[amb]

I am planning the prototype build for my β24 poweramp. Since it would be unwieldy to have separate PSU and amp cases for a large unit like this (and the high-currents involved would make an umbilical/connector difficult), it will be a single-chassis solution. My plan this time is to use a Hifi2000 Pesante Dissipante 4U 300mm case with integral heatsinks, 10mm front panel and milled handles:

Linky 1
Linky 2

The power transformers will be in relatively close proximity to the amp boards. So, I am looking for something better than raw toroids. A toroid built with shielding (and encapsulation if available) would be very desirable. I'm looking for a pair of 30V+30V 250VA+ units. I found the 630VA unit at diycable.com which looks interesting, but it's too big at 6.5" diameter. I'd prefer to use two smaller toroids to make the two channels truly independent, if possible.

Any suggestions where I could find something like this at reasonable cost?

 

[Pars]

Well, SumR, Plitron, maybe Victoria Magnetics, or Toroid Corp?. I know Plitron can do all these (shielding, etc.), and their 300VA transformers are 110mm, so about 4" in diameter. SumR 300VA was around 125mm IIRC.

As for reasonable... hehe, define reasonable? It won't match the $75 you were looking at on the 650VA (that is a nice price, btw).

 

[amb]

Quote:

Originally Posted by Pars /img/forum/go_quote.gif Well, SumR, Plitron, maybe Victoria Magnetics, or Toroid Corp?. I know Plitron can do all these (shielding, etc.), and their 300VA transformers are 110mm, so about 4" in diameter. SumR 300VA was around 125mm IIRC.

None of these lists shielding/encapculation in their standard production range with the needed specs. Getting a custom transformer made will be expensive. Looks like Plitron will add shielding at extra cost.

I think I am talking myself out of the twin-transformer idea. The diycables 630VA is looking better all the time, not only for its price, but the fact that it has extra secondaries for auxiliary circuits might come in handy. If I use only one of them it would fit the Pesante Dissipante case. The VA rating is high enough for both channels, and the β24 output stage has such high PSRR that a common supply for both channels should also be of no consequence...

 

[Ferrari]

amb, Schuro Germany has the toroidal transformers you are looking for:

V-RKT-MS-SW 225.30 (30V+30V, 250VA, encapsulated/shielded ), or
V-RKT-MS-SW 300.30 (30V+30V, 340VA, encapsulated/shielded )

BTW, Hifi-2000 Pesante Dissipante is an excellent choice for power amps.
I'm using one with 5U 400mm for my Super Leach power amp... very solid and beautiful enclosure.

 

[amb]

Thanks Ferrari. Schuro.de doesn't ship outside of Germany, but I think I found the perfect transformer. It's the Plitron 7391-B2-00 500VA unit. The nice thing about this one is that it has four 30VAC secondaries, so I could hook them up as two separate 30-0-30 circuits, one for each channel. This transformer is also encapsulated/potted/shielded. The price is a shocker at $307.48 Canadian, which at this time has roughly 1:1 exchange rate against US$. And that's before shipping. Gulp!

Edit: Just checked -- shipping to California is a whopping $54.43 CAD.

 

[Ferrari]

With four 30VAC secondaries, and only 166mm diameter.

Technically it's ideal for use in a Pesante Dissipante with effective inner dimensions of 360mm Width x 300mm Depth.
There is probably enough room left for everything else. Only the price is ...

One other thing that may need attention is the effective high of the Pesante Dissipante enclosure.
There are 2 brackets (how do we call that in english?) needed to mount the side heatsink to other parts of the enclosure. Since these 2 brackets are ~12mm each, the effective high (space) that can be used to mount the β24 amp board is ~24mm (2x12) less than the real high of the heatsink.
For a 4U enclosure, the effective high is thus 165 - 24 = 141mm. The picture below shows a 2U heatsink with brackets (the same are used on a 4U heatsink).

 

[amb]

Yes, I think there might still be enough room on the exposed portion of the 4U heatsink if I do a little creative lead-bending of the MOSFETs. Otherwise, I'll have to cut some notches into those brackets. I don't really want to go to a 5U-tall case if I could get away with a 4U one.

 

[Ferrari]

No doubt... a tinkerman of your calibre can make it works in a 4U one!

