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Old 14th January 2013, 02:12 AM   #1
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Default Number of output stages and dynamic output currents

why does pass amps has more output stages like having 20 pairs of Mosfets in the output stages still having less power like 250W? In general we would require 4 or 5 pairs of Mosfets at the output to achieve 250W in 8 ohms

I agree that the load on each output device will be reduced in terms of heat...

I believe the limitation on each output device collector Current is 1A when operated in SOA so lets consider if we have dynamic current demands from the speaker being 10Amps peak or 20Amps peak in that case the current is being limited by the number of output stages....

In case of Pass amps the output stages are being 20 so considering in class AB so only 10 will be on on the cycle so could max be 10Amps and hence the amp having more dynamic power? is it so? please tell me whats the use of the having more output devices?

adding 20 stages affects the amplifier stability?
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Old 14th January 2013, 02:32 AM   #2
mlise is offline mlise  United States
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Hi,
If you look down the data sheet for power transistors there's a graph of hfe vs collector current, or trans-conductance vs drain current. Nelson uses FET's, but I'm more familiar with BJT's, but the curves are similar. The curve peaks out at 2-ish amps for transistors that have a continuous current rating of like 10 amps. The flattest part of the curve is the top, so that's where you keep the amp. Variations in that curve cause unpleasant distortion. If the output needs 0 to 20 amps, to keep the transistors <2 amps you need 10 of them.
The other reason for gross parallelism is that the speaker at the ends of it's excursion looks like a dead short, not 8 ohms, so you need to kick it's but with way more current than you think. I designed bass guitar amplifiers that were less than 300 watts, but could drive 250 amps into a dead short. The high peak current capability really perks up the sound of an amplifier even though you're never near the power that should require that much current.

Cheers.
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Old 14th January 2013, 02:38 AM   #3
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Quote:
Originally Posted by mlise View Post
Hi,
I designed bass guitar amplifiers that were less than 300 watts, but could drive 250 amps into a dead short. The high peak current capability really perks up the sound of an amplifier even though you're never near the power that should require that much current.

Cheers.
how is 250amps into a dead short could be achieved? where is this peak current coming...

dead short? what is the duration in ms?
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Old 14th January 2013, 03:02 AM   #4
mlise is offline mlise  United States
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Hi,
The circuit had feedback, so if it had a load of, say 0.10 ohms instead of 8 ohms and the input suggested an output of 25 volts, the output transistors would drive 250 amps. The analog circuit would do it's best to be a voltage amplifier regardless of the load. I had a separate circuit looking at the SOA of the transistors and would very carefully let the amp drive for however many mS right up to the SOA curves on the data sheet before muting.
This was the Gallien-Krueger 700RB. Even though it wouldn't put out enough power to melt the voice coils on Electrovoice EVM15B's, it could brutally apply enough acceleration to fold the cones up like an accordion. The 1001RB did melt the voice coils in about 2 string pops. <shudder> The entire surface of my body felt like it was snapped by the corner of a towel.

Cheers.
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Old 27th January 2013, 04:46 PM   #5
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but if the output impedance stays pretty close to the nominal like may be 6 ohms or 7 then the current demands are also less. But the prob is all about the dynamic performance im just behind so even then if the current demands are high then whats the solution do we just need to increase the output devices?
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Old 27th January 2013, 05:18 PM   #6
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Quote:
Originally Posted by rhythmsandy View Post
but if the output impedance stays pretty close to the nominal like may be 6 ohms or 7 then the current demands are also less. But the prob is all about the dynamic performance im just behind so even then if the current demands are high then whats the solution do we just need to increase the output devices?
Dynamic impedances are all over the place. Speakers do not behave electrically like resistors. This was addressed above.

Reread what mlise said about the flat part of the curve. Transistor gain is not linear over its operating area. Understand it and you will understand the point.
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