Audio Asylum Thread Printer
Get a view of an entire thread on one page
|For Sale Ads|
Can someone recommend a decent DMM that has a max voltage hold function?
I want to connect it to a loudspeaker to estimate max power draw (speaker is a near-perfect resistive load).
Connect ANY true RMS DVM and play a low bass tone nearest 60hz.
Find a white / Grey / Fuscia noise on disc. They should have information on the 'mix' and you'll be able to see the meter without 'peak hold'.
Have a SPL meter, like one you can get for your iPhone and than you can estimate both power and what db louder peaks will draw.
Too much is never enough
By the way, I do own a scope, but am not so good at using it. It's fun to watch the waveform but determining the power level at clipping is not so easy.
This is a good opportunity to learn. Do a search on youtube. Tons of great info on basic oscilloscope use and setup. Besides being SO cool to play with it really is a very important piece of test equipment to understand and use.
A scope is basically a voltmeter that displays the voltage waveform. You can connect the probe to the positive speaker terminal. DO NOT CONNECT THE PROBE GROUND WIRE! Then set the time base to a low speed and turn up the intensity so you can see the transient peaks. Make sure you set the VOLTS-PER-DIVISION control so the peaks remain visible within the display.
The scope displays peak-to-peak voltage. Divide the peak-to-peak voltage in half to get peak voltage, which might be just what you want for determining peak power. If you want RMS voltage, multiply peak voltage by 0.707.
Try this method and see what you get. I'm sure that with a little practice you'll get the hang of it. You just have to keep watching the scope to get a sense of the amplitude of the transient voltage peaks. If you need instruction, I'll bet you can find it online. You can find just about anything on the internet using Google.
I've concluded that this:
is probably a very good method.
You noted: "speaker is a near-perfect resistive load"
I have never heard this before. In EE, we modeled a speaker as an RLC circuit where L was a dominant factor that changes with F. However, it was not very linear (perfectly resistive). Speakers are actually quite complex... I link to a loudspeaker model below, but also of note are:
Note in this presentation how R is parabolic in nature and no where near-perfect.
Modeling Large signals in microspeakers
ST Microelectronics compensation, see pg 24
Still, to get to your original question if you want a max hold, Fluke makes a series of handheld DMMs that will hold max, min or average. (in fact the Fluke 187, Fluke 189 and Fluke 87V can do this)
I should have mentioned: They are indeed a very flat four ohms in the bass, which is what will matter for power, so extrapolating power from voltage from bass-heavy music should work well enough.
I just want a rough idea of power drawn.
I did something like you propose several years ago with my actively biamped MGIIIa's, placing the DMM across the woofer. Typically my peak listening volume is around 85db, perhaps going to 90db at times. IIRC I measured a peak voltage around 12V which would calculate to 36 watts. The amplifier I was using produced 500 wpc at 4 ohms. I concluded that the measurement was pretty much useless but that my amplifier was more than capable for my listening levels.
I don't remember the brand of my DMM but I purchased it around 10 years ago for ~$50 on Ebay. It has a peak hold function but is not true RMS.
If I were to buy another DMM, I would get one with LCR function. For my use, the meter is accurate enough and I really don't need true RMS. You can peruse Ebay and find something suitable for under $70. FWIW I did buy a used scope a few years ago for around $250 (Tektronics 2336).
BTW I did measure the inductance of the woofer it was a fraction of a mH (I don't recall the exact value) so it is essentially purely resisitive.
I married the perfect woman. The downside is everything that goes wrong is my fault.
You might want to use a single sinewave for your measurement as capturing AC across a broad complex audio spectrum will require more than just a multi-meter. Many are good to about 1-KHz (1-ms) and some higher end models (that triode mentioned) are good only to about 4-KHz (250-uS).
BTW the decades old Fluke 187 & 189 were replaced years ago with with the Fluke 287 and 289 but these are in the $450 - $600 range.
The Fluke 87 V (5th gen) is around $350 - $400.
Some lower cost models that measure True RMS AC may do what you want. In Fluke terminology look for features like Min/Max, AutoHOLD, Fast Min/Max.
I don't think a multimeter will do the job. It takes time to sample the signal so you may never get a good reading of transient peaks. You will probably need an oscilloscope or maybe a storage oscilloscope.
Another approach would be to use a CD with tone-bursts. These are sinusoidal signals that produce a short burst and then turn off for 19-times the length of the tone-burst. You can feed the speaker a large voltage without damage because the average power would be only 1/20th the power of the tone-burst. I fed my Thiel CS3.7 speakers a 50-volt tone-burst at 1000-Hz to check the amplifier for clipping and also check the speaker for distorting. The amplifier just began to clip at 50-volts but the speaker had no audible distortion. This wasn't the case with a 400-Hz tone-burst. The speaker began audibly distorting above 36-volts although the amplifier exhibited no distortion at all.
Be sure to wear ear plugs for tone-burst testing.
400-Hz Tone-Burst ................................................... 1000-Hz Tone-Burst
These are magnetic planar speakers (Magnepan) and they are indeed frequently described as I said. I think a full-range mag-planar would be very close to that but, with crossovers as in the actual speaker, not as much. Still, they are much less reactive than cones & domes.
I found the Flukes last night. Nice, but they are very expensive.
I also found this, which does seem to have the max voltage hold feature:
At $47 may have to give it a try.
I don't want to spend a lot. I want to have my cake and eat it too. And after that, message my speakers.
Hey triode3, that link to "Loudspeaker model" is great! Thanks for that. It does an excellent job of describing parameters and behaviors in a scientific manner but without being overly math-oriented or pendantic. I have Harry Olson's "Acoustical Engineering" and it often makes my eyes glaze over. ;) Your link doesn't - it's very approachable even for the novice.
I haven't looked at your other links yet.
Post a Message!
This post is made possible by the generous support of people like you and our sponsors: