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Sunday, October 15, 2017

Isolation transformer effectiveness

Many had asked us, you have so many types of isolation transformers, which should I choose? 

Instead of repeating ourselves again and again (not that we're not willing to, but just to save everyone, and our time), here goes the effectiveness rating, from our perspective, from low (bad) to high (good), with some comments:
  1. Toroid isolation transformer (for HT, audio OK too but not warm and clean as C or EI)
    1. Up to 3.5KVA for highest grade toroid core (other grades with higher KVA are available but not recommended)
    2. >3.5KVA, go for parallel toroid for lower impedance 
  2. C core isolation transformer (for audio and HT) 
    1. Up to 1KVA (bigger size, can be custom made, up to 3KVA)
  3. EI isolation transformer (for audio and HT)
    1. Up to 3KVA
  4. C core ultra isolation transformer (for audio and HT)
    1. Up to 1KVA (bigger size, can be custom made, up to 3KVA)
  5. EI ultra isolation transformer (for audio and HT) 
    1. Up to 3KVA
Some articles about ultra isolation - article 1, article 2

OCC copper magnet wires (Ohno Continuous Cast) transformers are available for #2 to #5. OCC comes with a hefty price tag though. 

Benefits of OCC - lower DCR, better dynamics and clarity. We often use OCC for our output transformers, but for true audiophiles, those that hear a difference with high grade power cords, this is definitely better, and higher ROI compared to using OCC based power cords. 

* Balance or single ended outputs
* Multiple-taps isolation (explanation below) 
* Potting/frame
* Multiple shielding (electrostatic, primary/secondary, secondary/secondary
* Can be made as power transformers with isolation/ultra-isolation for SS and tube amplifiers


What is multitap-isolation? Normal isolation transformers have only 1 single set of outputs, say:

- Primary: 0-240V-G
- Secondary: 120V-0-120V (balanced)

Multitap-isolation is as follow:

- Primary: 0-240V-G
- Secondary 1: 120V-0-120V (balanced, isolated/shielded from 2/3)
- Secondary 2: 120V-0-120V (balanced, isolated/shielded from 1/3)

- Secondary 3: 120V-0-120V (balanced, isolated/shielded from 1/2)

This way, instead of having multiple gears connected to a single secondary, each output is connected to 1 set of secondary to make 3 sets of ISOLATED outputs. This way, the CDP, tube preamp, and DAC (for example), are isolated from one another as they are connected to different secondary windings instead of connected altogether in a normal isolation transformer unit where all the outputs are connected to a SINGLE secondary. 

This is a MUCH better isolation scheme than normal paralleled output on-the-shelf isolation transformer unit. Yet, it is more cost effective than having 3 units of separate isolation transformer, which is the ultimate way one should do it, eventually perhaps.

Go go go, custom made audio!

J&K Audio Design

Type 50 Amplifier

Supercharger amplifier with type 50 tube, anyone? Want to listen to how a pure type 50 tube amplifier really sounds like without other tubes to color it? Well, Sakuma style is a good example, using 50 to drive 50. Due to the scarcity of type 50 tubes, let's not waste the 50's. 

Note: One can use ANY power tube to replace type 50 tube in the below example, DHT, IDHT, pentode, tetrode, or any power tubes, even power transistors! Since we just heard the good news that Elrog is coming out with type 50 tube, now is a good time to look into this DHT tube!

Caution: Not any input transformers can be used in such circuit. Ordinary input transformers may not have enough core volume to sustain high voltage swing (usually <10Vrms). Ordinary input transformer primary winding may burn if driven by a power amplifier. Special input transformer is required for such purpose.

Some previous reading:
Supercharger Amplifier
Step-up input transformer

Type 50 tube amplifier

Take a look at the schematic above for type 50 tube. It is as simple as can be. Of course, you can complicate it a bit by making it fixed bias, add more bypass capacitors, LED bias, regulator tube bias, and etc. The power supplies for B+ and filament are kept out for the moment. Let's just keep it as is for now for simplicity sake.

There are 2 resistors that MAY be redundant too - the grid leak and the grid stopper. Grid stopper - prevent tube oscillations. Grid leak - as described in type 50 tube datasheet, needs to be <= 10K to protect the type 50 tube. In this case, it may not be necessary since the input transformer secondary acts as the grid leak path. The transformer secondary will be the least resistance path for the type 50 tube grid to take even if you connect a 10K resistor. Also, if the resultant waveform is not having severe ringing (even if it rings), the grid leak resistor, which act as the load for the input transformer, will tame the ringing. For type 50 tube, with input transformer, it is better not to have the grid leak resistor if possible.

Let's take the operating point of type 50 tube as 425Vdc B+, -70Vdc grid, 55mA, which is quite commonly used by a lot of folks. This needs about 140Vpp to drive type 50 tube to full power. That produces about 4W of undistorted power output to the load (your definition of undistorted may be different, especially the SS folks).

