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Sunday, October 16, 2016

Transformer Volume Control TVC

Transformer Volume Control, TVC

Transformer Volume Control (TVC)

What is the best attenuator in the world? There are many types available, such as digital chip based (PGA2310), analog brushed pot, discrete stepped attenuator, digital relay switching stepped attenuator, DSP based attenuator, Autotransformer Volume Control (AVC), and finally transformer volume control (TVC), which is our favorite. 

It is not the widest in terms of bandwidth but it has the best tonality, body, and harmonics that we look for - musicality in another term. Transformer volume control (TVC) became quite hot about 10 years ago but somehow it went rather lukewarm these few years. We heard some rumors or complaints on the durability of the unit being manufactured due to switch connectivity as that is one of the crucial component of the TVC, where mechanical connectivity will affect the outcome severely. High quality switchers such as Elma, Seiden or some milspec switchers will be desired in TVC application to ensure no mechanical issues arises after prolong (or even short) use.

No matter what, it is still a gem to be discovered if you have not ventured into TVC world. The sonic characteristics / benefits are not something other volume pots can reproduce easily. 

Different types

Low Level (signal, not quality) Input Transformer Volume Control (TVC) - install after source (CDP/DAC)
  • LLI TVC OFC - U$690 / matched pair, potted, 23/24 step
    • 23 step OCC - U$890
    • 33 step OFC - U$990 
    • 33 step OCC - U$1090
  • LLI Silver TVC - U$1090 / matched pair, potted, pure 4N Silver wires, 23/24 step 
    • 33 step - U$1390
  • Notes
    • Amorphous or Permalloy double C core
    • 2Vrms @ 20Hz

High Level (signal, not quality) Input Transformer Volume Control (TVC-XL) - install after preamp (SS or tube)
  • HLI TVC OFC - U$790 / matched pair, potted, 23/24 step
    • 23 step OCC - U$990
    • 33 step OFC - U$1090
    • 33 step OCC - U$1290
  • HLI Silver TVC - U$1190 / matched pair, potted, pure 4N Silver wires, 23/24 step  
    • 33 step - U$1490
  • Notes
    • Amorphous or Permalloy double C core
    • 5Vrms @ 20Hz (other input level / sizes at different pricing) 

Variations / Upgrades / Notes:
  • Options: balanced input, higher input level (bigger size), fully built in chassis version, higher/lower number of steps, with gain (step-up) options, and etc.
  • If higher signal level input is not required, 2Vrms @ 20Hz version will have better performance than 5Vrms @ 20Hz version in terms of bandwidth. Do not use truck engines in your sports car! 
  • These guys are hard to manufacturer - order early as they have long lead time (1 month generally).
  • Others not mentioned, please email us.
Application notes:
  • Grounding at primary or secondary depends on application. 
  • If it is going to act as a standalone preamp, secondary should not be grounded, and therefore it also acts as an isolation transformer to remove ground loop. 
  • If step-up (gain) is required, the load must be light to prevent bandwidth loss as output impedance will be increased with step-up. 
  • KISS rules - keep it (the design) sweet and simple, and it will serve you better. 
  • The load on the secondary will make or break the signal integrity. If you have a scope, try with different values of load on the secondary. If not, ask us for help, and mention it (the load) when you order and we will optimize for you.
  • High quality 23/24 step switch is a must. Cheap pots will be just a waste of time.
Milspec 24 Step Switch
Notice how thin those secondary wires are? Utmost care is required when soldering these wires. We tried to sell bare format TVC transformers (cheaper) but chose to do potted version instead. The success rate is too low for DIY folks for getting the soldering and order correct. Therefore, we chose the potted with solder tags format for easier handling and better re-usability.

Fully built TVC
This is a fully built TVC, with SE and Balanced input. It uses a special XXL double C core to handle very high level input (>5Vrms).

Double C Core TVC
A closer view of the double C core Transformer Volume Control.

Function Generator / Signal Generator
Every unit is characterized and matched before being assembled. Usage of function generator / signal generator is unavoidable. This function generator is capable of generating low distortion sine/square/triangle/saw-tooth 20Vpp of output for us to fully test the TVC for all the operating corners.

Testing Transformer Volume Control
  TVC test setup - function generator and high bandwidth oscilloscope.

TVC Output Waveform
One of the sine wave test - testing the input capability of the TVC - determine how large the signal can go before distorting.

TVC Primary Inductance
Matching of primary inductance. Take note of the 4 pictures here with the LCR bridge (green display). The LCR bridge is actually measuring the same TVC, but with different core air-gap. You can see that the primary inductance actually varies substantially with different core air-gap, from 50H to 187H. 

We are actually working on a next generation of MIND BOGGLING, FIRST IN THE WORLD NEVER BEING SEEN TVC EVOLUTION that is to be released very soon. The primary inductance measurement work is helping us to characterize the behavior of the EVO TVC to be made for a special purpose application.  

Stay tuned for very recent announcement - monitor or subscribe to our blog in order not to miss this special product.

Transformer Volume Control Primary Inductance
Low primary inductance - if cores left untouched - huge air-gap.

Core Air Gap Affecting Primary Inductance
Primary inductance surged to 187H if cores are tightened to almost 0 core air-gap. The difference is day and night, >100% change from 51H to 187H. The gain of primary inductance is not free though, something has to be sacrificed. Do remember what we say always, there is no free lunch in transformer design. All the parameters are working against one another.

Reduce air-gap, increase primary inductance
If we press the C-cores by hand, that's what we get! Interesting, right? This is like designing and characterizing single ended output transformer, where the size of the air-gap determines the maximum primary current / capacity of the SE output transformer. The gap has to be made exact with up to micrometer to have exact matching between pairs, and for accurate primary current loading capacity determination.

TVC Primary DCR
Matching the primary DCR.

Matching TVC
Since both units are winded at exactly the same methodology, matching is easier and the electrical parameters are very, very close. It was 771 Ohm and 772 Ohm between 2 units! That is like 0.1% matching!

TVC Design
Notice how thin the secondaries are? These are special Litz Teflon OFC magnet wire. These wires are giving us lots of headache in terms of winding (ultra thin, fragile and slippery) and soldering (very high soldering point to melt the insulation, no mechanical stripping allowed, it will break!), although they are, very very good in performance.

Double C Core TVC

TVC Winding

Transformer Volume Control
That's all for today! Thank you for reading. If you have any questions, email us for clarifications. Our customer service is always willing to help.

Hear the unheard!
J&K Audio Design

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