I have a subwoofer amp built with a LM3886, and currently power it from a 48V @ 1-2A (I'm not sure, but think it is closer to 1A) transformer, and I just got 2 transfomers with 12V output @ max 10A. If I were to connect these together, to get a little over probably +-15 (after rectifier), would it provide more or less power. I know that datasheet says that the higher voltage, the higher wattage, but I have always been told that Amperage affects the output power.
Thanks, Mike
Thanks, Mike
this is what's refered to as voltage or current limiting. you usually reach one limit before the other. basically you can make amps that could put out a ton of current, but cannot do so because to do so the voltage across the load would have to be higher.
so with the 10A +-15V supply you get less power. the transformer is basically just much more beefy then required.
so with the 10A +-15V supply you get less power. the transformer is basically just much more beefy then required.
So even there is only +-15V @ 10A, I would get less than +-24V @ 1A? I have been told that 1A limits the output power to 4W only, and also that this is a cause for distortion. Since it is in a subwoofer, I need the least distortion as possible. How much power would the +-15 supply put out?
Thanks, Mike
Thanks, Mike
with a lower supply voltage the gain should be reduced (by reducing the value of the feedback resistor) -- yes, you will get less power, that's the tradeoff. If you don't reduce the gain the amplifier will clip.
I looked at the datasheet, and was wondering if the output wattage change is linear with the voltage change. Anotherwords, if at +-28V, you can get 68W into 4-ohm, wouldn't +-15V give 34W into 4-ohms?
Also, I bought these transformers originally for a dual LM1875 amp. I think the 1875s need less power (need to check the datasheet) so would I be able to get the full 20W into each speaker with these?
Thanks, Mike
Also, I bought these transformers originally for a dual LM1875 amp. I think the 1875s need less power (need to check the datasheet) so would I be able to get the full 20W into each speaker with these?
Thanks, Mike
Mike you need the voltage as well as the current capabilities in order to acheive maximum output with minimal distortion....I have my GC on 26 volt rails with a 8 amp transformer and have found it to be the best scenario for the 3886 chip so far
check out the calculator on the Nat Semi website. this will reveal all:
http://www.national.com/appinfo/audio/files/Overture_Design_Guide13.xls
http://www.national.com/appinfo/audio/files/Overture_Design_Guide13.xls
Mike,
Many of these things are inetrrelated. It is true that 1A across a 4 Ohms speaker represents 4 Watts. But, this 1A is not required continuously. You have in the power supply reservoir capacitors charged to the supply voltage, and these can output high puls currents when necessary, even when the xformer cannot. So, even if the average available is 1A, if the music only requires that 1/5th of the time in peaks you have effectively 5A available.
Now lets look at the voltage. You have 24V. Say that the amp can deliver 20V peak (there's always a loss) that would, in the peaks, draw 5 Amps through the 4 Ohms. So, that seems OK. These 5A corresponds to 100W peak in those same 4Ohms. Trouble is, people mix up continuous power (which is 4W in this example) and average power (which would be 100W, in this example). Average power is often surprisingly low, sometimes not more than a few watts. Still, if you drive the subwoofer hard, 1A is a bit meager.
If you go to the 15V, even if the amp could output 12 V, across the 4Ohms, it would still draw not more than 12/4=3A. So, your peak level would be less (36W). It's nice for the xformer to have a 10A reserve, but you would never be able to use it. To draw 10Amps through 4 Ohms, you would need a signal voltage of 40V.
So, what to do? Are you experiencing distortion from the sub? If not, you probably are OK. If you want to be sure, get another 2x24V xformer, and make two rectifiers, each with its own xformer+bridge+caps etc, one +24, the other -24.
And yes, all you experts out there, there's more to it. Peak currents & watts should be translated into RMS etc. But as a first order COMPARISON, I think Mike should get a feel for it.
Jan Didden
Many of these things are inetrrelated. It is true that 1A across a 4 Ohms speaker represents 4 Watts. But, this 1A is not required continuously. You have in the power supply reservoir capacitors charged to the supply voltage, and these can output high puls currents when necessary, even when the xformer cannot. So, even if the average available is 1A, if the music only requires that 1/5th of the time in peaks you have effectively 5A available.
Now lets look at the voltage. You have 24V. Say that the amp can deliver 20V peak (there's always a loss) that would, in the peaks, draw 5 Amps through the 4 Ohms. So, that seems OK. These 5A corresponds to 100W peak in those same 4Ohms. Trouble is, people mix up continuous power (which is 4W in this example) and average power (which would be 100W, in this example). Average power is often surprisingly low, sometimes not more than a few watts. Still, if you drive the subwoofer hard, 1A is a bit meager.
