Measuring DC Offset on a HiFi Amplifier
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- Опубліковано 1 жов 2021
- This short video shows how to use your multimeter to check the DC offset of your stereo amplifier. A high DC offset measurement on the amplifier output can cause serious damage to your speakers. This simple technique is really useful if you are connecting a secondhand amplifier for the first time.
Please note, not all faults in amplifiers can be detected using this method, but many different faults do contribute to higher than desirable DC offset levels and so this is a test I would do on any amplifier when I first acquire them.
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Acknowledgements:
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~Video produced by Natalie Tynan at www.tynanmedia.com/
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Straight and simple info. No rambling. Thanks!
Thank you for your feedback.
Thank you very much for the detailed explanation.
Nice sir and beautiful demonstrate 👍
Good job! Thank you!
This was very useful, thanks! (I have several used amplifiers I have picked up over the years and haven’t had time to hook up speakers to them to try out and now I know when I do finally have time to check the DC offset before I hook up the speakers so I don’t damage them (I have several pairs of vintage speakers and would hate to have any of them ruined when a simple check with a voltmeter would tell me if the amp was ok or not).
Many thanks for your comments. I'm glad you found this useful. I wish you well with your future amp testing!
Great video and explanation.
thank you for you feedback.
Thank You!
Thanks!
I'm hooking up speakers to my receiver right this moment and I noticed the channels in the back have been going out on me to where they're not producing any power at all but been going out one at a time. Right now I am down to only one red and black left out of the four pair I have
Great video. Do you turn down the volume knob all the way when measuring dc offset or it doesn't matter?
thank you for the video. What should be a realistic value for the DC offset? Ideally it should be +/- 0mV, but that's theory.
This is an interesting and simple test. Would the test be more reliable if you would short out the imput?
Hello since you are dealing with amps can you help what’s the part CR100P it looks like a resistor but has the sign of a capacitor.
Hi, I note that you need to set volume to minimum for this test. I'm wanting to test a poweramp with no volume control (i.e. a fixed volume ouput): is it safe to test it in this manner all the same?
If there's no offset DC output in no source how about if there a source of sound is there a DC out but in my technics class A there's no DC offset even there a sound out in speaker.
Hi, thank you for the nice and clear explanation, just few questions: how do you set the volume pot for this test? does it have any meaning to this? if not, what can mean if dc reading continously increase taking volume pot from min to max? Thank you again.
Set the volume to minimum when doing the test. Perhaps your preamp circuit is introducing some DC offset ?
What would be a typical repair if the DC voltage level is high?
if it needs adjustment how do you do that?
After doing this encountered something odd. Mine were measuring single digits each and felt pretty good about that. I then checked the idle current on the main board which the manual says should be 7.4 so I dialed one from 15 to 8 and the other from 9 to 8. I checked DC offset after this and my numbers went up to around 22mv each. Why would lowering idle voltage and getting things in balance raise DC offset?
Is it a Sony?
If so it’s reversed ie opposite way to adjust
Hi I have a pair of Metaxas Solitaire power amplifiers. They both need restoration unfortunately. I'm sure you know that there is no volume control -just an on off switch .Is it safe to proceed with your procedure , without an 8 Ohm resistor (representing speaker impedance) across the speaker terminals? Thank you
I don't know what topology these amps use, but you should disconnect any input signal connections before testing. If they are valve amps then they should NOT be tested without a load. I wish you success.
Thanks. I didn’t hear anything about adjusting the volume control before testing. Can you please clarify? Sorry if I missed that
Set volume to zero.
Yep, set to min volume, with all input cables disconnected. Thanks.
Thanks mate. Although I'm a bit concerned my left speaker terminal is reading 14.5mv snd right is approx 35mv on my NAD C320BEE.
If you cannot notice any audio distortion in the speaker output then perhaps you can just leave it be.
What happenes when you cross the probes, I'm trying to find the polarity from a old guitar amp speaker wires, don't know what one is the hot wire or ground.
For DC, crossing probes simply changes the polarity of the reading. For AC, polarity has no meaning to the meter.
