Running some numbers by the pros

[sidehack]

Hi... yeah, I just got here.


I've been working up an idea for an amplifier for my floor here at college, and looked up some amp chips I remembered from a catalog I had some seven years ago or so. LM3875 and 3876. I didn't realize they were at the heart of a popular DIY amp... what a coincidence! In any case, my intentions are to construct a four-channel amplifier using either four LM3875 or two LM4780. I'll likely go with the latter, due to the lower unit price. My main question at this point is power.


I don't intend to maximize the chip, as in use all 56 (or 60) watts, but I wouldn't mind having the capacity to come close. I figure, if I give it enough room, it'll deliver very high volume for 8ohm (if we ever need it, which we probably won't), or be able to push 4ohm if we hook it up. I don't know what speakers it'll be driving, so I figure I'll plan for either impedance.

I've been eyeing transformers for a while, pricing mostly. And I've been running into a lot of material on this forum... actually I've been reading up on power supplies for about three hours just today.

Based on the 1.4*Vrms, if I run a 25VAC secondary that's a nominal 34V, right? Which by charts on 3875 and 4780 would drive more than what I'm looking for. If I want the capacity for 200W output, is a 25x2, 500VA toroid overkill, or appropriate? The next choice is a 25x2 330.


It seems that pretty much everyone here knows more than I do, as I'm merely a student and with little hands-on experience as to making things that actually work. I request advice.


Thanks.

[lineup]

LM4780 is, basically, two LM3886 in one chip.
This gives it a bit lower specs than separate LM3886.
( one chip can not cool as good as two )

The price difference between 4 x LM3886 ( or LM3875 ) and 2 x LM4780 means nothing
compared to all other stuff you will need.
Transformers, supply capacitors, heatsink, case etc.

I suggest you should build 4 separate amplifiers. ( mono amps )
Makes your use of them flexible.
And when you have built one successfully, you know how to build the other 3 in same way.

LM3886 is a higher power version of LM3875.
It has better current drive, so better for 4 Ohm, if you need extra push.
If you do not need maximal power,
LM3875 will drive 4 Ohm to a reasonable level.

[sidehack]

The size I have to fit the unit in is somewhat limited. Actually, it's intended to fit inside a hollowed-out redundant PSU case from a NetServer (9.5x4.5x7 inches, part number D4292A) with a 120mm fan mounted at one end for ventilation/cooling. With this space, I assume it'd be best to use a single transformer to power all four separate amplifier channels.

If I have to I'll cut it back to two channels for space/heat considerations.

The LM3886 is a good idea; I'd printed out the data sheet for that but forgot it in the drawer. I'll look into it.


I have a pair of 6800uF capacitors, rated 35V. With a 25V secondary, is this too close to the margin, or should I replace them?

[neutron7]

i wouldnt use those capacitors. they are a bit close for comfort. if the mains voltage gos up just a bit they will be out of spec.

[Wynand]

Allow for 10-20% headroom in your voltage rating

Go for 50V caps or 22v seconaries ( 22x1.414-diode drop for rectifiers = +-30vdc

30vdc leaves you with just under 20% headroom

[sidehack]

My main question so far is whether 500VA transformer is overkill for amp.


Seems it'd probably be alright; the next choice down is about 330VA and I'm sure that's probably too low.

[lineup]

Originally posted by sidehack
My main question so far is whether 500VA transformer is overkill for amp.
Seems it'd probably be alright; the next choice down is about 330VA and I'm sure that's probably too low.

I say you can use 330VA without running in to any problems.
Most would probably recommend 500VA for four (4) channels.
Some here use 400 VA for 2 channels, while many do well with 180-250VA.

As a rule, use 2-3 x Max Power for transformer.
Max 40 Watt amplifier should have 80-120VA per channel.


It is much dependent on what chip you use
and most of all how high DC voltage your transformer will put out.

A 2x30 Volt DC supply, will maybe have max 25 Volt peak output.

Power = peakVolt x peakVolt / ( 2 x Ohm )
For 30 Volt supply and 6 Ohm loudspeaker(a compromise between 4/8):
P = 25x25 / (2x6)
P = 52 Watt max power

[sidehack]

So 330VA would work, but 400 or 500 VA I could expect better performance (for regulation, at least) from the supply? Assuming I want 50W from four channels, at 200W total, 2-3 times that is 400 to 600VA, with 500 as a median.

2x22VAC would put out approximately 2x30VDV, which by the calculations would yeild roughly 50W through a compromise of 6ohm.

So a 2x22, 400VA would do the job... I'll start asking around then for something with those specs.

[lineup]

Yes, a 400 VA will do the job.
When you use 4 channels, at same supply,
it will never happen that all channels put out max power at same time.
Becuase they will not have exactly same signal at input.
And max power is only for short intervalls, while average output is at considerably lower level.
Even when you play loud.

I prefer using 4 separate rectifier bridge/diodes + supply capacitors.
Only sharing transformator.
But this is not necessary.
You can use one central high current rated rectifier + filter electrolytic capacitors,
because chips will have 1.000uF-2.200uF caps closer to chip.


Besides this Chip Amps forum
there are other websites where you can find good info
when building chip amps 'gainclones':
http://myweb.tiscali.co.uk/nuukspot/...loneindex.html
And the gainclone.com forum:
http://gainclone.com/phpBB2/index.php

[sidehack]

Just thought I'd toss this out there... I see a lot of swell custom PCBs on here. Where can you get that done, or do some of you guys have etch stuff at home...?