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Old 4th October 2013, 08:03 AM   #121
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Are you sure that isn't a bridge?

B.
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Old 4th October 2013, 12:57 PM   #122
troystg is offline troystg  United States
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Thanks for the illustration and explanation. For some reason since it was a direct response to Mr. Eraths feedback unit(when they started voiding warranties on their amps if Mr. Earths was added) I figured it would be more sophisticated than that.

This is a very interesting project and I will be following silently..
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Old 28th October 2013, 09:18 AM   #123
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Armand's work is too good to drop from view.

I notice in post #1, you have a miniDSP 2x8 in your toolbox. Seems to me (speaking only speculatively, not from experience), the place for DSP is inside the feedback loop. It can be used to band-limit (10 to 300 Hz), tweak naughty phases, and hand-tune any troublesome anomalies. Indeed, motional feedback in the past has been hobbled by having these tricks done feasibly (sharply, at several frequencies, within the loop, and at low frequencies) prior to DSP gizmos becoming available.

For using bridge-derived feedback, DSP also can convert velocity control into acceleration control, or do I have that backwards?

What would be a grand next step to this thread would be seeing a final sub using a quality sealed box (except also for IB, no other type of enclosure is possible), high quality driver, and quality amp....and mic traces with and without feedback.

Ben
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Old 4th November 2013, 05:30 AM   #124
superR is offline superR  Netherlands
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A miniDSP has a latency of over 1 ms. So that's a no go.
If you want to use the DSP route, I suggest to look for something like this; ADAU1772
Quote:
38 μs analog-to-analog latency
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Old 4th November 2013, 09:17 AM   #125
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Quote:
Originally Posted by superR View Post
A miniDSP has a latency of over 1 ms. So that's a no go.
If you want to use the DSP route, I suggest to look for something like this; ADAU1772
Bad me. You're right. But, I speculate, a DSP in the loop, capable of sharp non-ringing cut-offs, can correct for some of the Nyquist criteria (gain and phase) as expeditiously, I imagine, as the typical analog op-amp design with a very large bandwidth.

We are talking only of quite low frequencies, say below 100 Hz. But it is beyond my math ability to judge what speed is too slow. For sure, 1 m seems too slow. But what's your ballpark estimate?

Thanks for your correction.

Ben
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Old 11th November 2013, 05:45 AM   #126
superR is offline superR  Netherlands
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At 1 kHz, that is an open loop delay/phase of 360 degrees. 500 Hz, 180 degrees. 250 Hz, 90 degrees.

If I want to design a controller, in the continous time domain, my unity gain crossover should have (way) more phase margin than the delay added by latency.

Try and design such controller and see how large your THD suppression becomes.
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Old 11th November 2013, 08:53 AM   #127
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Originally Posted by superR View Post
If I want to design a controller, in the continous time domain, my unity gain crossover should have (way) more phase margin than the delay added by latency.

Try and design such controller and see how large your THD suppression becomes.
I am baffled in trying to parse those sentences. Can you please say again in other words.

So, if we are talking about subs playing south of 125 Hz, our 1 ms latency inside the motional feedback loop due to the miniDSP becomes just 45-degrees, at the worst freq, Moreover, the heart of the motional feedback action (and the region where the speaker behaviour starts to go wild) takes place two octaves below that with a phase angle delay of just 11-degrees.

My guess is that we wouldn't sweat that delay.

Another question: if there is no off-the-shelf program for a miniDSP suitable for providing the tricks needed within the motional feedback loop, is it feasible to write one (or is all the software written inside the miniDSP factory)?

Ben
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Old 11th November 2013, 12:49 PM   #128
troystg is offline troystg  United States
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I believe that mini-dsp writes all the software for their products however if a new plug-in could financially benefit them I'm sure they would write the new code.
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Old 11th November 2013, 06:25 PM   #129
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I believe that mini-dsp writes all the software for their products however if a new plug-in could financially benefit them I'm sure they would write the new code.
OK, down to the short strokes. Here are my vague impressions but I hope more knowledgeable others will speak up.

If you wanted to put a DSP inside the feedback loop to replace other component bits and pieces, you'd have it do the following tricks.

1. using very sharp slopes, ensure that nothing materially outside the speaker passband appears in the feedback loop

2. depending on various factors of driver size and whether you are using acceleration or velocity feedback, equalize and/or slope the signal to the driver

Ben
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Old 11th November 2013, 07:10 PM   #130
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negative feedback has problems with "sharp slopes" - they are always accompanied by large phase shifts

Quote:
...one thing Lurie does really well is show that the “conservation” relation for the total amount of feedback - the “Bode Integral” is exactly such a practical "good theory" - and has been the underpinning fundamental argument behind my posts in this thread

http://trs-new.jpl.nasa.gov/dspace/b.../1/98-0905.pdf
...
figure 11 specifically shows loop gain shaping to deal with a flexible structure resonant mode just beyond loop gain intercept


in well behaved systems it is possible to close digital loops with latency ~ 1/5 of the loop gain intercept frequency

but there's no excuse for only 1 kHz digital loops today - even running a "real operating system"; linux on GHz processors ($45 BeagleBone Black) can manage >10 kHz sampling/feedback loops

the problem may be I/O - USB uses 1 ms frame rate
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