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Old 2nd July 2011, 05:37 PM   #1121
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Well, let's do some fuse calculations based on a 2x PA150 bridged monoblock setup (500VA - 100.000uF per rail). I am using a softstart board, so they will be fast blow fuses:

For an 8ohm load (Each LM3886 seeing 12ohm, 8x3/2):Having measured a maximum +-30v output swing sine wave, thats 60v over 8ohm, so a total output current of 7.5A. That is 450W (is that right?). Divided across 6x LM3886 IC's, 75W per chip.

For a 4ohm load (Each LM3886 seeing 6ohm):+-30 swing its 60v over 4ohm, so a total output current of 15A. That is 900W.Divided across 6x LM3886 IC's, 150W per chip.

For a 2ohm load (Each LM3886 seeing 3ohm):+-30 swing its 60v over 2ohm, so an output current of 30A. That is 1800W.Divided across 6x LM3886 IC's, 300W per chip.
Hooking a 2ohm load could be a bit over the limit, as LM3886 are specified for 4ohm and here will be seeing 3ohm.


So are those peak currents or constant currents? I may imagine peak, as it cannot really go higher than +-30v.
Should I take worst case condition and make 1800W/220v= 8.18A fast fuse for the primary of the transformer? Or being a design aimed at 300W (BPA-300), should it be 300W/220v= 1.36A fast blow fuse?
I think having heard that LM3886's are limited to 50W per chip, so it shouldn't even be right at +-30v over 8ohm (75W dissipation per chip).


Thank you friends,
Regi
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Old 2nd July 2011, 09:58 PM   #1122
udailey is offline udailey  United States
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Regi,
The datasheet and AN1192 should shed light on your output power vs psu voltage. If I recall I dont think you are going to get that kind of wattage at all with only 30V. You can go to 42V for total of 84V. I think max is 68W per chip.
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Old 3rd July 2011, 04:07 PM   #1123
AndrewT is offline AndrewT  Scotland
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The mains fuse should be related to the transformer, not the amplifiers nor the speakers.

Normally rate the fuse at:-
three times the transformer power divided by the supply voltage.
For close rated fusing, making use of a soft start, aim for:-
transformer power divided by supply voltage. You may find that for either of the above cases that you will need a standard fuse or a slow blow fuse.
If the amp will start reliably with a standard fuse then it is worth trying a fast blow to see how many times it can start up the transformer. You may find that the transformer can start up reliably with an even lower fast fuse value. Once you encounter nuisance blowing then go back up one size.

The secondary fusing is determined completely differently depending on where in the secondary side you place the fuse/s.
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Old 3rd July 2011, 04:38 PM   #1124
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Quote:
Originally Posted by udailey View Post
If I recall I dont think you are going to get that kind of wattage at all with only 30V.
If it wasn't clear, 30v is the maximum output before clipping, not the supply voltage. Recalling or not, 30v over 8 ohm is a pretty straightforward calculation, I can't see where those 42v come from....

Regards,
Regi
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Old 3rd July 2011, 06:30 PM   #1125
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Quote:
Originally Posted by AndrewT View Post
The mains fuse should be related to the transformer, not the amplifiers nor the speakers.

Normally rate the fuse at:-
three times the transformer power divided by the supply voltage.
For close rated fusing, making use of a soft start, aim for:-
transformer power divided by supply voltage. You may find that for either of the above cases that you will need a standard fuse or a slow blow fuse.
If the amp will start reliably with a standard fuse then it is worth trying a fast blow to see how many times it can start up the transformer. You may find that the transformer can start up reliably with an even lower fast fuse value. Once you encounter nuisance blowing then go back up one size.

The secondary fusing is determined completely differently depending on where in the secondary side you place the fuse/s.
Considering the softstart board I am using, it places a power resistor of 100ohm in series with the primaries for the first second after starting it. So at startup, worst case is 2.2A through the primaries.

Then with a 500VA trafo, and 220v mains, that makes a 2.2A fast fuse. Seems like a nice match.

I will buy some different kind of fuses, with some different ratings above and under 2.2A, and will let them burn (just for the sake of cientific purposes).

