Check back regularly or subscribe for updates. Email us at jnkaudiodesign@gmail.com, or fill in the form on the left.

Saturday, July 23, 2016

输出变压器设计

输出变压器设计

It has been a while since we posted. Today, let's talk about output transformer design (输出变压器设计).

Output transformer is the heart of tube amplifiers (OTL - output transformer-less designs are exceptions of course). The quality of the output transformer determines the overall quality of the tube amplifier. It is much easier to get a power transformer made correctly than an output transformer with wide flat bandwidth.Good tube amp manufacturers often places a great deal of focus on their output transformer design, such as McIntosh, AN and ARC, which are famous for their high quality output transformers. For those 2nd or 3rd rate manufacturers with unknown transformers, or transformer designs, the transformer quality is something to be examined further and purchased with caution. 

So, let's think about what qualities should a high quality HIFI output transformer possesses? 
  1. High primary inductance - yes, on a surface,for better low frequency response and loading, matching the driving tube and circuit, but to a certain limit so as not to create other problems.
  2. Low leakage inductance, and capacitance, sometimes more accurately described as distributed inductance and inductance, because they are evenly distributed across all the winding of the transformer. 
  3. No resonances across audio bandwidth.
  4. No distortions.
  5. 0 phase delay across audio bandwidth (usually happens at both ends of the frequency spectrum.
For those that knows a bit or read about magnetic properties, it is very obvious that the above are very hard to achieve, and the parameters actually work against each other. 

In output transformer design, you get some, you loose some, and you can never have all. If someone tells you he can, think twice.

Magnetic cores, such as SiFe, or any cores, will introduce its' own distortion due to different and imperfect BH characteristics (curve). The input and output will be different due to the transformer core BH behavior, and therefore some sort distortions have been introduced.

We are often asked about having unusually high primary inductance for Single Ended output transformers for their orders. Yes, high primary inductance is good, but over-design is never ideal in output transformer design. In order to have high primary inductance, we need much longer wires, thinner wires, and more turns. These have the side effects of higher primary DCR, higher copper loss, higher leakage (higher distributed inductance & capacitance), and higher phase shift. This will cause narrowing of the bandwidth, phase shift at both audio frequency extremes, and decrease of dynamic range. 

That's why, often, for those with high primary inductance requests, we will usually ask them to trust us in our judgement to select the best design for their applications. If one blindly follows the "high primary inductance" rule, they are free to suffer from their own decision. Some just focus so much on the primary inductance, that they do not really care about the high frequency response, making their tube amplifiers sounded very smooth, warm and mellow. At first, it may sound soothing and nice, but after a while, listeners with good ears will start to feel something amiss - the HIFI component, i.e., the high frequency components and dynamics will be missing. It may sound dull and lifeless, but some may like it, mistaking it as fake warmth or vocal (mid-range) superiority. Such kind of tube amplifiers may be good for speakers with high frequency focus loudspeakers, with a drift that goes upwards in the frequency response, compensating this design defect.

We strive to design a wide bandwidth (frequency response) output transformer that excel at both frequency end, with very flat response at the standard 20-20kHz region, at +/- 1dB flatness, or better.

The way to achieve this is to fulfill the highest primary inductance requirement according to the design at stake, and try to lower the distributed capacitance and leakage inductance to achieve the high frequency response for modern playback/recording devices usages. The application of high quality transformer core materials such as high grade SiFe, amorphous cores, Nanocrystalline cores, Permalloy cores, and with complex winding methodology, we try to get as close as we can to a perfect output transformer. 

As we have mentioned, the parameters are contradicting each other and therefore the bandwidth will not be as wide as an output transformer-less tube amplifier or Solid State amplifiers. Still, we love and we are addicted tube amplifiers with output transformers!  

Sometimes, bandwidth limitation can be a blessing! You have heard of deployment of high quality vintage input transformers from the golden age of tube amplifiers, such as input transformers from Western Electric (which have attained a cult following, with insane sky high prices) makes the sound better, smoother, despite their limited audio frequency bandwidth. Such bandwidth limitation acts as a filter for the high frequency noises, interference from surrounding or playback electronics, or even high frequency component of music signals. Therefore, making the sound smoother, and less harsher compared to direct connection or equipment without coupling transformers. McIntosh, for one, uses such output transformer technology on their solid state amplifiers. 

The leakage inductance, distributed capacitance, the primary inductance, the tube plate impedance, the primary current, the plate load impedance, the primary/secondary DCR, primary/secondary turns ratio and the desired flatness in audio frequency range, has a semi-fixed relationship, and there is a formula (cannot disclose here, J&K proprietary, but can be found and derived from various magnetic formulas and textbooks) to ensure the design meets a certain HIFI criteria or requirements. The primary/secondary DCR cannot be too high, or else the dynamic range, resolution and transient responses will suffer. These all actually sums up to be the primary impedance of the output transformer, i.e. the load for the tube in use. Varying components at difference frequencies within the audio frequency range will affect the flatness of the response curve, and also the phase shift over the frequency range. Deviation of the phase response and response curve flatness generates distortions, and power loss or variations across frequencies. 

