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Saturday, August 4, 2018

Transformer selections for isolation transformer

We've seen some fully built isolation transformers appearing on the net that is quite close to what we're offering. I guess, imitation is a kind of artless! We'd like to emphasize: get only the originals, for peace of mind, for quality, and most importantly for excellent sound (pun intended)!

What's the recommended isolation transformer for my application? We've replied this question for countless times. Therefore, it is time for a post! 

Q&As:

High power SS amplifiers, home theater amplifiers (>200W class AB, or >40W class A)
* Do not use isolation transformers unless necessary, where you are having noise, sibilance and over-voltage issue that requires some step-down.
* Isolation transformers that you can consider using are toroid (for the size / efficiency 

Lower power SS amplifiers, all tube amplifiers
* EI isolation transformers, or Ultra isolation transformers (quieter, cleaner, better!)

Sources, such as CDP, DAC, preamp
* EI Ultra isolation transformers (highly recommended), standard isolation transformers 

Toroid vs EI
* Toroid has better efficiency and therefore is more suitable for high power applications. But, toroid has higher bandwidth and therefore is not a good filter / isolation device. Toroid has more noise that leaks through primary / secondary compared to EI.
* Most importantly, EI sounds warmer, cleaner, better bass definition, less sibilance, and better body when compared to toroid. EI is the one to use if the power required is not extremely high. 
* Toroid has lesser radiated magnetic field compared to EI and may be acceptable to use un-potted but once EI is potted in metal case or have comprehensive copper bands , it will have similar behavior on radiated noise.

Grades of cores
* For toroid, we only use the highest grade silicon steel toroid core from Japan, that is only available up to 3.5KVA for now. 
* There are lower grades toroid cores with higher VA out in the market that are less efficient and have high mechanical hum. We don't use them. 
* For EI, there are several types available too. We only use Z9 for higher end units (lesser loss, runs cooler, sounds better), and Z11 for standard units (still pretty high end compared to others out there).
* The H-grade EI cores are much lower end and we try not to use them unless budget is really a concern. 

Potted vs un-potted
* Definitely potted if possible 
* Potting dissipates heat better, is mechanically damped, protects the transformer from moist/rust, avoids misalignment of the cores (EI) from abuse, and etc

Input and output taps
* If step-up or step-down is required, multiple input/primaries can be made and users can select them based on incoming mains condition
* Secondary can be multi-tapped too but is not recommended. Try with only multi-tap primaries first, such as 250-240-230-0 or 120-110-100-0 (and etc combos) to tailor to various input mains levels. 
* Secondary is recommended to be configured as balanced, aka balanced-power, such as 120-0-120 for 240Vac, or 65-0-65 for 110Vac, and so forth.

Multiple isolated outputs
* Instead of all equipment sharing the secondary and cross-pollute one another, we can build isolated secondaries in a single isolation transformer so that one can still enjoy the benefits without paying too much for dedicated isolation transformer per equipment
* Secondaries can be of multiple similar or different voltages, similar VA or different VA, balance or un-balance power (there is a limit of how many secondaries are available, try to keep to 20 taps total). 
* Extreme example is as follow
-- Primary: 250-240-230-0, 2KVA total
-- Sec1: 120-0-120, 500VA (for 240Vac for British gears)
-- Sec2: 65-0-65, 500VA (for 110Vac USA gears)
-- Sec3: 110-0-110, 500VA (for 220Vac China gears)
-- Sec4: 50-0-50, 500VA (for 100Vac Japan gears)

Standard isolation versus Ultra isolation
* Ultra isolation transformers have multitudes better noise isolation properties than standard isolation transformers 
* Ultra isolation transformers size is ~2x of standard isolation transformer to fit all the additional secret sauce and is 50-80% more expensive


Magnet wire options
* OFC (standard)
* OCC Copper (recommended for high end gears and golden ears)
* OCC Silver (for millionaires and billionaires)

Outputs
* Wire/lead out with OFC
* Wire/lead out with OCC copper or OCC silver
* Wire/lead out with original magnet wire (not for small transformers with thin wires)
* Solder tags (be warned to not over-solder, or use too high power soldering irons)
* Binding posts 

Sizing or over-sizing isolation transformers (important)
* We are already oversizing the core for cooler operation and better sound, if you want further oversizing, please inform us.
* If is oversized, output voltage may be much higher when lightly or un-loaded 
* Inform us the estimated usage so that we can custom to reduce the output voltage (ex: rated voltage at 50% load or 25% load) - VERY IMPORTANT
* Over-voltage due to light load will burn your equipment, be warned! 
* We have often seen buyers of on-the-shelf isolation transformers to suffer from over-voltage issues since the on-the-shelf isolation transformers VA are higher than needed/used.
* Multi-tap primary may help to reduce the output voltage too, such as connecting the incoming mains of 240Vac to 260Vac input tap of the transformer will effectively lowers the output voltage. 

DC Blockers
* Highly recommended for toroid isolation transformers, and recommended for EI isolation transformers
* Removes DC from the mains, transformers run cooler and have less noise electrically and mechanically
* Price varies depending on the components used, long life electrolytic capacitors are recommended since these caps pass current constantly when equipment is switched on - do not skimp here 


Soft-start
* Not required for EI isolation transformers or toroid isolation transformers with <500VA capacity.
* Recommended for toroid isolation transformers >500VA 

EMI/RFI filter modules
* Optional, may be good or bad, very system dependent, add at your own risk
* Can be placed at input or output of isolation transformers
* Results will be 50-50 (option, have dual secondaries, one with and one without) 


Due to the higher grades of core being used, prices to produce these isolation transformers are higher compared to those out there. Don't use industrial grade core products to compare to ours! Low-ballers, please save your energy.

8/5/2018
J&K Audio Design

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