chip amp build for tower speakers... LM1875 vs 3886... - diyAudio
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Old 25th October 2010, 03:44 AM   #1
wankski is offline wankski  Australia
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Default chip amp build for tower speakers... LM1875 vs 3886...

Hi Guys,

I'm thinking of purchasing the chipamp kits to build a stereo amp for some floor speakers...

they are these:AEG LB 4710 4-Wege-Bassreflex-Lautsprecherboxen: Amazon.de: Elektronik

manual does not state sensitivity in dB/W but they are marked 8ohm... My concern is, while i don't buy the 200W rating, they coudl require some gas as they do have 2x4.5" + 1 tweeter plus a big 6.5" woofer in each tower, I'm guessing hooked up in series...

Do you think the 20w RMS power output of the LM1875 chipamp will be sufficient to power these babies??? or would i be better off on something like a sherwood RX4109 like so: RX-4109 Stereo Receiver

the chip am site states the LM1875 is a lower power version of the LM3886, but while it mentioned 20-25w x 2 is possible from the 1875, it does not mention what the 3886 puts out? anyone know?

If you are limited to a 18v+18v transformer in either build, is there any difference to what the 3886 will put out? would i be better off in that case w/ a 36v supply with the 1875?

Also i take it you could connect the 1875 at low volume to 600ohm headphones without issue?

Also if you wanted to connect 2 pairs of regular speakers in parallel, could you do so with these amps? or would the small 1875 be so overtaxed with the towers i have that it would not be a great idea?

also, how would the overall sound quality compare to the average 'cheap' commercial stereo receiver like i sherwood i linked to?


thanks, i apologise for the noob questions..

cheers

joe
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Old 25th October 2010, 06:21 PM   #2
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I recommend not to waste your money on those speakers. You don't trust the power specs? Well, don't trust the frequency response specs either. You cannot get 20 Hz at audible levels from a 6,5" woofer.

Why don't you build your own speakers? There are interesting examples in the speaker section of the Forum. The Needle, if you want it slim, the Viech, if you want it loud and punchy. If your first link is an indication that you live in Germany, check out the Mivoc SB 25 JM speaker kit, which costs about the same as the speakers in your link and will sound much better.

Don't worry about the power requirements.

With the same transformer you will get nearly the same output from either chip amp. The LM3886 has the advantage that at a later time you can use a different transformer and get something around 60 W out of it.

Yes, you can connect 600 Ohm headphones to it. A protective resistor in series would be a good idea.

When you connect two speakers in parallel, the resulting impedance will be half that of a single speaker (assuming they are both the same). Make sure that the resulting impedance remains above the recommended 4 Ohm.

A well-done chip amp should easily sound better than the amplifier section of that receiver.
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Old 25th October 2010, 06:26 PM   #3
AndrewT is offline AndrewT  Scotland
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Have you bought the speakers yet?

The drivers are very unlikely to use a series connected crossover.
The more usual parallel connected crossover is quite easy to separate into separate filters.
Each filter/driver combination can be driven with it's own amplifier.

The two 4.5inch are probably paralleled 8ohm units providing a 4ohm load to the amplifier. If these were driven separately then two 8ohm rated amplifiers could drive the separated pair of 4.5inch drivers.
Again, a separate amplifier could be used to drive the woofer.

this eases the load on the each amplifier, so they pass on the signal with less distortion and if the loading is higher also with less heat wasted.
This type of passive bi-amplifying or tri-amplifying can give very significant gains in performance when the complex load is too demanding for a single channel amplifier.
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Old 26th October 2010, 10:39 AM   #4
wankski is offline wankski  Australia
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thanks guys, that really helps!

Just to be clear i already have the speakers...

pacificblue, nein, ich komme aus australien!

amazon.de is the only reference i could find...

I like the idea tho of taking them apart and feeding them on seperate amps...

the 6.5" woofers could probably take their own amp and the rest on another...

so the question is, should i build two parallel amps and do that (LM1875) or shoudl i go with a 50w single lm3886?

I kinda like the simplicity of the single stereo amp... less fuss

so what PS would i need for a 50w lm3886? the manual is a bit vague... i take it a 2x25v 300VA transformer ought to do the job?

I know about halving the impedance by throwing in two sets of speakers in parallel... that's why the towers are 8ohm and any bookshelves i get as a second pair would be 8 as well so the total drops to 4ohm... altho with the limited power available i think i'll hold off doing so.... but that is an advantage of the lm3886, its got a bit more room for additions i guess...

thanks!
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Old 26th October 2010, 06:06 PM   #5
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If the woofers are 8 Ohm, the single LM3886 is the better solution. Then 2x25 V would be the right transformer to get ~50 W into 8 Ohm. 300 VA would be enough for several amplification channels.
With a 4 Ohm load you could run into heatsinking problems with that supply voltage however. In that case you should rather use separate amplifier channels for the bookshelf speakers or use a transformer with 2x18 V. That will also give ~50 W, but this time into 4 Ohm.

You are aware that you can more or less forget about stereo imaging, when you use two pairs of speakers in parallel?
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Old 27th October 2010, 10:08 AM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by wankski View Post
.... but that is an advantage of the lm3886, its got a bit more room for additions i guess...
your guessing is misguided.
That's the problem with chipamps. They are severely current limited and cannot easily drive lower impedance loads.
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Old 27th October 2010, 12:39 PM   #7
wankski is offline wankski  Australia
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ok great, thanks guys! really helpful info there..

I will run just these 2 8ohm speakers and keep it at 8ohm....

for the purposes of only driving these two tower speakers (left and right), pacificblue, you seem to be saying that the 300VA powersupply is more than sufficient for this purpose?

is it ok to drop it down to a 2x25V 160VA transformer, or is this too low? the manual for the lower spec lm1875 specifies a 250VA trans (at 2x18v)...

sorry for the annoying questions, I just want to get these speakers running!

cheers!
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Old 27th October 2010, 12:48 PM   #8
AndrewT is offline AndrewT  Scotland
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recommended transformer size for good performance is VA equal to 1 to 2 times the total maximum output power.

Two channels of 50+50W can use a transformer rated from 100VA to 200VA.
The amplifier will work with a transformer outside this range, both bigger and smaller.
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Old 27th October 2010, 06:06 PM   #9
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Quote:
Originally Posted by wankski View Post
the manual for the lower spec lm1875 specifies a 250VA trans (at 2x18v)...
Which manual is that?
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Old 28th October 2010, 04:53 AM   #10
wankski is offline wankski  Australia
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sorry the instructions:

DIY Chip Amplifier Kits, PCB's, Components and Information.

clicking on the suggested trans link reveales 2x18v 250VA here:

Avel Y236651 250VA 18V+18V Toroidal Transformer | Parts-Express.com
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