Season’s Greetings to all the AoS members, especially those who have been so supportive over the last few years.
Hi Marko,
I am no transformer specialist and the information I give here is based on discussions with specialists when I have been looking for exceptional performance for my projects. In particular Terry Monaghan from Canterbury Windings has been very helpful in my quest for a very high performance transformer for the SR5. Terry has been in the industry for a long time and specialises in Audio Grade transformers. His products are excellent.
You are right about size matters however there is a lot more to the SR5 power transformer than being oversize for the job.
Firstly, all the ‘off the shelf’ transformer ranges you can find in the component catalogues are built to a price in a very competitive market, primarily for industrial use. They are usually wound with a high flux density for efficiency. If you switch them on when the mains voltage is high they can easily go into core saturation and can take a long time to recover from this. The materials used in the construction are, to be kind, ‘cheap and cheerless’. They are wound on automatic machines with little care and attention to detail during the winding process. They are running on the edge all the time because of these manufacturing conditions. Most of these transformers are very noisy mechanically as they often have loose cores and windings. This type of transformer has no place in a high-end system, where background noise can be a serious distraction, so what I am about to say does not relate to these low quality transformers.
Regarding the size issue, back to my family saloon/Bugatti Veyron analogy. Both will take you where you want to go but the Veyron will do it effortlessly. Put your foot down and the family saloon will struggle but will get you there eventually. Trying to stop rapidly is also a struggle. The Veyron will go like a bat out of hell and stop on a sixpence. It’s just coasting with far more energy available and a heavily engineered system to keep things under control when the going gets tough. Yes it is over-specified for the job, but I know which I would prefer, and it is not just about looks and flash. The performance is exemplary.
Increasing the transformer size allows for reduced secondary impedance, which in turn allows more current to be available for transient demands. The flux density in the primary drops with increasing size and this reduces the sensitivity to core saturation overload. There is also less leakage flux from winding to winding, which reduces interaction between the windings. The winding technique is critical to high performance and this is an area where information is carefully guarded due to proprietary rights protection.
The SR5 transformer is one of Terry’s specials. The winding is carefully controlled on a manually operated winding machine. High quality materials are used and the transformer is wound on an oversize core with inherently low flux density so there are no core saturation problems even when the mains voltage is higher than usual. The transformer is designed for silent running so no annoying buzzes in the listening room.
The transformer combined with the regulator module I use for this supply gives an exceptionally quiet power supply both electrically and mechanically.
Hi John,
‘Interesting Power supply Paul.
Do you have a spec sheet and price for it?
Is that phosphor bronze?’
The regulator is a proprietary discrete component topology using error amplifier devices with a noise spec of 0.5 nanovolts root Hz and a noise corner of 1 Hz. The regulator output noise measured 35 nanovolts root Hz on an HP3561a spectrum analyser. This is half the noise level (6 dB lower) of the low noise version of the ALW regulator that many audio enthusiasts have used in their DIY projects. 35 nanovolts root Hz is around 5 nanovolts rms over a 20 KHz bandwidth. A National Semiconductor LM317 would typically have around 630 nanovolts rms output noise in standard configuration and in the region of 63 nanovolts rms output noise with the low noise configuration when set for an output voltage of 21 volts.
Cascode operation of the SR5 regulator error amplifier, on a higher supply rail than the output stage, extends the bandwidth out to around 400 Mhz allowing high-speed drive to the mosfet output device gate, which in turn enables rapid, very high transient current delivery, into the load.
The output mosfet is a very rugged IXYS device capable of delivering large transient currents, well in excess of 60 amps, into the load. The continuous current delivery is 4 amps. Both of these current delivery specs comfortably exceed the SL1200/SL1210 requirements with Mike New’s excellent Platter and bearing system fitted to the deck.
Output impedance of the regulator measured <3 milliohms from DC to 80 KHz and the supply line rejection >80 dB from DC to 80 KHz.
The output Mosfet is mounted on a copper bracket to enable fast heat removal under high current conditions.
Mark,
The SR5 is a high-speed series regulated power supply not a shunt regulated one. I have a high current shunt regulator design that would work with the SL1200/SL1210 but it is not as quiet as the series regulator (around 12 dB higher noise level). Also the available transient current is restricted to the current limit setting of the current source feeding the shunt element of the regulator. This regulator requires heavy thermal engineering and is more costly to implement.
I have to go now, as food is ready (and wine I hope).
Regards
Paul
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Originally Posted by Paul Hynes
I didn't think there was enough heatsinking & i couldn't see a series dissipating element such as a power resistor or possibly another mosfet used as a resistor 
Isaac Bashevis Singer
, although just like a class A amplifier, you can use them for background heating as well as powering equipment.
