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Driving 8 pairs of MJL21194/93
Driving 8 pairs of MJL21194/93
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Old 4th January 2019, 08:16 PM   #21
BrianL is offline BrianL  United States
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Driving 8 pairs of MJL21194/93
There are definitely a lot of sardines crammed into that can, so to speak. Unfortunately not enough details in the pictures to understand exactly what was going on either before or after.
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Old 4th January 2019, 11:58 PM   #22
Mark Johnson is offline Mark Johnson  United States
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Driving 8 pairs of MJL21194/93
We used to buy those insulated buss bars to distribute +5V across big BIG cards full of wire wrap DIPs. You can see them on the Jeff Rowland Model 10 amplifier photos, look for the orange sticks. They're actually a distributed transmission line (supply and ground) with ceramic capacitors built-in throughout. Notice that the orange sticks connect to two different PCB networks: supply and ground.

Jeff Rowland LM3886 Amplifiers
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Old 5th January 2019, 05:07 AM   #23
poldaaudio is offline poldaaudio
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Quote:
Originally Posted by Bob Cordell View Post
<snip> The great Bart Locanthi taught us the value of using triple output stages in the late '60s when I was still in High School, and a great many top designers since followed his advice, including Barney Oliver of HP. Yes, transistors have improved since those days of the 2N3055 and the MJ802, but Bart's advice still rings true.

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Who is arguing that a triple EF isn't technically better than a double EF? It's the evangelism and pretentiousness here that is absurd. So Self perhaps has not put a design into production with a triple EF OPS and you allegedly haven't built an amplifier with anything less since high school. Does that make you intellectually superior? Is Self now not "a serious designer" and are all of these designs put out by Self over what must be a few decades now, presumably enjoyed by tens of thousands, despite their wholly adequate objective performance, now revealed to be unfit for the originally intended purpose?

Even Self has a whole chapter in one of the later editions of his own book (which I think might even predate the publications of yours) on triple output stages (with actual amplifier measurements as opposed to SPICE simulations) in which he appears quite enthusiastic about the overall reduction in distortion.

And to put your original comments back into context you suggested a minimum output transistor beta of 20 for a proposed design delivering 16A peak into a 4R load with 6 to 8 parallel pairs.

Last edited by poldaaudio; 5th January 2019 at 05:22 AM.
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Old 5th January 2019, 06:20 AM   #24
rhythmsandy is offline rhythmsandy
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I think its always better to be very conservative on the beta of transistor especially in the OPS I see alot of transistors droop badly and one forgets the temperature rise of the die when such beta droop comes in play. No matter what I strongly feel even thermal compression in transistors play a significant role. Coming to the triple I have tested triple with large driver TO 264 to drive 5 pairs of output transistors and just with 0.5V of input and with gain of 50 to the amp i was blown away by the way it delivers the power it was hardly 25V peak on the CRO and the way it was able to hold the driver was amazing at that voltage output. I will never build an amp without a triple from now.
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Old 5th January 2019, 06:43 AM   #25
davada is offline davada  Canada
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I don't buy the dew point stuff. I've had these amplifiers on my bench. It is just too much to close. The moister from the dew mixes with sulfur dioxide in the air forming sulfuric acid which is conductive. What causes acid rain.
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Old 5th January 2019, 06:44 AM   #26
keantoken is offline keantoken  United States
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It could be that the condensation was collecting and then causing the arc when dripping off an electrode?
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Old 5th January 2019, 07:17 AM   #27
davada is offline davada  Canada
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Dry air has dielectric constant of 20KV per centimeter. 150V even with moist air.
No way. Unless the moister is carrying ions. And it's forced air. How is it going to condensate.
Especially in an amplifier running that hot. The heat raises the dew point. That air would dry out so quickly.

The story is totally illogical.
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Last edited by davada; 5th January 2019 at 07:23 AM.
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Old 5th January 2019, 09:51 AM   #28
rhythmsandy is offline rhythmsandy
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ok in general what is the clearance required for

1> upto 100V of operation without issues
A> Pad to pad distance , B> Route to Route distance
2> Upto 200V of operation same cases as above
A> Pad to pad distance , B> Route to Route distance

Bob I have a doubt regarding the heatsink / thermal dissipation from the OPS transistors.

Consider I have a sub amplifier at +/-70V DC with MJL21194/93 5 pairs and I have been driving it at 1/3rd of the maximum clipping voltage output. Approx 20V or so then if the heat dissipated through the output transistors is making the heatsink just warm then would that be dissipating lesser heat at 70% or 90% of the output? Its just to understand that my heatsink would be sufficient to deal with the heat at higher output power?

on the LTspice sim im able to see the dissipation curve is like half sinewave at max 90W at about 1/3rd the output and there is a notch on the curve at higher power like M shape but not crossing more than 100W.

Regarding the Mosfet driver if we use it in conventional EF I believe the loading of the VAS transistor will be substantial because of the mosfet ciss. Now considering triple for the same application where a pre driver will be eventually be able to drive the mosfet with ease even the IRFp240 by 2sc5171 should be able to drive it with full potential. Now what is the use of having Mosfet in the driver stage? As per the simulation the THD dropped even further but sonically does it offers any benefits especially in bass or mid or treble?
as Mosfets are not as linear as transistors but using a mosfet to drive the BJT OPS would that inject the mosfet signature in the amp?
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Old 5th January 2019, 05:45 PM   #29
davada is offline davada  Canada
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Can the mosfet ciss be tamed using cascode bootstrap?
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Old 6th January 2019, 03:23 AM   #30
rhythmsandy is offline rhythmsandy
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Quote:
Originally Posted by davada View Post
Can the mosfet ciss be tamed using cascode bootstrap?
I think its better to drive the mosfet with a better pre driver than directly driving from VAS.

Hi Bob I have a doubt regarding the SOA with restpect to Ic vs Vce where in the following example where you can see that green being the output voltage swing of the amplifier. Red curve is Voltage across Collector and Emitter of the output transistor and Blue being the current through the transistor.

Now I have a doubt that at full swing the voltage across Collector and Emitter is dropped as much as 20V and thats where the Ic is at its peak so what is the use of selecting High SOA transistor when the Vce is low enough and Ic being at its peak which will be about 7Amp DC at 20V for 2sc5200.
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