audio testing of amplifiers...espically home audio and PA...

[Audiotexan]

I was just wondering why somebody would use test tones.....the music store uses them to test their PA systems...

IMHO, most of the time it's either:

A. To impress the client with something like: "FEEL how low this subwoofer cabinet goes!!"

B. To prove/disprove to the "nay-sayer" of: "Will this speaker cabinet REALLY do a-z frequencies?"


Once in a while, actual customer education takes place:

C. To educate someone starting out, the importance of, and the difference in frequencies, and how certain cabinets/crossover types interact (null/enhance/rollover/etc), how power is distributed via frequency (think: bi/tri-amping/etc -why horns only need say 200w, where your subs want 2000w)


Other than that...test tone cd's can be good to (loosely) dial in a room (with EQ in mind), or car stereo to the particular vehicle...but, as a tech testing a completed repair's "functionality"? -I'll always go with music.

[bsco]

IMHO, most of the time it's either:

A. To impress the client with something like: "FEEL how low this subwoofer cabinet goes!!"

B. To prove/disprove to the "nay-sayer" of: "Will this speaker cabinet REALLY do a-z frequencies?"


Once in a while, actual customer education takes place:

C. To educate someone starting out, the importance of, and the difference in frequencies, and how certain cabinets/crossover types interact (null/enhance/rollover/etc), how power is distributed via frequency (think: bi/tri-amping/etc -why horns only need say 200w, where your subs want 2000w)


Other than that...test tone cd's can be good to (loosely) dial in a room (with EQ in mind), or car stereo to the particular vehicle...but, as a tech testing a completed repair's "functionality"? -I'll always go with music.

I have had some powered PA cabs come in for problems with freq response....usually a distorted sound at a certain point....or some kind of a weird sound that they can't really explain.....it turns out that they are listening to the crossover point....or trying to drive the cabinet so low that it sounds like there are problems.......they usually say the speaker sounds blown......they are usually newbies that just got hired..even though they are musicians......I can see using the tones to find cabinet rattles, etc......I always use music when signal tracing...I just stick in a cd and set up repeat and go to it....and I usually pick a song that has lots of music content....if I have trouble with a horn ckt, then I might use a sine wave but alot of times I still use music......I have also seen people use their cell phones for testing this stuff.....but they usually don't set the levels up right...... a good old fashioned cd player or tape deck is a very handy signal source to have and is very cheap as well....

[g1]

The guys in rental dept. will often use a simple 4 or 5 freq. test tone for the cabs. It's mostly a go/no go test for woofer/mid/high, or maybe a blown bulb for tweet protection. It's an easy reference for quick basic function tests.
The little teac hand held ones were fairly common, now the Rolls MO2020 is probably more likely.

[Enzo]

Sure, I have one of these sitting out inthe warehouse, where we check larger stuff out.

https://reverb.com/item/575145-foste...cording-studio

[km6xz]

I guess I am the odd man out. I use sine waves for testing, because they give the best reference for distortion and visual indication on the scope and spectrum analyzer. Only after an amp is meeting design performance does music go through it. The type and character of distortion is hard to tell with complex waveforms of fully known quality themselves. I prefer not to guess in evaluations and complex material with lots of compression can mask a lot of ills. I want to know not only the distortion but spectral distribution of the artifacts and that is pretty much impossible with recorded music that has had time, and amplitude and harmonic traits altered in the recording process as often as not. An amp can't be called "repaired" until it meets spec on signal quality and sine waves are the key to knowing whether the amp is functioning properly. Whether an amp "sounds" good is very subjective and has little to do with whether it is working as intended. After it is repair you can worry about tweaking to the subjective tastes of the owner. Intermod distortion tells you more about linearity than listening to music of unknown characteristics.
Now with pulse testing of Class D amps, sinewaves are finally giving way to 4 cycle burst of precise zero crossing settling to zero. Most shops are no set up for testing D amps for power but they are the future and a reality for commercial repair shops.

