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Old 21st June 2009, 01:18 AM   #11
kscharf is offline kscharf  United States
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Quote:
Originally posted by ruijorgemoreno
That was the answer I was waiting for!
thank you so much!
your answer is what I though I would hear in the first place..

So, even if possible, it's more pratical and reliable to have chips in parallel to achieve this kind of ratings, right?

And, having two 3886 in parallel, how much amps are needed from the power supply? How can you tell? I was told 2-3A each chip is enough...but I don't know...

And by the way, (example: 3886 @ 30V) the 3886 vs 3876. I noticed the difference between them (when in parallel) is not that much.. (about 16W) which one should we choose in this case?

I'm sorry for making so many questions, I really want this thing to be as good as I can make it... Plus, I'm very curious!

Thanks!
RJ.
Putting chips in parallel will make the amp more reliable as the chips share the load. But you won't get more power if the amp is voltage limited, as is the case with at 8 ohms. Note that in a parallel configuration a series resister is required in the output of each amp to help equalize the load (about .1 to .5 ohms depending on the number of amps in parallel). Also the gain setting resistors need to be .1% types to keep all amps very close together in gain.
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Old 21st June 2009, 12:23 PM   #12
AndrewT is offline AndrewT  Scotland
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the TF is designed for cheap assembly.
If you require high power and particularly high current for low impedance loads then the "F" is a severe restriction to keeping the chip cool.

Are your speakers really 6ohm?
These are often mis-specified 4 to 8ohm speakers that have a 4ohm bass/mid driver combined with an 8ohm treble driver via an appropriate crossover.
If your speaker is a 4 to 8ohm type then you must design the chipamp to drive 4ohm loading.

The maximum power from a 3886 chipamp driving 8r0 should be limited ~65W.
If you try to design to 60W into 4r0 you will find that the chipamp is very near it's current limit when cold. As the chip gets warmer or dare I say "hotter" the current limit drops and the chip will enter protection mode trying to drive a reactive 4ohm load.
I recommend that for 4ohm reactive speaker loading you aim for a maximum power output of <=40W into 4r0.
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Old 21st June 2009, 12:26 PM   #13
AndrewT is offline AndrewT  Scotland
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post9 shows a table of powers and impedances.
please note these impedances are resistive loads only.
National specify resistive loads for all their data. It makes their amps look good on paper.
National specify 25degC for the chip temperature for all their data. It makes their amps look good on paper.
You MUST de-rate for reactive loads and for higher temperatures.
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Old 21st June 2009, 07:26 PM   #14
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Yes, my speakers ARE 6ohm each.

Each speaker or enclousure if you like has a tweeter and a woofer..
Each woofer has 6 ohm. each tweeter has 6 ohm. Really!

So my idea is to make (considering only one side) a 90-100W for the woofer and a 40-50W fot the tweeter, using lm3886's to the woofers and LM3876's for the tweeters..

For example: Supply voltage: 30 V
each tweeter (6 ohm): LM3876 (single) - 51W
each woofer (6 ohm): LM3886 (2 parallel ) - 116W
'T' packages, of course!!!

Is this good or would you advise me to do it in another way?

Thank you!
RJ.
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Old 22nd June 2009, 03:17 AM   #15
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I would suggest you check out the sensitivity of the woofer and tweeter, and decide how loud you want it to go. 50W to the tweeter is going to be way overpowering compared to 100W to the woofer.
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Old 22nd June 2009, 10:03 PM   #16
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Output Sound Pressure Level: 86dB/W

Is this what you mean?

RJ
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Old 24th June 2009, 01:02 AM   #17
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While reading and searching some stuff, I came to the conclusion that 100W is way too much for my woofer.. So is 50W for the tweet.. :s

Sorry people.. Anyway, I decided to go on a +/-60W for the bass and 25-30 for treble..

Well now I have another issue..
My bookshelf 2-way speakers have 2 inputs. high-freq and low-freq.
The high input has a capacitor to the tweet..

I need a 5KHz x-over!!!

My problem is:

Should I mount a passive crossover after the amp?
Ou sould I go onto an active crossover before the amp, using an amp per tweeter/woofer?

(forgetting if one takes more work than another)

What do you people think?

(I am soooo annoying sorry... )
Thanks!
RJ.
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Old 24th June 2009, 08:01 AM   #18
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If you don't want to use two power supplies, your amplifiers will have the same power for woofer and tweeter.

If your speakers already come with a cross-over, the easiest solution is to leave them as is, even if it is only a single capacitor. Should you decide to go for a new cross-over, the active solution is more promising. It results in an easier load for each amplifier channel, which is good for the sonic performance.
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Old 24th June 2009, 10:20 AM   #19
AndrewT is offline AndrewT  Scotland
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if your speakers already have two sets of input terminals then we call them bi-wireable.
The crossover has been split for the user to all two inputs to be used to drive the speakers.
In single wire mode the two sets of terminals are paralleled and both halves of the crossover receive an identical signal.
When one uses the bi-wire ability then one amplifier sends an identical signal into each pair of cables to the two halves of the crossover.

One can choose to bi-amplify these bi-wireable speakers.
Connect a pair of identical amplifiers each fed with an identical wideband signal to feed the two halves of the crossover with an identical wideband input.

The bass driver and the treble driver each with their own half of the crossover are designed to receive an identical input, that is why they can be used with a single amplifier with a single set of cables.

For a first effort at bi-amplifying, don't make it any more complicated.
Listen to the speaker with a pair of 50 to 60W chipamps, compare the speaker with a single 50 to 60W chipamp driving the paralleled terminals.
Tell us how the comparison sounds.
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Old 24th June 2009, 09:13 PM   #20
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Quote:
If you don't want to use two power supplies, your amplifiers will have the same power for woofer and tweeter.
Not necessarily! I can go with a 3886 for each woofer (about 60W) and a 4766 (stereo) for the tweeters (about 40 W).. Vcc=+-30V
I also read that this chip is kind of crappy but one can make a "derivation" from the power supply and add it a voltage regulator and so on...

Anyway, AndrewT, I think I'll go for your advice..
I'll order the parts and I'll get started asap.

Thank you very much people! You have been a great help for me!

RJ.
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