Hi,
I've been planning on building a BrianGT LM3886 for my father... The one hook I've become tripped up on is finding an affordable transformer.
What I was wondering is can I use two center-tapped 24v 5a transformers in the same way that you would use a transformer with dual-secondaries? I can get them for fairly cheap.
Any better suggestions?
I've been planning on building a BrianGT LM3886 for my father... The one hook I've become tripped up on is finding an affordable transformer.
What I was wondering is can I use two center-tapped 24v 5a transformers in the same way that you would use a transformer with dual-secondaries? I can get them for fairly cheap.
Any better suggestions?
I did search but I'm still a bit confused as to whether or not two center tapped transformers can be used in this configuration.
From my (rather minor) understanding of electrical theory it should... But I'm basically just doing a quick error check in my thinking.
Edit: Just saw Steeno's post thanks for the answer... I owe ya.
From my (rather minor) understanding of electrical theory it should... But I'm basically just doing a quick error check in my thinking.
Edit: Just saw Steeno's post thanks for the answer... I owe ya.
I just built an amp using dual transformers with singal outputs in series. The rails differ slightly in voltage. It should work although the transformers will be worked hard if the amp is running flat out. Also check the output voltages match otherwise one transformer will be workrd harder than the other.
Nuuk said:I suspect the results would be better using one transformer for each channel!
Really? I thought +/-12v was awefully low for LM3886... Maybe I mis-read the data sheet. That would certainly make my life easier!
Anyway, in the end, I may just pick up a toriod from Parts Express (they seem to have the best prices for the US).... It may be worth it instead of messing around with these transformers.
Yes, you can run the 3875 from that voltage.
I wouldnt recommend running 3875 from 25volt secondaries. That is too much. Take a look here, RJM has something to say about that subject: http://www.geocities.com/rjm003.geo/rjmaudio/diy_gc.html
That article is by the way supposed to be a fairly close 'cloning',
of the original Gaincard from 47 Labs. Nice article. (and a nice homepage)
Steen.
I have run the 3875 with +- 39 V DC (was 60Vac ct) with no ill effects. But only because thats what I had kickin' around to try.
I'm currently using 2 x 48 vct transformers (one for each channel).
DC filtered is about +-36v. This setup also works well with no ill effects and sounds very good.
If I were buying toroids I would stay in the 2x18 - 2x22 v range tho.
You could easily use 2 bridges with your 24's (cneter taps not connected) to make a single +- 36 dc to run both amps. I wouldn't complicate issues by trying to use more bridges tho.
My 2 cents worth.
Hugh
I'm currently using 2 x 48 vct transformers (one for each channel).
DC filtered is about +-36v. This setup also works well with no ill effects and sounds very good.
If I were buying toroids I would stay in the 2x18 - 2x22 v range tho.
You could easily use 2 bridges with your 24's (cneter taps not connected) to make a single +- 36 dc to run both amps. I wouldn't complicate issues by trying to use more bridges tho.
My 2 cents worth.
Hugh
Don't bother trying to run a LM3875, LM3876 or LM3886 at +/-12V. You won't get much for output power at that level. The ideas already given are better. Run at +/-24V by using each xformer as one side of the supply rail. The LM3886 will like this amount of voltage and as long as you use a respectable heat sink or don't try to drive sine waves you will be OK. They will get pretty hot if you are trying to drive a 4 ohm load. Are the chips you are using the 'TA' or 'TF' type?
The chips are the insulated version. TF I believe.
So how much heatsinking are we talking about? (I'll work it out from the application notes later, but if someone has a rough estimate, it would be appreciated).
I had planned on just using a length of copper pipe for the heatsink... but I can easily upgrade that to something larger. (Or I could toss in a $5 PICAXE driven temperature monitoring/fan system... Been wanting to build one of those for a while.)
So how much heatsinking are we talking about? (I'll work it out from the application notes later, but if someone has a rough estimate, it would be appreciated).
I had planned on just using a length of copper pipe for the heatsink... but I can easily upgrade that to something larger. (Or I could toss in a $5 PICAXE driven temperature monitoring/fan system... Been wanting to build one of those for a while.)
My VBIGC (3875 chips) uses just 3/4 of an Athlon heatsink for each chip without any problem. There are vents below and above each heatsink.
You can see one of the heatsinks in this picture.
An externally hosted image should be here but it was not working when we last tested it.
That should do it also. At some point, I wanted to find out how much heatsinking thats needed for these chips. (Being used to ClassA amps made it a bit hard to understand
) So to find out, a chipamp was made in a small chassis and both chips mounted on a Athlon heatsink. I let it run at full throttle for an hour with no excess heat. The chassis did get handwarm, but not hot. No vent-holes Have a look at the infamous "Ugly duckling" amp Steen.
Btw Nice amp on the picture, Nuuk.
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That's pretty much my findings with the 3875 Steen! If you get the circuit right, they run fairly cool!
Even the two 3886 amps that I have made so far don't seem to produce much heat although I will continue to play safe with larger heatsinks!
From your experiment it would seem that a PC CPU heatsink is more than adeqate for one 3875 although we could get away with one for two chips if space was very tight!
Even the two 3886 amps that I have made so far don't seem to produce much heat although I will continue to play safe with larger heatsinks!
From your experiment it would seem that a PC CPU heatsink is more than adeqate for one 3875 although we could get away with one for two chips if space was very tight!
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