ARTA

If T/S measurements depend on power level the DUT must be quite non-linear. Can you post some link?
Possibly they are, I do not know. Will check this later.

Btw, for measurements of speaker sound output an amp is vital, and I stock a reference latFET amp for just this purpose.

I don't have a link, but have some results myself.
I thought I posted it somewhere else.
Can't find it atm, but otherwise I will post it again.

But anyway.
DUT non-linear; yes and no/ maybe.
After a certain voltage it stays quite linear again.

I have seen this behavior with a few newer drivers van Scan Speaker, Seas and SB Acoustics.

But that is not relevant.
What IS extremely relevant and important, is that one is only able to detect certain quirks when there is more output available.
Or in other words, without some more output power you're already crippling yourself.

Would never be my advice to do.

It doesn't have to be some fancy amp, unless you really care about (very) low distortion numbers. Just one that is reliable.
 
1. Do I benefit from an ARTA box? First, I have DATS v2 for impedance measurements and ZMA files already. Is ARTA any more accurate than DATS v3, which upgrade I'm considering? Second, is it really necessary to do full dual-channel measurements including the voltage probe to take the amp itself into account as long as the amp is of good quality, or is it sufficient to do just semi-dual channel measurements like I'm used to? This especially since, and I quote from the relevant application note: "The quality of measurement with this setup can be better than with ARTA Measuring Box, especially when measuring impedance as there is no need for voltage dividers, and the dynamic range is larger (usually the divider reduces input level 20 dB or more)". If still considered useful, I'd get the ready-made ARTA 4.1 module but obviously not if considered superfluous in my case. If it matters, my interface is the Focusrite Clarett 2Pre USB (or EMU 0404 USB if I could just find it…).

Accurate? Depends on the application. Versatile, of course. The complete 2 channel jig allows for T/S at adjustable voltage level, as well as ability to analyze other circuits as well such as active filters if you are clever.

Semi-dual vs full dual channel is up to you. For loudspeaker measurement, full dual channel measurement would be my recommendation. For example if you want to measure some delicate ribbon tweeter with a transformer that requires a capacitor, you can locate the reference channel probe on the speaker side of the capacitor and compensate completely for it. Keep in mind as well that the difference between the dual channel and semi-dual channel is just a couple cheap resistors and some wire, so why take the lazy approach?

2. If I choose to do full dual-channel measurements, I understand that the negative amplifier output needs to have true ground (zero potential between it and input ground) which rules out almost all modern digital amps and bridged Class AB etc. But I see the Thomann PM40C being recommended which LM3886 chip is a "Class AB-A (conjugate) amplifier that has a fully symmetrical structure (push-pull), meaning that the sine waves produced, will produce a +, - output". Not sure what that means exactly, if it's a deal killer, but I'd like a confirmation that it has true ground on the negative output (single chip, not two in parallel bridged) before going down this route.

No bridged amplifier is correct, at least if you plan to use the jig as all schematics online show. This would mean to avoid most low cost class-D solutions, including some Icepower solutions. Hypex modules are just fine to use, as well the LM3886 chip is perfectly usable device for measurement setup, I use a DIY My_Ref amp myself a lot of the time. The description above sounds like some fancy description of "it's an amp". Specs for the Thomann is 50W/4ohm so I have no doubt that is is a single LM3886 in a box.
 
If T/S measurements depend on power level the DUT must be quite non-linear. Can you post some link?
Possibly they are, I do not know. Will check this later.

Unfortunately this is not true. (as I said before)
I can show you quite some comparisons I did, also with references from manufactures. (who show the same behavior)

There is a need for highers voltage in some cases.
Otherwise your T/S can be off by 30-40%
Actually, this is not a correct understanding of the situation - the measurements are not "off by 30-40%", they are a true reflection of the T/S parameters of the driver at that drive level.

Bucks bunny is correct - the compliance (Cms) of a driver (all drivers) is non-linear, this means all T/S parameters dependent on Cms change with drive level - this includes Fs and Vas.

The resonant frequency (Fs) of a woofer changes with drive level, so there is no one correct value! How much it changes depends on how non linear the suspension is. On some drivers it may only change a few Hz, on others it can be much more, but it changes with level on all woofers I've measured.

Due to the non-linear restoring force from the suspension, a driver is a non-linear harmonic oscillator and with any non-linear harmonic oscillator the resonant frequency changes depending on the amplitude of the oscillation...

This probably comes as a shock to anyone who assumes T/S values are absolute and that provided you measure them "accurately" you will get the same result. No, some of the T/S parameters are dynamic due to this nonlinearity of Cms and they are constantly changing as your music plays and changes in amplitude.

The question then becomes - what drive level should T/S parameters be measured at ? And unfortunately there is no consensus on this that I can find.

Some sources suggest measuring drivers at 1v RMS to "standardise" the measurements, and generally suggest that you measure the T/S parameters with a "small" excursion. But I don't think this is very representative of a woofer where the Fs will go down quite a bit with increased excursion.

