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Old 17th October 2017, 11:08 PM   #1
GeorgeM59 is offline GeorgeM59
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Default Help with DIY amplifier

Hi guy's, I'm new here and was hoping for a little help with an amp project for live PA work.
I while back I built what I know as a Studio 350 kit and am very pleased with it's performance but I am in need of a bit more power. I have 2 toroidals 30+30 300va and was thinking of using them in series to get 60+60 600va and after rectifying should give 84volt dc rails.
Can I build a studio 350 with MJL4281/4302 transistors using the above power supply without any other changes and will it give me a decent improvement.
I have seen some schematics online for 1200 to 1600 watt amps that claim to only use 65 volt rails but can't see this is possible as my 350 uses +-70 volts to give 350 into 4 ohms and around 250 into 8 ohms.
From my calculations to get more power into 4 or 8 ohms which is the load I am using the rail voltage needs to be higher, am I missing something.
If the studio 350 design is not suitable does anyone know of a good design/schematic that I could build to give around 400/500 watts into 8ohms.
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Old 19th October 2017, 11:07 AM   #2
indianajo is offline indianajo  United States
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You can figure backwards.
500w into 8 ohm requires 7.9 amps. MJL4302/4281 have a 50 v 2nd breakdown of 4.5 A and 100 v of 1. That is 1 second non-repetative, so you have to be lower for full time use. The datasheet I have has no SOA chart showing DC soa, so you'll have to guess. If soa is 2 amps, you could get away with 4 pairs output transistors. Safer would be 5 pairs. Don't forget adequate heat sinking and fan at this high power.
Minimum gain MJL4302/4281 @ 3 amps or 1 amp either one is 80. So @ 2 amps a single pair requires 25 ma drive current, 5 pairs requires .125 A.
Can your driver transistor pair of the studio 350 provide 125 ma continuously? Is the heat sink adequate? Is it SOA rated at 85 v for that current?
If that is okay, back up one stage. How much drive current does the driver transistor need? can the input op amp or VAS transistor produce that current?
You're lucky with the MJL4302/4281 they have rather high gain and don't need much drive current compared to other output transistors. Of course, that gain is at 25 deg C, may be lower at actual operating temperature.
Wattage ratings vary. Many amps these days have an advertised 4 ohm rating, and a lower 8 ohm rating. E bay rating amps tend to have a 1 second rating, ie if you put out that wattage for two seconds, the outputs short out. Successful PA quality amps have a 24 hour/7 day a week rating, not enforced by any government body. Only by reputation. Other repairmen have noted succesful PA amps include Crown, Peavey, QSC, Yamaha. So look at their designs and see what the successful companies with conservative watt ratings are doing. My Peavey PV-1.3K has that 600 w/ch rating at 4 ohms, at 8 it is 360 w/ch. It has 84 v rails and 5 pairs output transistors. Heat sinks and fan, heat sinks on the drivers, which are MJE15032/33 in the latest revision bulletin.
Have fun calculating and building.
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Last edited by indianajo; 19th October 2017 at 11:14 AM.
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Old 19th October 2017, 11:53 AM   #3
GeorgeM59 is offline GeorgeM59
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Thanks for that information. It's been a while since I built my Studio 350 but I'll drag out the schematic and do some checking on the driver components.
Question on the torodial, is the output current directly related to the speaker current. The ones I have are rated at 5 amps which will probably be the power limiting factor?
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Old 19th October 2017, 01:25 PM   #4
indianajo is offline indianajo  United States
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With those toroids, you could get music power (peak) ratings higher than 500 W due to the storage in the capacitors, but not RMS rating. You can't get more current out than the transformers put in.
That limit makes 4 pairs MJ4302/4281 more viable. The voltage collapses as the current goes up to maximum. 2nd breakdown limit current goes down as the voltage goes up, on other ON semi datasheets.
Really, at that wattage I'd consider starting fresh with a honeybadger PCB or maybe kit. Has several pairs of 2sa1943/2sc5200 but I think the On semi parts have more current soa. Then you could listen to music with the 350 while you were building/debugging the new device. See diyaudio store button up on the top.
Or make an offer for a dead PA amp of that class at a repair shop. Gets you heat sinks, case, fan, maybe transformer for $80-100. Enzo hauled a trailer of those to the scrapyard last year when he closed his business. No market for class AB transformer amp hulks, too heavy for the road when switcher supplies are half the weight.
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Last edited by indianajo; 19th October 2017 at 01:44 PM.
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Old 20th October 2017, 12:12 AM   #5
GeorgeM59 is offline GeorgeM59
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I'll see if I can give the toroids a load test , they were used as the power supply for large stepper motors in CNC plasmer cutters and are quite heavy for 300va transformers.
I have found the schematic for my 350 amp but I'll have to work out how to post it here.
The repair off a dead one sounds good I'll take a look. Currently working on a pair of Allen & Heath mixers I picked up for $100, one GL2000 and a Gl2200. Got the 2000 working ok but the 2200 has a weird problem with the stereo returns.
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Old 21st October 2017, 01:03 AM   #6
wg_ski is offline wg_ski  United States
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Quote:
Originally Posted by GeorgeM59 View Post
Thanks for that information. It's been a while since I built my Studio 350 but I'll drag out the schematic and do some checking on the driver components.
Question on the torodial, is the output current directly related to the speaker current. The ones I have are rated at 5 amps which will probably be the power limiting factor?
It's actually quite a bit more complicated than that. Power will be limited by what the voltage drops to under load. With +/-84 and 2 channels at 4 ohms, with transformers of that size, it will likely drop to about +/-70V. After realistic losses you'll get about 500 watts per channel at full signal sine wave. At that point you'll be drawing about 11 amps D.C, which is why the supply drops so much.
With music, the supply will be varying somewhere in between the two extremes.

