|   Serious technical help needed | Tony Blair
Oct 13, 2003 2:48 PM | | I originally posted this in the tweaks room, but thought I'd come here to find the experts...
I've been upgrading my system, so have spent some loving fiddling time lately and seem to have uncovered a problem. I appear to have a voltage imbalance in my amp. I say it's a problem, because I noticed it as an audible imbalance in the channels on one of my amps and I think it might be responsible for blowing the voice coil on one of my tweeters (more on this later).
Also I should mention, I'm not a technician and have never done electronics repair, so my techniques might be cringe-worthy. That's why I've come to the experts.
Trusty multimeter in hand, I went ahead and tried to measure the problem. First I measured my main amp and saw the imbalance(and could hear a sligtht - nothing like with the amp that ate my tweeter - volume imbalance that I hadn't noticed before). I've since went around and tested all of my othe amplifiers in the house and found that they all have the same thing...leading me to believe this is relatively normal. But then, I've got no idea!
Here are the kind of measurements I'm talking about.
(measured with RadioShack digital multimeter set to DCv, measures approximate based on memory, when tested in ACv, average measurements were similar)
Quad 909
l=1mv
r=.05mv
Nakamichi AV10
l=50mv
r=23mv
center=23mv
rear l=23mv
rear r=23mv
Marantz mini system
l=20mv
r=5mv
Yamaha integrated
l=16
r=4
As far as possible, I tested these as isolated systems. The home cinema amp I measured using the setup test tone (no speakers or source hooked up, just the amp). The others, I used a just a CD source and, just to be sure it wasn't the source, I tested with 2 different sources and the results were exactly the same. I also tested things with the speakers hooked up to present a load, and no change.
I also tried some of the systems in different rooms with different mains sockets, one of which has a basic RFI filter with no change to the results.
So, to my mind, I've eliminated the source, the speakers, the cabling, the mains supply, and the tone and balance adjustments (all set to defeat). Yet, I've got a house full of amplifiers determined to do their own thing.
Is this normal? Can one man be so unlucky as to have all his electronics fry together in a show of solidarity? What other tests should I be running? How do I fix it?
Now, a quick word on the offending tweeter. My girlfriend swears she didn't touch it, but I came home the other day and it was dimpled in. I teased it out with masking tape and thought everything was fine. But a couple of days later (definitely not the girlfriend, it was in again). All of this was happening during my testing sessions, so lots of different amps online including the, I believe, offending amp, the Nakamichi. I didn't know an amp could cause a dimple, but the guy at the electronics shop mentioned faulty electronics as a cause even before hearing my story.
I've since replaced the voice coil (thank you Morel for making replaceable voice coils) but when looking at the old voice coil, I noticed some black liquid on the inside. Now, the Morel MD 28 is liquid cooled, so I'm wondering if I didn't fry more than the voice coil. Or, again, is a little bit of liquid perfectly normal. When I had the guts out, I didn't see a slick of liquid anywhere, so it's only small scale. Have I munched more audio equipment? Do I need a new tweeter? If so, do I need to replace the pair of tweeters or just the one?
Advice and assistance is greatly appreciated. |
| re: Serious technical help needed | skeptic
Oct 13, 2003 5:24 PM | | I'm not quite sure I understand what you measured. If this is the speaker output terminals with no input signal, then this is the residual noise of the amplifier. (I assume that the Yamaha was in mv also.) It is virtually insignificant. What you measured with the meter set to DC is called the DC offset. This is the dc voltage leaking through the output capacitors or if these are direct coupled amplifiers, the dc voltage leaking through the output transistors. To see how much wattage this represents at 8 ohms, use the formula P = Vsquared/8. Even at 50mv with no load, this is at worst about 1.25 milliwatts. (Try it with the speaker connected and see if it isn't even lower.) The others are a fraction of a milliwatt. The difference between the dc and ac measurements indicates the actual ac residual noise. Even if this were all in the tweeter's range, it is far too small to cause a problem for any tweeter. Put your ear up close and see how much of this noise you can actually hear. It should sound like a slight hiss. The dimple in the tweeter was almost surely caused by someone sticking their finger in it IMO.
To determine if there is actually something wrong with your amplifiers (not likely IMO), you would have to take the cover off and measure the dc bias voltages at various corresponding points of the two channels and see if there are any differences. If you try this, be very careful not to short anything out or you may cause significant damage yourself. Then to verify if any performance differences between channels exist, you would have to compare the test tone output traces on an oscilloscope to see if there are any significant amplitude differences. A distortion analyzer would help too. One audible test to try is to set these units for mono (use a y cable at the input if you don't have a switch) and see if both loudspeakers sound the same loudness with the balance control centered. If one sounds louder than the other, switch the speakers to see if the change is due to the speakers themselves or the amplifier.
BTW, you will probably have to replace the damaged tweeter. |
| Tony Blair? | Kursun
Oct 14, 2003 11:25 AM | | The voltages you mention are very typical DC offset voltages at the outputs of most solid state power amplifiers.
PS. When asking for help, don't you think you'll have better luck under another name? |
| Tony Blair? | Tony Blair
Oct 15, 2003 4:44 AM | | Hmmm, good point. Haven't bothered to change it. Maybe I'll go back to our buddy Bill. |
| Tony Blair? | Kursun
Oct 15, 2003 11:00 AM | | I must say the impeccable response from skeptic (as always) leaves nothing to add...
The AC measurements probably show the noise floor of your equipment. You did a good job of measuring the DC offset and residual noise (AC) figures of amplifiers. I hope to hear more, maybe some lab test reports from you in the future. |
| Bit rough on the newbie... | Bill Clinton
Oct 16, 2003 11:18 AM | | ...but advice is appreciated all the same. I'm happy to humbly learn from my mistakes.
Armed with this new info, I'll put on my lab coat and try some new measurements using my sophisticated test equipment (multimeter plus ears).
I'll report back duly...under my new alias Bill Clinton! Long live the king. |
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