Heatsink compound used to ensure that any gaps between the two surfaces you are mating (ie the component and the heatsink itself) are filled with something that will transfer heat, so that the heat is consistently(and efficiently) transferred from the component to the heatsink.
If there is already a non-electrically conductive pad like a silicon pad being used, it will do that job by itself (fill any gaps that is) without additional heatsink compound.
I have read that you must install heat sinking grease with the l 25 d in addition to the silicone pads. I have read that on this forum as well as from zoe........as I understand these get very hot when operating at higher levels. The listing I bought these from says you need both....confirmation from LJM, please?
The silver compound may be electrically conductive.....any concerns?
The silver compound may be electrically conductive.....any concerns?
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Well if we use P = V*I, then you'll be able to supply just over 250 watts with that with no wiggle room. The L25D's are supposed to be 97% efficient at 100% load, so you should get pretty close to that out..
That's not accounting for losses in the rest of the power supply though..
Thanks MrSlim... Should I not base this formula on the voltage after rectification (+-78v x 2.5A) or am I dreaming?
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Yes, you dream
Your transformer can deliver 2.5amp ac max.
Considering the amp efficiency close to 100%, the max rms power you can get in 8ohms is P=R*I²=50Wrms
Your amp will not be able to use the full 78V, because the available current from power supply will be limited first.
So you 're saying I'd need 5.6A (give or take) to get 250 watts RMS? (more or less, a 600VA transformer)
not really that much for one amp
To get 250Wrms in 8ohm you need 44.6Vrms and 5.6Arms.
To get 44.6Vrms music, the amp must be powered at about 68-70Vdc or above (to take into account voltage drop of power transistors).
+-70Vdc translates back into about 2x50Vac transformer. It should be rated 50*5.6=280VA or more.
For a stereo amp, 2x250Wrms, you indeed need a 2x50V 600VA transformer.
not really that much for one amp
To get 250Wrms in 8ohm you need 44.6Vrms and 5.6Arms.
To get 44.6Vrms music, the amp must be powered at about 68-70Vdc or above (to take into account voltage drop of power transistors).
+-70Vdc translates back into about 2x50Vac transformer. It should be rated 50*5.6=280VA or more.
For a stereo amp, 2x250Wrms, you indeed need a 2x50V 600VA transformer.
Wouldn't the VA rating on a 100Vct / 5.6A be 560? Pardon my ignorance
ah yes of course
For the use in this context, a dual secondary transformer (each secondary independant) or center tapped (center wire of both secondaries is common) are equivalent. For audio, the usual is to use toroidals (or R-core) where the secondaries are separate.
2X50Vac 560VA is the same rating as 100Vct 560VA.
to summarize,
to get 250Wrms (mono) in 8 ohm, a 100Vct 280VA transformer is ok
to get 2x250Wrms (stereo) in 8ohm, a 100Vct 560VA is needed (or better, 600VA to take into account the losses here and there in the amp)
note: It's quite rare to need 2x250Wrms to listen music, unless you do public adress or this kind of things
If one board (mono) requires +-72vDC * 5.6A to produce approx. 250W, does this 100VCT transformer @ 5.6A not only cover one channel? Would I not then require a second identical transformer to supply 250W to the second channel?
I fear 6mths from now I'm going to look back at this post with shame, wondering "what the hell was I thinking?"
Anyway, I know that 250W/ch is lots of juice. It's for 2, 12" subs... I'd like to be able to work them out every now and then.
Thanks for your patience.
You can either use one 100Vct 560VA or two 100Vct 280VA transformers to achieve 2x250Wrms in 8ohms. Now that I know it's for two subs, it's better to use two transformers, one in each sub if the amp goes into the sub
Note that the calculations above were considering 100% amp efficiency.
Also, many speakers rated 8ohms do actually go down to 5 or 6 ohms. This requires more current to achieve same power.
If you think you'll really use the 250Wrms, I would recomend to chose 350VA transformers to have some margin.
These are the transformers that I bought at a surplus store in Toronto. I bought one for each L25D board. (I wound back the secondaries until I got 55-0-55 on each transformer)
As you can see the label states 575VA. I figure it's a misprint because, 1) it's much smaller than the 420VA toroids that I have. 2) The label then says "62-0-62 (2.5A)"
My thinking is that it should be approx 310VA... After winding the secondaries back to 55v-0-55v, I figured it's about 275VA... Am I way off?
As you can see the label states 575VA. I figure it's a misprint because, 1) it's much smaller than the 420VA toroids that I have. 2) The label then says "62-0-62 (2.5A)"
My thinking is that it should be approx 310VA... After winding the secondaries back to 55v-0-55v, I figured it's about 275VA... Am I way off?
Yes if you use the transformer as a single 100Vac supply, No if you use it as a dual rail +- supply served by 50Vac each.Wouldn't one 100Vct 280VA transformers be only 2.8A?
So, using the 100VCT transformer in a dual rail +- configuration I'll be getting the required 5.6A? & to apply this to my modified transformers above, I'll get +-55v and 5A (Which at the end of the day is what I wanted anyway)