[Enzo]

It depends on what you are trying to accomplish Stan. If I want to look at unwanted distortion, sure, I want a sine wave. But for general checkout of an amp, music is a full range signal, I can hear instantly that the tone controls are having proper effect, I can tell if there is some hole in the response. I don't have to adjust anything to hear if a crossover is functioning. Sure, I need a precise set of signals if I want to verify that crossover is operating at the intended freq points. Clearly, if I am listening to the OD channel on a MArshall, my country music station sounds like hell. But it is readily apparent that when I kick in the OD, I can hear that the gain goes way up and distortion increases. A sine wave through that turns into a spikey, squarish sort of goo.

[km6xz]

Of course a listening test is made but it is backwards. By setting up to listen at all levels to determine if further diagnostics is needed, you have to reconnect everything for testing on instruments to find out WHAT is wrong. By doing a quick look at waveform, spectrum, power, you get to the heart of the problem is seeing what is going on, not just that something is going on. Only very gross deviations in circuit operation will swamp variations in listening position, level, content etc, where moving the ears 6 inches changes the hf response impression more then 10 db or more, you have to reconnect the instrument set up to verify that what you think you here is actually an artifact of the amp instead of variations in room acoustics, listening position and orientation, mood, etc. Besides, a before reference plot compared to the post repair/adjustment plot proves your work had a positive impact. I always challenged my techs to prove a problem was taken care of and that they actually saw a problem. If they can't reproduce the reported defect and just started doing the routine pm items, they could never be sure they took care of a problem regardless of what they subjectively thought was a problem. We have very poor memory of sound, but long term excellent memory of our impressions of sound. This is what trips up the Golden Ears in hi-fi and guitar amps, they remember their impression or opinion of the sound character but in reality they have almost no retention of the sound memory over a second or two. I have done so many double blind tests with sound gurus who fail more than random chance when isolating the only variable to whatever they are listening for.

[Enzo]

Well, if the job gets done, I won't worry if I took a taste from my soup or my sandwich first. And if listening position enters into it, I fear we are no longer working on a Hot Rod DeVille.

[nevetslab]

pink, 1/3-Oct Pink, burst pink, guitar signal sources

I tend to use pink noise as my signal source on testing what I have on the bench, or burst pink noise, allowing me to hear the behavior of the reverb or compressor circuits when checking things out. Usually start with nothing plugged in, and listen to the self-generated noise, rack all the pots back and forth to find the obvious gritty ones....but will also find them when I plug my bass in.

Depending on what I'm working on, if it's a high wattage self-powered sub, like a Mackie SWA 1801, I use 1/3 Oct Pink (B & K 1027/1617), tuned to 63-80Hz for starters. At low frequency, the cyclic amplitude fluctuation of that 1/3 oct pink signal looks like low freq modulated sine wave, so it's very effective to see how the upper power stages turn on and off, and will also reveal how the speaker is behaving. I will usually check the speakers first when suspect, so I'm not complicating matters with a defective or blown speaker.

With most guitar and bass amps, after initial evaluation for listening for noisy tubes, noisy pots, I'll feed it pink noise, which allows you to hear the tonal change on all the tone controls. I very rarely bother with a mating spectrum analyzer. Have it here, but seldom required. My burst pink noise source (GenRad 1382) is followed by a GenRad 1396B Tone Burst Generator & Krohn-Hite 3550 BP filter, so I can shape the noise spectrum as desired.. It and guitar / bass work well in checking anything having dynamics circuits and reverb/echo circuits.

Large power amps...like QSC Powerlight 9.0 or 6.0, I use a burst sine wave generator (Sound Technology 3100B, where you can set the frequency, 0dB on time, attenuated on-time, 4kW load bank and a 60A AC Mains source/40A variac. It will allow you to see full tilt power level for short durations, and usually set the attenuated portion -20dB or -30dB below the 0dB level. When all is well, it gets fed to a woofer bank that can handle the power. Amps behave differently with loudspeakers.....protection circuits can bark when not working correctly....activated by back-emf, which you'll rarely see on resistive load banks.

In all, I tend to rely on my ears first, before breaking out any heavy artillery.

When I"m done with PA gear, I'll send music from a CD player as a final check.

[bsco]

Wow....thanks guys ..lots of ideas here for sure....much appreciated......
Cheers