Transformer help with LM3886

windforce · 12th January 2010, 07:51 PM

Hello all, I am looking for some advice.

I bought a LM3886 amp with a 6N11 tube. The power requirements are listed at 28v-0-28v AC and 6.3v-8v AC. Attached is a picture of the amp.

A few questions:

In reading it seems the general consensus is that 28V would be considered too high for an LM3886 running 4 ohm speakers. Am I correct here?

Second question, if I need a 28v-0-28v ac and 8v ac transformer where might I find one? If I need smaller what would be the recommended size, and what might a a good source for the smaller one?

Is 160va enough or should I look at something closer to 250va for the transformer?

PartsExpress offers a variety of 18v-30v transformers, but not a 28v one, I also dont know for sure if they offer 8v in combo either.

Please advise, thanks for the help...

windforce · 04:02 PM

In doing some searching I came across a transformer that is very close to what I need. Specifially it has a 0-12v for speaker protection, I think this is what I need for the amp in my previous post.

What do you think?

150W 22-0-22V+12V Ultra-Thin Toroid Power Transformer - eBay (item 310192836777 end time Feb-05-10 07:19:43 PST)

Power, AC: 150W
Voltages, primary: 220V(Red)
Voltages, secondary: 22-0-22V(Blue-Green-Blue), 0-12V(Yellow; for speaker protection circuit)

cjkpkg · 05:20 PM

I would use 2 Xformers - one for the chipamp supply 22-25VAC and then a separate 6.3V Xformer for the tube filament. The chipamp voltage I would presume is rectified onboard to DC but the 6.3V can stay as AC for the filaments for ease of the build. Many people here use AC for tube heaters with no ill effects - just make sure to twist the wire pairs of all AC wiring.

I built a tube preamp that required 3 different voltages - the 250V and the 5V were from the main xformer and I used a cheap Radioshack 6.3V filament transformer for the required 6.3V

I used an Antek AN-3225 300VA 25V for my LM3886 chipamp and it puts out pleanty of power. I am getting about 34VDC at the rails.

The heater on the tube - if comperable to a 6DJ8 draws .365 amps alone...

Minion · 13th January 2010, 10:38 PM

The speaker protection doesn"t use a seperate supply , it runs off of the positive Power rail , it uses a UPC1237 which has a Voltage range of between 24v-60vDC so the 12v tap is useless for this job , the circuit uses a Zener regulator to power the Relay, the 6.3v -8v is for the heater of the tube .....

So for 4 ohm loads you need a Transformer with between 2 x 18v to 2 x 20v and with a 6.3v Tap , you can also use a seperate small 6.3v transformer if you can"t find one with the 6.3v and the 2 x 18 .....

Cheers

13th January 2010, 03:39 PM	#2
windforce diyAudio Member Join Date: Dec 2009	Follow up... In doing some searching I came across a transformer that is very close to what I need. Specifially it has a 0-12v for speaker protection, I think this is what I need for the amp in my previous post. What do you think? 150W 22-0-22V+12V Ultra-Thin Toroid Power Transformer - eBay (item 310192836777 end time Feb-05-10 07:19:43 PST) Power, AC: 150W Voltages, primary: 220V(Red) Voltages, secondary: 22-0-22V(Blue-Green-Blue), 0-12V(Yellow; for speaker protection circuit) Last edited by windforce; 13th January 2010 at 04:02 PM.

13th January 2010, 05:15 PM	#3
cjkpkg diyAudio Member Join Date: Aug 2008 Location: Roswell GA	I would use 2 Xformers - one for the chipamp supply 22-25VAC and then a separate 6.3V Xformer for the tube filament. The chipamp voltage I would presume is rectified onboard to DC but the 6.3V can stay as AC for the filaments for ease of the build. Many people here use AC for tube heaters with no ill effects - just make sure to twist the wire pairs of all AC wiring. I built a tube preamp that required 3 different voltages - the 250V and the 5V were from the main xformer and I used a cheap Radioshack 6.3V filament transformer for the required 6.3V I used an Antek AN-3225 300VA 25V for my LM3886 chipamp and it puts out pleanty of power. I am getting about 34VDC at the rails. The heater on the tube - if comperable to a 6DJ8 draws .365 amps alone... Last edited by cjkpkg; 13th January 2010 at 05:20 PM.

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13th January 2010, 10:38 PM	#4
Minion diyAudio Member Join Date: May 2006	The speaker protection doesn"t use a seperate supply , it runs off of the positive Power rail , it uses a UPC1237 which has a Voltage range of between 24v-60vDC so the 12v tap is useless for this job , the circuit uses a Zener regulator to power the Relay, the 6.3v -8v is for the heater of the tube ..... So for 4 ohm loads you need a Transformer with between 2 x 18v to 2 x 20v and with a 6.3v Tap , you can also use a seperate small 6.3v transformer if you can"t find one with the 6.3v and the 2 x 18 ..... Cheers