O-scope question

[daz]

I need to know how i can look at the waveform on the scope and tell whether the output level is loud enough to be acceptable for a 2xel34 amp. After the mod i just did adding a tube to the preamp i have a couple gain trimmers that i've been playing with and when i get the best tone I can't help but wonder if the amp is loud enough to gig with. Not that i do anymore aside from rare occasion. But i want to know it is, and just turning it up is impossible to tell. I've done gigs where an amp turned out to be far lower in volume than i thought from my experience with it at home.

At the moment, where i have these trimmers set for the best tone, the wavform looks like the one below which i posted a while back. And heres the thing that worries me....even with the master on 10 and gain set high it looks like that. So while it sounds quite loud, is there anyway to tell from the scope whether it will put out it's full volume? The way I'm looking at this is maybe the output is simply showing that it's at max CLEAN volume and anything past that would only be output clipping, which i don't necessarily want anyways. Any advice on how to tell? remember, i'm new to the o-scope and know nothing about it.

[Jazz P Bass]

I am a bit confused as to what the question is.

I use a DVM to measure the output voltage, RMS.
That squared times divided by the speaker resistance is the output wattage.

Is that posted waveform the output?
If so, it is anything but a clean signal.

It appears that it would sound nice and crunchy.

What frequency is the waveform?
I find it helps to sweep the freq & see just how the amp reacts to those frequencies.

[daz]

Is that posted waveform the output?
If so, it is anything but a clean signal.

It's preamp OD and that wave is what it looks like with master low or on 10.

[g1]

I'll assume you are using a sine wave for the input. Set it's level for 50mV RMS. Set amp gain at zero, master at 10. Bring the gain up a bit till you can see a sine wave at the output. You want it to be the same shape as what is going into the input, you may have to adjust the tone controls to get the proper shape.
Assuming you are running into a load, and scoping the output, increase the gain till the top and bottom of the sine wave just start to flatten. Back it off till just before the flattening (clipping) occurs.
What is the peak to peak voltage at this point? What is the load resistor impedance?

[daz]

I'll assume you are using a sine wave for the input. Set it's level for 50mV RMS. Set amp gain at zero, master at 10. Bring the gain up a bit till you can see a sine wave at the output. You want it to be the same shape as what is going into the input, you may have to adjust the tone controls to get the proper shape.
Assuming you are running into a load, and scoping the output, increase the gain till the top and bottom of the sine wave just start to flatten. Back it off till just before the flattening (clipping) occurs.
What is the peak to peak voltage at this point? What is the load resistor impedance?

1k sine wav, 100mv, 8 ohm load, PP....i dunno how to check that. I basically did it as you describe, turning the trimmers till just before the wave started to flatten with master on 10 and gain up as high as i ever use it. Except with 50mv i guess the amp would have taken more gain to get to that point so i assume as it is it would not be 100% output b4 clipping. Thanks.

[g1]

Did the waveform look the same as the sine wave at the input? This is why you should set the gain as low as possible (and master at 10), to get a clean sinewave at the input. Then once you know what your clean power level is, you can have an idea of how loud you will be.
If you can get a clean sinewave at full power out with a low setting of gain pot and 100mV input, you will be louder than the clean output when clipping. The peak to peak reading is how you determine the output wattage.
In your picture of your clipped waveform above, there are about 4.5 divisions from top to bottom of the waveform. One of the knobs on your scope will be the volts per division. If for example it is set for 10v/div that would be a 45V p.p. waveform.
You can also read RMS volts across the load with your meter.

[teemuk]

You can also read RMS volts across the load with your meter.

But note that unless your meter is "True RMS" it will show false results with anything but a sine wave. I would use the scope because it provides visual clues of waveform's integrity and there's no point not to use a scope when you have one.

But you can forget the scope for a moment. The first thing is to get that rig ready for testing.

Ouput power is calculated from sinusoidal wave with very low distortion, few percents at best. The higher this "total harmonic distortion" is in the signal, the more power that signal will also produce. Since we are interested in clean power there's no point to measure at insane distortion levels.

So, as the first step, input sinusoidal waveform to your amplifier. The goal is to get that sinusoidal wave as clean as possible in the output too so understandably you turn down any gain controls in the preamp that would otherwise cause too early signal clipping. Since preamp input signal levels, especially ones that do not get overdriven quickly, are low I suggest you start from 20mV or 40mV amplitude and advance higher if the signal seems too low for the purpose. If it still clips/distorts too soon turn down the amplitude of the input signal. Turn off all effects, reverb, FX loops, etc. Then turn the master volume control higher until that sinusoidal wave just barely begins to clip in the output. Now back off a little bit so that clipping barely dissapears. You now have full output power of your amp at low distortion, at least at low distortion calibrated by eye. I assume, though, that you have no spectrum analyzers to measure the actual % of THD, so for now we have to stick just to that "visual" calibration. (And that is why you need that scope, a plain multimeter won't show how much distortion is in the output signal or if it clips).

