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Driving 8 pairs of MJL21194/93
Driving 8 pairs of MJL21194/93
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Old 15th January 2019, 01:00 PM   #131
Bob Cordell is offline Bob Cordell  United States
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Quote:
Originally Posted by rhythmsandy View Post
It was all about the current delivery capability into a tough reactance load. <snip>
It is true that not all class D amplifiers are created equal, especially those that use switching power supplies (most). To first order, SMPS and class D amplifiers are a match made in heaven. However, both SMPS and class D amplifiers have their own quirks and characteristics, and some of these can reveal themselves at very low frequencies and very high power levels.

Bus pumping is one phenomena of class D amplifiers worth keeping in mind. While it can be a bit difficult to describe in short, simple terms, it refers to the fact that a class D amplifier when driving a load can deliver energy from the positive supply to the negative supply, and vice versa. This is partly because the output stage of a class D amplifier is not unlike a switching power supply. This can cause strange things to happen if the rail receiving the extra energy cannot absorb that energy in some way. Fortunately, most high-power class D amplifiers are so-called Bridge-Tied-Load in nature, a class-D way of describing what is essentially a bridged amplifier configuration. When properly applied, bus pumping effects are significantly mitigated because the energy pumped into a given rail by one side of the output is used (sunk) by the other side of the BTL output.

Another phenomena is that some class D amplifiers have rather poor power supply rejection, depending on details of their design. If the rails are moving around a lot due to delivery of very high current into a load, some degradation can occur.

Finally, energy storage in an SMPS used to power a class D amplifier can be an important issue. Because the ripple frequency in an SMPS is at a high frequency, large reservoir capacitors are not required to get ripple small. However, this can bring on complacency in reservoir capacitor sizing in regard to energy storage needed to sustain very high power bursts over a long cycle. Those 100,000 uF capacitors charged to 100V or more in our good old high-power class AB/G amplifiers store a lot of energy! Where is that amount of energy storage in an SMPS used for that same amplifier? This is the question. It may be there, or it may be made up for in some SMPS designs, but it may not be there in all SMPS designs for class D power amplifiers.

In many SMPS, really large reservoir capacitors are not used on the secondary side where the rails are. However, the SMPS first rectifies the incoming AC to a high-voltage intermediate bus prior to the isolating SMPS-proper. Often it is there, on the primary side, where there may be a lot of energy storage, where the DC voltage may be on the order of 350V. That supply side may even be made stronger by the proper use of active power factor correction prior to the intermediate bus. PFC does a better job of extracting the most power out of the line, since it does not just take power in the form of very large rectifier transients, but rather extracts power over most of the cycle.

However, even if there is really good power and energy storage on the primary side, large bursts of current drain must be supplied in real time by the main SMPS itself if there is not a lot of energy storage on the secondary side. This means that the SMPS may need to be very over-designed to sustain those very high current bursts. SMPS also sometimes do funny things when asked to deliver more current than their design maximum. They do not often sag or "fail to deliver" gracefully (like a good old linear supply with a big transformer and big reservoir capacitors). Most SMPS want to create a stiff, regulated output until they reach their maximum current output. Beyond that, all bets are off as to what they will do.

Cheers,
Bob
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Old 15th January 2019, 01:11 PM   #132
indianajo is offline indianajo  United States
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Thanks for the experience written down in post 126 rhythmsandy. I've recently been offered an 18" Allen organ speaker, about the same time I noticed that piano & organ notes go to 27 hz, not the 54 my speakers roll off at. The huge organ boxes are port tuned down to 20 hz so I'll be diving off into the subwoofer world soon. No wonder my wood piano sounds better even with unprofessional me pushing the keys. A use for the PV-1.3k I picked up for $55, which has 5 pairs MJ21193/94 now it is repaired.
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Old 16th January 2019, 06:49 AM   #133
rhythmsandy is offline rhythmsandy
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Bob Im thinking of a situation in using Balanced amplifier configuration. There are two cases I thought of using 5 pairs of MJL21194/93 in the +/-70V amplifier for a 500W RMS into 4ohm load with current in load being at roughly 20Amps. Now calculating the current demand from each MJL transistor is about 4Amps for which it can deliver with ease ( Now look at the beta droop at 4Amp which is about 60 and for 1Amp delivery the beta is about 75. Now consider when using the amplifier in fully balanced amplifier at +/-40V where I thought of using 3 pairs of MJLs on each half. Now if you look at the current delivery from each MJL transistor is about 6.7Amps consider about 7 amps with little headroom. The amount of beta drop at 7Amp is about 40. Now considering the fact of driving such low beta requires more demand on the drivers hence consider we need more current from drivers and little bit more current even from pre drivers. Now my question is if we do use MJLtransistors in the above case where beta might drop to 40 or 30 in worst case and if the driver is able to deliver enough current to the OPS then the beta sag wont effect the performance? Im just thinking that wouldnt that be like transistor struggling to deliver the current? Like its capability of delivering the current is degrading and hence might not deliver the current as effectively as when 4amp is demanded at higher 70V?

