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Class D Switching Power Amplifiers and Power D/A conversion

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Old 2nd June 2004, 12:13 PM   #181
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Quote:
linear actuators






tweeters !

Regards

Charles
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Old 2nd June 2004, 12:54 PM   #182
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Hi,

I spent some hours simulating similar idea. I used fixed frequency average current mode control loop on load current. Actually it was not load current but filter capacitor current substracted from switching stage current. Results were not so spectactular, maybe it would be a good idea to sum with feedforward of capacitor current like in Mueta. Phase shift oscillator would maybe have better bandwidth, but would not work into noload condition. Also highly inductive load may be problematic.
But I think it is idea worth of consideration.

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Jaka Racman
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Old 2nd June 2004, 01:09 PM   #183
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If your aim was effectively to construct a current source it's quite normal for it to become unstable when the load is taken off. Any good current source would. The i/v converse is a ucd type amp with a screwdriver short on the output. Output shorted -> no feedback -> amp goes crazy.
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Old 2nd June 2004, 01:28 PM   #184
Pabo is offline Pabo  Sweden
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The amplifier that I have designed is a voltage controlled current source. It is based on a self oscillating hysteretic current loop which uses the return current from the load and capacitor in order to turn on the hi and lo switches. The performance is pretty good but I have had to work on it for quite some time in order to optimize the regulation loop. The THD at 1kHz/1W is about -90dB and about -83dB at full power 1kHz. At 7kHz/1W the THD is at about -80dB. The bandwidth is about 70kHz in 8ohms and it is completely flat up til about 30kHz.

I have received very good listening comments on it so it is definitely possible to design with this technique allthough the measurement results arenīt as good as with ucd.

I have also designed a fully working transient model of a similar circuit but with the current sense point moved to the actual load current. This current is then fed back to a zero hysteresis comparator in order to create phase modulation. The advantage is that the switching frequency varies less than with hysteretic loops and thereby I can apply more feedback at higher frequencies. It should ideally have about the same performance as ucd or what do you say Bruno?
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Old 2nd June 2004, 01:42 PM   #185
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Quote:
Originally posted by Pabo
It should ideally have about the same performance as ucd
If your dead time and loop gain are the same I should hope the THD results are similar too.
I can't quite estimate these factors from your description. UcD's loop gain is around 34dB (f<=20kHz) and dead time is on the order of 20..50ns.

What interests me most is: if your output is a current source, have you modified the speakers in order to have a flat frequency response under this condition?
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Old 2nd June 2004, 02:06 PM   #186
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An interesting thing would be something that behaves like a current source at high frequencies and like a voltage source around resonance(edit: = driver's fs).

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Charles
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Old 2nd June 2004, 02:08 PM   #187
Pabo is offline Pabo  Sweden
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Sorry, forgot to explain that.

I have a voltage loop around the output filter correcting the output amplitude and linearising it further. This voltage loop is of integrator type and it applies about 15dB of feedback at 20kHz giving low enough output impedance for most applications. It is also combined with a small zobel network in order to reduce filter peaking at high inductive loading. At 1kHz and down the feedback is about 37db giving about 10mohms of output impedance down to 20Hz.
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Old 2nd June 2004, 04:30 PM   #188
Pabo is offline Pabo  Sweden
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By the way, does anyone know a lot about the distorsion factors in self oscillating classd amp? I can imagine why there is symmetric distorsion like third order but I donīt understand where the second harmonc comes from. How does the propagation delay affect the performance? Does it only add phase lag or does it cause some kind of nonlinearity as it affects higher amplitudes less where the switching frequency is lower? I know that deadtime affects the behaviour. In my amp I have about 20ns of deadtime but I also use gateresistor/diode in order to control the flank rise and fall times. I have seen that decreasing the shoot through to a certain level decreases the distorsion and the other way around. There seem to be a minimum.

I have tried both koolu and ferrite cores without any differences in performance. I have also tried to change the output capacitor without any improvements. What can be done about distorsion?

Would you care to share your experiences.
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Old 4th June 2004, 07:54 AM   #189
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Hi All,
right now i checked the circuit which was posted several days ago by me here. Well.. at 10khz THD is 0.002%, ie my sound card have exactly same, when "in" to "out" itself and only 48khz sample rate is possible (no AP -no happy:-), so i can reduce 2nd harmonic to zero almost. However optimal adjusting for 7khz slightly other, and for 5khz vs 10khz just a very other! At finally..for different output power optimal adjusting is different too. So this circuit haven't any THD improvement yet, but, by the way, it isn't spoiling the sound (and no improving).
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Old 7th June 2004, 07:21 PM   #190
Sheriff is offline Sheriff  United Kingdom
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Default Got the modules...

Hi All,

received the modules UcD from Jan-Peter late last week.

I was hoping to get them up and running over the weekend, but i have got some other stuff that needed my attention first.

Off to the Far East for the rest of this week so 'feedback' will have to be applied to this thread when i have closed the loop and returned home.

I also need to build an AES17 filter so that the AP sys1 will compunicate with me in english.......

Cheers

Sheriff
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