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[Bradford "BJ" James]

I have a customer coming to pick up some TH118's on Monday. He's using the SC48 dsp, and PS DIGAM5000 (1 amp per pair of TH118). I remember a few months back we made some presets for some MTL-2 subs and the same amps. These amps do not like a continuous input signal, they clamp down the output voltage hard when presented with a sine wave. Looking for a few easy suggestions to get an accurate measurement to set limiters.
Would frequency "warbles" fool the amp? I have a bunch of low short sweep warbles from Bink's audio files I could try. ??

[Chris Grimshaw]

Pink noise would do it for checking peak limiters.

Chris

[Mark Wilkinson]

I have a customer coming to pick up some TH118's on Monday. He's using the SC48 dsp, and PS DIGAM5000 (1 amp per pair of TH118). I remember a few months back we made some presets for some MTL-2 subs and the same amps. These amps do not like a continuous input signal, they clamp down the output voltage hard when presented with a sine wave. Looking for a few easy suggestions to get an accurate measurement to set limiters.
Would frequency "warbles" fool the amp? I have a bunch of low short sweep warbles from Bink's audio files I could try. ??

I really think you need to use sine waves.
Something sounds wrong if the amp is balking....I'd get to the bottom before trying to set anything.

To solve amp issue.....
Looking briefly at the Digam 5000 manual two things pop out:
The default sensitivity is pretty dang sensitive......... 1.1V..... I'd lower it 6dB,  probably even 12 dB.
2nd....and something I don't understand...the manual says the amp's output is a bridged output, not referenced to ground.
It says output has DC voltage to ground and recommends a blocking capacitor in situations. 
I'd dig into this if lowering sensitivity doesn't solve the problem.

As far as the SC48.....https://www.danleysoundlabs.com/dna-system-engineer/
In that software download is a folder called "Loudspeaker Presets".  In that folder is a PDF for setting up the limiters.

[Bradford "BJ" James]

I don’t think pink would make any difference. It’s the continuous signal the amp doesn’t like. It’s inherent in the design, not a defect. It’s not the ideal amp for the application, but it’s what the customer has.
Just wondered if there was another way to do this.

[Len Zenith Jr]

Do you have any AFS (automatic feedback suppression) going on in your DSP? That is usually the case when something like that happens, double check.

[Chris Grimshaw]

I don’t think pink would make any difference. It’s the continuous signal the amp doesn’t like. It’s inherent in the design, not a defect. It’s not the ideal amp for the application, but it’s what the customer has.
Just wondered if there was another way to do this.

Pink noise, by definition, is not a continuous signal. The peaks are 9dB above the RMS level, so it's something similar to compressed music, instead of sine waves.

If an amp won't put out pink noise solidly, I'd think seriously about putting it in the bin.

Chris

[Ivan Beaver]

Pink noise, by definition, is not a continuous signal. The peaks are 9dB above the RMS level,

There are several different varieties of pink noise in terms of crest factor.

I have seen/measured 6dB, 10dB, and 12dB.

So that MUST be known.

But I would not use any form of pink noise for setting limiters

Sine wave is the only way.

[Ivan Beaver]

If the amp will not provide full signal sine wave, here is a way to do it.

Simply adjust the limiters for 20dB below what you want.

So if it is 100V, then adjust to 10V  (1/10th) the voltage.

That is a 20dB difference.  Now raise the limiter threshold 20dB.

If everything is accurate, it should now be set for 100V.

[Frank Koenig]

There are 4 types of limiters that you are likely to encounter on various modern DSP amplifiers.

1. Absolute voltage limiter. This is not a limiter in the usual dynamic envelope control sense but rather equivalent to setting the supply rail voltage of the output stage. Driving the amplifier beyond this voltage will simply cause the amplifier to clip.

2. Peak limiter. This is a true limiter and dynamically reduces the gain of the amplifier so as to prevent the (positive or negative) peaks from exceeding some threshold. The peak detection requires a specified attack and release time. The attack time may be set to zero. This is typically used in an attempt to prevent over-excursion. To be effective it needs to take frequency into account or, better, uses an accurate dynamic model of the speaker.

3. RMS voltage limiter. This dynamically reduces the gain of the amplifier to prevent the root-mean-square (RMS) voltage from exceeding some threshold in an attempt to prevent overheating of the voice coil. The RMS voltage serves as a proxy for power which is accurate for a constant and non-reactive load resistance. This may be preferable to a true power limiter in situations where multiple drivers are powered in parallel from a single channel. The averager has adjustable dynamics that are set with regard to the thermal time constant of the voice coil.

4. True power limiter. This averages the instantaneous product of the voltage and the current and provides a good estimate of the actual power delivered to the speaker. This may not always be desirable. Consider the case of two drivers in parallel running near their limits. If one blows (open circuit) the other will have twice the power available. As with the RMS voltage limiter, dynamics should be set for the voice coil's thermal behavior.

Absolute voltage, peak, and RMS voltage limiters can, in general, be tested with no load. This prevents the amplifier from power limiting as the current is zero. Connect a voltmeter or oscilloscope to the output and run the input up to verify limiting at the desired voltage. For absolute voltage and peak limiters a scope lets you see the peak value directly and any test signal will do. If using a meter (RMS or full-wave rectified average) use a sine wave test signal and multiply the meter reading by sqrt(2). For RMS voltage use a sine wave and meter and read the voltage directly.

To test a true power limiter you must use a load. As most amplifiers will not put out high power continuously a tone burst (gated sine wave) is a good test signal. It is not necessary to gate the sine wave at its zero crossings but it make the display less confusing. Tone bursts also let you examine the dynamics of the various limiters.

When connecting test instruments to amplifier outputs be mindful that some are not ground referenced and a floating instrument or differential inputs of sufficient common-mode range must be used (transformers can be used if they are calibrated). This is one advantage of using a battery-powered meter. I generally like to use a scope, however, just to verify that nothing weird is going on. Tone burst tests, of course, require a scope.

Hope some of this helps.

--Frank

[Mark Wilkinson]

It’s the continuous signal the amp doesn’t like. It’s inherent in the design, not a defect.

No, simply can't be a problem with a continuous signal....something else is going on.

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