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PR-800 ClassA/AB Amplifier Kit
PR-800 ClassA/AB Amplifier Kit
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Old 29th January 2016, 06:35 PM   #1
roclite is offline roclite  United Kingdom
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PR-800 ClassA/AB Amplifier Kit
Default PR-800 ClassA/AB Amplifier Kit

Hi,

Ready to embark on my next amplifier build and chose this one from eBay:

PR-800 Class A /AB Professional power amplifier board ?no heatatsink) CL135 | eBay

Would be good to hear from anyone who has built/bought previously and has any words of wisdom before I throw the power switch. Or will I be out there my own again, path-finding another classic good amp design (Locanthi-T topology?) that has numerous opportunities for component/circuit tweaks that might turn it into a half-decent amplifier. I know one thing for sure, I wont be running it at 2.5A quiescent current per channel as I already have a perfectly good toaster in the kitchen
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Old 30th January 2016, 04:45 AM   #2
djk is offline djk
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It might do 300W/4Ω from 70V in class AB without blowing up.
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Old 30th January 2016, 01:27 PM   #3
roclite is offline roclite  United Kingdom
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PR-800 ClassA/AB Amplifier Kit
djk, personal experience or finger-in-the air calculation? Cant find any near schematics or "pseudo-schematics" out there but the all-transistor, multi-parallel output stage should be fairly easy to sketch up. The huge 10,000uF/rail on-board electrolytic's might be a bit over-kill but would form a useful part of a larger high-current, filtered power supply. Needs a separate ac/dc supply for the pre-amp or bias section though, which is not convenient. I only use 8 ohm speakers and my listening space is not huge but I would be interested to see if this design can deliver a deeper bass response, with my existing gear.
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Old 31st January 2016, 09:39 AM   #4
AndrewT is offline AndrewT  Scotland
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300W into 4ohms requires a maximum output of 49Vpk.

You should be able to get that from a poorly built/designed +-70Vdc fed amplifier.

A really good design/build would probably get a maximum output of around 60Vpk into 4r0 test load.
Check the SOA of the 2sa1943/c5200 @ 70Vce. They are terrible compared to good transistors @ that voltage.

+-10mF (10000uF) is low for a power amplifier, especially one claiming to be 4ohms capable.
I use +-20mF for 8ohms speaker capability and recommend +-40mF for 4ohms capability.
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Last edited by AndrewT; 31st January 2016 at 09:42 AM.
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Old 31st January 2016, 09:56 PM   #5
roclite is offline roclite  United Kingdom
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PR-800 ClassA/AB Amplifier Kit
Hi Andrew, I wont dispute your calculations but might take issue with your blanket assumption that this is also a poorly designed amplifier for reasons mentioned. Have you built, tested and listened to it? I am not interested in building a high power "disco" amp but will probably run it conservatively at 55 - 60V supplies. I normally use about 30 mF filter caps in my amplifier builds so we can agree on that, I would RC filter the on-board capacitors from the larger main power supply. Not sure that your SOA concerns are a major issue when you consider there are 4 parallel pairs of output transistors, and they will be moderately biased Class AB. Sure, there are probably much better transistors available but the ones supplied should be good enough.
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Old 31st January 2016, 11:36 PM   #6
TonyTecson is offline TonyTecson  Philippines
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PR-800 ClassA/AB Amplifier Kit
when your psu comes to real life, then all calculations will have actual meaning...
when confronted with questions like this, i always answer with a question,
what does your psu look like?
after all the psu s 50% of any amp and just as important....
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Old 1st February 2016, 08:28 AM   #7
AndrewT is offline AndrewT  Scotland
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Originally Posted by roclite View Post
.................. but will probably run it conservatively at 55 - 60V supplies. I normally use about 30 mF filter caps in my amplifier builds so we can agree on that, ...................
expect the supply to sag when under load. A volt or two droop when the amplifier is quiescent, another 3V to 10V when under maximum load testing. That gets your Vpk down to 40Vpk to 50Vpk
Quote:
...............Not sure that your SOA concerns are a major issue when you consider there are 4 parallel pairs of output transistors, and they will be moderately biased Class AB. Sure, there are probably much better transistors available but the ones supplied should be good enough.
You should be concerned about SOA. You need to temperature de-rate as well.
The 1943/5200 are terrible at the higher voltages. Look at the datasheet and compare to an MJW0281/0301. Both are 150W devices.
@ 70Vce & 25C, the Toshiba has a rating of ~68W whereas the ONsemi has a rating of ~102W

If you run @ +-55Vdc, then the Toshiba compare much more closely.
I have recommended in the past that for ClassAB, the Toshiba should not be used @ >+-55Vdc. ONsemi and Sanken are more suitable.
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Last edited by AndrewT; 1st February 2016 at 08:30 AM.
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Old 1st February 2016, 08:11 PM   #8
roclite is offline roclite  United Kingdom
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PR-800 ClassA/AB Amplifier Kit
Andrew, thanks for elaborating on the importance of SOA and temperature derating. The supplied transistors should perform OK at the nominal +/- 55V supply rail. The amplifier PSU design will hopefully minimise sag and droop. As we know, the amplifier sellers quoted 'head-line' performance figures are more mythical than practical. I will post results (good or bad) but still happy to hear from anyone who has built and tested this module.
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Old 2nd February 2016, 03:53 AM   #9
Ian Finch is offline Ian Finch  Australia
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Anyone looking to buy or even browse at the OP's Ebay link now will be disappointed as Along1986090 and many other stores are on a Chinese New Year holiday for a month.
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Old 2nd February 2016, 06:44 AM   #10
AndrewT is offline AndrewT  Scotland
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Originally Posted by roclite View Post
................ The supplied transistors should perform OK at the nominal +/- 55V supply rail............
That is a more reasonable PSU voltage for the 1943/5200 outputs
Expect 4pr to give around 200W for maximum reliable power.
You should get around 150W to 170W into 8ohms when using a 230:40+40Vac transformer. That transformer gives me +-58.5Vdc, when mains is 240Vac

Do not try to use 4ohms speakers with a 40+40Vac transformer. I recommend a maximum of 35+35Vac for a 4ohms speaker.
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