Saturday, August 4, 2018
Temperature is too high
- Short circuit in the winding, at primary or secondary. Once shorted, the current will be high and cause rise in temperature. If this continues, not only the insulation layers will be burnt, and the adjacent winding will be burnt due to high temperature, and the vicious circles continues, until the whole transformer is burnt. This high temperature happens whether it is loaded or unloaded. The only fix is to remove the bobbin and remove and replace the burnt winding(s), or all altogether.
- High temperature may happen at small transformers and goes un-noticed, where there are a lot of turns in a single winding. If the adjacent magnet wire got short-circuited, since the voltage is low, wires are long and thin, the temperature may not reach the exterior, but it will not work for long.
Note: When there are shorts in the transformer, the no-load current will be higher than normal.
- Magnetic flux density (B) set too high - especially when using unknown transformer core/lamination. If B is set too high, causing the turns to be lesser than ideal. With reduced primary turns, the no load current will be too high, causing increase of transformer temperature. Even if secondary are unloaded, the transformer temperature will rise high. If you reduce the primary voltage up to a certain point, the temperature will be at normal temp, for such kind of transformers.
- Overloaded transformer - self explanatory
- Magnet wire is too thin - high DCR causing more power dissipated at the wires.
Electrocute feeling when touching the core
- If one touches the core and feels a slight tingle or eletrocute feeling, this is due to parasitic capacitance between the winding and core. You can measure a certain Vac value on the core with respect to ground. This may be resolved by adding grounding to the core or mount it at a grounded plate.
Magnetic flux leakage is high - causing noise
-When powered on, the field generated by the winding should follow the path formed by the core, and should have low magnetic path resistance, to enable a high efficiency transformer. Due to various reasons, if the path is not smooth, leakage form where magnetic flux to goes through the air instead (magnetic flux leakage). That could happen to uneven surface between the cores (gap), or cores not tighten enough with the E/I lamination, or the air-gap laminate is too thick, and various other reasons.
- If the transformer works alone, the impact is not much and may not be noticeable. If it is mounted in jam-packed equipment with lots of sensitive electronics, the magnet flux leakage will be detrimental, causing AC hum to the adjacent components (therefore, giving EI transformer a bad name).
- To check for magnetic flux leakage, one can wind a 200-300 turns of wire on an iron nail, connecting it to a high impedance sensitive headphone, and move this to a powered transformer with magnetic flux leakage, and see if there are audible hum generated in the headphone.
- To avoid this, the lamination must be even so that the interface is as close as possible. This is even more important when using refurbished cores. It has to be clean enough and polished to ensure the closest interface. OTOH, transformer designer should not design it close to the maximum magnetic flux density. Also, the copper band will come in handy to prevent such leakage from affecting the environment. Distance may help too since the density drops as it moves further away from the transformer.
Noise when powered
- Ringing / hum when powered - this could be due to loose lamination. Lamination could get loose in many ways - abuse, drops, not tightened enough, and etc. Low grade core with un-eveness in the cores (not flat enough).
- Short circuit in the transformer, where there is high current flowing., causing the lamination to rattle, and heate up.
- Misaligned lamination - you can see the un-eveness alignment of the lamination when the whole transformer is constructed. Such zig-zag lamination will cause noise. This should be easy to see externally.
So, you have been empowered now with the knowledge to know the general transformer problems. Choose wisely.
J&K Audio Design
What's the recommended isolation transformer for my application? We've replied this question for countless times. Therefore, it is time for a post!
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)
* 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.
* 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
* 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.
J&K Audio Design
Thursday, July 26, 2018
Wednesday, June 20, 2018
It uses SIT (Static Induction Transistor). SIT characteristics is quite close to vacuum tube Triode. Based on many user's feedback online is that SIT sounds quite close to Triodes!
SIT, THF-51S, 2SK180, 2SK180ES and such are very high power devices with PD of ~400-1KW. Say we take 15% as efficiency of class A amplification, we can easily get 60W - 150W out of SIT! This is definitely more economical and simpler than to draw 50W out of a 212 tube.
A level 1 OPT, tailored for this above application
- Z11 - U$490/pair, Z9 - U$590/pair.
- Pri: 250 Ohm (configurable), inductance ~1H
- Pri DC: 1A (others are available)
- Second: 0-4-8-16R (any)
- Output power: 70W
Some samples bias settings (not tested):
A sample circuit from Nelson Pass, in his SIT Nemesis article:
This looks alike to a normal SPUD circuit and is simple enough for DIYers with experience on building tube amplifiers.
It would be quite interesting and ear opening to try out such designs.
J&K Audio Design
Monday, June 11, 2018
The answer(s) to why transformer / autoformer based volume control is better than resistive volume pots, is quite complex, as there are many factors to it. But, one that stands out, that many may not be aware of, is the Miller capacitance + volume pot effect, that caused the amplifier bandwidth to be reduced significantly when used with a resistive attenuator, or resistive volume pot. This is especially significant if a high value volume pot is used, such as 100K-250K pot.
Everyone knows about Miller capacitance, and why grid stoppers are required, right? But, is he aware that volume pot is part of grid stopper too?
The resistance from the pot has the same effect as a grid stopper (The resistance, starting from input of source, to the volume pot viper section, is part of the grid stopper!), creating a RC low pass filter with the input capacitance of the tube connecting to the volume pot. This RC low pass filter will attenuate any content above the cut-off frequency.
The cut-off will be worsen by higher gain tube as the Miller capacitance is multiplied by the gain, in simple calculation sense, a good example, will be 12AX7. (Could it be the same reason where some prefers lower gain tubes in their amplifier instead of a high gain ones?)
Take 12AX7 as an example, the Cin (Miller capacitance) will be ~200pf, give a take a few or tens of pf.
Say, with 100K Ohm resistive volume pot, the setting now is at mid point of 50K, indicating that there's a 50K+ "effective grid stopper" connected to 12AX7. Ignore the actual grid-stopper connected for the moment. With that, the resultant cut-off shall be:
f cut-off = 1 / (2 * pi * Rg * C) = 1 / (2*3.142*50,000*200pf) = 16kHz (!!!)
That's a freaking 16kHz low pass RC filter in the amplifier without one even realizing it. That is sad...
That's why a lot of folks prefer TVC or AVC versus resistive volume pot. That's why most people will say that the realism is better, the highs are airier and with more body once they migrated to TVC or AVC. Once they tried TVC or AVC, they will never go back to resistive volume pot.
Till then. Go figure.
J&K Audio Design
Level 0: link 1
Level 3: depends on type, size and complexity. Email us for details.
Power-trans: link 1
* Finished amplifiers, DACs, audio gadgets, upgrades and repairs - this is not our core business and we do it out of passion. We do not have fixed models, fixed price and we customize for each individuals. The sky is limit of creativity.
* Our product lines are always improving and increasing. If you do not see what you want, contact us!
* Please email for volume discounts, distributor and OEM pricing.