Alternative method is bypassing the preamplifier completely and feeding the sinusoidal input signal directly to the power amp. This is useful when you want to test the power amplifier alone or if you feel that preamp's signal processing is obstructing measurements too much. e.g. the preamp clips so excessively that you can't get a reasonably high clean input signal to power amp no matter what. In such cases you can test the power amp for its capabilities and then settle for a fact that you never get its fullest output power due to preamp design. Whatever is the case, you once again use a sinusoidal input signal and tune for cleanest maximum signal swing in the output.

Since you now know the amplitude of output signal in volts and load in ohms you can calculate the output power as usual. The measurment part is the only part where you ever need to use that scope. If you know how to measure voltage with it then you're set to go.


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And with that said, in my opinion the output power is the worst spec ever to evaluate how loud your amplifier is going to be.

[daz]

This amp's clean using the gain pot isn't very clean and i only use it with the gain at least 1/2 way up. If i test output power with a clean preamp signal and adjust those trimmers till the output is just starting to clip, there tone will go to sheet because i will have to crank those trimmers quite a bit and i know how that sound from playing with them. Thats why i wanted to test it with that dirty preamp tone as in the wave i posted.

[teemuk]

If i test output power with a clean preamp signal and adjust those trimmers till the output is just starting to clip, there tone will go to sheet because i will have to crank those trimmers quite a bit and i know how that sound from playing with them.

We don't care how it sounds when we test output power. We only care how much power it puts out. Why do you think we even use sine waves for measurement? Because they sound nice? ...Or because a sine wave consists of only the fundamental harmonic and nothing else, making it a perfect test signal because we can easily even hear any distortion it it. (Which is something that can't be said of most signals).

There's no point in measuring output power at ridiculously high distortion levels because the result will not be comparable to any of those results where amplifiers were measured at THD levels of 0.1% - 1%. If you want a meaningful figure you must measure it according to usual standards of measuring output power. Proper "cleanness" of output signal is one of those standards.

Sure, you can measure the output power with practically a square wave output signal. The THD may be close to 100% and the measured power will be twice as high as when measuring a clean output. Not much point in that, IMO. Such measurment isn't going to tell anything meaningful about the amp and isn't even slightly comparable to "standard" power ratings.

So the clean output fairly quickly stops being clean and distorts... So what? You measure the output power on that clean level and that is your result, whether its hundred watts or a single watt. That's your amplifier's output power at reasonably clean output signal. Period. We know that the amplifier can output more power than that when its driven to distortion... But so can any other amp.

And yes, this dilemma is exactly one of those reasons why output power rating is a very poor measurement of perceived loudness.

[g1]

I understand you do not use clean settings, we are trying to establish a reference. Once you know what the clean output capability is, you can get at least a rough idea how loud it will sound.
Looking at a distorted waveform makes it almost impossible to tell the limits of the power amp.

[daz]

I understand you do not use clean settings, we are trying to establish a reference. Once you know what the clean output capability is, you can get at least a rough idea how loud it will sound.
Looking at a distorted waveform makes it almost impossible to tell the limits of the power amp.

I get that. But the point is, to get a clean wave from the preamp with this amp, the gain must be down to literally 8:30-9:00 thereabouts. If i set the power amp to just start to clip at that point, if it even will, then where i normally keep the gain is going to overdrive the outputs insanely. My goal here is to know that *with my normal preamp setting at the lowest i ever use it* there is just enough signal at the PI so that any more than that will not result in more volume, only compression/OD. In other words, where i set my gain normally i want the output to get as loud as possible without beginning to clip. The only reason i'm even concerned is because on those rare occasions where i do do a gig i want to know i'm not going to find that in the band mix the volume can't even keep up. My ears are very bad at determining that at home. But to tell you the truth, as much of a pain as this is i'd rather just put a switch or pot on the back to get more gain from the trim pot so if that ever happens i can just reach in back and take care of the problem.

[g1]

I'm hoping I'm understanding correctly . Seems you want to set it up so with your standard gain settings you just get to full power of the power amp but don't overdrive the power amp.
But to find that point of full power is what I'm getting at and for that we need a clean signal.

[daz]

I'm hoping I'm understanding correctly . Seems you want to set it up so with your standard gain settings you just get to full power of the power amp but don't overdrive the power amp.
But to find that point of full power is what I'm getting at and for that we need a clean signal.

Yes, that was what i was getting at, if rather unclear. But no worries. I'm just going to do as i described and add a external switch or pot. Thanks.

[Dave H]

If you don’t want to deviate from your standard preamp settings you could measure the sensitivity of the power amp and compare it to the preamp output at the standard settings.

Disconnect the preamp from the power amp at the PI and inject a sine wave at the PI input. Turn the signal up until it is just clipping at the power amp output then measure the peak to peak input voltage to the PI (Vp say). Reconnect the preamp, plug in your guitar, set all controls to ’standard’ and measure the peak to peak voltage at the PI input as you play. If it is less than Vp you are not getting full power from the power amp. If it’s greater than or equal to Vp you’ll get full power.