Optimum for 4Ω is 25v rails, 18v AC from the trafo, 300VA is overkill for a stereo amplifier.
dave
In fact even the 300VA one is oversized,go figure.
But at last still reasonable.
But at last still reasonable.
I may change my order for 2 300va transformers, if there's no benefit to going stupid big. All this talk of inrush current and soft start stuff has me thinking it's not worth the hassle, plus I can save a few bucks for a couple amp building beers and make my enclosures a little bit more smaller.
Larger enclosure becomes kind of a pain, though I think I've got a cost effective solution, plus the extra work of coming up with a soft start thing.... I've emailed antek and if it's not too much hassle I'll just change it. Plus I can change the secondaries to get something better for the 4780 chip, sounds like 20v is on the low side there.
Exactly. This'll be my first foray into proper hifi amps coming from vintage receivers (which I do love) during college and I kind of want to go all out with it, I would hate to get finished with this whole frikkin thing and find that the transformer was oversized but still not giving the optimal voltage.
Soft start is always a nice feature though. At 300VA, you are right at the threshold I think for a toroid soft start. Soft-start stops the lights from dimming at turn on, and helps the power switch. The DH-200 Halfers were notorious for eating power switches. I took the switch out of mine after it died. I've added soft start to my 4-way system in a box the power amps plug into. No light dimming now.
Since the typo is yours and is the first in the thread, you should see an EDIT link under your post.
Here is what I see when I post the first post in a thread:
However you are very new and may still be limited in your powers.
An alternate, for important typos, is the "(!)Report" link under any post. This goes to a Moderator, who can edit any message, but don't annoy them.
True, and FWIW "just" VA isn´t the only parameter to consider, type is too: for the same rating Toroids in general will have lower DCR and higher magnetizing current than equivalent EI types.
The price you pay for having better regulation.
Yes that is my understanding, and I thought 300VA is when they recommend the soft start for toroid. Not sure where the E-I is recommended. Soft start is pretty new (relative to a DH-200, and all my 70's early 80's amps) and was probably expensive to implement back then. Did they even have power NTC's back then? And if I recall correctly, a relay today is less expensive that it was back then not even factoring inflation.
Like this: Neurochrome: The Ultimate Guide to Soft Start Design. On that page I also explain why you'd want a soft start.
Tom
tomchr: Very nice writeup.
Also some very good articles on inrush current limiting at: https://sound-au.com/
Hal
Also some very good articles on inrush current limiting at: https://sound-au.com/
Hal
Back in the day (late 70s) when I was into PA rental, I used AB Systems amps which had a unique soft start system: not only each amplifier had a (if I remember correctly) 2 ohm 50W resistor in series with mains wiring, shorted by a relay some 5 seconds later, but also you could cascade them so not all would start at exact same time.
Each amp first started and then "allowed" the next one to start and so on.
I had 7 of them in a wheeled rack, it was funny to turn it on and amps go click click click top to bottom 🙂
I also used stacked BGW750, those you had to manually turn on one by one, using the combined Power/Circuit Breaker switch, otherwise you would trigger the venue´s main circuit breakers, very annoying.
Sounds like this all be in vain, as peter has no chips and i cant find any 3875s or 4780s. I do have a 4780 PCB coming, for whatever that's worth. Now I'm tempted to go the neurochrome 3886 Done Right but IDK if I can source any of those chips either.
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As the others have indicated there's a right finger in the air for soft start being needed. Although I have 120+120+80 they are in parallel and I use a big manual NTC that has a manual bypass short switch. That way the startup occurs with current going through the NTC then when I'm ready I'll switch in the bypass. Fine for my use.
There's also Rod Elliot's soft start guide: https://sound-au.com/project39.htm
The LM3875 and LM3886 share a lot of features and quirks. The LM4780 is basically two LM3886 in one package. So for OP's questions about the optimal supply voltage, I suggest reading here: https://neurochrome.com/pages/output-power
Note that the difference in build cost for a Modulus-86 vs LM3886DR adds up to maybe 10% of the total build budget once the power supply, chassis, connectors, etc. are factored in but the Modulus-86 provides vastly better performance due to its built-in error correction circuitry.
Tom
I have a small stash of LM3886T/NOPB that I'm selling as an off-the-menu item with my boards. Max one per LM3886DR or MOD86 board. I'll get you hooked up.
Note that the difference in build cost for a Modulus-86 vs LM3886DR adds up to maybe 10% of the total build budget once the power supply, chassis, connectors, etc. are factored in but the Modulus-86 provides vastly better performance due to its built-in error correction circuitry.
Tom
I've got a pair of 4780s that I doubt I'll ever use. I have no idea of current value but If you're interested PM me.
If you want to save money / have more fun, by building a soft start yourself instead of paying someone else their hourly rate to build it for you, the diyAudio Store sells a bare PCB for it. You buy the electronic components yourself and solder everything together yourself. Another different possibility is a user contributed soft start PCB with free Gerber files you can download. You either mooch/wheedle/cadge a spare board from a diyAudio member who previously ordered 5 PCBs from a fab, and has extras, or you send off the Gerber files yourself, to a PCB fab of your choice. Again you buy the electronic components yourself and solder everything together yourself.