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Tuesday, October 18, 2016

OPTVC

OPTVC - Output Transformer Volume Control (OP TVC)

What is OPTVC? What is the difference between TVC and OPTVC? Please read on.

What we're going to share today is something special for the purist. We're not the first but definitely this is something not very common.

It is the simplest form, purist approach, preamp with volume control!

Before we go there, here's how a traditional TVC (transformer volume control) looks like.


TVC
Below two pictures are how traditional simplest single stage tube preamp with volume pot look like, with or without output transformer (just remove the OPT and replace with a resistor). There are lots of components in this simple preamp.

For parafeed preamp with OPT: 
  • Volume pot
  • Load resistor
  • Coupling capacitor
  • Preamp Output transformer
Parafeed output tube preamp
Traditional tube preamp WOT


To make things simpler, we go with traditional non-parafeed preamp with output transformer above. We still have:
  • Volume pot
  • Preamp Output Transformer
It has become quite simple already, right? 

The resistive brushed (yucky) volume pot in front can be replaced with a TVC to significantly improve the sound, and below is how it looks like. It is still not as simple we want - we still have TVC + Preamp OPT, two components.
We can simplify it further!
  • Remove the TVC at input!
  • Replace the preamp OPT with a OPTVC - output transformer volume control! 
Now we are left with only the OPTVC and the triode! That is what we call TRUE simple tube preamp!  (Don't try this with normal TVC! This requires a special type of TVC!)
OPTVC on a simple Preamp

The simplest, the PURIST-EST design! The tube uses the TVC as a load, with multiple (23-24 step) secondaries as volume control! That's it! No more additional volume pot at the input. We are left with ONLY 1 transformer in the circuit. The preamp OPT is combined with TVC to become what we call OPTVC - Output Transformer Volume Control!

The differences between TVC and OPTVC are:
  • TVC usually has low primary impedance for direct CDP/DAC drive
  • OPTVC needs high primary impedance for tube loading 
  • TVC has no core air-gap, therefore DC at primary is not permitted
  • OPTVC has air-gap, and DC loading is permitted 
  • OPTVC requires significantly larger core due to DC loading
The compounding effects of having two signal transformers, where the high and low frequencies are attenuated twice, are now gone! You get the benefits of both TVC and preamp OPT in one shot! 

Compounding attenuation example, for a design with traditional TVC + preamp OPT
  • preamp OPT has -3dB at 20Hz
  • TVC has -3dB at 20Hz. 
  • TVC + preamp OPT = -6dB at 20Hz
The compounding effect of 2 transformers is actually making it -6dB at 20Hz, a much steeper slope than what we thought. That applies for high frequency attenuation too!

You SAVE MONEY on spending 1 less component, either an expensive volume pot, or an expensive preamp OPT! What's saved can be used to get an ultimate OPTVC and you actually have lesser coloration since component count and signal path are significantly shorter/simpler/lesser! 

Copper OPTVC (Output Transformer Volume Control)

  • Primary: 5K (other values, please ask)
  • Primary DC: 10mA (standard, other values available)
  • Secondary: user defined impedance (600 Ohm usually as preamp)
  • Matched pairs, potted - must 
    • 23 steps U$690 OFC
    • 23 steps U$890 OCC (Ohno Continuous Cast)
    • 33 steps U$990 OFC
    • 33 steps U$1090 OCC  

Silver OPTVC (Output Transformer Volume Control)

  • Primary: 5K (other values, please ask)
  • Primary DC: 10mA (standard, other values available)
  • Secondary: user defined impedance (600 Ohm usually as preamp) 
  • Matched pairs, potted - must 
    • 23 steps U$1090 4N pure silver wires
    • 33 steps: U$1390 4N pure silver wires

If more steps are needed (49, etc) please email us. 

Order a pair of OPTVC now, and experience the magic of simplicity!

Hear the unheard! 
10/18/2016
J&K Audio Design

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