| unitgain |
Hi everyone! I have a question, please forgive me if it's stupid.
I have four LM3886s and I want to make a little amp to power a subwofer (one channel). The speaker is the Shiva, it has two voice coils 8 Ohm each, so I can run with the load impedance of 4, 8, or 16 Ohm.
However, the only power transformer I currently have is 2x50V DC after rectifying and filtering. This doesn't go very well together with 3886s! Unfortunately, power toroids are few and far between where I live. There's a little chance I'll find something more convenient.
I can do one of the following:
1. Regulate the excessive 8 to 10 volt away, run into 16 Ohm load (8 Ohm per side).
2. Parallel the secondaries to get 1x50V, run into 8 Ohm load (4 Ohm per side). Since it's a bridged configuration, I don't need the centre tap (I think). I can get the ground reference with two resistors.
3. Ditch the idea to use the chips.
4. Try to get a different trafo anyway :(
5. Anything else?
Also, will two chips be enough or do I need to use all four (in parallel)?
Any idea or advice will be greatly appreciated! |
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| analog_sa |
| I like 2. Seems like a good idea and at least i can't see any reason to shoot it down. |
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| leadbelly |
| quote: | Originally posted by unitgain
I can do one of the following:
1. Regulate the excessive 8 to 10 volt away, run into 16 Ohm load (8 Ohm per side).
2. Parallel the secondaries to get 1x50V, run into 8 Ohm load (4 Ohm per side). Since it's a bridged configuration, I don't need the centre tap (I think). I can get the ground reference with two resistors.
3. Ditch the idea to use the chips.
4. Try to get a different trafo anyway :(
5. Anything else?
Also, will two chips be enough or do I need to use all four (in parallel)?
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Well, FWIW, my opinion, in order of preference:
4
2
5: parallel the secondaries, build the single-supply circuit version in the National datasheet times 2, use a DRV134 to drive the inputs
As for whether 2 or 4 chips, only you can answer whether your transformer can power them and you are willing to source 0.1% resistors, etc. It is much more work, but the driver can handle the power. |
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| ve2lui |
A 3886 has a better performance at higher impedance.
I made much experimentation and comparision with
a tube amplifier and I learned much from this.
I would use a 16 ohms load, inverted amplifier.
You may get less than 0.00001% THD!
More, I would try a small value resistor in series at amp output, like one or two ohms.
How it sounds? It does not sound! It is a "perfect amplifier".
I am pretty sure that any difference, at such a perfect performance level is from
interaction of speakers with what is offered to them by the amplifier.
I have built a very precise tube amplifier and after tweaking to have the same "presentation" to the speakers, it sounded exactly
the same, using a switch to A / B compare. No one could see a difference. Without the "tweaking" there was one for sure! |
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| unitgain |
leadbelly,
I'm trying to source some toroids but without much luck.
The one that I have is 500VA which I think is enough to power four chips, and then some. As for 0.1% resistors I don't think I need them, because if I decide to use four chips I can run two independent channels, each to its own voice coil. So there's no need to parallel them. |
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| leadbelly |
| quote: | Originally posted by unitgain
The one that I have is 500VA which I think is enough to power four chips, and then some. As for 0.1% resistors I don't think I need them, because if I decide to use four chips I can run two independent channels, each to its own voice coil. So there's no need to parallel them. |
Actually, 500VA is only marginal for 4 chips since the general guideline is 150VA per chip, but you should still be OK.
Yes, when you first said parallel, I assumed you meant paralleling chips, and not paralleling amps.
You may just want to try yet another option: just build 2 of the single-supply version circuits, and run 1 to each voice coil. (1) low risk, sure to work (2) if you're not satisfied with the power, you can build 2 more of the same circuit, and put 2 DRV134's in front of these 4 chips and run them bridged, without having to undo the original 2 circuits |
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| daatkins |
Another alternative is to un-wind some of the turns from the secondary of the transformer - that would lower the voltage.
Nice one,
David. |
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| unitgain |
| quote: | Originally posted by leadbelly
Actually, 500VA is only marginal for 4 chips since the general guideline is 150VA per chip, but you should still be OK.
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Hm... This App Note says one 385VA transformer per channel in the BPA200 amp. It is my understanding that they mean a stereo amp and each trafo powers four chips, am I wrong? |
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| leadbelly |
| quote: | Originally posted by unitgain
Hm... This App Note says one 385VA transformer per channel in the BPA200 amp. It is my understanding that they mean a stereo amp and each trafo powers four chips, am I wrong? |
National's app notes are written for the audience of a designer developing a commercial product with typical economic constraints, in this case they are speaking of the example of meeting the spec of 200W into 8ohms for a commercial product, so they spec a 385VA xfrmr and 40000uF per rail. The GC experience on this forum is to go bigger on xfrmr and smaller on caps to get best performance. |
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| unitgain |
| quote: | Originally posted by daatkins
Another alternative is to un-wind some of the turns from the secondary of the |
David, have you done such a thing? I'm a bit afraid, the winding looks uniform and symmetric, what will happen if I break the symmetry? And what will happen with the rated power? I'll need to remove about 20% of the winding! |
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