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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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In Reply to: How to match line level connects? posted by Chris on June 21, 1999 at 13:52:04:
Chris,For two sentences, you require a small book as an answer. I'll try to be brief.
1. Most interconnects do not affect level enough to exceed the +/- 0.1 dB criteria that the ABXers hold dear. In reality, if you are within +/- 0.2 dB, this would be good enough, and except for certain combinations of tube output stages and 30 foot or longer interconnects, there should be no level differences or HF roll-offs that would invalidate a direct comparision.
0.1 dB is approx. a 1% voltage change, and 0.2 dB is a 2% voltage change.
2. Carefully check the RS digital voltmeter owners manual/instrcution booklet for the specs. MOST cheaper digital VOMs only have FR up to about 400 Hz, above which they go crazy and totally inconsistent. Unless there is a spec in the booklet that SAYS it has better, do not assume it is good for anything higher than AC line frequency! Even the ones that do have a spec to 10 or 20 kHz, the tolerance for this range is often +/- 1 dB or more.
3. In order to check the cables properly, they would have to be under the load of the device they were attached to. Lets say they run from the CD player to a preamp. You would have to measure the signal WITH THEM ATTACHED TO THE PREAMP, and here is the rub. Inexpensive digital voltmeters often have a low input impedance, which will affect the measurement. One of the first rules of measuring is that the measurement device must not affect the signal or entity being measured. In order to do this, the usual rule of thumb says that the input impedance of a measurement device should be a MINIMUM of ten times the circuit impedance.
Unfortunately, this only prevents the worst of a potential error, as an input impedance 10 times that of the circuit being measured would allow a 1 db droop in the signal. Hard to accurately measure 0.1 dB when you meter is causing a 1 dB droop, and the droop changes with circuit impedances. In order to accurately measure a circuit impedance to 0.1 dB, the measurement device should have an input impedance of MORE than 100 times that of the circuit being measured.
Check that spec sheet again, and that of your preamp, power amp, etc. and see if the Digital VOM has a high enough input impedance.
You may find that a digital meter that can do what you would want to take an accurate measurement may cost much more than you willing to spend.
Ironically, an moderately expensive analog meter usually can have a decent FR, and a fairly decent input Z at costs far below that of a digital meter that can do the job. The ones to look for are the ones with an electronic front end, usually refered to as a FET buffered input VOM.
4. Calibration of the meter. You will need to use a CD that has test tones on it, at spot frequencies across the audio band. There are several such CD's, including the CBS CD-1 Test disc, and the Stereophile test disc #2 and #3, and the Sheffield "My Disc", as well as others.
Then read the output of the cd player directly into the meter by sticking the probes onto the RCA output jack and recording the deviations from nominal or 0, or the output level at 1 kHz. Once you have the meter calibration chart, which now includes the FR of your CD player and meter combo, you can check any other item in the system, as long as you do it under actual loaded conditions.
This may require making a special harness, probe or cable splitter to allow you read the voltages while the cable is loaded by the unit it connects to.
5. Once you measure the CD player and the meter, then measure the addition of a cable or other audio device, you must subtract the original deviations from the total deviations. This will require the old subtraction rule that when you subtract a negative number, it gets added to the first number. The final end result of all this is the actual deviation of the cable or other audio component.
Surprisingly enough, most interconnect cables are pretty flat, while power amps, preamps and CD players may exhibit more deviations than the cables!
It is rare for them to individually exceed +/- 0.2 dB, and for a total system to exceed +/- 1 dB, exclusive of speakers.
Now aren't you glad that was the short version?
Jon Risch
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Follow Ups
- Re: How to match line level connects? - Jon Risch 06/21/9920:05:42 06/21/99 (0)
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