wanted to I could drive it into clipping but the volume gain running it is not even noticeable , well except for the position of the volume knob to get to full steam. Guess I need to get a bigger amp and bigger SDAs :cheesygrin:

Thanks guys

heiney9

I see it's a Parasound HCA 1000A.

As I suspected it has a DC servo so the offset will always be very low unless the servo malfunctions and there is DC present.

FTGV

heiney9 wrote: »

My guess is those readings are so low this is a capacitor coupled amp or has active correction circuitry.

heiney9 wrote: »

As I suspected it has a DC servo ...

He He not quite you were purely guessing it either one of the above.While yes a servo will keep offset levels in check your coupling capacitor statement was not factual.Also low offset(without the aid of a servo) can be achieved by very close matching of transistors especially those in the input section and the voltages feeeding them.

heiney9

I know very closely matched at the input will give low offset.....he...he, but I looked up the specs on the amp and it has a DC SERVO.

H9

FTGV

heiney9 wrote: »

I looked up the specs on the amp and it has a DC SERVO.

H9

Ummm yeah obviously your statement indicated that you looked it up.Previous to that it was pure speculation.