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Looking for a transformer

Mike Hanson

Mike Hanson

Trying to understand...
I'm planning to upgrade a dual mono NCC300 amp to Voyager configuration. I'm using a 40-0-40VAC transformers for the output stages. Given I need to provide at least 5VDC more for the input stages (to accommodate the regulation), I'm looking for a transformer with dual secondaries (or equivalent centre tap) in the range of 44-50VAC. I don't expect much current to be drawn by the input stages, so 50-100VA would probably be fine. As for configuration, a toroid would probably do, but I'm open to suggestions.

Oh yeah, it's for North America, so the primary(ies) must be 115VAC.

I've checked around, and these specs are proving to be a challenge. Any recommendations for suppliers/models?
 
I’m sure tiger toroids or airlink in uk would build to custom spec, shipping might be the issue though. Are there any equivalent tx companies out there?
 
Not for cheap one-offs, no. The ones I've contact want to charge an exorbitant design fee, and even then they balk, because apparently it's a big deal to switch up their machinery to wind something strange. They prefer to work with customers who order hundreds of the same thing, not just two. :rolleyes:
 
You might want to consider an SMPS supply. There are excellent ones around, like the Connex one I use in my Quad 405, since its transformer started to hum again (I was on my second from Quad themselves).
 
I used Demeter Transformers, based in Essex somewhere. They charged £90 about 10 years ago for a valve amp trafo, forget the exact spec but we specifically worked out a spec (how else do you work out a specification apart from specifically?) for 240V ac when the manufacturer one had been 220 and had gone pop. It's been perfect. They sized it to fit, no problem. The design process is simple, after all. Anyone charging large sums for that is pulling your leg. A 110 primary is trivial, just double the ratio of turns cf. a 220. We all remember that from O level Physics.
 
You want 48V secondaries so two transformers with independent 24v secondaries, wired in series would be workable
 
chartz said:
You might want to consider an SMPS supply. There are excellent ones around, like the Connex one I use in my Quad 405, since its transformer started to hum again (I was on my second from Quad themselves).
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I've had good luck with SMPS supplies in some use cases. In this situation it's for the input stage on the board, and I need it to be as quiet and stable as possible. Therefore, I think I would avoid an SMPS here.
 
FYI, I've been offered something in a private message that would suit my needs perfectly. Hopefully that pans out.
 
Mike Hanson said:
That looked promising, but I don't think 30VA would be enough. When I looked for the bigger ones, they didn't have the required voltage. :(
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330mA ought to be ample? Typical NCC200 front is only about 12mA total, not sure about NCC300. In fact, IIRC I used a pair of 2.3VA 30-0-30 transformers for my modded NCC200 front end....
 
a.palfreyman said:
330mA ought to be ample? Typical NCC200 front is only about 12mA total, not sure about NCC300. In fact, IIRC I used a pair of 2.3VA 30-0-30 transformers for my modded NCC200 front end....
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You're probably right. I wasn't sure of the current requirements in this case, and I always tend to over-spec my components, just in case. If this private thing doesn't work out, then I may go for that after all.
 
Mike go for a tiger toroid. 0.5A primaries should be fine, the transformer won't be that heavy plus you'll get an audiophile quality transformer potted and screened. Current requirements are no more than 200 or 220 ie just over 10mA

When I ordered mine I just paid for the transformer then organised my own shipping through one of the online courier companies. Mine was an 800VA weighed nearly 10kg, 5 secondaries and cost £120. Shipping was about £90. That was a couple of years back probably a bit more now. I'm guessing maybe 60 for the transformer and 50 for courier?
 
a.palfreyman said:
This?
www.toroidal-transformer.com

2x 45V toroidal transformer 30VA

Specificaties : Vermogen : 30VA Primaire wikkeling : 2x 115V Secundaire wikkeling : 2x 45V - 0.33A Secundaire wikkelingen in serie geeft : 90V
www.toroidal-transformer.com www.toroidal-transformer.com
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I bought two of these (one for each channel), and fit them into my ongoing build. With 45VAC secondaries wired as 45-0-45, I expected to get a rectified and smoothed voltage of roughly ±70V (with my somewhat high mains), but I'm actually getting ±77VDC. Methinks it's not 45VAC as labelled. :rolleyes:

I'm probably OK with that extra voltage going into my input stage regulators (given the very low current). Regardless, I'm wondering whether it would be prudent to use a simple Zener regulation circuit to knock down some of that extra voltage, bringing it down to the 70V where I expected it originally. It's probably going to be a challenge to find a 70V Zener, though.
 
Easy; use 5-6 , 12v zeners in series. or even 10 x 6.9Vz.

There's a significant noise advantage to doing this.*
Drive them fairly rich, 8-10mA, constant-current, for best effect.
NB that also means the dissipation is divided across teh string.

You'll probably only need 5 at that sort of bias current. Or trim with 5.6-10v zener in last position of 6 to suit.

* a statisical thing: because the zeners noise contributions are random, the addition is RMS... so you'll only have about SQRT(5) x the noise of a single 12v zener at that current; or SQRT(10) = 3.162 x the voltage noise of a 6.9V** zener , for a 70-72VDC voltage reference - and /not/ the full noise of a single c. 70v zener (which will be considerably worse, and need to dissipate up towards ~1W.)

** It's right around this value, maybe a little lower, that treu Zener action happens, not avalanche (higher voltages): and that's both best regulation, and - least noise. 10 zeners at 10mA = 70mW each, buy 20, cost about a buck or two .. about as good as you'll get.

Use a dab of RC following c 470-1K/ 47-100uF, to drive the base/gate of your pass device - and the whole thing is beyond contention. Broadband output noise/ripple at c -100dB, maybe a bit more - easily achieved with good layout.
 
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