Say, let's use a standard source, which is an ordinary CD player, or DAC, with 2.8Vpp output and 600 Ohm of output impedance. To get sufficient Vswing to drive type 50 tube to full power, one needs a gain of = 140Vpp/2.8Vpp = 50. Wow, that's a lot of gain required, and is not feasible at all here. Let's drop the standard source.

Let's go for a preamp instead, say a 5687 simple tube preamp with output transformer, having output impedance of say 150 Ohm. 5687 has a u of 18, and is usually loaded with 5K primary impedance. With that, the resultant gain would be root of 150/5000 * 18 = 3. Not so much eh, but is good enough perhaps? Let's check.

With 50 / 3 (gain of tube preamp), we get to gain of 16. Hooray! STU-001 here has a gain of 1:18! Just close enough, with a little bit more for headroom or circuit tolerance. 

Conclusion? As previously mentioned, a very high drive strength preamp is required if a supercharger is driven. If tube preamp is used, something with output transformer and higher gain is required. The above example of using a simple 5687 with output transformer looks like a bare minimum supercharger amplifier can tolerate. 

Enjoy tube tasting! 

J&K Audio Design

Tuesday, October 10, 2017

Easy Parcel and Parcel Hero

Hi guys,

Great news! 

We are engaging Easy Parcel and Parcel Hero now to ship out transformers and products. In addition to that, local post office has included EMS in the delivery options recently. 

With this, we hope to offer faster, safer and cheaper options of delivery the transformers to our customers! 

Instead of 8-16 days of port-to-port standard delivery duration, the new vendors offer much faster service, of 4-8 days, sometime faster.


J&K Audio Design

Thursday, October 5, 2017

NOS Tubes Sound

What is true sound of NOS tubes? (Rants warning!)

Most NOS tubes players like to ask among themselves, what tubes do you roll on your amplifier? Or what tubes you like to play? How do you like their sound? 

Often, answers would be the manufacturer of the tubes, such as Western Electric, RCA, GE, Sylvania, Tung-Sol, GEC, and etc; or the model of the tubes, such as 300B, 211, 845, KT88, 2A3, and etc. 

For those with more advanced tubes knowledge, they would go into the year of manufacture, factory location, getter type/quantity, filament suspension style, plate structure, base type, special tube structure support, and such. 

Rarely, do people asks, what transformers are you playing with? People rolls tubes. They do not roll transformers. That's how it is now. I wish, some day, people would start rolling transformers too! 

One of the most important thing, other than the amplifier design itself, is the quality of the transformers, especially the output transformers! Without high quality transformers, no matter how good the tube are, the true sound of NOS tubes, or NOS tubes sound, can never be fully achieved or heard. 

When we talk about transformers, we are referring to things such as power trans, chokes, input / output trans, interstage and driver transformers as the main categories. 

All of them, play very vital roles in a tube amplifier. Don't even talk about NOS tubes sound if your transformers aren't done right in the first place. The quality of transformers will determine the sound quality, voicing, noise floor, linearity, scale, power delivery, transparency, tonality and all the parameters of having HI-FI sound. 

When we talk about transformers, all these below matter, just like NOS tubes.

* capacity
* lamination type and quality
* magnet wire type and quality 
* transformer electrical and mechanical design
* bandwidth 
* safety and longevity 

Something as simple as a choke, also varies in performance and sound. The grade of materials (core, wire, potting method, potting compound, bobbin, insulation, and etc), the design, the DCR, the inductance variances across frequency and load, and etc, are all different, and will affect the sound and electrical performance substantially. If one things that all chokes are built the same, think again. 

Try ordering a choke made with NanoCrystalline core with ultra thin lamination thickness, extreme low DCR design (example 5H 10 Ohm, compared to ordinary 5H 50 Ohm), OCC magnet wire, high grade potting compound/bobbin/insulation, adequately but not extremely oversized core, proper mechanical design (perfect wire strain and alignment), and compare it with a standard choke, one will witness the vast improvements one small thing like that will bring to the sound. 

If this is done on an output transformer, the difference will be much more significant! When we go into more complex transformers, such as the audio signal transformers, the factors impacting the sound increased exponentially, so as the sound. People often thinks about primary inductance as the basis for good low frequency response. That is not all. The primary and secondary winding DCR is very important as well. 

A 3.5K:8 Ohm standard output transformer often carries the DCR of 150-200 Ohm for primary, and ~1 Ohm for secondary. Try ordering a pair made with DCR of 30-50 Ohm for primary, and 0.1-2 Ohm for secondary, and with everything else staying the same, such as the primary inductance and bandwidth (Not all can do so. There's a penalty in price and size.). The result differences will be ultra significant! Hints: increase in dynamics, better power delivery, improve control, increase transparency, lower distortion, and etc). 

What I bring up here is just the tip of the iceberg and to get you to start thinking deeper. There are more than meets the eyes. 

From today onward, start thinking about rolling transformers, on top of rolling tubes. You'd be rewarded handsomely (better ROI in a lot of cases), since rolling tubes can be significantly more expensive than rolling transformers! Don't waste the NOS tubes if you don't play high quality transformers. 


