I was considering converting a stereo LM3886 amp I built into a quad amp. First, some background. I am using +/- 28v rails for roughly 38w into 8ohms and 68w into 4ohms almost identical to the app note in the datasheet. I have only 2 concerns:
1) The heatsink:http://bgm.bgmicro.com/prodinfo.asp...255237&prodid=ACS1395&page=1&cri=sink&stype=3
I am currently using one chip per sink. They are attatched to the case with their fins running horizontally(the less efficient way). Do you think I can fit 2 per sink?
2) The transformer. I am using a plitron 300va transformer(20-0-20, 7.5A). I think this would definately be okay if all 4 loads are 8ohms. It might be a little weak if 4 ohm loads are used, and all four amps are cranked up.(which will never happen) Comments?
Since the tranny is pretty oversized for a stereo +/-28v amp, I thought it would be cool (and cheap) to turn it into a quad, for biamping.(I would add an active crossover)
Please share any comments you may have...🙂
1) The heatsink:http://bgm.bgmicro.com/prodinfo.asp...255237&prodid=ACS1395&page=1&cri=sink&stype=3
I am currently using one chip per sink. They are attatched to the case with their fins running horizontally(the less efficient way). Do you think I can fit 2 per sink?
2) The transformer. I am using a plitron 300va transformer(20-0-20, 7.5A). I think this would definately be okay if all 4 loads are 8ohms. It might be a little weak if 4 ohm loads are used, and all four amps are cranked up.(which will never happen) Comments?
Since the tranny is pretty oversized for a stereo +/-28v amp, I thought it would be cool (and cheap) to turn it into a quad, for biamping.(I would add an active crossover)
Please share any comments you may have...🙂
At 38W into 8 ohms with those rails, it looks like you'll draw about 70 watts per channels (from the datasheet). Each chip dissipates about 32W. If somebody chips in with a formula to calculate C/W for your heatsink dimensions we can complete the calculation for temperature rise.
70 W/ch * 4 = 280W, your transformer is probably too small. Common rule of thumb is double the W to get VA.
You can still use your transformer, you just won't be able to get full power from the chips (which might be a problem with your heatsinks anyway).
The chips have thermal protection, so they should just shut down. But I guess when they do blow, it can be spectacular (search around here a bit).
Why don't you use all 4 channels to drive your girlfriend's new woofer?
70 W/ch * 4 = 280W, your transformer is probably too small. Common rule of thumb is double the W to get VA.
You can still use your transformer, you just won't be able to get full power from the chips (which might be a problem with your heatsinks anyway).
The chips have thermal protection, so they should just shut down. But I guess when they do blow, it can be spectacular (search around here a bit).
Why don't you use all 4 channels to drive your girlfriend's new woofer?
Where exactly in the datasheet did you get these numbers?
At the top of p.11, (and from calculations on pgs.13/14) the chart looks like at a Vcc of 56v (+/-28v), the PD(dissipated power) into 8ohms is roughly 19w, and into 4ohms is roughly 40w. That would mean 38w or 80w per sink.
If anyone has built an LM3886 amp, could you post pics of your heatsinks and comment on how hot they get?
Also, if any could guess at the C/W of my sink, it would be appreciated.
At the top of p.11, (and from calculations on pgs.13/14) the chart looks like at a Vcc of 56v (+/-28v), the PD(dissipated power) into 8ohms is roughly 19w, and into 4ohms is roughly 40w. That would mean 38w or 80w per sink.
If anyone has built an LM3886 amp, could you post pics of your heatsinks and comment on how hot they get?
Also, if any could guess at the C/W of my sink, it would be appreciated.
I used two LM3876TF and it only gets warm with +- 36 volts, heatsink is 5.75" by 7.75" by 1.125". Mounted both to one heatsink.
My first amp, LM3886s have heatsinks inside the 19" case with air circulated by a couple of 80mm fans. Doesnt get too warm.
My other LM3886 amp is running on +-40V supply from two 500VA torids. The LM3886s in that got so hot i had to find a way around using tab insulators (mica, silicon, etc) as they where then overheating and self clipping. I ended up mounting the LM3886s on 6mm alu bar which was then insulated from the heatsink.
My other LM3886 amp is running on +-40V supply from two 500VA torids. The LM3886s in that got so hot i had to find a way around using tab insulators (mica, silicon, etc) as they where then overheating and self clipping. I ended up mounting the LM3886s on 6mm alu bar which was then insulated from the heatsink.
On the transformer rating thing - I'm building a twin 3886 amp and crossover module at the moment. Using +/-25V rails, I got my friend who has been designing amps for a pro-audio company for years to help me work out what rating transformer i needed 'cos I thought this was going to be a bit of a problem. After he'd compensated for duty cycles and various other things, he worked out that I'd be fine with one of the 30VA transformers we have lying around at work. I've used it and had absolutely no problems at all. If I do run into any problems, I'll be the first to go and have words with my mate about giving me dodgy advice.
So, on this basis, I'd say a 300VA transformer will be more than enough for a quad 3886 on those rails. I know of MUCH bigger pro amps that run off smaller transformers than that.
So, on this basis, I'd say a 300VA transformer will be more than enough for a quad 3886 on those rails. I know of MUCH bigger pro amps that run off smaller transformers than that.
Gonna go for it...
Well, thought about it, did some math, talked to a few other people, and I've decided I'm gonna go for it.
I figure that it should be okay unless I am running 4 loads of 4 ohms each, cranked up all the way. So, since I am planning on using 8 ohm loads and I am not looking to give myself hearing damage, I think it will be just fine. The tranny will be fine. The heatsinks are still a minor concern. If they get too hot, I have a couple of big (50cfm) fans to blow the sink.
I had oversized sinks, psu caps and transformer. All I need to buy is a few caps and output and input jacks. It's too cheap to resist.
I'll let you guys know how it turns out...
Well, thought about it, did some math, talked to a few other people, and I've decided I'm gonna go for it.
I figure that it should be okay unless I am running 4 loads of 4 ohms each, cranked up all the way. So, since I am planning on using 8 ohm loads and I am not looking to give myself hearing damage, I think it will be just fine. The tranny will be fine. The heatsinks are still a minor concern. If they get too hot, I have a couple of big (50cfm) fans to blow the sink.
I had oversized sinks, psu caps and transformer. All I need to buy is a few caps and output and input jacks. It's too cheap to resist.
I'll let you guys know how it turns out...
Using the +/-30V curve I see about 25W max at 8 ohms and maybe 48W into 4 ohms (I think I read from the +/- 35V curve before).seangoesbonk said:
It'll work. Go for it.
Re: Gonna go for it...
Cool, i'll look foward to it
What schematic are you using by the way?
seangoesbonk said:
Cool, i'll look foward to it
What schematic are you using by the way?
Re: Re: Gonna go for it...
If you're interested, the values I'm using are:
Rb and Ri = 1K
Rin and Rf = 21K
Rm = 28K
Cs = 22uF and 100nF in parallel
Ci = 47uF
Cm = 100uF
Nothing too fancy. I'll see how I like it, and make changes if I feel the need.
I should have this done by the end of next weekend.(I'm waiting for one last part)
I'm just using the "Typical Application" (fig.1) from the datasheet, slightly modified to eliminate the mute switch.Optical said:
If you're interested, the values I'm using are:
Rb and Ri = 1K
Rin and Rf = 21K
Rm = 28K
Cs = 22uF and 100nF in parallel
Ci = 47uF
Cm = 100uF
Nothing too fancy. I'll see how I like it, and make changes if I feel the need.
I should have this done by the end of next weekend.(I'm waiting for one last part)
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