Looking towards Mosfet here as I found a IRFP240 Fairchild Datasheet and its states the DC current delivery of the IRFP240 is half that of MJL21194. So there is no beta droop in mosfets so is it ideal to choose 5 pairs of mosfets in each half in the case? Or using an 3 pair of MJL21194/93 per each half of the balanced amp?
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Old 16th January 2019, 12:11 PM   #134
amplidude is offline amplidude  Denmark
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Join Date: Feb 2016
Quote:
Originally Posted by Bob Cordell View Post
It is true that not all class D amplifiers are created equal, especially those that use switching power supplies (most). To first order, SMPS and class D amplifiers are a match made in heaven. However, both SMPS and class D amplifiers have their own quirks and characteristics, and some of these can reveal themselves at very low frequencies and very high power levels.

Bus pumping is one phenomena of class D amplifiers worth keeping in mind. While it can be a bit difficult to describe in short, simple terms, it refers to the fact that a class D amplifier when driving a load can deliver energy from the positive supply to the negative supply, and vice versa. This is partly because the output stage of a class D amplifier is not unlike a switching power supply. This can cause strange things to happen if the rail receiving the extra energy cannot absorb that energy in some way. Fortunately, most high-power class D amplifiers are so-called Bridge-Tied-Load in nature, a class-D way of describing what is essentially a bridged amplifier configuration. When properly applied, bus pumping effects are significantly mitigated because the energy pumped into a given rail by one side of the output is used (sunk) by the other side of the BTL output.

Another phenomena is that some class D amplifiers have rather poor power supply rejection, depending on details of their design. If the rails are moving around a lot due to delivery of very high current into a load, some degradation can occur.

Finally, energy storage in an SMPS used to power a class D amplifier can be an important issue. Because the ripple frequency in an SMPS is at a high frequency, large reservoir capacitors are not required to get ripple small. However, this can bring on complacency in reservoir capacitor sizing in regard to energy storage needed to sustain very high power bursts over a long cycle. Those 100,000 uF capacitors charged to 100V or more in our good old high-power class AB/G amplifiers store a lot of energy! Where is that amount of energy storage in an SMPS used for that same amplifier? This is the question. It may be there, or it may be made up for in some SMPS designs, but it may not be there in all SMPS designs for class D power amplifiers.

In many SMPS, really large reservoir capacitors are not used on the secondary side where the rails are. However, the SMPS first rectifies the incoming AC to a high-voltage intermediate bus prior to the isolating SMPS-proper. Often it is there, on the primary side, where there may be a lot of energy storage, where the DC voltage may be on the order of 350V. That supply side may even be made stronger by the proper use of active power factor correction prior to the intermediate bus. PFC does a better job of extracting the most power out of the line, since it does not just take power in the form of very large rectifier transients, but rather extracts power over most of the cycle.

However, even if there is really good power and energy storage on the primary side, large bursts of current drain must be supplied in real time by the main SMPS itself if there is not a lot of energy storage on the secondary side. This means that the SMPS may need to be very over-designed to sustain those very high current bursts. SMPS also sometimes do funny things when asked to deliver more current than their design maximum. They do not often sag or "fail to deliver" gracefully (like a good old linear supply with a big transformer and big reservoir capacitors). Most SMPS want to create a stiff, regulated output until they reach their maximum current output. Beyond that, all bets are off as to what they will do.

Cheers,
Bob
Class d in btl and bus pumping issue is past, run it from boat anchor toroid and huge cap bank.. did that with 4 irfp4227 rated at 65 amps each.. current ad libitum
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Old 16th January 2019, 12:47 PM   #135
amplidude is offline amplidude  Denmark
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Originally Posted by amplidude View Post
Class d in btl and bus pumping issue is past, run it from boat anchor toroid and huge cap bank.. did that with 4 irfp4227 rated at 65 amps each.. current ad libitum
Something like this..
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Old 16th January 2019, 02:13 PM   #136
rhythmsandy is offline rhythmsandy
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I initially thought of Class D in the same way but was not confident of IRS2092 for its stability and bass slam. Im going through the linear route right now till I get a decent performing Class D. How did sub bass sound with the above circuit? What was the voltage with which it was running. The DF is a bit lower i think in 150 or so but I think that is still ok.
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Old 16th January 2019, 02:49 PM   #137
amplidude is offline amplidude  Denmark
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Quote:
Originally Posted by rhythmsandy View Post
I initially thought of Class D in the same way but was not confident of IRS2092 for its stability and bass slam. Im going through the linear route right now till I get a decent performing Class D. How did sub bass sound with the above circuit? What was the voltage with which it was running. The DF is a bit lower i think in 150 or so but I think that is still ok.
The shown schematic was just an example, but mine is similar, it's running on +/- 75 volt, for bass duty only working frequency could maybe be lowered also output inductor could be modified as it is designed for full audio spectrum, but that would be way above my paygrade calculating those
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Old 17th January 2019, 06:08 PM   #138
rhythmsandy is offline rhythmsandy
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of what if to drive 10 pairs of IRFP240s to be driven? which driver configuration to be used? Any existing ideas to drive a large bank of mosfets? How stable will they be?
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Old 18th January 2019, 01:34 PM   #139
amplidude is offline amplidude  Denmark
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Originally Posted by rhythmsandy View Post
of what if to drive 10 pairs of IRFP240s to be driven? which driver configuration to be used? Any existing ideas to drive a large bank of mosfets? How stable will they be?
Regarding the mjl21193/mjl21194, i have 8 of each from an scrapped d.a.s audio amp, a guy in swapmeet sold lme49830 driver chips so I bought 4, so that's my approach, I'm not much for theorizing I like building stuff, the succes rate is so so, but I'm having fun.
I don't own a big high end stereo setup, I'm not building to reach some glorified goal, it's the road that drives me.
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