So what I do is take two sets of readings - one at a very low signal where the cone is not visibly moving, and a second set of measurements where the cone is reaching about 1/2 Xmax.

This will give two extremes. I then model the in-box response with both sets of measurements and the change in the response will show what the "sensitivity" to the change in Cms with drive level is - some bass alignments (particularly small closed boxes) will be very insensitive to a change in Cms and result in hardly any change, while some other alignments (particularly large box bass reflex) can be more sensitive to variations in Cms.

The above only really applies to woofers. Midrange drivers not producing bass won't have visible excursion so the "small signal" T/S parameters are the right ones to use if you're calculating a midrange enclosure. (Although if its a small closed box it will be fairly insensitive to Cms variations anyway)
 
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Long story for something I already said before: point is that you simply can't investigate these issues without the proper voltage coming out of an amp.

I deliberately didn't go into the details.
1 - they are not relevant for the discussion here.
In fact they show that only very small signal isn't enough.
2 - I have had countless discussions about this with manufacturers. They cherry pick specs.

So compared to their " claimed" specs they are of by 30-40%.
Yes they change, that is extremely obvious.
But it is sometimes important to know how much they change.

I was involved in a case were this was even very audible. Yet again, you always get into the same chicken egg story with the manufacturer.

Point is, this can only be seen and measured when higher output voltages (and power) is being used.
Fyi, in this case it was barely anything, around 2V.
Even the difference between pink noise vs stepped sine wave was very significant.

Btw, I understand the situation extremely well.
It has been my daily work now for over a decade.
Bumped into many different issues when it comes down to measuring loudspeakers.
From weird cherry picked specs, till datasheets that are full of typos, companies using twisted or debatable standards, till companies that are flat out lying.

But I deliberately didn't go into that direction because it's heavily offtopic.
People just need to know that a small pink noise signal isn't enough to investigate or troubleshoot problems.
 
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The question then becomes - what drive level should T/S parameters be measured at ? And unfortunately there is no consensus on this that I can find.

T/S parameters are actually "low level" by definition. They are meant to represent the characteristics of a driver "at rest". You can of course determine the parameters at higher drive level, but it may be a bit of a stretch to define those values as T/S parameters.

Using T/S for a cabinet model, if we understand that T/S is defined at low level only, it should be clear that the box model is only accurate at low level simulation as well. As you increase drive level, the box model becomes increasingly inaccurate due to missing data such as change in motor strength and suspension stiffness.

The problem as I see it is that T/S are a bit overly simplistic representation for modern Hi-Fi, so T/S should be included with semi-inductance model, BL(x) curve, Kms(x) curve, Le(x) curve, etc.

To throw another wrench into the mix, T/S parameters can change noticeably by simple change in ambient temperature by a couple degrees. How many of us, or manufacturers, specify or maintain strict environmental control when comparing T/S parameters?
 
Accurate? Depends on the application. Versatile, of course. The complete 2 channel jig allows for T/S at adjustable voltage level, as well as ability to analyze other circuits as well such as active filters if you are clever.

Thanks, DcibeL. Don't need to measure T/S at different voltage level (the whole definition of the parameters is "small" voltage below 1V). Analyzing the transfer function of circuits I can do anyway. So nothing in it there for me, really.

Semi-dual vs full dual channel is up to you. For loudspeaker measurement, full dual channel measurement would be my recommendation. For example if you want to measure some delicate ribbon tweeter with a transformer that requires a capacitor, you can locate the reference channel probe on the speaker side of the capacitor and compensate completely for it. Keep in mind as well that the difference between the dual channel and semi-dual channel is just a couple cheap resistors and some wire, so why take the lazy approach?

This is more interesting, especially the convenience of neutralising the response and phase impact of a protective cap. But then again, like you say, two resistors would achieve the same thing. If I ever want to take the "lazy approach", could instead do Linear Audio's Autoranger I suppose.

No bridged amplifier is correct, at least if you plan to use the jig as all schematics online show. This would mean to avoid most low cost class-D solutions, including some Icepower solutions. Hypex modules are just fine to use, as well the LM3886 chip is perfectly usable device for measurement setup, I use a DIY My_Ref amp myself a lot of the time. The description above sounds like some fancy description of "it's an amp". Specs for the Thomann is 50W/4ohm so I have no doubt that is is a single LM3886 in a box.

Thanks - any thoughts on operating an LM3886 module off a +/- 26VDC supply like the one linked? The Thomann doesn't lend itself to portability. Surprised NC400 are OK since differential topology but got them at home so so can use when mains AC is available.
 
My solution to the problem of measuring the output of an amplifier or passive crossover is to use a microphone transformer to provide galvanic isolation and voltage step down.

The one I use has several taps but I use the 1200 ohm impedance tap on the input side and 30 ohm output impedance.

The impedance ratio gives just the right amount of voltage step down from a ~100w amplifier to a line level input without needing to use a diode protection network, nor is a resistive divider needed.

