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Old 1st August 2009, 01:56 AM   #1
diyAudio Member
Join Date: Jun 2008
Default 6 A transformer for Gainclone ?....help

If this has been answered elsewhere - I looked but could not find it !

I've bought an Antrim 6 amp rated 25 volt toroidal transformer with the hope I can use it for an LM 3886 x 2 stereo gainclone P amp.
No VA rating info yet.

Question is....can I ?

Also will it ' POP ' my spkr drive units to destruction without a soft start circuit due to inrush ?

Thanks in advance

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Old 1st August 2009, 03:56 AM   #2
ryan750 is offline ryan750  Philippines
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Join Date: Oct 2007

(25 x 2) x (6A) = 300VA. am i right??

btw, i also use a 18v x 2 6A for my 2 lm3886 and works pretty fine,
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Old 1st August 2009, 04:59 AM   #3
tresch is offline tresch  United States
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Join Date: Dec 2008
question is if it's single or dual secondary.

If it's a single secondary center tapped (-12.5, 0, +12.5) If I remember right those voltages are definitely on the low side for the 3886 and you won't get near the rated power out of the chip. Capacity wise 25*6 = 150VA which definitely is cutting it close for a 2x60 watt amp.

If it's single secondary.. regular tapped(?).. (0, +25) you still have the same problems as above, but you also need to rework the schematic to run the amp in "single supply" mode

If it's dual secondary, and EACH is 25v, then that seems just about right.. but you'll want to double check the datasheet for the chip to make sure your voltages are within spec
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Old 1st August 2009, 10:41 AM   #4
diyAudio Member
Join Date: May 2008
Default Re: 6 A transformer for Gainclone ?....help

If it is 2x25 V then it is okay for 8 Ohm, but probably too much for 4 Ohm speakers.

Originally posted by AndrewGM
Also will it ' POP ' my spkr drive units to destruction without a soft start circuit due to inrush ?
Those two things are not related. The inrush current may trip the fuses or circuit-breakers before the transformer. One way of dealing with it is, use correctly rated fuses on the secondary side and overrated fuses on the primaries as small as possible, but big enough to avoid convenience tripping. The other is, use a soft-start circuit and correctly dimensioned fuses on the primary side.

The two most common sources for popping are contact arcing, when you switch something on or off and the fact that different parts of the amplifier switch on or off at different times, as the supply voltage builds up in the power supply. The cure for the first are capacitors or RC snubbers across all contacts. The cure for the latter is to delay the amplifier activation until the supply voltage is high enough and stable. In the LM3886 you can use the mute pin for that task. Or you can add a relay to the output that can do the job and at the same time be used for additional protection circuits.

If it pops, when other devices are switched, plugged into or unplugged from mains, you should add some mains filtering. A small capacitor between the input pins of the amplifier can do the job. Ferrite beads on the mains cable also help. Or use a mains filter with the correct current rating.
If you've always done it like that, then it's probably wrong. (Henry Ford)
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Old 1st August 2009, 02:12 PM   #5
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Join Date: Jun 2008
Ryan Hi

Thanks for that - I learned something in your maths lesson there
and won't forget that tip !
Looks like it should be OK then - thank you.

Tresch Hi

Your contribution left me scratching my head a bit.
I have Red, Grey, Yellow and Blue wires as secondry.
I'm going to do it in series - Red / Sec 1 and Grey / Sec 2
and the yellow and blue will be the linked centre tap.
Will this be OK / adequate ?

and to Pacific Blue....

Good advice for me.
I can do those things and will before I power it up.
Great help.

Thank you all - it's great to have this help out there as a novice.

Many thanks
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