If you are trying to separate the live and "ground" of an AC line, then measure both lines against a known ground the live line will still show a voltage difference, the "ground" line will not.
I have a separate amplifier and preamplifier. Do I keep the preamp connected to the amp when checking DC offset?
I would say no. Do not keep the preamp connected. The reason being is some preamps/amps have the "ZOMG U KANT PUT TEH CAPACITOR IN TEH SIGNAL PATH OR U GET TEH BAD SOUND!!!!1!1!11one!1111" philosophy which means depending on the model of preamp and amp the dc offset from the preamp will get amplified by the power amp. This could lead you to an erroneous conclusion about the state of the power amp.
I would test each component separately.
I'm a beginner and please excuse my banal question. Are the speakers powered by alternating current or direct current? If Google translated your statement correctly, you are talking about fluctuating direct current? Which is correct, alternating current or direct current? Thanks!
gracias muy bien si activas los subtitulos mejor
Please activate subtitles so our friend TECNIS DAIMOND M.G can understand better your videos.
Thanks you for your message. I have added subtitles to this video now.
Thank you. I have added subtitles to this video.
The Volume Knob should be turned all the way down, all the way up, centered, the position of the Volume Knob doesn't matter? If the DC Offset is incorrect? How does one adjust it? If the E Capacitators have not been replace as of yet, will this affect the DC Offset? Thank-you.
The volume control should be set to the minimum level and the tone controls to the middle of their range. The method of correction of an overly high DC offset depends on the design of the amplifier - some have built in adjustments and the procedures are detailed in their service manuals. Other amps require components to be replaced in order to reduce DC offset - and require a little more effort to identify the actual source of the problem. Replacing the relevant capacitors may or may not solve the problem.
@@northbalwynrepaircafenbrc8202 Thank-you very much for the information and replying back to my question.
@@northbalwynrepaircafenbrc8202 yes myn needed adjusting using the trimming pots that correspond to offset/bias. Pretty struck by the right terminal at 12v! Although I think the last time I did it I must have had the wrong setting on the multimeter. Silly me. Hope I haven't done any long term damage to my speakers. It's only just be aligned and I've had this amp rigged up with my newish triangle bro3's for over 3 months, some heavy loud listening times too 🤔 checking the tweeters and driver would be a right pain so I'm just hoping everything is 👍
Thanks for posting this but can the offset be measured with 8 ohm load resistors across the speaker terminals or should they be removed first?
Measure the DC offset without a load connected (assuming you are not using a tube amp which I understand should not be run without a load, and this video is not relevant to testing tube amps). I guess if you measured the voltage drop across an 8ohm load then you may be able to get a better estimate of the DC power being dissipated in the speakers, but to keep it simple, measure without a load, minimum volume, tone and balance controls centered/flat. I hope your measurements go well.
@@northbalwynrepaircafenbrc8202 I just found out the speakers in the Harman Kardon 330A I'm working on are capacitor coupled and the procedure is done differently. I've downloaded the service manual for it so I'll study it later today and see what it says. Thanks and cheers from Victoria BC Canada!
@@mumbles552 I look forward to hearing how you go. Yes the DC-coupled amps SHOULD block the DC from reaching the speaker outputs, but these capacitors can fail. For interest, I have had a capacitor-coupled amplifier fail in a way that I measured close to 60V DC offset on one of the speaker outputs - not good at all, but it was an easy fix - replace the output coupling capacitor(s). If you learn some new techniques from the Harmon Kardon service manual, I'd appreciate an update.
Also for interest - here's a discussion containing a list of the amps/receivers that have capacitor-coupled output - may be of use for future repair projects (with thanks to Audiokarma, a great source for advice!): audiokarma.org/forums/index.php?threads/list-of-capacitor-coupled-solid-state-receivers-and-integrated-amplifiers.912075/
Would this procedure work for a behringer ep4000 power amplifier?
