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Old 18th October 2013, 09:34 PM   #2141
jcx is offline jcx  United States
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Yes the "Bus pumping" is the resultant effect from recycling energy. Also if the output stage is of Full-Bridge topology[in my experience], you can eliminate the pumping completely.
sounds like breaking thermodynamic laws - with low loss, reverse conducting output stage, absorbing stored energy in the speaker the recoverd joules have to go somewhere
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Old 18th October 2013, 10:44 PM   #2142
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Originally Posted by Bob Cordell View Post
Hi Kanwar,

What I was referring to is the loudspeaker system as a whole, including the passive crossovers. And you are correct: the passive crossovers can also create quite high currents.

In Class-D amplifiers the recycling of reactive energy results in what is called "bus pumping". Indeed, bus pumping can occur even when there is no reactive load, due to the energy storage in the output inductor. Bus pumping is explained in my book in Section 29.3 and Figure 29.4. Bus pumping can increase the rail voltage above its nominal level. A large reservoir capacitor mitigates this effect.

Cheers,
Bob
I think I saw exactly that today when doing a 47 kHz test . Well above 34 Vrms when 30 Vrms at 1 kHz and clipping ( Hypex Ucd 180 ) . The idle PSU voltage +/- 47 V . That is not to say the PSU being switch mode wasn't doing similar stuff . The inductance of the resistor must be part of it . However it is the same set I use all the time and never saw this before . The amplifier is about 3 db down at this frequency so doubly odd .
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Old 19th October 2013, 05:50 AM   #2143
Defiant is offline Defiant  United States
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Originally Posted by jcx View Post
sounds like breaking thermodynamic laws - with low loss, reverse conducting output stage, absorbing stored energy in the speaker the recoverd joules have to go somewhere
The stored energy is in the output inductor, not the speaker. The side switching at a longer duty cycle puts energy into the inductor, which is then transferred into the opposite supply rail through the opposite side switches. Look at the diagram posted above. You're not "creating" energy, you are simply transferring it from one rail to another.
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Old 19th October 2013, 10:40 AM   #2144
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Measured the Hypex SMPS 400/180 . Not the quietest of devices . Made an RF probe and found the fuse end to be quietest . 3 cm is the optimum minimum spacing . Good news is the RF didn't carry up the mains cable .

As I understand it the Hypex PSU is not fixed voltage . It tracks the mains voltage . Rather a shame because I want to try an optimum class H design . I do see 48 V switch-modes now . That would be ideal . My brief look at this says to me that the Hypex 180's deserve a conventional PSU . Mine I have been trying is 500 VA only as it was what I had handy . I suspect 160 VA would power a pair of UCD 180's if the Hypex 1/8 crest factor and 92 % efficiency rating is factored in ( less if space requires it ) . From my measurements that would even allow continuous 70 Wrms if heat-sinked on a biscuit tin or better . I would suggest 0 -25 0 - 25 transformer as a excellent choice if so . I used 2 x 22 000 uF . I was testing on a piece of 2.6 mm aluminum about the size of A4 paper . Slightly more as it has 70 mm right-angle folds at each end with no side pieces . Even after 15 minutes at 70 W rms the heat build up was fine .

Anyone know of a simple cheap temperature switch ( over temperature ) . The ones Naim audio used need high current , I don't trust a toaster switch to work at uA . If the toaster switch ran a 5 V relay at 70 mA I guess that might be OK ? It could have a shunt resistor to help . I guess a thermistor and comparator might be OK ? It would switch the 12 V logic of the modules . 0V = ON .
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Old 19th October 2013, 03:09 PM   #2145
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Originally Posted by Defiant View Post
The stored energy is in the output inductor, not the speaker. The side switching at a longer duty cycle puts energy into the inductor, which is then transferred into the opposite supply rail through the opposite side switches. Look at the diagram posted above. You're not "creating" energy, you are simply transferring it from one rail to another.
Exactly right. And it is usually true that in a bridged configuration bus pumping is not a problem.

Cheers,
Bob
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Old 1st November 2013, 08:55 AM   #2146
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Default compensation of the feed-trough capacitance of CCS

Referring to APAD5, page 262, Fig 8.49 and APAD6, page 394, Fig. 15.7
The feed-trough effect can also be reduced by the use of the variable capacitance of a reverse biased diode or the BC-junction of a transistor of the same type than the one used in the CCS, connected in lieu of the speed-up capacitor Cs. By doing so, the compensation capacitance would vary approximately in the same way than Cbc of the CCS transistor (Fig. 8.48 and 15.6 respectively).
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Old 1st November 2013, 09:50 AM   #2147
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Nigel, I've used the Airpax thermal switches like this one: http://www.farnell.com/datasheets/1561970.pdf They are rated for switching logic signals.

I believe mechanical energy stored in a woofer would cause bus pumping just the same as energy stored in the output inductor. That is the great thing about Class-D amps. A linear amp has to dissipate all the power returned to it by a reactive load, but a Class-D amp can recycle it to the DC bus.
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Last edited by scopeboy; 1st November 2013 at 09:53 AM.
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Old 1st November 2013, 01:50 PM   #2148
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Originally Posted by scopeboy View Post
Nigel, I've used the Airpax thermal switches like this one: http://www.farnell.com/datasheets/1561970.pdf They are rated for switching logic signals.

I believe mechanical energy stored in a woofer would cause bus pumping just the same as energy stored in the output inductor. That is the great thing about Class-D amps. A linear amp has to dissipate all the power returned to it by a reactive load, but a Class-D amp can recycle it to the DC bus.
Hi scopeboy,

Yes, this is true.

Cheers,
Bob
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Old 1st November 2013, 03:44 PM   #2149
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Originally Posted by scopeboy View Post
Nigel, I've used the Airpax thermal switches like this one: http://www.farnell.com/datasheets/1561970.pdf They are rated for switching logic signals.

I believe mechanical energy stored in a woofer would cause bus pumping just the same as energy stored in the output inductor. That is the great thing about Class-D amps. A linear amp has to dissipate all the power returned to it by a reactive load, but a Class-D amp can recycle it to the DC bus.
Excellent . Thanks . That's the company that made motors for the LP12 I suspect ? Part of Philips if so .
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Old 1st November 2013, 07:22 PM   #2150
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Originally Posted by franzm View Post
Referring to APAD5, page 262, Fig 8.49 and APAD6, page 394, Fig. 15.7
The feed-trough effect can also be reduced by the use of the variable capacitance of a reverse biased diode or the BC-junction of a transistor of the same type than the one used in the CCS, connected in lieu of the speed-up capacitor Cs. By doing so, the compensation capacitance would vary approximately in the same way than Cbc of the CCS transistor (Fig. 8.48 and 15.6 respectively).
Could you post a pic for those of us beach bums who only have older versions and are too indigent to buy the latest?
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