Regards,
Regi
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Old 4th July 2011, 08:32 AM   #1126
AndrewT is offline AndrewT  Scotland
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try 2A and 1.6A, they may work for long.
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Old 11th September 2011, 04:42 PM   #1127
bcmbob is offline bcmbob  United States
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BPA-150 success!

One of the simplest amps has given me fits for a couple months. Finally got everything up and running.

I have to tip my hat to Andrew T who insisted I build and use a dim bulb tester. With the bulb I was able to track the problem down to two non-functioning LM3886 chips. This picture shows the initial surge when the switch is toggled on.

Click the image to open in full size.


These are the heavy duty power supplies running off one Antek 2224 115V transformer. They both produce 33.4DVC on the money, despite several mini fireworks displays that occurred before I built a second bulb rig to dissipate the PS caps.

Click the image to open in full size.

Here is the tester in the process of dimming as the current stabilizes. It goes to full off with higher wattage bulbs and during the process of setting each of the three segments of the BPA150. I found it absolutely necessary to set DC offset to zero one at a time by removing the 0.2R resistors in the other two segments. Not doing so resulted in over voltage that destroyed several caps on the amp board. I never did get one segment lower than 24 on its own, but all three stabilized to 11 after the interactive pot adjustments. That's better than the first board which equalized at 17. I don't know if zero is possible with all three chips active.

Click the image to open in full size.

Running at full volume from a Sony Walkman CD player the sound is clean, dynamic and with a full and solid tight low end. After running for about an hour, the caps stay at room temp and the small aluminum plate never goes above 101F/38C. I'll use a real heat sink when it goes in the case.

Click the image to open in full size.

The power supply and amp PCBs were purchased at Jim's audio on ebay. Their customer support during all my good and bad moves was nothing less than first rate.

The components came from both Mouser and DigiKey. This will be the bottom half of my bi-amp project and I'll post again when they are matched up with the MyRefs

Thanks Stanton Tin. And thanks again Andrew for insisting on the dim bulb.

P.S. This was my first shot with surface mount soldering and I'm not afraid any more
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Last edited by bcmbob; 11th September 2011 at 04:45 PM.
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Old 11th September 2011, 09:47 PM   #1128
udailey is offline udailey  United States
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Bob, would be interested in comparison with myrefc.
Uriah
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Old 11th September 2011, 11:22 PM   #1129
bcmbob is offline bcmbob  United States
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Hey Uriah,

You're reading my mind again. I just hooked them up to a pair of transmission line speakers with high quality drivers (Tang & Band 5" driver and Vifa XT25SC90 1" ring radiator tweeter) that I built last winter.

Click the image to open in full size.

Without a preamp - feeding the amps direct from the Sony Walkman - I have got to say the sound is excellent. Darn close to the MyRef when listening at the end of a 5' triangle. The placement of all the instruments on a Holst "The Planets" CD is as good as I have gotten from some earphones. Some of the tracks include those bottom organ pipes and they are there, though not what I would call powerful. I am sure the addition of a preamp/buffer will fill in what's missing. The depth of stage is immediately noticeable. On jazz and vocals the BPA150s produce more punch in the mid range and a very solid and responsive low end without any hint of mud.

I am still running with the dim bulb in circuit for a burn-in period before I hook up to the three way Sunflowers for a full comparison. When I take the bulb out I'll have to reset the DC offset as suggested buy Stanton and other builders. At this point I'm guessing the MyRefs will win in the refinement and sparkle comparison, but it's going to be close.

I haven't hooked up the BrianGTs in quite some time but these amps push out a more solid experience than the single LM3886 used on that build. Hopefully I'll have everything in place by tomorrow night and will post again.

P.S. I'm beginning to believe one could combine the LM3886 with rocks, bananas and string and the results would be a good sounding amp.

later...........
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Old 12th September 2011, 12:45 AM   #1130
bcmbob is offline bcmbob  United States
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For those who may be interested:

LM3886 x3 150W amplifier PCB Reliable Design ! | eBay

Heavy duty power supply PCB for Pass amplifiers diy ! | eBay
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