Hear the unheard! Sail the uncharted! 
Long live custom made audio! 

J&K Audio Design

Thursday, September 21, 2017

Tamura STU-001 Input Transformer

Tamura STU-001 Input Transformer

Tamura Input Transformer - STU-001, 150,300: 100KCT (Push PULL). Tamura STU-001 is widely used in designs by Sakuma-san, in Sunaudio amplifiers, and designs in MJ Audio Technology magazine. 

Unfortunately, STU-001 input transformers are out-of-production now(at time of publication), and is quite rare to be found now. We've received quite a lot of inquiries on similar high step-up ratio input transformer such as the Tamura STU-001 and have decided to open it up for mass market instead of making them quietly in the background. 

Below are some applications designed and built by HIFI enthusiasts from the East. It can be used as single ended step-up, or phase splitting for push-pull use. You will also notice that directly after the Tamura STU-001 input transformer, is power tubes, such as 211, or 300B, meaning that it is some sort of a Supercharger amplifier, or power amplifier. 

We have blogged about supercharger amplifier in the past - you can refer to this post:

Level 2 Step-Up Transformer  STU-001 for Supercharger Amplifier use - U$345/pc. 
- 150,300: 100K-CT (standard)
- Permalloy cores only
- SE, or PP input/outputs, configurable, customization allowed
- potted with silver plated copper turrets 
- bandwidth, depends on source driving impedance, the lower, the wider the bandwidth  
- Larger STU with bigger core, bigger swing, is available


- transformer is a ratio machines, with such high step-up ratio, the secondary load needs to be very high impedance, and the primary source needs to be very low impedance, to keep the bandwidth wide enough for audio applications
- 300:100,000 equals a step-up ratio of 1:18, or impedance ratio of 1:333. Load will be reflected accordingly to the primary and the source will need significant drive strength to ensure satisfactory performance
- volume pot is not recommended at the primary of such input transformers for higher fidelity

Let's take 250K and 500K (maximum grid resistance 300B and 211 can tolerate in self-bias circuit, fixed bias will be much lower) secondary load for STU-001 as example.

- primary reflected impedance = 500K / 333 = 1.5K Ohm 
- primary reflected impedance = 250K / 333 = 750 Ohm

You can see that it is not a really friendly load for weak (high output impedance) tube preamplifiers. Most common-cathode topology simple tube preamps will not apply since the output impedance will be high. We would need tube preamps with low output impedance, such as those with Cathode Follower output stage to drive such STU-001 input transformers satisfactorily. Or, tube preamps with output transformers, that has low secondary impedance, such as 50-100 Ohm, or maybe lower, will work nicely. That reminds us of tube headphone amplifiers!

On the other hand, high swing low output impedance solid state preamplifier will come in nicely here. Therefore, choices are plenty. 

In some cases, secondary load may not be required, if there are no oscillations / overshoot or undershoot at the output of STU-001, and therefore, the secondary of STU-001 acts as a grid-choke for the driven tubes! 

One may asks, so what's the benefit of doing so? You can set the power tube free from the coloration of the driving stage! You get to hear the real sound, real character, true flavor of the power tube with such high gain input transformers. Imagine if the power tubes are WE WE 300B, 242, WE 211, WE 212, GEC & Telefunken power tubes and various other exotic power tubes. Such input input transformers such as Tamura STU-001 and the likes will come in very handy.

Note: Standard STU-001 may not have that much swing for high power tubes. STU-001 with larger cores (diff price) is needed for big power tubes. 

Hear the unheard! Sail the uncharted! 
Long live custom made audio! 

J&K Audio Design

Wednesday, September 20, 2017

Autoformer Volume Control

Autoformer Volume Control, or Transformer Volume Control

We think that AVC, or TVC are one of the many great things that happened in audio industry in the past 10 years. The sound signature brought forward by AVC, or TVC can be stated as - better fluidity, warmer, wider bandwidth, and more analogous.  

For those that are unaware (not following us on Facebook, it is a good time to do so now!), we've found a new casing to house our TVC (Transformer Volume Control), or AVC (Autoformer Volume Control). 

Here are some pictures of the casing. The matte version is still available if one still want those. 

Some past articles on TVC and AVC. We can do both, but TVC is preferred due to various reasons, but we can still do AVC if customer prefers AVC. There's no right or wrong in audio, just what one prefers. 

Our preference:

** brushed pot -> stepped attenuator -> digital volume control -> AVC -> TVC

Some tips on using TVC / AVC: 
* low output impedance source is preferred but not 
* use a very high quality switch - it make or breaks the AVC / TVC
* durability of the switch is very important - you will be switching them quite frequently
* encased / sealed switch is preferred over open casing type
* step down type of TVC will have lower output impedance and will be capable of driving heavier loads compared to AVC 
* invest on good low melting point solder and good soldering irons - very swift soldering, with low melting point solder is preferred - TVC / AVC are made with very thin wires and are very fragile 

Hear the unheard! Sail the uncharted! 
Long live custom made audio! 

J&K Audio Design

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