Of course the transformer itself has its own frequency response limits - the one I use measures nearly ruler flat from 5Hz up to about 30Khz so is more than good enough for me.

I use it for the 2nd (reference) channel on dual channel microphone measurements, and also use it to measure the output of passive crossovers.

Due to the full isolation you can connect to any part of any crossover - it doesn't matter if its a series crossover, whether it has an all pass filter (that doesn't share a common input and output "earth") etc.

In my experience even if you use a simple crossover where the negative is passed through to the driver directly and the negative terminal of the amplifier is common to signal ground, directly connecting the line input of a sound card (via a resistive divider) to the amplifier / crossover causes earth loops that cause noise pickup and inaccuracies.

This is particularly apparent if you try to measure a passive high pass or low pass filter - instead of seeing the expected continuous rolloff the response will often rise again due to the ground loop!

So I find a step down transformer very useful for measuring low impedance high level signals with full isolation.

Thanks, DBMandrake. This is quite an interesting approach. Could you share the make and model of your transformer, please. And do you measure with a DMM everytime (trial-and-error) to determine which tap is giving you an acceptable voltage for the line-in on your soundcard?
 
To throw another wrench into the mix, T/S parameters can change noticeably by simple change in ambient temperature by a couple degrees. How many of us, or manufacturers, specify or maintain strict environmental control when comparing T/S parameters?

To throw the wrench back, that difference in temperature is only very small.
Just a couple of percent at the very most.
All things one can aspect.

Anyway, the topic was about what is needed for a decent ARTA measuring system.
So I think it's better to open a new topic for this discussion.
Point is, that having a decent amp will help troubleshooting such things to come.
 
Change in drive voltage can also be a couple percent, if you are running QC I'm sure you will maintain a constant drive voltage, so environmental control should be maintained as well. Something to be aware of especially when your new driver arrives in the mail and you bring it in from outside eager to test it out.

Back to the topic at hand, of course, full dual channel configuration provides more versatility to the measurement system use for very little extra complexity of the jig, just a couple more resistors. As well, the full channel jig can be configured as semi-dual if there appears to be some advantage in doing so.
 
Change in drive voltage can also be a couple percent,

Yes, that is what I just literally said straight from the beginning as well as that it some cases it is actually a lot more than a few percent. :rolleyes:

I am not talking about cheap stuff, but from SB Acoustics for example (and the like)

Anyway, I will start a new topic, so this discussion can be continued over there.
 
Help, i am refreshing my memory (last time used several years ago) about how to set up Arta on W10 laptop with Arta modules installed and with a focusrite 2i2 2gen soundcard & microphone pre-amp connected with usb cable, and arta measuring box and amplifier.

Studied the Arta manual (1.9.4) but cannot get my head around what to do.
I tried to do a loop back test between mic input and line output, but cannot get it to work properly, basically i cannot figure out how to correctly control the levels of output and input. Perhaps it is also due to the incorrect settings in Windows 10 audio panels for recoding and playback.

Tried to find some info in this forum and on the internet, but cannot find it.

The purpose is to measure drivers,
i have a arta license, downlaoaded 1.9.4 (latest)
i have a microphone that needs 48V (is being calibrated right now),
i have a arta measurement box,
and an amplifier of course.
In Windows 10 i use WDM as i read that the Asio driver gives issues.

Regards, JanRSmit
 
help in setting up Arta on W10 with Focusrite Scarlett 2i2 2gen

I am picking up the loudspeaker measurements again after several years of "silence" , using Arta on W10 and a focusrite scarlett 2i2 2gen (soundcard, microphone pre-amp & 48V )
Looked at the Arta manual, and searched this forum and the internet for the How-To's, but cannot get it correct, nor do i remember how i did it in the past.

With the 2i2 i do a loop test from output to input, but cannot get good control over level out and level in. It is either too little or too much according to the Windows sound set up.

On W10 i use the WDM driver as it is reported to be more stable than the Asio driver.
I have a Arta license, downloaded the latest (1.9.4) and latest driver of the 2i2.

Is there some step by step instruction or similar?
 
A very long time ago, I asked the developer.
He said that the stepped sine wave in STEPS should be more accurate.

From a practical point of view I don't know why that would be. I have used other programs in such "fast mode" that give excellent results.

I have to admit that I never looked further into it.
Not really any particular reason, just not interested since I haven't bumped into any issues lol.
 
At the very least, I ran a comparison between ARTA and REW, and they both provided nearly identical results for the sine sweep distortion plot. Differences between ARTA sine sweep and STEPS, at least on the surface, appear to be mostly related to the plot resolution, the short sine sweep contains higher resolution than 1/24 octave steps.

I can see a lower noise floor as a result of a longer stepped sine measurement, but I would think a similar reduction in noise floor should be achievable by running multiple sine sweeps with averaging, which unfortunately doesn't appear to be an option.