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Old 30th March 2016, 11:35 AM   #1
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Default 5200/1943 SOA voltage limit

Hi, I was wondering would it be safe to run an amp that uses 2sa5200/2sc1943 outputs on 100-0-100 voltage levels?
I have 4 channels , two use original Toshibas with rated max CE voltage of 230v, the other two use Fairchild's analog's with CE rated at 250v.

what do you think?
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Old 30th March 2016, 12:56 PM   #2
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You can but would be limited to 32 ohm speakers unless you used maybe 4 pairs.
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Old 30th March 2016, 01:01 PM   #3
xrk971 is offline xrk971  United States
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Quote:
Originally Posted by crazymechanic View Post
Hi, I was wondering would it be safe to run an amp that uses 2sa5200/2sc1943 outputs on 100-0-100 voltage levels?
I have 4 channels , two use original Toshibas with rated max CE voltage of 230v, the other two use Fairchild's analog's with CE rated at 250v.

what do you think?
I tried running mine at 53v with 8ohm speakers and it worked for 3 minutes and then catastrophically failed in shorted out mode and blew fuses on the PSU. So no it won't work. They are cheap to replace though.
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Old 30th March 2016, 01:05 PM   #4
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I think that if you are considering component choices for a new design, you should know the answer already. The maximum Vceo rating is like a breakdown voltage rating that applies at very small currents. It has little to do with the much lower Vce limit that would apply at maximum dissipation or output power. Look at the SAFE OPERATING AREA graphs for for the max. voltage at say, 2A current using the 100ms line on the datasheets. e.g: 2SC5200 Datasheet pdf - NPN Epitaxial Silicon Transistor - Fairchild Semiconductor
For a single pair of power transistors, maximum power (4 ohm load) is achieved with just 50V rails max. That equates to a Vce of about 90V. Any more voltage and you risk failure. Increasing the number of pairs or the load impedance does allow them to operate at higher voltage/lower current but the law of diminishing returns applies, in my estimation, as you approach 70V supply rails.

The best reason to use these lightweight semis is they are cheap but they are nowhere as powerful as you might think. In $/W terms, MJL4281/4302 would be better and have a much higher SOA limit that will tolerate higher rail voltages. Yes, you do see numbers of naive designs with 2SC5200/A1943 output transistors posted here but we can only assume they are simulated designs that have never been tested at rated maximum power.
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Old 30th March 2016, 05:22 PM   #5
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well I have 4 similar channels , each channel has 4 pairs of these transistors , previously I ran them on 90-0-90 and they were fine,
I'm just thinking because I will have to check how much voltage I'm getting from my self made smps transformer , I was just a bit lazy and thought that maybe if it's not over +-200 it should be ok.

also what do you think with 4 pairs whats the lowest impedance I could run , could it go for 2 ohms? probably not 4 would be safe i assume.
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Old 30th March 2016, 10:51 PM   #6
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Not a chance with 2 ohms, even 4 ohms would not be reliable.
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Old 30th March 2016, 11:58 PM   #7
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Did you test the amplifiers you built at full rated power with 90V rails or just use them at low power and assume they would survive at max. power? Maximum power means full output voltage at the onset of clipping with a standard signal like 1kHz sine wave. The load needs to specified but should be resistive only at 4, 8 ohms etc.

Few constructors want to know the weakness in their designs and find load testing too difficult or too cruel on their project. If you don't test though, you can't claim an amplifier "works" at any particular supply voltage and power level.
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Old 31st March 2016, 03:20 AM   #8
wg_ski is offline wg_ski  United States
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Quote:
Originally Posted by crazymechanic View Post
well I have 4 similar channels , each channel has 4 pairs of these transistors , previously I ran them on 90-0-90 and they were fine,
I'm just thinking because I will have to check how much voltage I'm getting from my self made smps transformer , I was just a bit lazy and thought that maybe if it's not over +-200 it should be ok.

also what do you think with 4 pairs whats the lowest impedance I could run , could it go for 2 ohms? probably not 4 would be safe i assume.
It would take 6 pairs to do 8 ohms, 12 to do 4. If you are making your own SMPS transformer, consider running H-class with 100 and 50V taps. The taps dont cost anything - just an extra set of rectifiers and caps, and you have to wind it anyway.Then you can use 4 pairs and its perfectly reliable at 4 ohms.
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Old 31st March 2016, 04:40 AM   #9
MOER is offline MOER  United States
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Default 2SC5200/2SA1943

Secondary breakdown is a serious problem at these higher rails.

If you want to drive a 4 ohm speaker with +/-100v rails you will require 7+7 devices at 25 deg C

At 70 deg C (A better design with SOA temperature derating) you require 10+10 devices.

Now if you run your devices in series, secondary breakdown is pretty much eliminated as you are effectively running each series stack at +/- 50v, you now need 2+2 devices per stack (A total of 8 devices as compared to 20 and little chance of secondary breakdown.

Yes you need extra drivers and predrivers for the series stack but in total you have a better design.

We have been using these devices for over 30 years (We of course used the older 2SC3281/2SA1302 in those years) and never ran them over a +/-55v rail per stack. In lower powered amplifiers we did not stack them of course.

ALL bipolars used for audio benefit from lower supplies and higher current

Steve Mantz

Zed Audio Corp.
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Old 31st March 2016, 08:09 AM   #10
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Well I'm not a fan of class H , these were actually class H first but they also had the lower power version if you take the mosfet drivers out it turns into a class AB design.It's the apex h900/500 version I'm talking about right here from the forum , I just made it some years back and now finally decided to put it in a box.
haven't load tested yet.
So if I would instead used the more rugged MJL4281/4302 pairs I could then probably do 100v rails with no problem.
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