Hi all, I'm getting ready to build an audio sector gainclone in monoblock configuration (ordered the lm3875 kit but sounds like Peter may be sold out of those, so maybe lm4780 instead?). I see many people recommending 300va transformers for these, however being a firm believer than anything worth doing is worth overdoing I ordered 2400va [Edit: Two 400VA transformers!] units. I'm now having concerns that these might cause issues due to being too big. Are these fears unfounded? Is it safe to oversize transformers so much? Thank you!
Last edited:
The one in mine is 225VA, 25VDC rails, more than enuff. Very nice amplifier.
What speakers will you be driving?
dave
What speakers will you be driving?
dave
I don't see any issue. You will want to soft start one that big though.
At the moment I have some vintage dynacos but will be building some s2000 mtms soon. They are reasonably sensitive 4 ohm speakers.
Even with a huge transformer like that you will get no more out of the amp so it’s wasted money. A little bigger is good but 8x is just ridiculous! Twice is way more than enough. Spend the money on a few more ps caps.
Whew - that was a considerable oversize.
Bad things that could happen from this? - let me try to list some:
Cheers, Martin
Bad things that could happen from this? - let me try to list some:
- Possibly a higher primary-secondary capacitance
- A much (!!) deeper hole in your wallet 🙂
- Expect higher iron losses (increasing your electricity bill when used for many hours).
- Add increased risk of a back injury while carrying the amplifier around.🙂
- Expect the transformer to deliver its output voltage specified for idle load all the time no matter how hard you drive the amplifier.
- You may also consider the inrush current the capacitors in the PSU will have to cope with - this may actually carry a risk of component failures.
Cheers, Martin
I can't figure out how to edit my post but it should be 2 400va units, not 2400!! It was a whopping 7 dollars extra so I figured why not put some extra gravity in there.
Last edited:
Typo, it's 2 400va transformers, not 2400va! It's a minimal price increase over 300va
I can't edit my post, but this is 2x400va transformers, not 2400va! Thank you for the reply, I just wanted to clarify that it's not quite such a dramatic oversized part!
One 400VA toroid per channel (monoblock setup). I'm gathering that it won't be much of a problem going 30% larger than the recommended transformer so I think I'll just go ahead with it at this point.
I'd have to sit down and do some math, it's got 20v secondaries so will somewhat limit the full output power of the chip. I got 20v because I thought I'd be running the lm3875 into 4ohms but now it's looking like I'll probably get the lm4780 chips instead which evidently do fine into 4 ohms without limiting rail voltage.
Soft start is really easy. Basically a negative temp thermister in series with the primary. Hotter it gets, less resistance. And usually what is done is at turn on, thermistor is at max resistance and quickly heats up. After a few seconds, most short the thermistor with a relay to completely eliminate the thermistor resistance in the primary circuit.. There are lots of circuits around. Just google soft start. Because you are using a toroid inrush currents will be higher than an E-I trafo. You'll soften the blow to the diodes and caps in the power supply. The power switch will like it better too. Also as someone else mentioned, look at the spec sheet for the trafo. It will spec both the idle and full load voltages. You'll be much closer to the idle voltage. Much.
2 x 400va is a whole different story! That is totally within reason.
Sounds good, I'll have a look at how to do that. Thanks very much! I'm looking forward to getting this build started, as soon as I figure out what kit Peter is shipping.
Yep, that's why I ordered 20v secondaries. Though the lm4780 apparently does better with 4 ohm loads.