Furthermore, these inductance and capacitance that exist in the transformer forms a LC (RLC) circuit and causes resonances at certain frequencies. At resonances, the impedance at such frequencies are either 0 or infinite, causing peaks and dips, abnormality/gradients on the frequency response curve. It may cause premature roll-off at both ends of the frequency spectrum if resonances fall at those 2 extremes. Therefore, one other factor to consider is to design the output transformer so as to reduce these artifacts, or make them fall FAR outside of the audible frequency spectrum. 

We often stress on the quality of transformer cores because of the effect is has on the dynamic ranges (of course there are other effects). The signal amplitude, or the responsiveness to response to minute or major signal changes, highly relies on the induction density and permeance rate of change, which varies according to the signal amplitude, and therefore affects the dynamic range, or how well it responds to small/big signal changes. 

Another factor that is related to signal amplitude is the transformer core saturation. If the signal is large, it may be working at the saturation region of the B-H curve and therefore further increase in signal amplifier will be saturated and output be distorted. That is why some magnetic cores are for small signal usages, and some are for power output usages due to their core magnetic flux density limitation. 

We have not talked about DC magnetization of the cores, such as in single ended tube amplifier application, where we need to add air-gap in the cores to prevent saturation. The control of the air-gap size will affect the magnetic properties of the core. Sometimes, some air-gap is used on push-pull output transformers too due to mismatch in both arms of the push-pull output transformer caused by mismatched tubes or unbalanced loading (difference in DC bias). 

We've discussed a lot in this session. Till then. 

Hear the unheard!
J&K Audio Design.
7/23/2016 

Thursday, July 7, 2016

Father's day sale - counting down...

Guys - last reminder on date to secure your order is on 7/19/2016

*Tips: You can place a 50% deposit to secure the price 3 months too if you are still unsure on the final specifications. 

Father's day sale!

Apologies for no postings for some time. We are very busy preparing the orders from Father's day sale!

Hear the Unheard.
Hear the Difference.
Only at J&K Audio Design.

Saturday, June 25, 2016

Isolation Transformer Bandwidth

How effective is isolation transformer? 

Not all isolation transformers are built the same, and hence the effectiveness of filtering noise varies significantly between design and implementations. It is the bandwidth that counts, or the limitation of the isolation transformer that filters the noise outside of the pass-band.



Benefits of isolation transformers – we have mentioned it several times. We are not going to repeat it here. What we have now is our continuous effort in seeking for perfection in each and every products.
For J&K followers, we believe they have already accustomed to our grading of transformers, from low level to high level. Every step one goes up the ladder, the quality and performance of the product is raised significantly. Do not be-little these grading. The end results – sonic improvements is substantial. Ignorant folks can still go on with their lives using low grade transformers, that’s their lost, your gain (Pun intended! LOL), especially important for those that make tube amplifiers for sale – to gain an edge over others.
 
Back to isolation transformers, even they have grades in terms of winding and core quality. An ideal isolation transformer is a bandwidth limited passive device. Passive as in it does not have any active circuitries with it. Isolation transformers last long too (almost indefinitely) if properly built and maintained (no accidents too). Bandwidth limited as in it does not pass signal (noises) below and above the pass-band, therefore filtering all those noises / interference / unwanted signals away. 
 Here are the specifications of how good isolation transformer should have:
  • Efficiency ~95%
  • Regulation ~5%
  • Common mode rejection ratio (CMRR) >= 120dB
  • Differential mode rejection ration (DMRR) 1kHz >= 30dB, 200kHz => 45dB
  • Primary – secondary capacitance ~0.006pF
  • Peak attenuation ~30dB 
Our power line should be only filled with 50-60Hz sine wave, which is the base frequency of electricity transmission. Due to various noises being generated by connected equipment on the same power line, as well as radiation from surrounds (radio signals, EMI/RFI, cellphone signals and etc), it is very, very polluted. They come in various forms and are riding on the 50-60Hz line into our audio system. Isolation transformer will effectively filter out all those unwanted noises. 

There are two types of noise - differential and common mode noise on the power line and both can be filtered by balance isolation transformer. Single ended isolation transformer can only filter differential mode noises.



A truly balanced isolation transformer will have 120dB of rejection at 100Hz! Less balance isolation (not fully balance on winding) will not work as good and as effective. 

Putting the numbers with dB might not look significant since dB is not something we work with every day. Let's convert it to gain and see how much noise and spikes attenuation can it actually get! 



40dB of attenuation means the noise is reduced by 100x! 




Hear the Unheard. Hear the Difference.
Only at J&K Audio Design
6/26/2016

Search This Blog

Loading...

Product Lists

Product Lists

Level 0: link 1

Level 1: link 1, link 2, link 3, link 4

Level 2: link 1, link 2, link 3, link 4, link 5, link 6, link 7, Link 8, link 9

Level 3: depends on type, size and complexity. Email us for details.

Iso-trans: link 1, link 2

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.

Follow by Email

Follow us using Google+ Badge to get instant updates!

Google+ Followers