Measuring amp voltage, where did I go wrong?

[Tim A]

Ok, I finally got to try this out last night. The results were surprising, but I'm not sure I did it right.

I set up a pink noise CD through my board and ran the outputs almost to clipping. Set the rest of the gain structure flat. Meters on the EQ indicate it was getting the same signal as the board.

First amp, the QSC RMX 1450, no load, taken at the speaker output. It measured at 28-30v. With no load I have no idea how to do the math to determine how many watts that is.

Second amp the SHS SA2500, same setup, it measured at 40v.

That didn't seem right. My understanding is with no load it should be higher, giving you a margin of safety when there's a load. If the QSC is capable of 280 w, or 47 v @ 8 ohms, shouldn't the reading have been much higher?

On the SHS, I'll be running 2 cabs per side. If I figure 450w X 4ohms I get about 42 volts. So it would appear by testing the top amp needs no limiter in this situation. For that matter, neither does the QSC running the 3 subs.

From a 'by ear' standpoint, I need to cut the tops back at the crossover for the subs to keep up. This is 4 DR200's and 3 T-39's. That would indicate the readings are pretty close.

I also tried to measure the QSC at the subs with all 3 hooked up for 3.33ohms. I couldn't get it to go past about 2 or 3 volts without shaking the garage off the foundation.

So, what did I miss? I have a feeling my understanding of the test method isn't complete.

[Bill Fitzmaurice]

A direct coupled SS amp is a constant voltage device, unless it's operating into a dead short the load impedance, or even lack of a load, will have no effect on the output voltage.

[wallywally]

With pink noise you won't be able to reach "full voltage". The amps are rated using 1khz sine wave. This is the reason I use sine waves when setting limiters. If the amps maximum rating is very close to the voltage you are trying to limit, you will never reach that value with pink noise. Remember we are trying to limit peaks to protect speakers. Pink noise represents an average of music not peaks. Your amps can produce a higher voltage. You do need limiting. Download Audacity and run some sine waves to set your limiters. JMHO

[LelandCrooks]

I rest my case.
http://billfitzmaurice.com/phpBB2/viewt ... =14&t=6146

[SoundInMotionDJ]

With pink noise you won't be able to reach "full voltage". The amps are rated using 1khz sine wave.

Actually my amps include both ratings in the spec sheet. There is about 50W more output with a 1kHz sine wave than with pink noise.

--Stan Graves

[Sydney]

There is about 50W more output with a 1kHz sine wave than with pink noise.

Energy is focused @ 1 frequency and not distributed.
Testing an amp into a fixed frequency into a fixed resistor allows for a bigger power number.
Hardly reflective of the real use - kind of like evaluating a car based upon horsepower on an engine stand instead of wheel horsepower.
Sine wave the same amp on a low impedance reactive load at frequency points where you have -45 degree phase angle and see how much power you get out - and for how long

[Tim A]

ALRIGHTY THEN!

Please try and remember you're communicating with a Neanderthal.

So what does al that mean?

A) I did it right and the results were to be anticipated. I still need to set my limiter to protect against spikes.

B) I'm a complete idiot and I did it wrong.

If I did it right, how am I to set the voltage with the limiter using the meter if I can't get it to the voltage I want to begin with?

Example: 3 subs. Say I want to set the limiter at 450w, or 150w/sub. 150 x 3 = 450 x 3.33ohms, Sq. rt. = 38 or so volts. The amp is only reading about 28v. How do I know when I've reached the correct setting? Do I adjust the limiter until it starts to affect the signal, then back it off a touch? Seems that'd set it for 28v. But since that's all the amp is putting out, is that the right setting anyway? What is the capital of Sri-Lanka?

[wallywally]

For subs run a 100hz sine wave. Your voltage will be higher than with pink noise. For tops run a 1khz tone. The most correct way is the way you did it. But, your amps couldn't reach limiting voltage. That is why I recomend using sine waves, Your amps should be able to reach limiting voltage allowing you to set limiters. It's not quite as accurate this way but it is far more accurate than no limiters at all. Were You driving the amps to clip? I set mine with the clip lights just starting to flicker.

[Bill Fitzmaurice]

Example: 3 subs. Say I want to set the limiter at 450w, or 150w/sub. 150 x 3 = 450 x 3.33ohms, Sq. rt. = 38 or so volts.

Making it more complicated than you need to. 150 w into 10 ohms is 38 volts. So is 300 w into 5 ohms and 450 w into 3.3 ohms. You only need calculate the max voltage into one cab, it remains the same with more parallel wired.

The amp is only reading about 28v

Then the input level is too low, assuming it's rated more than 100 w/8 ohms.

[SoundInMotionDJ]

Ok, I finally got to try this out last night. The results were surprising, but I'm not sure I did it right.

I set up a pink noise CD through my board and ran the outputs almost to clipping. Set the rest of the gain structure flat. Meters on the EQ indicate it was getting the same signal as the board.

First amp, the QSC RMX 1450, no load, taken at the speaker output. It measured at 28-30v. With no load I have no idea how to do the math to determine how many watts that is.

The specs for the 1450 claim that full input sensitivity is based on 1.15v, and that the gain is 40x (32db). That should give you a final output of 46v. Did you measure the input voltage from the limiter? Are you getting anywhere near 1.15v?

Was the input gain knob on the front all the way up? If not, then crank it up and re-check. Are any of the dip switches on the amp set weird? (bridged, mono, 50Hz high pass?)

--Stan Graves

[Sydney]

I've not had to deal with limiters yet...
My question is related

Then the input level is too low,

Did you measure the input voltage from the limiter? Are you getting anywhere near 1.15v?

For a Odb meter reading on a mix console - what is the appropriate Pro Industry Standard Reference Voltage (output)?
I thought the answer was in in The Yamaha Sound Reinforcement Handbook

[Bill Fitzmaurice]

I've not had to deal with limiters yet...
My question is related

Then the input level is too low,

Did you measure the input voltage from the limiter? Are you getting anywhere near 1.15v?

For a Odb meter reading on a mix console - what is the appropriate Pro Industry Standard Reference Voltage (output)?
I thought the answer was in in The Yamaha Sound Reinforcement Handbook

+4dBu/1.23v. But you have to consult the manual for the mixer in question, if referenced to 0dbV it's only 1v. It might even be -10dBV, 0.316v ,

[Paul Norman]

First amp, the QSC RMX 1450, no load, taken at the speaker output. It measured at 28-30v. With no load I have no idea how to do the math to determine how many watts that is.


I also tried to measure the QSC at the subs with all 3 hooked up for 3.33ohms. I couldn't get it to go past about 2 or 3 volts without shaking the garage off the foundation.

So, what did I miss? I have a feeling my understanding of the test method isn't complete.

I use RMX 1450's on the my Titans Bridged. 2 subs 4ohms rated at 1400 watts. My voltage test were done as you did and came out the same 28 to 32 volts. I set the limiters, but have never had to push close to clip to get the sound level I wanted.