Amplifier Distortion and DC-Offset Info
Posted: Thu Oct 11, 2007 1:20 pm
As many of you know I have asked some of you from time to time to check your DC offset of your amps prior to sending them to me.
I have used this simple testing method for years as a means to a end. The end being a quick judgment call on the overall condition of any amplifier placed in front of me. Its a simple test method but yet it yields tons of real info about what you have in front of you, and you don't need to be a engineer to understand the relevance to your audio experience.
And as for perspective amplifier purchases I have found this simple test a tell all of what I was getting into buying a amp from another person's use.
I hope you will find this as helpful to each and everyone of you as it has been for me over the years.
I am quoting another post I found from a another source to explain why I ask such things of you.
I figure if you see other learned peoples thinking perhaps mine will not be in question.
I hope you find this tech post a bit more down to earth and informative then my last. I ran across this some time ago and found it both amusing and informative at someone else's expense.
I myself use a much tighter standard of offset than the one listed in this other person text, but I am a GDPIA about this sort stuff so I strive for the absolute best possible numbers.
Again here I am using someone else's data to avoid confrontation issues. But I think you will like this one and find some solace in why I ask the things I do....C
PS the link inside the post will take you to techno land of graph charts and Calculus explanations, if this is not your bag then please don't click the link...c
I have used this simple testing method for years as a means to a end. The end being a quick judgment call on the overall condition of any amplifier placed in front of me. Its a simple test method but yet it yields tons of real info about what you have in front of you, and you don't need to be a engineer to understand the relevance to your audio experience.
And as for perspective amplifier purchases I have found this simple test a tell all of what I was getting into buying a amp from another person's use.
I hope you will find this as helpful to each and everyone of you as it has been for me over the years.
I am quoting another post I found from a another source to explain why I ask such things of you.
I figure if you see other learned peoples thinking perhaps mine will not be in question.
I hope you find this tech post a bit more down to earth and informative then my last. I ran across this some time ago and found it both amusing and informative at someone else's expense.
I myself use a much tighter standard of offset than the one listed in this other person text, but I am a GDPIA about this sort stuff so I strive for the absolute best possible numbers.
Again here I am using someone else's data to avoid confrontation issues. But I think you will like this one and find some solace in why I ask the things I do....C
PS the link inside the post will take you to techno land of graph charts and Calculus explanations, if this is not your bag then please don't click the link...c
Amplifier Distortion, DC-Offset, and You!
Time for one of those long, boring semi-technical posts that no one here reads...
As a few of you know, I bought a rare Kenwood 700M amplifier a few weeks ago on eBay. It arrived with a weak channel which was taken care of by replacing a bypass cap. Since then, I have gone through the entire amp and replaced all the electrolytic capacitors with the exception of the big power supply caps (not that expensive...maybe $20 in caps).
The previous owner described the amp as 'perfect' sounding, and compared with the big Mac's and Krell's and such. Before buying it, he described his current system which was quite high-end. I assumed that since he owned expensive equipment that he knew what he was listening to.
Got the amp back together today (parts finally in), and fired it up with my small Dynaudio's (can't use the Heil/Dynaudio's for this, as they are bi-amped). Ummm...it sounded like ass. OK, it didn't always sound like ass, but at low volume levels it was obvious that something was wrong. I had a pretty good idea what was going on, so I grabbed my meter. Sure enough, there was 100mV of DC offset in the left channel, and almost 250mV (!!!) in the right! 250mV is almost enough for the protection circuitry to kick in!! Not good. I pulled the driver boards out and replaced the amp input differential pairs for both channels with new Zetex HG PNP's ($5 in transistors, no biggie). DC offset is now about 12mV in both channels. An input pair being as unbalanced as the Kenwood was when it arrived probably generates 10x the distortion as a properly balanced pair, especially at low volume. If you would like to read more about the benefits of a balanced differential pair, read here. http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm#5
OK, sit down for another listen. NOW we're cookin'!! Amazing night and day difference. I can honestly say that it is without a doubt the nicest amp I have ever listened to, and there have been more than a few that impressed me. The bottom end on this thing is as clear as spring water, and it has an openess that is difficult to describe. As for power, my dummy loads cannot take the power output of this thing, but I can crank it for 10-15 seconds without destroying them. Left channel - 218W before clip, right channel - 220W, this into 8-ohms. Totally cool!! I just have never heard bass like this...wow...
Bottom line...if you expect to hear great sound, you just might...regardless of the reality. The guy I bought this from was well-meaning, but did not know how to listen subjectively. His new spendy amps could be performing horribly, and his expectations of what he felt he was supposed to hear would rule out anything to the contrary.
Your own subjectivity could be suffering too, so give yourself a reality-check.
As a semi-poll, I'd like to see those on this board whip out their multimeters and take a look at the DC that is being presented to the speakers. This means..
1. Speakers disconnected (or connect the meter to the 'B' speakers and set the front panel speaker control accordingly)
2. Input set to an unusued position (not Phono)
3. Volume control at minimum.
4. Balance in center
5. Tone controls either defeated or set to mid position
6. Set your meter to read DC, and set to a low scale (300mV scale is common) Connect directly to the Pos and Neg of the speaker terminals
7. Give the amp 10 minutes to settle. Report back...I'd like to see how healthy all these old amps are.
If you read:
0 - 15mV: Damn good!! If you read '0V', you may have a capacitor output, or your meter is set wrong
16mV - 50mV: An acceptable value, especially at the lower end of this range. 2nd harmonic distortion is probably twice to four times what manufacturer's spec calls for at higher frequencies. Probably not audible, as the distortion is mostly in the upper octaves. At the upper end of this range I begin to raise an eyebrow.
50 - 85mV: Something is certainly amiss, and while this is not enough to put your speakers or equipment in jeopardy, the amp is running nowhere near where it should. I'd venture to guess that most of the DC-coupled amps that are in use by forum members here fall into this range.
100mV to ?: A high enough voltage will cause the DC protection to kick in. This happens at a level determined by the designer, but is usually equivalent to about a diode drop (600mV)or so. Needless to say, if you are listening to an amp with 100mV or more of DC offset, you have no idea what the amp really is supposed to sound like. Indeed, some amps without a differential input are actually designed to have a bit of DC at the outputs, but this is triple-rare, and I don't think anyone here owns one. (in my book it's piss-poor design, but if you can sell it WTH..)
Soooooo...go grab a meter and tell me what you find...
Quote:
Edit:***WARNING!!!***
Since the start of this thread, which was simply intended to have people LOOK at offset, many have decided that they want to try and ADJUST it.***DO NOT TRY TO ADJUST OFFSET OR ANY OTHER DAMN THING INSIDE YOUR AMP OR RECEIVER UNLESS YOU REALLY REALLY REALLY KNOW WHAT YOU ARE DOING!!!***
I can forward you emails and thread links concerning guys who have destroyed their amps/receivers because they thought that all that they has to do was 'a little tweak'. Instead, they cranked up the bias pot instead of the offset pot, either because their amp/receiver HAS no offset adjust (many don't), or because they picked the wrong pot to tweak. OR, a slip of the probe lead and POOF! goes the output transistors and many other parts. So read and re-read the ***ALL CAPS*** statement above until you fully comprehend it.
...and buy the service manual!
Guess i'll have to wait till 5pm rolls around., lol
