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apex b500 merging 4 channels into 2
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Old 4th May 2017, 07:41 PM   #11
Ian Finch is offline Ian Finch  Australia
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Don't assume that there will be equal current sharing between the output devices, as you would need equal die temperatures and close matched sets of transistors to stay within about an 8% current sharing error range and that's unlikely - you may need more safety margin unless you have a good matching jig and test set-up. Why not ask Apexaudio for advice on how he would go about driving 8 Prs of output transistors in the B500 design?
If only I had listened....
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Old 5th May 2017, 03:22 AM   #12
PRR is offline PRR  United States
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It is "simpler" to use an extra "output" device as a driver. Your workers don't put the wrong part in the wrong place. You need half as many inventory bins. You may get a better price on a larger number of outputs than a small number of small drivers.

Yes, an output device as the driver is "over-qualified". Like the small shack I am buying, which has a massive foundation. That's not necessarily bad.

85V rail each side. Simple resistor math says about 10 Amps peak. With four outputs, about 2.5A each; with eight, 1.25A each. (Though sharing is uncertain.)

> I think its the total current for all 8 devices

Yes. I think "1.5 ohms" is awful low. I figure 10A peak, min hFE 20, half Amp total base current. Assuming hFE does not vary much between 1.25A and 2.5A, eight or four devices take about the same base current. Perhaps the hFE is a hair higher with 1.25A, so the eight-device scheme will use a hair less base current.

Yes, scaling to 1.5 Ohms and hFE>50 does give around 1 Amp total. At this high current I would look closer at the hFE curve. Many devices will be falling hFE at 7 Amps, so the 8-part final may be noticeably less base current. Back when hFE was failling by 1 Amp, we sometimes paralleled for better current gain rather than dissipation or collector current limit.

However 7 Amps peak would worry me about SOA. Until recently few parts could stand much over 1A at 40V. On resistor you could swing 80V or 2A peak, but oval load-lines would cross the SOA line and melt your bits.

IMHO, commercial hard-duty (PA) amps got a lot more reliable when the price of transistors fell so low a designer could reasonably use a dozen devices instead of four. Kill an original Phase Linear 400 (not hard) then go pop the hood of a modern PA-market equivalent: long rows of final devices.

Last edited by PRR; 5th May 2017 at 03:25 AM.
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Old 6th May 2017, 09:44 PM   #13
crazymechanic is offline crazymechanic  Latvia
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well the problem is APEX never responds via PM's I've tried years ago and a few days back - the same, as for the amplifier , I did join the extra transistors and merged the channels and now I have two channels running 16 output devices each on 80-0-80v rails and as far as listening experience I would say nothing has changed, sadly I dont have large enough speakers to test this amp to its full ability in a reactive load with some scope probes attached but i'm sure I will do that or the same into a resistive dummy load some time in the future.

ill post some info or pics if anyones interested later as I have them on my phone but write from pc.

what do you think could this extra output transistor while using the same driver could influence the sonics and frequency response of the amp?

I noticed the bias current is now lower since it is shared among twice as much transistors as before but the goodside is that the heatsinks say colder as before they got i'd say too warm at idle with time.
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