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Driving 8 pairs of MJL21194/93
Driving 8 pairs of MJL21194/93
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Old 11th January 2019, 03:12 PM   #81
Bob Cordell is offline Bob Cordell  United States
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Quote:
Originally Posted by Kay Pirinha View Post
Bob,
thanks. I'm scratching my head, though. How often do we want to listen to clipping amplifiers and notice that this clipping is modulated due to a dramatically decreased PSRR in this state?
Best regards!
The simple answer is that we NEVER want to listen to amplifiers that are clipping. I wish every amplifier had an accurate clip indicator LED. It would be an eye-opener for some. Amplifiers clip more often than we think, especially on well-recorded music with little compression and high crest factor, and especially when driving speakers of lower efficiency, sometimes on the order of 83 dB SPL one watt/one meter. I would especially like it if a clip indicator was available to reviewers when they review amplifiers.

Cheers,
Bob
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Old 11th January 2019, 04:12 PM   #82
rhythmsandy is offline rhythmsandy
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Very true amps clips much earlier in many cases.
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Old 11th January 2019, 04:30 PM   #83
rhythmsandy is offline rhythmsandy
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Originally Posted by Mark Johnson View Post
The final output of the Crown amp, swings 2*VBE less than the 2nd stage ("VAS" stage) swings.

That's not much of an improvement over a "Locanthi T" / "3EF" amplifier, whose final output swings 3*VBE less than the 2nd stage swings.

(VAS - 2*VBE) versus (VAS - 3*VBE) is insignificant when the supply rails are 105V DC, to deliver 700 watts RMS per channel into an 8 ohm load, as the Crown does.

Oh and Crown seems not to have heeded Bob Cordell's advice to use one pair of output transistors per 75W RMS of max RMS output power. (Crown uses 4 pairs for 700W --> 175W RMS per pair.) I imagine Crown installs quite large fans with much MUCH greater airflow than Bob contemplated.

_
The other day when I saw the inside of Crown amplifier I was shocked to see punny heatsinks I didnt understand the math involved in driving the transistor at edge. Why do they have to do it? Is it precise engineering or cost cutting factor or both? Even in QSC RMX i see 4 pairs of 2sc5200/1943 transistors driven at 78v rails. I dont know how they are calculating. Consider its super force air cooling but how does SOA is justified? I agree the OPS current will be limited but upto what extent?
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Old 11th January 2019, 05:24 PM   #84
jwilhelm is offline jwilhelm  Canada
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The MJL21193/4 devices were tried a few times in Ostripper's Slewmaster EF3 design and they were catastrophically failing from cross conduction. We had to go to faster output devices to run reliably. If I recall correctly he recommended minimum 10MHz fT devices be used.
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Old 11th January 2019, 05:38 PM   #85
Bob Cordell is offline Bob Cordell  United States
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The drivers need to be run quite hot to reduce cross-conduction. In a 3EF Locanthi triple, it is not unreasonable to run the drivers at 60 mA, so that 60 mA is available to pull minority carriers out of the output transistor bases. In fact, in a big amplifier with numerous output pairs, a TO-247 or TO-264 device can be used for the driver and even more bias can be run through the driver. These measures help with cross-conduction with lower or higher ft output devices.

By the way, by the time you get to serious cross-conduction, you have already suffered fairly high amounts of dynamic crossover distortion in the output stage.

In a simulation, one way to look for cross-conduction in a Locanthi Triple is to look at the waveform of the driver collector current. If and when the driver collector current goes to zero, the driver has lost control of the output transistor and you are pretty much in the cross-conduction region.

Cheers,
Bob
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Old 11th January 2019, 06:17 PM   #86
rhythmsandy is offline rhythmsandy
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When It comes to driver conduction current I believe its more applicable in mid and high frequencies or will the distortion be much higher in lower frequencies also? for sub bass do we need to worry about the cross conduction?

Please find the following waveform from the collector current where its not hitting zero. Is that correct way?
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File Type: png cross cond.PNG (14.0 KB, 125 views)
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Old 11th January 2019, 06:27 PM   #87
rhythmsandy is offline rhythmsandy
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Here is another pic with load being 1ohm both the above waveform and the one attached in the post is with class A driver stage. Its not touching zero. Both the above cases the driver is biased at 10ma. Hence I believe that will reduce the dissipation in the driver transistors yet doesnt turn off even at 1ohm load. Please correct me If Im wrong here.
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File Type: png cross cond_driver current.PNG (26.6 KB, 124 views)
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Old 12th January 2019, 02:43 AM   #88
rhythmsandy is offline rhythmsandy
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Quote:
Originally Posted by jwilhelm View Post
The MJL21193/4 devices were tried a few times in Ostripper's Slewmaster EF3 design and they were catastrophically failing from cross conduction. We had to go to faster output devices to run reliably. If I recall correctly he recommended minimum 10MHz fT devices be used.
Were they getting burnt?
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Old 12th January 2019, 02:53 AM   #89
jwilhelm is offline jwilhelm  Canada
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They were disintegrating from what I understood.
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Old 12th January 2019, 09:52 AM   #90
rhythmsandy is offline rhythmsandy
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When we use for sub application the input signal itself is limited at max it might reach 300Hz to 500Hz so what is the problem? Even then the cross conduction occurs? If so then there has to be a super fast charge discharging method for removing the minority carriers.
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