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Chip-Audiophile 7th May 2008 07:29 AM

Transformers for LM3875 Monoblocks
I have built one of's LM3875 Premium Kits into 2 x monoblock amps. I plan to run 1 x transformer and rectifier board for each amplifier. I was planning to use 2 x 300VA 25-0-25 toroidals [one for each monoblock] but think that I might use 300VA 18-0-18 instead. I will be using the amps to drive my Sonique Slim 66 speakers which are 6 ohm so I thought that 18-0-18 would be more suitable. I also have some B&W 603 S2 which are 8 ohm that I might use too.

Basically I want the amplifier to be a fexible as poosible for various speakers [they will be used primarily with the Sonique's though] and also wondered weather 300VA per channel is overkill. Should I just go for 160VA? I want to make the monoblocks the best that they can be and 300VA toroids are not really much more expensive than the 160VA ones. And should I go for an 18-0-18 or 25-0-25 transformer? Thanks

danielwritesbac 7th May 2008 07:40 AM

Re: Transformers for LM3875 Monoblocks
Do you have available a 20 + 20 or a 21 + 21 transformer with dual secondaries?

Chip-Audiophile 7th May 2008 07:58 AM

The 18-0-18 and 25-0-25 seem to be more common. I could possibly source 20-0-20 or 21-0-21 or have some custom ones made.

danielwritesbac 7th May 2008 08:22 AM

2 Attachment(s)
So, the transformers that you described earlier are perfect for either 4 ohm speakers or 8 ohm speakers, but not both.

See attached National Semiconductor Overture Design Guide spreadsheet.

Nuuk 7th May 2008 08:38 AM

Chip, from my own experience, the 18 volt transformers do work much better with 6 ohm speakers than the 25 volt versions.

You can get away with 80VA per channel as a minimum but more power won't hurt. In fact, for reasons I can't go into here, a 300 VA transformer (in a typical set-up) is better than a 160 VA for reasons not related to power output. :att'n:

Nordic 7th May 2008 09:07 AM

Just to throw some cents in here... I use a 250VA 24V (pretty common voltage) trany to power a chipamp with 8 ohm drivers... I would say anything larger is likely overkill. For monoblocks 160VA should do it...

AndrewT 7th May 2008 09:31 AM

18Vac or so is ideal for a 40hm speaker load.
25Vac or so for 8ohm.
For 6ohm somewhere in between would get maximum reliable power.

Checkout the B&W. Although you say 8ohm they may be 4 to 8ohm, which would require the amp to be designed for the 4ohm part of the load.

For a single channel monoblock the minimum is 60VA for 60W output.
I and many others recommend around VA=1.5Times output power.
But transformers with this low a VA also have a high regulation.
To avoid some of the down side of poor regulation specify a higher VA.
Thus instead of 90VA use 150VA.
Above 150VA, the potential gains are quite small, but could be audible. I suggest 300VA as an absolute max for 60W.
applying the 1.5Times rule would indicate that 300VA is suitable for around 200W of output power. Many will rubbish the suggestion that 300VA is useful for a 60W ClassAB amp. There's only one way to find out. Try it.

Nuuk 7th May 2008 09:42 AM

A couple more thoughts.

I find dual mono produces a clear improvement in sound quality so always recommend two transformers over one.

If you go with 300VA, which I agree with Andrew is the sensible maximum for a chip amp, and put both in the same enclosure then switch them both on and off independently, ie with a power switch for each transformer. That way you can get away without a soft-start.

And, DON'T put them both in the same enclosure if it is made of metal. Don't ask me why not, I don't know what happens but the sound is better when you don't. Perhaps the magnetic fields are somehow 'shorted' by the metal case, I really don't know but I can hear the difference even removing the metal lid of a metal case.

Chip-Audiophile 8th May 2008 07:39 AM

I plan on making 4 x separate enclosures for the monoblocks. Two for each channel - one housing the toroidal, rectifier and filter caps and the other housing the amp, heatsink etc.

So it sounds like 160VA would probably be plenty however the 300VA is only about $20 extra per toroidal...

It also sounds as though neither the 18v or 25v is going to be the most flexible... I probably need 21V or 22V secondaries for the best of both worlds... Would 25V [35V rectified] be too much for a 6 ohm load even if the heatsinks were large enough? The Sonique speakers may dip down to 4 ohm too... something to keep in mind.

AndrewT 8th May 2008 08:05 AM

Hi Chip,
the complexities of separating the PSU from the amplifier are not easy for a newbie to comprehend.
This proposed separation does not help overcoming the " two amplifiers in a single box grounding" problem.
I recommend that each monoblock be stand alone with PSU + Amplifier combined for your first foray into amp building.

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