The continuous capability of the trafos is somewhere in the neighborhood of 400 watts. Capacitor input supplies have a power factor of about 65%. That is enough to power a kW worth of audio power at any realistic distortion level, where the long term average over hours of use will be well below that. When I'm designing for abusive conditions, I'll oversize the transformers (and speakers) appropriately.

Since you have two trafos you intend to put in series to get the voltage, I'd consider class H. You can have quad 42v rails, you may as well make use of them. Only need 50v caps and less heat sink. Peak stresses on the output transistors are exactly the same as class AB for resistive loads, but much less for reactive loads. You could get away with four pair of C5200/A1943, where the same would be insufficient for class AB. Or use the On Semi parts and have it be bulletproof. The Apex A900 would work fine, or at least be a good starting point.
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Old 21st October 2017, 08:11 AM   #7
GeorgeM59 is offline GeorgeM59
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Quote:
Originally Posted by wg_ski View Post
It's actually quite a bit more complicated than that. Power will be limited by what the voltage drops to under load. With +/-84 and 2 channels at 4 ohms, with transformers of that size, it will likely drop to about +/-70V. After realistic losses you'll get about 500 watts per channel at full signal sine wave. At that point you'll be drawing about 11 amps D.C, which is why the supply drops so much.
With music, the supply will be varying somewhere in between the two extremes.

The continuous capability of the trafos is somewhere in the neighborhood of 400 watts. Capacitor input supplies have a power factor of about 65%. That is enough to power a kW worth of audio power at any realistic distortion level, where the long term average over hours of use will be well below that. When I'm designing for abusive conditions, I'll oversize the transformers (and speakers) appropriately.

Since you have two trafos you intend to put in series to get the voltage, I'd consider class H. You can have quad 42v rails, you may as well make use of them. Only need 50v caps and less heat sink. Peak stresses on the output transistors are exactly the same as class AB for resistive loads, but much less for reactive loads. You could get away with four pair of C5200/A1943, where the same would be insufficient for class AB. Or use the On Semi parts and have it be bulletproof. The Apex A900 would work fine, or at least be a good starting point.
Thanks for the reply.
I was planning to use 2 of those trannies per channel but just went through the box and found I have 6 in total. As I'm getting older and have a bad back it was going to be 2 mono amps in separate cases to keep the weight down but I could build a power supply box the all 6 in it and build a separate 2 channel amp with just main boards, heat sinks, speaker protection etc.
I will take a look at the Apex A900.

Here is a copy of the Studio 350 Schematic I built from a kit. Also built a lower voltage version (56v rails) and cut it back to 2 MJI5003/5004's per side, now lives in an old Fender MA6 powered mixer amd works very well.
Feed back no it is's design with the increased rail voltage would be handy as I have full instructions for it and already have a lot of the parts.
Click the image to open in full size.
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Old 21st October 2017, 08:45 AM   #8
GeorgeM59 is offline GeorgeM59
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Where do i find the Apex A900? I found an Apex BA1000 design on this forum, is it worth looking at.
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