Thanks for the video. I have a question, I obtained a pair of pioneer speakers and checked the impedance of the speakers. One speaker gave me a reading of 6.9 ohms the other 6.1 ohms. I was wondering if the reason for the lower impedance is because the speaker itself was damaged by a previous owner through excess dc on the output to the speaker. Is this possible and if not what do you think could have caused such a difference.
Thanks for your question. I do not think that the difference you have measured necessarily points to a speaker problem. It is worth remembering that speaker impedance is not constant - it varies greatly across the frequency range. A good description of speaker impedance can be found on UA-cam on "Steve Guttenberg Audiophiliac" channel - video titled "Everything you need to know about speaker impedance". You might have some some slight difference in response of the speaker drivers themselves, the cross-over components, or even the wiring/connectors, but it might not (probably wont) even be noticeable in use. So in summary, I'm not sure you could definitely point to a high DC offset as being the cause of the slightly differing resistance measurement you have quoted, and hopefully it is not going to adversely affect your listening pleasure!
I would welcome other peoples comments.
@@northbalwynrepaircafenbrc8202 ok thanks I will have a look at the video link. Just wanted to add the impedance I measured was for the bass drivers after they were disconnected from the crossover boards. The mid range and tweeters were fine.
@@NZfabrock OK, you can ignore my comments re the crossover and wiring components then! I'd be interested in what you find out in your research. I did a quick check on one of my mid range drivers and the resistance measurements did fluctuate a lot (multiple ohms) as I put a little light intermittent pressure on the driver cone and it did take a little while to settle back down to a resting reading.
@@NZfabrock If I may jump in... you might have a problem. Maybe. And it's not necessarily because of DC offset; it could be, but might have just been normal loudness. If a speaker is driven hard, such that the heat builds up in the voice coil (because, by design, it lives in a small gap and doesn't have a lot of clear air around it), what can happen is all the voice coil glue melts.
The power is applied to the speaker's voice coil, and becomes an electromagnetic force pushing against the fixed magnet around it. It's actually the *coil* that is being forced back and forth, the coil is firmly attached to the bobbin, the bobbin is firmly attached to the speaker cone, and the speaker cone gets pushed in an out, and makes sound -- everything's fine.
But if so much power is being pumped in, turning to heat, the voice coil glue can melt, and the coil windings are no longer firmly attached to anything -- they're floating in a puddle of goopy melted glue. So the coil can change shape and position, the winds of wire get pushed around, the glue gets moved around and fills up the gap the coil lives in. The wire is still intact, still sorta make sound, but doesn't seem quite right. Then it all cools down, the glue hardens up, and everything is still sorta there and still sorta works, and you say "Phew! I thought I'd lost it -- but I guess it's fine." Lingering doubts! And you can hear (and feel) the glue scraping on the walls of the magnetic gap when you push gently and evenly on the cone to move the coil in and out of the gap, you get the sense there's something not quite right, and ... the impedance measurement may have changed a bit.
This is especially possible with woofers and subwoofers, where lots of power lives in the low frequencies.
Midrange drivers and tweeters tend to just go "poof!" when overloaded, because the voice coils are so light and are wound with such fine wire. There is an assumption that there won't ever be much power in them, and with typical music (for example), that's true, because of the relationships between frequency and power and loudness -- the higher the frequency, the less power you need to make it sound loud. Radically so.
Surely, there can be other reasons for the impedances to be different, but that's one possible scenario. I think it warrants further investigation. Prepare yourself emotionally for whatever you might find... I hope you get lucky. :)
@@ydonl That's what happened to my Soma speakers years ago, my Marantz amp blew the mids and pop they went. Soma replaced the speakers. I think it was something to do with the loudness being on and treble high? Damn though, I miss that amp.
Nice explanation, thanks
ho hum, my amp has DIN speaker sockets and the ends of multi-meter probes are too thick to fit inside to touch the speaker terminals.
get me probes then. or use a paperclip.
geez
The problem with testing that open circuit is that you have no idea what the impedance of the DC offset is. It could just be a so called "ghost" or "phantom" voltage.
