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Old 21st July 2013, 06:05 PM   #1
davym is offline davym  Scotland
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Default Transformer voltage for lm3886 kit

Hi, i'm wondering if this 115/230v 300va 0 25v 0 25v transformer would be ok for this kit when running 6 ohm speakers, or is it to high a voltage?

Toroidal Transformer 300va 0 25v 0 25v

The next down is quite a drop to dual 18v which I feel may be too low a voltage.

I'm also wondering about a suitable volume pot value, would 20k do?

The source will be Musical Fidelity's V-DAC & V-LPS which have low output impedance, the v-dac measures max 78 ohms and 2.08v max output according to a Stereophile review.

Cheers

Davy
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Old 21st July 2013, 06:31 PM   #2
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I would use that 25vac transformer with not problem. and for the pot I would use a 10K TKD potentiometer http://www.partsconnexion.com/tkd_73334.html
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Last edited by lanchile; 21st July 2013 at 06:35 PM.
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Old 21st July 2013, 06:39 PM   #3
Mooly is offline Mooly  United Kingdom
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The big problem with any chip amp is getting the heat away from the device. 25/0/25 sounds OK on the face of it but if the speakers are demanding current wise then the 18V may be a safer option. The voltages will be a bit higher anyway when the regulation figure of the tranny is factored in.

There's no right and wrong answer for this one. If you really need the higher voltage swing then use the 25V, if you don't then use the 18V

Try this first,
A Test. How much Voltage (power) do your speakers need?

A 10K or 20K pot is fine in practice.
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Old 21st July 2013, 07:00 PM   #4
davym is offline davym  Scotland
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Cheers Mooly! The speakers are Rega R3's mini floorstanders which are quite easy to drive to a high volume without a lot of power. My main concern is that the amp gets enough voltage to produce a decent bass, I hate soggy bass and one of the reasons I chose this kit was I read it had a nice tight bass. If an 18v transformer will make no difference to a 25v one in this regard, then i'd be as well with the 18v one. I'd value your opinion on this because i'm about to buy the lm3886 kit.

Davy
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Old 21st July 2013, 07:20 PM   #5
davym is offline davym  Scotland
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An interesting test! I once tried the Rega's on an Icon Audio valve headphone amp with a 3 wpc speaker output. I was amazed how loud it went but it had no bass power or control. Nice mid and highs though if a little bit veiled.

The transformers i'm looking at are rated for 230v, I get 245v as a norm so I suppose the 2x18v might reach 2x20v at times.

Think i'll go for the 2x18 to be safe.

Last edited by davym; 21st July 2013 at 07:24 PM.
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Old 21st July 2013, 07:21 PM   #6
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http://chipamp.com/docs/lm3886-manual.pdf
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Old 21st July 2013, 07:26 PM   #7
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if I "had" to build a chip amp for me, I would build an LM3875 chip amp!
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Old 21st July 2013, 07:30 PM   #8
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Quote:
Originally Posted by davym View Post
An interesting test! I once tried the Rega's on an Icon Audio valve headphone amp with a 3 wpc speaker output. I was amazed how loud it went but it had no bass power or control. Nice mid and highs though if a little bit veiled.

The transformers i'm looking at are rated for 230v, I get 245v as a norm so I suppose the 2x18v might reach 2x20v at times.

Think i'll go for the 2x18 to be safe.
2x18vac will be 2x25.2vdc after rectifier
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Old 21st July 2013, 07:31 PM   #9
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by davym View Post
An interesting test! I once tried the Rega's on an Icon Audio valve headphone amp with a 3 wpc speaker output. I was amazed how loud it went but it had no bass power or control. Nice mid and highs though if a little bit veiled.

The transformers i'm looking at are rated for 230v, I get 245v as a norm so I suppose the 2x18v might reach 2x20v at times.

Think i'll go for the 2x18 to be safe.
Yes, the secondary voltage will be higher by that primary voltage percentage difference and you must also add the regulation figure percentage to the secondary voltage as well. That will be in the data sheet and probably around 6%. That's the rise in voltage at light loading.

The chip amp voltage shouldn't really affect the bass as such provided the output isn't near clipping. I would do that test, its a real eye (ear ) opener and you might be surprised at the result.
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Old 21st July 2013, 07:52 PM   #10
davym is offline davym  Scotland
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Cheers folks that's settled it, 2x18v should do fine.
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