Need advice to chose best toroidal transformer for LM3875 monoblock?

[Norris Wilson]

Hi everyone,


I am in need of advice for finding the best possible toroidal transformer to drive a monoblock LM 3875 chip amplifier from Scott Nixon? I plan on using the transformer inside the same chassis as the amplifier module with a shield between the two. So, I feel that the transformer with the best control of stray magnetic feilds and that has the least mechanical vibrations would reduce my efforts to control these problems.



I have looked at Avel Lindberg and Plitron products, but would like to know if there is a difference in quality between these companies products? I also would like to hear any suggestions of other transformer companies products that are equal or superior to the ones mentioned, and possible URL links to those companies?



Is there any advantages to using a larger rated power supply transformer for the monoblock LM3875 amplifier than needed, like a 22V - 22V 250 VA, versus a 160 VA unit?



And my final question is about the Panasonic Pureism electrolytic capacitor listed on the diyclub.biz website. Has anyone here used them, and what are your experiences when using these capacitors for audio purposes, good, bad, excellent, where in the circuit do they work the best?

http://www.diyclub.biz/catalog/default.php?cPath=154_80



Thank you to all who respond.

Norris Wilson

[BrianGT]

I have been getting my transformers from Victoria Magnetics and have been quite happy with their performance.

--
Brian

[MWP]

For choosing transformer size, a quick rule is take your max combined power output (for stereo, 50W per channel = 100W), and double it (so 200VA).

[la9]

Not a torroidal but what I've been using is 2 radio shack 25.2 volt transformers. They are $10 a piece and I hook them together to get a +-25 volts at 4amps for $20. Part #273-1512.

Hope this helps.

[falcott]

I have recently bought 2 torroidals from Farnell for my valve buffered GC project: a 300va 2x25v and a 30va 2x25v (part nos. 433-5351 and 433-5041 respectively). Both have 230v single primaries. I haven't checked the 300va yet, but today I fired up the 30va. Rather than 25v, I get 29v (from 235v ac). From the rectifier I am getting 41v dc, which is okay for the valve anyway. But this is hardly the 1% tolerance that Farnell advertise.

[squadra]

Originally posted by falcott
Rather than 25v, I get 29v (from 235v ac). From the rectifier I am getting 41v dc, which is okay for the valve anyway. But this is hardly the 1% tolerance that Farnell advertise.

Without load the output voltage normally is higher then under load.
The difference is lower for higher VA torroids.

The 1% is probably measured under load.

Pj

[falcott]

Originally posted by squadra
Without load the output voltage normally is higher then under load.

Oh yeah, that's right... duhh
Anyway, I'm just happy that my rectifier is working!