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Naim style rectifier: why?

Avon said:
Could you please explain which of these four circuits you’ve built and compared?
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Naim uses one transformer in the olive NAP250, and the transformer has 4 secondary windings: the first of my diagrams shows that configuration. I compared that to my second drawing.

Avon said:
So you do need two transformers here then?
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As you requested, claiming it would be the same as two of my second circuits - but it would work with one transformer as shown in "A".
 
Thanks for the redraw in post #58. Splitting the two halves of different circuits seems like an odd thing to do as I don’t see how that’s different from not splitting them. Again though, it’s your call.

I do intend to build a power-amp front end at some point. Given the effort you put into optimising the grounding scheme of your power-amp, your findings are enough to make me look for a transformer with two, centre-tapped secondaries. Or even to use two transformers, depending on availability. That is, to build the first circuit rather than the second. Many thanks for regreasing a squeaky cog.
 
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I find this subject interesting, but it always remains unresolved, so I’ll attempt to summarise. The Naim rectification scheme is unusual as it involves using two centre-tapped windings on the one transformer (see post #1). It provides either (1) a single +V/0V/-V output to a power-amp, or (2) two +V/0V outputs to (say) a Hicap. You could vary that scheme of course e.g. four centre taps to double the number of outputs, the windings could be on different transformers, or a single centre-tap could produce a single +V/0V supply. EDIT: This is classed as full wave rectification.

I remember asking Naim why they did this, and was told bluntly “it’s a secret”. Various comments on this thread seem to imply that Naim didn’t know what they were doing. Martin Clark’s plausible answer is that it makes no difference, but only in so far as it doesn’t “waste” half the power of the transformer. A few people do say it’s better sounding, but they’re rather unusual and it’s obviously more expensive to implement. In consequence, I wrote it off as unimportant, long ago.

As with many things hi-fi, any advantage (probably) cannot be measured and therefore it’s unproven. If it reduces noise though, which seems likely, then that could be measured. It’s a matter therefore of believing someone else’s ears before deciding to build. As a result of this thread, I’m now prepared to try it next time I do. Whether I try doing an A/B comparison is another matter.
 
In my diy dual 24v Naim PSU, moving from a single transformer and full wave rectification to adding a 2nd transformer and going half wave was a major improvement - no idea why, but as above, I also suspect it’s the removal of 2 diode junctions.
 
Note that the Naim scheme counts as "full wave rectification" though: https://en.wikipedia.org/wiki/Rectifier.

From the link above "For single-phase AC, if the transformer is center-tapped, then two diodes back-to-back (cathode-to-cathode or anode-to-anode, depending on output polarity required) can form a full-wave rectifier. Twice as many turns are required on the transformer secondary to obtain the same output voltage than for a bridge rectifier, but the power rating is unchanged."
 
Thanks for the summary! So there is no difference in regard to 100Hz vs 50Hz: both circuits are full wave and thus 100Hz. And there is no waste of transformer VA: Martin Clarke explained this well here. Here some more answers about possible advantages of the Naim style rectifier:
And here some posts about JV/Naim on this topic:
Avon said:
Thanks for the redraw in post #58. Splitting the two halves of different circuits seems like an odd thing to do as I don’t see how that’s different from not splitting them. Again though, it’s your call.
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That splitting wasn't intentional - it's a result of your request to redraw the circuit - could that bury another answer...?
 
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Hi All,

Martin's post makes sense to me, it's the iron in the core that sets the "VA" capability of the transformer. At the end of the day its irrelevant if you use a center tapped winding (at twice the voltage) with two halve wave rectifiers or a single full wave rectifier with no center tap.

I have my own understanding on why naim have done it this way. Procurement & supply chain plays a much bigger role in product development than many outside the industry can appreciate. Custom components are exceptionally expensive and require huge volumes before the production of custom parts can become commercially viable. It's always more cost efficient to use off the shelf parts that have a reliable and repeatable source. I’m guessing that the design may have been driven by the pricing and availability rather than any technical merit. Talema, who are naim’s main transformer supplier were probably able to supply a transformer at a better price when configured as naim use it in this circuit. Also consider that naim share several transformers across different designs in their product range. Doing this allows the procurement team to negotiate a better piece price as the total purchased quantity on a particular item will be more.

In a medium sized enterprise like naim, supply chain and commercial factors have to drive engineering decisions otherwise the company could not survive. A good engineer knows how to work within these commercial boundaries and still design a product that maximises the resources available. At the end of the day naim may have done it this way based purely on a commercial decision. Anybody can build a good amplifier, it takes skill and knowledge to build a good amplifier on mass with consistent quality, within a budget and on schedule.


LPSpinner

EDIT:
PS: In addition to the above thread, to those who want a better grip in the technical aspects, have a read of Linear Power Supply Design (sound-au.com).

Elliott Sound pages is good read and source of sensible information regards to DIY Audio. This site is a good starting point for those who are interested in such things. Warning though, he doesn't do "audiofoo" and comes at it from an engineers perspective. He also debunks a few audiophile scared cows, you have been warned !!!
 
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But I can't imagine how twice as much copper on the secondary would cost less even in larger batches. Also supply chain decades ago played not the same role as today. BTW before Talema Holden+Fisher was the supplier of the transformers.

Regarding Naim not knowing what they did: there are several quotes on JV that seem to indicate they knew exactly what they were doing. Generally: looking at old solutions from a modern point of view without understanding them completely doesn't prove old wisdom to be stupid.

And last not least note that all the forum members who actually compared the circuits reported a significantly better sound with the Naim arrangement.
 
I thought he prefers the dual bridge approach - isn't this what Naim did in the NAP250, using two bridge rectifier devices, wired as shown in the very first diagram of this thread? But I might have misunderstand him.
 
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Hello! Joining in here - I have experimented with this and here are some ideas. Consider that the transformer is not an ideal component that behaves like the conceptual representation on as seen in a schematic symbol. You have capacitance's, inductance's mutual couplings etc. a true equivalent schematic representation of a transformer would look downright ugly! Anyhow - with a typical bridge the current flow path of the primary:secondary capacitance for instance will be different as compared to the "Naim style" (or old tube amp) where you have a center tap "constant". If you float the chassis (cut the ground to power connector) and measure the ac volts in the chassis and ground (or the current flow to ground) you will note differences. A bit TLDR but my point is there is a lot going on and I think old Julian figured (some) stuff out he did not shout out - but that was clever, sounded good and was based in science.
 
On DIY audio there is a thread "dual-bridge-rectifiers-in-psu-why." (can't seem to post url) this is "Nelson Pass" variant of using two rectifiers - this has advantages too - Though not the same as with the Naim style topology.
 
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