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Old 29th July 2013, 09:52 PM   #11
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Originally Posted by bolserst View Post
snip

The only other suggested I would have is to try a larger accelerometer coupling capacitor(C2) to gain a bit more phase margin at your low frequency oscillation region.

Keep up the good work!
snip
Yup - we all love your good work! And you are lucky to have an informed person like Bolserst critique your posts.

Or you might consider a smaller capacitor (or other intervention) to cut down the loop gain at that point, depending on the Nyquist status of phase and gain. Better to end up a good stable system than to shoot for the moon and miss it. Quite a balancing act where the ups-and-downs are so dramatic.

After all these years of having speakers not in a feedback loop, any modest improvement is a big step forward. Unless we are talking about electrostatics, ahem, ahem, getting speaker distortion comparable to amp distortion is a hopeless effort. But then current amp distortion is way below perceptible levels while speaker distortion is not.

Ben
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Old 29th July 2013, 10:55 PM   #12
Armand is offline Armand  Norway
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Thank you all for your response. I am going to answer your questions and do your suggested measurements and post my findings. But it will have to wait a few days. Wife and two kids....

Armand
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Old 30th July 2013, 06:44 AM   #13
superR is offline superR  Netherlands
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Thanks,

Like Bolserst I am interested in the actual sub 20Hz behaviour. Physics don't agree with the phase and magnitude The overlay looks more like the dynamics I would expect.
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Old 31st July 2013, 09:19 AM   #14
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"Loop gain" should really be something that R8-R9 adjusts (replacing the two resistors with a pot). Thus the pot would control the amount of input from the accelerometer into the amp, instead of being fixed. The term refers to the amount of control exercised by the accelerometer feedback loop, not the gain of the whole amp.

In practice, you have the input level control pot (input_gain, 10k). But more logical to control the feedback loop rather than the input level (or both).

Do I have that right?

Ben
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Last edited by bentoronto; 31st July 2013 at 09:24 AM.
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Old 31st July 2013, 09:28 AM   #15
superR is offline superR  Netherlands
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Thanks,

Like Bolserst I am interested in the actual sub 20Hz behaviour. Physics don't agree with the phase and magnitude The overlay looks more like the dynamics I would expect.
Wait a minute! I thought you were using a current feedback amplifier instead of a voltage feedback amp. Let's think this over
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Old 31st July 2013, 02:09 PM   #16
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Quote:
Originally Posted by bentoronto View Post
"Loop gain" should really be something that R8-R9 adjusts (replacing the two resistors with a pot). Thus the pot would control the amount of input from the accelerometer into the amp, instead of being fixed. The term refers to the amount of control exercised by the accelerometer feedback loop, not the gain of the whole amp.

In practice, you have the input level control pot (input_gain, 10k). But more logical to control the feedback loop rather than the input level (or both).

Do I have that right?
Loop Gain is the gain around the feedback loop path. Generally to measure it you break the loop at the red X in the figure and inject signal at "Input" and measure resulting "Output". Loop Gain in dB would then be 20*log(Output/Input). Since it is the total gain around the loop that matters, you can adjust the gain of opamp U2 with R8-R9 as you suggest. But it is just as valid to set the loop gain by adjusting the gain of opamps U3A, U3B, or the poweramp LM3886 for the matter. All that matters is the total gain. In practice it is best to divide the gain between the stages in a manner that maximizes headroom and S/N ratio.

The only stage whose gain should not be considered is that of the input buffer U1, since it is not inside of the the feedback loop.

Quote:
Originally Posted by superR View Post
Wait a minute! I thought you were using a current feedback amplifier instead of a voltage feedback amp. Let's think this over
That measurement was "open loop", so no motional feedback was used in that test case.
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Last edited by bolserst; 31st July 2013 at 02:11 PM.
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Old 31st July 2013, 07:48 PM   #17
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snip Since it is the total gain around the loop that matters, you can adjust the gain of opamp U2 with R8-R9 as you suggest. But it is just as valid to set the loop gain by adjusting the gain of opamps U3A, U3B, or the poweramp LM3886 for the matter. All that matters is the total gain. In practice it is best to divide the gain between the stages in a manner that maximizes headroom and S/N ratio.

The only stage whose gain should not be considered is that of the input buffer U1, since it is not inside of the the feedback loop.


That measurement was "open loop", so no motional feedback was used in that test case.
We're getting lost in terminology. I think we are looking for something called "the feedback fraction" or something like that. Sometimes (but not here), the gain of the sub-assembly represents what we are interested in and we look at the difference between with-accelerometer-feedback and without-accelerometer-feedback.

You want to know, "how much negative feedback are you using from the accelerometer?" And the next steps are to adjust the amount of feedback and phase, where troublesome. So working on R8-9 makes sense - even though in a purely disputative sense you could argue about it.

Ben
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Old 31st July 2013, 08:43 PM   #18
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We're getting lost in terminology. I think we are looking for something called "the feedback fraction" or something like that
In feedback or control analysis Loop gain = β AOL (feedback fraction x open loop gain)

The feedback fraction(β) is a useful term to use in amplifier design as it is the fraction of the voltage output that is fed back to the input. But the term is troublesome to use in MF woofer design where the "output" is SPL and the feedback signal is "voltage". Talking in terms of loop gain tends to make more sense.

Quote:
You want to know, "how much negative feedback are you using from the accelerometer?" And the next steps are to adjust the amount of feedback and phase, where troublesome. So working on R8-9 makes sense - even though in a purely disputative sense you could argue about it.
I wasn't trying to be argumentative. Rather, I was attempting to point out that doubling(or halving) the gain of U2 with R8-R9 will change the loop gain, phase, SPL, distortion etc exactly the same as if U3A or U3B gain had been doubled(or halved) instead.
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Old 1st August 2013, 11:13 AM   #19
superR is offline superR  Netherlands
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Quote:
Originally Posted by bolserst View Post
That measurement was "open loop", so no motional feedback was used in that test case.
I was talking about the amplifier. I thought he used a current feedback amplifier, but instead he used a voltage feedback amp....
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Old 3rd August 2013, 08:49 PM   #20
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