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Old 1st August 2008, 10:37 AM   #1
Atilla is offline Atilla  Norway
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Default LM3886 based amp- questions

So, I've decided to go for the dual-mono kit from chipamp.com. For one - because it's been a long time since I've played with creating and assemblin my own boards. Also, it turned out, it was quite a nice way to get all necessary bits and pieces, from financial point of view. So - a couple of questions, to make sure I've got things straight, while waiting for the kit to arrive.

1: I'm picking transformers right now. The best option is 120VA transformers from Farnell 2x25V output. They're really nice and on a discount. As far as I get it, for a dual-mono system two of those would be great. Is 25V too much though? I plan on using 8ohm speakers only, but who knows, I might be running 4ohms as well. The other option is quite low though - 18V output.

2: I've noticed on the schematics that there are no capacitors neither on the input, nor the output sinal lines. The suggested schematic from National Semiconductor provides caps on both. Is there any specific reason they're omitted in this schematic, I'm just wondering?

3: Rz and Cz is the Zobel network, correct?

4: Can I reduce the DC offset to zero? http://myweb.tiscali.co.uk/nuukspot/...ncloneFAQ.html suggests connecting leg 7 to the ground trough a resistor matching the feedback resistor used. However - that is for a different schematic and a different chip, so I don't think the same logic would apply.
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Old 1st August 2008, 01:13 PM   #2
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2x 25V is going to result in about +/- 35VDC at no load -- depending up on the amount of filter capacitance this will be lower as you load the power supply. I would say that this is quite a bit on the high side.

You can access the National Semi Design from their website. They put it back (after a brief absence) here :

http://www.national.com/appinfo/audi...gn_Guide15.xls
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Old 1st August 2008, 02:37 PM   #3
Nordic is offline Nordic  South Africa
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18VAC is perfect and at the top end for 4ohms... and still good for about 40W into 8ohms, i.e. plenty loud.
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Old 1st August 2008, 08:08 PM   #4
Keebz is offline Keebz  United States
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Hi, Thanks for posting this question. I just ordered the same kit and am also looking for suitable transformers.

I read the assembly pdf for this particular kit and it talks about using 160VA-330VA transformers, 2x18-22V secondaries, for stereo configurations, but I didn't see anything about dual mono.

It seems like smaller transformers would be fine with this kind of configuration(even for techno?), but do these transformers need dual secondaries?
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Old 2nd August 2008, 09:49 AM   #5
Dxvideo is offline Dxvideo  Turkey
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You will need two 2x18V tranformers for dual mono configutation. Also you will need two independent secondaries and two independent rectifiers for each toroids.
With 18v AC you will get ~24.5v DC after rectifying. That makes ~55W into 4R and ~28W into 8R. I think 160VA for each channel is overkill! May be 100 or 120VA will be enough..
For techno music; I dont know what loud level you like and what kind of speakers you have? But with high sensitive speakers (like Klipsch RF and RB series) this power levels are satisfactory.
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Old 2nd August 2008, 04:20 PM   #6
Atilla is offline Atilla  Norway
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I think 120VA should be more than enough, tha manual for the kit suggests about 80-100VA per channel. I still hope to find a middle ground - 20 to 22 V transformer, but the 25V ones are really much cheaper *sigh*.
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Old 2nd August 2008, 04:29 PM   #7
croat47 is offline croat47  United States
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Antek has a 50VA with two 9V secondaries. Running that series... the 50VA too low? For me, low level listening in a small room (12x16) on 2-way monitor types, 4 ohms.
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Old 3rd August 2008, 10:37 AM   #8
AndrewT is offline AndrewT  Scotland
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what's the minimum voltage a chipamp can operate from?
How low will the ripple take the DC down to when trying to reproduce sustained bass notes?
Will this bring in the undervoltage protection?
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Old 3rd August 2008, 10:57 AM   #9
Nordic is offline Nordic  South Africa
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The datasheet claims 20V minimum.

I remember running one off an 11VAC tranny once...

There is a note ;
"Supply under-voltage protection, not allowing internal
biasing to occur when |V+| + |V−| 12V, thus eliminating
turn-on and turn-off transients", so I reccon a bit north of that should be ok.
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Old 3rd August 2008, 11:12 AM   #10
Nuuk is offline Nuuk  United Kingdom
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I ran one off a 12v SMPS for quite a while. Of course, my speakers are very efficient!
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