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Realistic LM3886 transformer requirement (120VA for 40-50W max?)
Realistic LM3886 transformer requirement (120VA for 40-50W max?)
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Old 13th June 2018, 04:45 PM   #1
lost eden is offline lost eden  United Kingdom
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Default Realistic LM3886 transformer requirement (120VA for 40-50W max?)

I built a pair of LM3886 monoblocks back in 2011, each with a 120VA 2x25V transformer & 2x 8200uF filter capacitors. They power a pair of 5" 2-way bookshelf speakers from ~80Hz up while I have a separate TDA8920BTH amp (bridged) powering an 8" sub under 80Hz.

I would like to downsize this setup - the LM3886 amps currently sit in a huge 3U 19" enclosure, with another 1U 19" for the active crossover & then a separate project box on top of the 6U 19" rack for the TDA8920 amp. It's kinda ridiculous.

My plan is to build a third LM3886 amp (specifically two LM3886 in parallel) to power the sub & then to house all three LM3886 amps along with a crossover PCB into a single enclosure.

My question is whether I could realistically use the two 120VA transformers I already have to power all three LM3886 amps - using the first to power both of the single LM3886 amps I already have & the second to power the new parallel LM3886 that I plan to make (so each 120VA transformer would be powering two LM3886).

Realistically speaking, I will never want more than ~20W of clean power from each of the single amps & ~50W from the parallel amp - & even that would be on rare occasions. These are all 8Ω speakers. My understanding of the VA to W relationship isn't the best, so can anybody weigh in on whether this is realistic?

If this idea isn't even remotely realistic, I can dismantle the TDA8920 amp for its 225VA 2x18V transformer & use that for the parallel LM3886, but if it's plausible to run the whole setup from the two 120VA transformers I would prefer to do that & keep the size/weight/complexity of the project to a minimum.
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Old 13th June 2018, 04:49 PM   #2
DPH is offline DPH  United States
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I believe there's a calculator or two hanging around for just this concern, but, realistically, that should work just fine, especially on modest needs. 25v rails before rectification is towards the high end of the 3886, right?
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Old 13th June 2018, 04:52 PM   #3
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Maximum voltage of +-35volts so 1.414 * 25 = 35.35volts assuming the mains is not on the high side. Be aware of the voltages.
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Old 13th June 2018, 05:08 PM   #4
lost eden is offline lost eden  United Kingdom
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Quote:
Originally Posted by DPH View Post
I believe there's a calculator or two hanging around for just this concern, but, realistically, that should work just fine, especially on modest needs. 25v rails before rectification is towards the high end of the 3886, right?
I did a quick search for a calculator, but couldn't find one I could understand :3 My intuition said that 120VA should be fine for 50W of output, especially toward the upper end of the IC's voltage handling & thus efficiency, but it's good to hear that your intuition is on the same track!

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Originally Posted by JonSnell Electronic View Post
Maximum voltage of +-35volts so 1.414 * 25 = 35.35volts assuming the mains is not on the high side. Be aware of the voltages.
The datasheet states 84V as the absolute maximum operating supply voltage (94V with no signal) & IIRC I measured something like 76V when I built them back in 2011. I remember this being higher than I was expecting, but I probably didn't realise at the time that the transformers ratings assume 230V across the primary & the mains where I live is usually more like 245V. They've been running fine like this since 2011 so I'm not concerned about voltage, just whether the VA rating of the transformers will be sufficient.
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Old 13th June 2018, 05:31 PM   #5
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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+-35volts is the safe operating voltage under power. As per the Ti data sheet. The peak voltages quoted are with no signal and no load.
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Old 13th June 2018, 05:49 PM   #6
lost eden is offline lost eden  United Kingdom
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The datasheet clearly specifies 84V as the maximum operating voltage with signal. They have used +/-35V as one of their test conditions when filling out the tables with other data such as power output, but nowhere that I can see is +/-35V stated as the maximum operating voltage with signal.
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Old 13th June 2018, 06:09 PM   #7
DPH is offline DPH  United States
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It's probably fine, albeit less than ideal. Especially if you're not stressing the SOA of the 3886. Certainly higher voltage than I'd like, but I'm sympathetic to using what time got.
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Old 13th June 2018, 06:48 PM   #8
asuslover is online now asuslover  Romania
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Realistic LM3886 transformer requirement (120VA for 40-50W max?)
I know lm3886 loves to sing when is fed +/-20v - +/-38v . If you go over that he will sing but will burp/clip a lot, terrible harsh sounds.... But of course speaker impedance is important, 4ohms 28v , 8 ohms 35v , 12 ohms ....well, no more than 42v.
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Old 13th June 2018, 09:52 PM   #9
lost eden is offline lost eden  United Kingdom
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Just measured the mains voltage in my current apartment & it's steady at 247V, which means just a hair under +/-38V on paper before any voltage sag. As I said previously, I remember metering around 76V from the output of the rectifiers when I built them back in 2011 at a different apartment. So they've been running for 7 years like this with zero problems & have always sounded lovely

So if nobody thinks it is implausible to power two LM3886 from each 120VA transformer for ~50W max load, I think I will go ahead & start planning a new enclosure?
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Old 13th June 2018, 11:07 PM   #10
00940 is offline 00940  Belgium
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You've got more than enough transformer on hand. The calculator sheet is at the bottom of that page: LM3886 chip power amplifier power supply design. The section on crest factor is well worth reading.
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