 

[n_maher]

Ti,

Back when I was fighting the hum issues in my first single ended beta22 I spoke with Avel-Lindberg about getting custom trafos made. Given the prices that you're finding for pre-made units I think that custom really may be the only option. I'm sure that there would be at least a few of us who would go in with you on a small group buy to try and help offset the prototyping costs so that you're not just ordering 1 or 2.

 

[luvdunhill]

Quote:

Originally Posted by amb /img/forum/go_quote.gif And that's before shipping. Gulp! Edit: Just checked -- shipping to California is a whopping $54.43 CAD.

don't forget that Plitron only uses UPS and you'll have to deal with customs/brokerage fees. At least I did when I bought $500 worth of toroids from them...

I talked to the guy at Victoria Magnetics and he suggested that I pot the transformers myself. The potting substance was dirt cheap and the only issue was finding an plastic or metal enclosure. These seemed easy enough to find.

 

[amb]

Nate, that's a good idea but I had already pulled the trigger on the Plitron transformer. Oh well, I never expected the prototype to be cheap...

When β24 reaches production phase, I think a GB of custom transformers might be a feasible thing to do.

luvdunhill, potting is a nicety but the more important thing I was looking for is magnetic shielding. The right way to do this is during manufacture and isn't easily "added on" afterwards. To quote Plitron about the Low Noise Power Transformers for Solid State Amplifiers:

Quote:

Magnetic Shielding: To attenuate stray magnetic fields. Multiple layers of grain oriented silicon steel laminated between insulation. Wrapped around circumference of transformer under case.

 

[Nebby]

Quote:

Originally Posted by n_maher /img/forum/go_quote.gif Ti, Back when I was fighting the hum issues in my first single ended beta22 I spoke with Avel-Lindberg about getting custom trafos made. Given the prices that you're finding for pre-made units I think that custom really may be the only option. I'm sure that there would be at least a few of us who would go in with you on a small group buy to try and help offset the prototyping costs so that you're not just ordering 1 or 2.

When the time comes I'd be in for one or two transformers also

I'm a fan of shielded transformers.

 

[steaxauce]

Quote:

Originally Posted by amb /img/forum/go_quote.gif I'd prefer to use two smaller toroids to make the two channels truly independent, if possible.

Ti,

I'm curious as to why you'd want to do this. Will it improve channel separation? I'm in the process of building a beta22, and if I end up building the balanced configuration and using multiple transformers and s22 boards, I planned on connecting the two transformers together before the s22 boards, so that they share the load of two boards instead of each supplying the current for one board, because the average of two s22's is a more consistent load than a single board, and the average voltage of two transformers is more consistent than that of one transformer. I was going to do the same thing between the s22 boards and the b22 boards. Would this be a bad idea?

 

[amb]

On the β24 and β22, using two transformers will probably not result in improved channel separation. On the β24, everything other than the output stage is tightly regulated; on the β22, everything is regulated. Moreover, both amps have very high PSRR, so any supply rail ripple as a result of one channel's current draw will not produce appreciable leakage into any other channel's output.

My reason for wanting separate transformers per channel is more for pedantic reasons (keeping the dual-mono theme). Also, I thought that using two smaller transformers might give me a little more flexibility with mounting within the case. But instead of doing that, I had since changed my mind and went with the Plitron 4-secondary transformer instead. This is almost the same as two transformers, and it will fit the target case nicely.

As for your thought about paralleling two transformers and then using them to feed two σ22s, I wouldn't recommend it. If you're using two transformers, then keep them separate and feed each σ22 independently.

Also, never parallel the outputs of two σ22s. One σ22 will "see" the output of the other, which will appear as a short circuit and the two will fight. You'll have some blown parts as a result.

 

[steaxauce]

Quote:

Originally Posted by amb /img/forum/go_quote.gif As for your thought about paralleling two transformers and then using them to feed two σ22s, I wouldn't recommend it. If you're using two transformers, then keep them separate and feed each σ22 independently. Also, never parallel the outputs of two σ22s. One σ22 will "see" the output of the other, which will appear as a short circuit and the two will fight. You'll have some blown parts as a result.

Oh, wow. Glad I asked. I didn't even think about the possibility of that happening, but it makes sense. As to why I shouldn't parallel two transformers, is this because the two won't necessarily draw the same current?