If you go to the 15V, even if the amp could output 12 V, across the 4Ohms, it would still draw not more than 12/4=3A. So, your peak level would be less (36W). It's nice for the xformer to have a 10A reserve, but you would never be able to use it. To draw 10Amps through 4 Ohms, you would need a signal voltage of 40V.
So, what to do? Are you experiencing distortion from the sub? If not, you probably are OK. If you want to be sure, get another 2x24V xformer, and make two rectifiers, each with its own xformer+bridge+caps etc, one +24, the other -24.
And yes, all you experts out there, there's more to it. Peak currents & watts should be translated into RMS etc. But as a first order COMPARISON, I think Mike should get a feel for it.
Jan Didden
Thanks, Jan, for the explanation, and yes, the reason I asked is because I am experiencing distortion from my subwoofer. And, yes, I do drive it hard usually. So, how would I get 24V using another transformer and another rectifier? Do you mean connect the 2 outputs together to get 24V at twice the amperage and use the point where the two wires come together as one output and the center tap as another?
Yes, it is 48VCT transformer, which is why I was a bit confused about getting 24V off the entire thing, no just one side
I measured the voltages out with my DMM, and what I was told by the company I bought them from was wrong. It really puts out 15VAC and 8VAC. So, I then thought to tie the outputs together to get 22VAC on each side. After running through the rectifier it brings the power down to about 21VDC, then through the caps it brings it up to about +-23.5, for a total supply voltage of 47VDC. It now sounds consiterably louder and has less distortion than before. I hope that it will sound even better when I get larger caps too. Right now, all that I had two of were 1,000 uf caps, s I had to use those. I have heard that getting larger caps should increase the sound quaity greatly.
Oh, another thing, is it possible that the power cord connecting to the AC outlet be limiting the actual amperage the transfomer can put out? The case it is in was originally from an EQ, so the power cord isn't what would be called heavy duty. Or, could the wires connecting from the secondary to the rectifier to the caps and to the amp be another cause? they are not very thick.
How can I measure the amperage out of the rectifer with my DMM. I remember trying this before on a 1A supply and got a nice spark and a 12A rating, which I know is wrong. How can I test the actual amperage out of the supply?
Thanks, Mike
Oh, another thing, is it possible that the power cord connecting to the AC outlet be limiting the actual amperage the transfomer can put out? The case it is in was originally from an EQ, so the power cord isn't what would be called heavy duty. Or, could the wires connecting from the secondary to the rectifier to the caps and to the amp be another cause? they are not very thick.
How can I measure the amperage out of the rectifer with my DMM. I remember trying this before on a 1A supply and got a nice spark and a 12A rating, which I know is wrong. How can I test the actual amperage out of the supply?
Thanks, Mike
Well I finally really listened to it, sounds much better than before. I noticed a large problem, though, is that since the 3886 heatsink touches the metal bottom plate, I got a nice spark when it made contact with the metal casing of my dvd player.
But anyway, any help with the questions in my last post would be appreciated.
Thanks, Mike
But anyway, any help with the questions in my last post would be appreciated.
Thanks, Mike
etrmz...you could measure the current drawn by your amp...but measuring the total current produced by the trafo..I'm not too sure...I think u have to put a shunt or something...
li_gangyi,
Any ideas on the power cord being a limitation? Or can any power cable produce enough power to run this? And will getting larger caps for thr power supply really help as I think it will?
Any ideas on the power cord being a limitation? Or can any power cable produce enough power to run this? And will getting larger caps for thr power supply really help as I think it will?
power cord...a cable that can take 13A is more than enough...I guess with bigger caps...it will give tighter bass...try it out and hear for yourself...chip amps don't follow the basic rules of a "normal" amp whereby more caps means better sound or something...I'm not sure what would happen..I have tried 6800uF...the bass is tighter and highs are more muddied...
Well, I decided to try and see if the distortion is the amp, so I took a onkyo 2 channel amp and connected the speaker to the output, and the crossover to the input. I tried it, and the same thing happens: distortion at the same volume. So, I now know that the distortion now is caused by the speaker. But one more thing, I have asked about popping sounds in the speaker with loud volumes. I don't really get them that much anymore, and they are nowhere near as loud as before but I do sometimes. Would it help to reduce the gain of the amp?
Thanks, Mike
Thanks, Mike
Oh, and how again would I be able to test the amperage out of the supply? I can disconnect the amp. Where do I connect the amps meter?
Thanks again, Mike
Thanks again, Mike
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