Suppose that I measure a 1V DC offset open circuit; but, further suppose that the impedance of that offset is 100 kilohms. If I have a speaker that has a DC resistance of 10 milliohms, then the 1V is divided across the the 100 kilohms and the 10 milliohms. The speaker only receives:
10mΩ/(10mΩ + 100kΩ) ≈ 100nV
The current drawn will be:
100nV/10mΩ ≈ 10µA
That's not enough to damage a speaker.
Some meters have low impedance modes to help eliminate "ghost" voltages. If not, you will need to test the DC offset under some load to ensure that it is not simply a "ghost" voltage.
HI, is it same as power amplifier? Thanks
Yes, a faulty power amplifier can have a high DC offset across the speaker output which could be damaging to your speakers.
When checking DC offset do you set the volume to its lowest level?
The volume control should be set to the minimum level and the tone controls to the middle of their range. I wish you success.
thank you...I bought a used unrestored reciever...this is first thing I'll do....
@@northbalwynrepaircafenbrc8202So the lower DC offset the better right?
Thank you
@SDsailor7 Generally, although there are likely to be practical limits at to how low you can achieve. The amp manufacturers will often quote a level in there service manuals (perhaps around 10-25mV range) and some designs will provide adjustment procedures if you are outside these limits. My suggestion would be to try to source the service manual relevant to your amp and check whether it has any relevant DC-offset information. Thanks for your post.
Helpful video, thank you! Appreciate the posts in the comments as well.
What setting do you have you multimeter set at.
lowest voltage
@@mat.b.And then he proceeded to put in in the VDC setting instead of mVDC . . . that I totally didn't understand.
Thanks. FYI, probably a good idea to test red probe to + and black probe to - speaker connections (I noticed one measurement was positive and the other negative).
No, for this sort of quick test the polarity of the measured DC offset doesn't matter - just the magnitude. You can connect the probes either way and ignore the polarity of the multimeter measurement.
Oh boy, some of these fella in the comments are diving in head first and even giving advice with little to no knowledge of what they write about. Good luck indeed. 😅
You would think positive to positive, but apparently is not the case.
You are correct. The thing you are measuring is the voltage across the terminals - either positive or negative - and so the orientation of the multimeter probes is unimportant for this measurement.
My output transformers prevent this. Else there would certainly be 700VDC 😊
Yes indeed - transformer-coupled amps will prevent the DC offset from being seen at the speaker terminals. Thanks for you comment
i don't get it; say DC offset is 100 mV;
100Mv on a 4 Ohm is a power of only V*V/R , or 2 mW;
what diff. does that make??
He told that he'll show "what we can check before connecting speakers".
Passing high DC voltage through a speaker may be fatal (speakers don't like DC).
And there is another problem. Even a 100 mV voltage may not harm the speakers, but it will displace the woofer moving coil from is center position by a few millimeters. And so, the maximum excursion of the cone in both directions will have different limits.
The tweeter is itself protected from DC by the capacitors in the crossover filter.
A speaker impedance rating is for AC and it changes with frequency -- sometimes in complex ways due to the crossover.
At the heart of most speakers is a coil. At DC, an ideal coil is a dead short. In a real world coil, the only impedance of the coil will be provided by the series resistance of the wire that the coil is made from and will likely be far less than a single Ohm.
That said, since he is measuring the offset open circuit, we don't know that the output impedance of the DC offset is. I'm not convinced that it is low enough impedance to do any harm. The voltage could easily collapse under and load. See my top level comment for details.
Passbook
I set my dc offset to 5v and charge my phone.
🤣 very nice
I mean, I guess alternatively, you could keep a few cheap speakers around that you don’t care very much if they blow. Also, to the best of my recollection, I don’t think all amplifiers are designed to be run without a load, and might be damaged if left running without speakers connected,… such as certain tube amps. I dunno, but, I reckon I’ll just keep a few dumpster salvaged crappy Chinese, or salvaged speakers around for testing amps and receivers, like I’ve always done. I can always alligator clip ‘em into the appropriate impedance,… and pack em away, when I’m done, or scrap em, if I blow em up.
monster
buy a better mic and save my ears bro